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Functional Organization of Motoneuron Pool and its Inputs

Elwood Henneman, Lorne M. Mendell

10.1002/cphy.cp010211

Source: Supplement 2: Handbook of Physiology, The Nervous System, Motor Control

Originally published: 1981

Published online: January 2011

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 Morphological Considerations
1.1 Columnar Arrangement of Motoneuron Pool
1.2 Dimensions of α‐Motoneurons and Distribution of Cell Size
1.3 Scaling of Motoneurons
1.4 Initial Segment of α‐Motor Axons
1.5 Axon Collaterals of α‐Motoneurons
1.6 Recurrent Inhibitory Feedback from Motoneurons
1.7 Direct Synaptic Interconnections Between Spinal Motoneurons
1.8 Species of Motoneurons
1.9 Terminals of Motoneurons in Muscle
1.10 Morphology of Neuromuscular Junctions
1.11 Matching the Properties of Motoneurons and the Muscle Fibers They Supply
1.12 Concluding Comments
2 Firing Patterns of Individual Motoneurons and Motor Units
2.1 Functional Significance of Size of Motoneurons
2.2 Measurement of Total Output of Motoneuron Pools
2.3 Critical Firing Levels of Motoneurons
2.4 Relation of Critical Firing Level to Axon Diameter and Motoneuron Size
2.5 Effects of Inhibitory Inputs on Critical Firing Level and Rank Order During Repetitive Firing
2.6 Recruitment of Motor Units in Humans
2.7 Evidence Regarding Alternative Patterns of Recruitment
2.8 Evidence Regarding Voluntary Selective Control of Motor Units
2.9 Size Principle in Other Species
2.10 Modulation of Firing Rate
3 Organization of Input to Motoneuron Pools
3.1 Anatomical Studies
3.2 Techniques Used to Study EPSPs Elicited by Impulses in Single Afferent Fibers
3.3 Amplitudes of EPSPs Elicited by Impulses in Single Fibers
3.4 Boutons of Ia‐Fibers on Motoneurons
3.5 Physiology of Ia‐Terminals
3.6 Distribution of Ia Excitation to Motoneuron Pools
3.7 Comparison of Projections to Homonymous and Heteronymous Motoneurons
3.8 Correlations Between Morphology and Function
3.9 Latency of EPSPs
3.10 Other Examples of Divergence in Inputs to Motoneurons
3.11 Ia‐Projections to Motoneurons Controlling Other Parts of the Body
3.12 Group II Input From Secondary Endings in Muscle Spindles
3.13 Inhibitory Inputs to Motoneurons
3.14 Group Ib Input From Golgi Tendon Organs
3.15 Monosynaptic Input From Descending Pathways
3.16 Topographic Factors Governing Development of Connections of Ia‐Fibers to Motoneurons
3.17 Concluding Comments
4 Nonuniformity of Motoneurons
4.1 Early Classification of Tonic and Phasic Types of Motoneurons
4.2 Significance of Nonuniformity of Muscle Fibers
4.3 Motoneuron Properties Independent of Size
4.4 Differential Responses of Motoneurons to Injected Currents
4.5 Influence of Muscle on Developing and Mature Motoneurons
4.6 Evidence From Human Disease
4.7 Concluding Comments
5 How Size of Motoneurons Determines their Susceptibility to Discharge
5.1 Properties of Motoneurons That Influence Susceptibility to Discharge
5.2 Role of Input in Determining Susceptibility to Discharge
6 Some Principles Underlying Organization of Motoneuron Pools
6.1 How Sensitivity in Gradation of Tension is Achieved
6.2 Basis for Relation Between Motoneuron Size and the Force Its Motor Unit Develops
6.3 Actual Sensitivity in Grading Muscular Tension
6.4 Mathematical Derivation of a “Principle of Maximum Grading Sensitivity”
6.5 Recruitment Order and Minimum Energy Principle
6.6 Collective Action of Motoneuron Pool: Role of Input
6.7 The Size Principle in Ia and Group II Sensory Fibers
6.8 How Does the Central Nervous System Use the Motoneuron Pool?
Figure 1. Figure 1.

Reconstructions of medial gastrocnemius (MG) and soleus (SOL) nuclei from serial sagittal sections in cat. Right, dorsal view of spinal cord outline (white matter–pia boundary in heavy lines) on which are superimposed positions (dots) of MG (left hemicord) and SOL motoneuron cell bodies (right hemicord). Boundaries between L6, L7, and S1 segments (identified by dorsal root entry zones), are indicated by horizontal heavy lines; midline denoted by vertical dashed line. Dashed lines across the cord denote levels (labeled A–E) at which reconstructions of cross sections were made. Left, reconstructions of cross sections at levels A–E showing white matter–pia boundary in light lines and gray‐white matter boundary in heavy lines. Dashed lines at most lateral parts of cross sections indicate estimated outline; the most lateral parts were lost in sectioning. All diagrams drawn on same scale. Neurons indicated on each cross‐section diagram are cells located within 300 μm rostral and caudal to that level.

From Burke et al. 47
Figure 2. Figure 2.

Distribution of motoneuron sizes in the 7th cervical segment of 6 normal rhesus monkeys.

Adapted from Hodes et al. 161
Figure 3. Figure 3.

A: diameters of sciatic α‐motor axons of cat at initial segment (IS) are plotted against mean diameters of their parent cell bodies (n = 68). Linear correlation coefficient (r) = +0.522 (P < 0.001). B: mean diameters of sciatic α‐motor axons in white matter are plotted against mean diameters of their parent cell bodies (n = 42). Linear correlation coefficient (r) = +0.516 (P < 0.001).

From Cullheim 80
Figure 4. Figure 4.

Axon conduction velocities in cat are plotted in A against size of parent cell bodies (n = 24), in B against axon diameters at initial segment IS (n = 24), and in C against axon diameters in white matter (n = 33). In A the linear correlation coefficient (r) = +0.673 (P < 0.001); in B, r = +0.869 (P < 0.001); and in C, r = +0.804 (P < 0.001).

From Cullheim 80
Figure 5. Figure 5.

Relation between axonal diameters in spinal cord white matter and diameters of initial segments (IS) for 39 sciatic α‐motor axons in cat. Linear correlation coefficient (r) = +0.698 (P < 0.0001).

From Cullheim and Kellerth 82
Figure 6. Figure 6.

Relation between maximum tetanic tension of individual motor units in cat soleus muscle and conduction velocity of their axons in 3 different experiments.

From McPhedran, Wuerker, and Henneman 253
Figure 7. Figure 7.

Relation between logarithm of tetanic tensions developed by motor units and the conduction velocities of their motor axons in 4 leg muscles of the cat. Heavy dots on vertical bars indicate mean value of tension ± SD. For peroneus longus and tibialis anterior, intervals corresponding to conduction velocities under 85 m/s contained too few motor units to allow valid calculation of mean tension. Range of tensions developed by these motor units are represented by a vertical bar. Open circle represents tension of a single unit. For soleus, 4 motor units innervated by axons, either faster than 90 m/s or slower than 60 m/s. are not represented.

From Jami and Petit 185
Figure 8. Figure 8.

Distribution of maximal tetanic tensions of motor units of cat's soleus (A) and medial gastrocnemius (B).

A, from McPhedran, Wuerker, and Henneman 253; B from Wuerker, McPhedran, and Henneman 353
Figure 9. Figure 9.

Smoothed version of histogram in Figure 8B, illustrating distribution of some of the properties of motor units in typical limb muscles. Height of columns indicates relative numbers of motor units with the properties listed below.

From Henneman 154. Skeletal muscle: the servant of the nervous system. In: Medical Physiology (14th ed.), edited by V. B. Mountcastle. St. Louis, MO: The C. V. Mosby Co., 1980
Figure 10. Figure 10.

Reflex discharges recorded from a filament of 7th lumbar ventral root in response to stimulation of ipsilateral sciatic nerve of cat. Numbers at right of each tracing indicate relative intensity of stimulation. Initial deflection on left of each tracing is the early discharge referred to in text.

From Henneman 153
Figure 11. Figure 11.

Stretch‐evoked responses of 2 α‐motoneurons recorded from a filament of 7th lumbar ventral root of cat. Amount of tension applied and developed reflexly is indicated by separation of 2 top beams in each frame. Several seconds elapsed between successive frames while muscle was stretched 1,2,3,4,5 and released 6,7,8,9.

From Henneman et al. 159
Figure 12. Figure 12.

Stretch‐evoked responses of 5 α‐motoneurons recorded from a filament of the 1st sacral ventral root of cat. Numbers above action potentials indicate rank of units according to size.

From Henneman et al. 159
Figure 13. Figure 13.

Frequency distribution of thresholds of tonic responses to stretch in 3 experiments on cats. Abscissa, threshold tension of deefferented triceps muscle. Ordinate, number of units whose thresholds fell between values indicated on abscissa.

From Henneman et al. 159
Figure 14. Figure 14.

A: orderly recruitment in cat of 3 triceps motoneurons of different sizes in response to increasing degrees of stretch. Tension produced by stretch of triceps muscle indicated by separation of top traces in each frame. B: orderly inhibition of same 3 units during a constant stretch of 4 kg. Records 1–4 on left show control responses of stretch before, between and after 3 inhibitory stimulations. Records on right were obtained during 100/s stimulation of ipsilateral deep peroneal nerve at 3 intensities. Largest unit was silenced first (line 1), intermediate unit next (line 2), and smallest unit last (line 3). Note lasting effects produced by brief inhibitions, i.e., failure to recover to previous levels in lines 2 and 4.

From Henneman et al. 160
Figure 15. Figure 15.

Graphic representation of discharge frequency of 3 motoneurons of different sizes (A–C) in triceps surae of cat, and of contractile tension developed by the muscle itself (D) in response to varying degrees of excitation and inhibition: x‐axis, intensity of inhibitory stimulation applied to ipsilateral deep peroneal nerve; y‐axis, frequency of discharge in A–C and contractile tension in D; z‐axis, stretch of triceps surae, in millimeters. Data for all 4 graphs obtained simultaneously. Cells were subjected simultaneously to various mixtures of excitatory (z‐axis) and inhibitory (x‐axis) stimuli. Each plotted point represents mean of 2 successive determinations. Inhibitory stimuli were 100/s shocks applied for 0.9‐s periods at 4‐s intervals. Note that unit A (smallest) fired spontaneously without stretch, unit B began to discharge between 0 and 5 mm of stretch, and unit C (the largest) between 5 and 10 mm.

From Henneman et al. 160
Figure 16. Figure 16.

Scheme of experiment used to measure maximal monosynaptic discharge of a motoneuron pool. DR, dorsal root; VR, ventral root.

From Clamann, Henneman, et al. 59
Figure 17. Figure 17.

Pairs of responses in cat recorded as in Figure 16 from proximal and distal halves of 1st sacral ventral root in response to single‐shock stimulation of combined nerves to medial and lateral gastrocnemius (MG and LG). Left and right deflections of each pair are antidromic and reflex responses, respectively, displayed on superimposed traces of a 2‐beam oscilloscope. First 10 pairs in top row recorded before 12‐s tetanus (500 shocks/s) to MG and LG nerves. Subsequent pairs recorded at 2‐s intervals after tetanus.

From Clamann, Henneman, et al. 59
Figure 18. Figure 18.

Simultaneous recordings in cat of a series of monosynaptic reflexes of triceps surae pool (top) and a single triceps motoneuron (bottom) showing critical firing level of the latter. Top trace, time integrals of monosynaptic reflexes recorded at 1/s from proximal half of 1st sacral ventral root. Height of each vertical line measures size of population response. Bottom trace, monosynaptic reflexes of a single triceps motoneuron recorded from a small filament of 7th lumbar ventral root. Records were made during decline in posttetanic potentiation following a brief train of conditioning shocks that were applied to the combined medial gastrocnemius and lateral gastrocnemius and soleus nerves at 500/s.

From Henneman et al. 155
Figure 19. Figure 19.

Critical firing levels of 3 triceps motoneurons in cat. Single unit in bottom left had a sharp threshold at 39%. The 2 units in bottom right tracing had thresholds of 60% and 33%. Plateau in top left record indicates 100% discharge level of triceps pool.

From Henneman et al. 155
Figure 20. Figure 20.

Susceptibilities of 2 pairs of cat plantaris motoneurons with known critical firing levels (CFLs) to repetitive discharge in cat. Electrical stimulation of plantaris nerve at 300 stimuli/s in both cases. Stimulus intensity (i.e., voltage) indicated by level of trace a with reference to base line d. Responses of plantaris units recorded on traces b and c. CFLs indicated at right end of these traces. Time marks at 1‐s intervals for both top and bottom records.

From Henneman et al. 155
Figure 21. Figure 21.

Differences in susceptibility to repetitive firing of 2 cat plantaris motoneurons whose critical firing levels were 12.0% (top) and 15.5% (bottom). A: shocks applied to plantaris nerve were supramaximal for all group Ia‐fibers. Frequency was switched from 20 shocks/s initially to 30 shocks/s at 1st arrow, and to 40 shocks/s at 2nd arrow. B: frequency of stimulation was 500 shocks/s throughout and intensity was increased gradually as trace moved from left to right.

From Henneman et al. 155
Figure 22. Figure 22.

Relation between axon size and critical firing level (rank order) of 21 cat plantaris motoneurons isolated in a single experiment.

From Clamann and Henneman 60
Figure 23. Figure 23.

Critical firing level (CFL) of a cat plantaris motoneuron with and without inhibitory input from lateral popliteal nerve. Top, time integrals of monosynaptic reflexes of plantaris population elicited at 2‐s intervals during declining phase of posttetanic potentiation. Bottom, simultaneous responses of a plantaris unit with a critical firing level of 61%. Alternate reflexes preceded by a 200‐ms train of pulses to lateral popliteal nerve. Horizontal line indicating critical firing level drawn as described in text.

From Clamann, Gillies, and Henneman 58
Figure 24. Figure 24.

Effects of recurrent inhibition (A) and lateral popliteal inhibition (B) in cat on monosynaptic reflexes (traces 1, 2) and repetitive firing (traces 3, 4) of a pair of plantaris motoneurons with critical firing levels of 18.6% (top) and 18.4% (bottom). A: in traces 1, 2 the underlined reflexes were preceded and accompanied by a 100‐ms train of shocks (100/s) to proximal portion of 7th lumbar ventral root. Antidromic volleys set up by stimulation in A1, A2 resulted in recurrent inhibition mediated by Renshaw cells. Of the 18 reflexes in trace 1, 9 were inhibited by this input; in trace 2 only 1 reflex was silenced. In A3, A4 the same pair of units is seen responding rhythmically to repetitive stimulation of the plantaris nerve. When continuous recurrent inhibition was begun, indicated by thick base line in A4, top unit ceased firing and bottom unit slowed to about one‐half original rate of discharge. B: same pair of units was subjected to inhibitory input elicited by stimulating lateral popliteal nerve at 100/s. In B1, B2 the underlined monosynaptic reflexes were somewhat more susceptible to this inhibition than to the recurrent inhibition. Of the 16 reflexes receiving this inhibition, 15 in trace 1 and 4 in trace 2 were silenced. In B3, B4 the same pair of units firing rhythmically in response to 100/s stimulation of the plantaris nerve was subjected to an inhibitory input whose intensity was gradually increased as the traces moved across the oscilloscope. The top unit was finally suppressed by this input whereas the bottom unit was only slowed in rate.

Clamann, Gillies, and Henneman 58
Figure 25. Figure 25.

Records illustrating quantitative relationships between intensity of excitatory stimulus causing 2 cat motoneurons with critical firing levels of 18.8% and 12.7% to discharge repetitively and intensity of inhibitory stimulus required to silence them. Stimulus strength of excitatory input (100/s stimulation of a plantaris nerve), indicated by level of line E with reference to base line B, was constant during each test but varied in different tests. Stimulus strength of inhibitory input (100/s stimulation of lateral popliteal nerve), indicated by line I, was increased as trace moved across screen. Intensity of inhibition required to silence each unit was measured at time of their last discharges, indicated by dashed lines. Time signals at top of record represent 1‐s intervals.

From Clamann, Gillies, and Henneman 58
Figure 26. Figure 26.

Graph showing linear relation between strength of excitatory stimuli causing each of 2 cat motoneurons to discharge repetitively and strength of inhibitory stimuli required to silence them. Data obtained as illustrated in Figure 25. Data for lower unit, with critical firing level of 12.7%, are plotted with filled circles and solid line; data for upper unit, with a critical firing level of 18.8%, are represented by open circles and dashed line.

From Clamann, Gillies, and Henneman 58
Figure 27. Figure 27.

Number of human motor units having the twitch tensions indicated. A: linear scale. B: logarithmic scale. Distributions are similar for all 3 subjects. The computed best‐fitting line on the semilog plot (B) indicates an approximately exponential relation between number of motor units and twitch tension.

From Milner‐Brown et al. 258
Figure 28. Figure 28.

A: firing pattern of a high‐threshold unit recorded from human extensor indicis during isometric contractions with different rates of rise of tension (A–D). Each registration shows (top to bottom) the spike record, isometric force, and tracking signal which was visually displayed to the subject. Time scale in bottom 2 records is one‐half that of top 2 records. During increasingly faster contractions, unit starts firing at successively lower force levels. B: dependence of time between beginning of muscle contraction and firing onset of 3 motor units on rate of rise of tension. Double logarithmic scale. Faster contraction causes earlier recruitment. C: change of threshold force of recruitment (ordinate) associated with variation of rate of rise of isometric tension. Recordings from 7 motor units of extensor indicis of 3 subjects (▪, ▴ •). D: regression line of change of threshold force of recruitment on rate of rise of isometric tension (y1 = −0.067x + 198.66) calculated from 12 motor units of extensor indicis. The increase of muscle force that occurs within the mean contraction time of the muscle is shown for comparison (y2 = 0.060x).

From Büdingen and Freund 30
Figure 29. Figure 29.

Records of EMG activity and force outputs of medial gastrocnemius (top 2 traces) and soleus (bottom 2 traces) muscles of cat, together with simultaneous record of activity of a small number of medial gastrocnemius (MG) motor axons in a natural filament of MG muscle nerve (3rd trace). During MG vibration (160 Hz, 90 μm) activity in whole MG increased and that in soleus (SOL) showed a smaller increase. Pinching the skin over the lateral ankle with toothed forceps produced 2 bursts of MG activity, with recruitment of large amplitude motor axons in the MG filament, but caused quite complete suppression of SOL activity. Isolated activity of 5 identifiable MG motoneuron axons is shown in A–E. Note slowing of unit A discharge during each burst of activity in the higher threshold units B–E.

From Kanda et al. 192
Figure 30. Figure 30.

Simultaneous recording of one continuously firing long‐interval motor unit and one intermittently firing short‐interval motor unit in human. A: increasing contraction. B: prolonged maximum effort. C, D: rapid twitches. Time bar, 100 ms.

From Grimby and Hannerz 139
Figure 31. Figure 31.

Properties of a small (A), medium (B), and large (C) motor neuron of lobster innervating the main power stroke muscle. 1, Soma diameters; 2, axonal conduction velocities; 3, amplitudes of action potentials recorded with 2 extracellular electrodes at different positions on the motor nerve; 4, amplitudes of excitatory junctional potentials (EJPs) recorded from a single muscle fiber (top trace in each record); 5, adaptation to a maintained intracellular depolarizing current; and 6, facilitation properties of extracellular EJPs (top trace in each record) during 50‐Hz stimulation. Note antifacilitation produced by the largest motoneuron. Time marks in row 5 (lowest trace), 0.1 ms.

From Davis 85
Figure 32. Figure 32.

Morphology of Ia‐collaterals to motoneurons and arrangements of boutons on dendrites. A: sagittal section through lumbosacral cord of 40‐day‐old kitten (section shown at right). Primary fibers in dorsal columns (a) send ventrally directed branches to ventral horn (b) to terminate in region of motoneurons (c and d) that have sagitally running dendrites. Primary afferent branches also give off branches to dorsal horn and intermediate zone; these are described in more detail in the original illustration of Scheibel and Scheibel 310 from which this figure is modified. B: scale drawing illustrating morphology of collaterals of a single triceps surae Ia‐fiber as revealed by horseradish peroxidase (HRP) filling. This 3‐dimensional representation provides both a cross‐sectional (transverse) and sagittal perspective. The Ia‐fiber in the dorsal columns is drawn as a thick black line along with the dorsal root ganglion cell and dorsal root portion of the cell. Four collaterals emanating from the dorsal column region are illustrated, each one coursing through the dorsal horn (in a ventrallateral and rostral direction) to terminate in the cylindrical motoneuron region (lamina IX). Other terminals are given off by the same fiber in laminae VI and VII. C: morphology of a single soleus Ia‐fiber in the dorsal columns with 9 branches given off to ventral horn; HRP technique. Two motoneurons—soleus (SOL), which innervates type S motor unit, and lateral gastrocnemius (LG), which innervates a type F, fatigue‐resistant motor unit (R)—were also filled with HRP. Circles and stippling denote somata and maximum dendritic extents of these neurons. Note that at most only 2 of 9 Ia‐fiber branches have access to each of these motoneurons. Also ventrally directed collaterals of this fiber do not exhibit rostral component shown by Brown and Fyffe (Fig. 32B). Top and bottom solid lines denote dorsal and ventral borders of cord. Dotted lines represent dorsal and ventral limits of ventral horn. D: origins of primary collaterals of MG, LG‐S, and posterior tibial (PT) Ia‐fibers. E: schematic drawing of disposition of Ia‐fiber boutons on α‐motoneurons. In each case, both Ia‐fiber and motoneuron filled with HRP. Note in SOL motoneuron that bouton A is on proximal dendrites in contrast to B‐F, which are on distal dendrites. Terminals on LG‐type FR motoneuron are of en passant variety (triplet) and of terminal variety.

A, adapted from Scheibel and Scheibel 310; B and D, from Brown and Fyffe 26; C and E, from Burke et al. 51
Figure 33. Figure 33.

A: arrangement for spike‐triggered averaging. Muscle‐stretch activated group Ia impulse recorded from dorsal rootlet (a) is used to trigger an averaging computer, using a pulse‐height analyzer to select impulses of constant height produced by a single fiber. Each trigger pulse causes averager to process the next several milliseconds of signal recorded from the microelectrode impaling the motoneuron. Motoneuron is identified by antidromic invasion following muscle nerve stimulation (b). If Ia‐fiber acting as source of trigger signal sends terminals to motoneuron, then an EPSP is averaged. B: examples of single 20‐ms records (sweeps) from a medial gastrocnemius (MG) motoneuron triggered by an MG group Ia impulse. The EPSP is seen at start of sweep superimposed on highly variable synaptic activity. Note variability in amplitude of Ia‐evoked EPSP. C–F: 4, 16, 64, and 256 sweeps are averaged, respectively. Note improvement in resolution of EPSP as number of sweeps averaged is increased. Only potentials synchronized with the MG afferent are preserved. Others are asynchronous with respect to the trigger signal and so are averaged out. G, H: dorsal root recording (G) and simultaneous intracellular recording from motoneuron (H) at slower sweep speed. Note 2 Ia impulses in G and synaptic noise in H. Calibration, 500 μV for B–F, and H; 0.7 ms for B–F; 5 ms for H.

A, adapted from Mendell and Henneman 255
Figure 34. Figure 34.

Amplitude histograms of single‐fiber EPSPs in homonymous and heteronymous motoneurons. A: histogram obtained using electrical stimulation method. B: histogram with spike‐triggered averaging method. Arrows represent values of less than 1% of N. Afferents and motoneurons are mainly from triceps surae and semitendinosus muscle groups in A and exclusively so in B.

A, from Jack et al. 82; B, from data of Mendell and Henneman 255, Nelson and Mendell 269,270, Scott and Mendell 315
Figure 35. Figure 35.

Plot of mean amplitudes of postsynaptic population potentials (PSPPs) elicited by afferent impulses with mean condition velocities shown on abscissa. The PSPPs were evoked by impulses in single group Ia and II fibers of cat medial gastrocnemius muscle, using spike‐triggered averaging. Responses recorded from 1st sacral ventral root and lower part of 7th lumbar ventral root. The 153 data points were grouped in pentads of conduction velocity and mean and standard deviations of PSPPs for each group are shown. Mean data points fell into 2 groups, each of which was fitted with a straight line by method of least squares, using point at 72.5 m/s for both calculations.

From Lüscher, Henneman, et al. 244
Figure 36. Figure 36.

Dependence of amplitude of single‐fiber EPSPs on size of motoneuron in cat. Top, relationship between EPSP amplitude (obtained by spike‐triggered averaging) and motor axon conduction velocity determined for 4 separate group Ia afferent fibers. Axonal conduction velocity determined by antidromic stimulation. Medial gastrocnemius (MG) Ia‐fibers and motoneurons in each case. Note inverse relationship. Bottom, each point represents median value of amplitude distribution of a presumed single‐fiber EPSP evoked in a motoneuron during stretch of triceps surae. The EPSPs are not restricted to those generated by Ia‐fibers. Many of these EPSPs are probably produced by interneurons. It is assumed that variation of these EPSPs with motoneuron input resistance as shown would also occur in the subpopulation of Ia‐evoked EPSPs. Small cells with large input resistance 203 generate larger EPSPs. ○, Gastrocnemius motoneurons supplying type F (rapidly contracting) motor units; ⊕, MG motoneurons supplying type S (slowly contracting) motor units; and •, soleus motoneurons.

Top from Mendell and Henneman 255; bottom from Burke 36
Figure 37. Figure 37.

A: EPSP shapes computed for input restricted to specific regions of the dendritic tree of cat. All EPSPs assumed to be recorded from compartment 1 (the soma) of a 10‐compartment motoneuron. Compartments 2–10 represent dendritic regions of equal electrotonic length but increasing distance from the soma. Top, time course of the conductance transient generating EPSPs is given by dotted line; a is EPSP expected for a transient delivered to all compartments simultaneously. Bottom, b–d are EPSPs expected for transients delivered to compartments 1, 4, and 8, respectively. Ordinate is in arbitrary units; to produce EPSPs with equal peak amplitudes as shown, intensity of conductance changes for d > c > b > a. Note differences in time course of EPSPs, and for b–d, note differences in time of their onset. B: end plate potentials at different locations in same muscle fiber (frog sartorius in vitro). Microelectrode was inserted sequentially into curarized fiber and end plate potential in response to motor nerve stimulation was measured. Numbers at left of each trace represent distance in millimeters from end plate focus. S is the stimulus artifact. Time is in ms.

A, from Rall 287; B, from Fatt and Katz 112
Figure 38. Figure 38.

A: single‐fiber excitatory postsynaptic potentials (EPSPs) obtained using spike‐triggered averaging produced by the same medial gastrocnemius (MG) Ia‐afferent fiber in 6 different MG motoneurons in cat. Note different shapes of the 6 EPSPs. Calibration pulse at the end of each sweep is 200 μV. B: single‐fiber EPSPs (by spike‐triggered averaging) produced in the same LG motoneuron by an LG Ia‐fiber (top) and a soleus Ia‐fiber (bottom). Calibration pulse is 100 μV, 1 ms. Note differences in shape of these EPSPs. C: example of a single fiber EPSP (MG Ia‐fiber and MG motoneuron) with compound shape (spike‐triggered averaging). Calibration is 1 ms, 50 μV.

A, from Mendell and Henneman 255; B, from Scott and Mendell 315; C, from Mendell and Henneman 255
Figure 39. Figure 39.

A: plot of ratio of excitatory postsynaptic potential (EPSP) amplitudes against ratio of their rise times (RT2/RT1). Each point was obtained by consideration of 2 averaged single‐fiber EPSPs (spike‐triggered averaging) evoked by different Ia‐fibers in the same motoneuron. Medial gastrocnemius (MG) Ia‐fibers and MG motoneurons throughout. By definition RT1 < RT2 so that RT2/RT1> 1. Furthermore , the ratio of the amplitudes, is defined so that amplitude of faster rising EPSP is in numerator. Majority of points have suggesting a tendency for faster rising EPSPs to be larger than slower rising ones. Correlation between and RT2/RT1 is very weak. B: plot of single‐fiber EPSP amplitude (obtained by electrical stimulation) against distance (in units of space constant δ) between the terminals and the electrode (assumed to be in soma). The EPSPs are chiefly from hamstring and triceps surae motoneurons with both homonymous and heteronymous combinations.

A, from Mendell and Weiner 256; B, from Iansek and Redman 176
Figure 40. Figure 40.

Model of projection of single Ia‐fiber to homonymous and heteronymous motoneurons (circles). Systematic differences in mean number of boutons provided by single Ia‐fibers to homonymous and heteronymous motoneurons would lead to differences in projection frequency and excitatory postsynaptic potential (EPSP) amplitude in these 2 types of projections. Mean bouton number was chosen using estimates of Iles 177 as a guide. Numbers below each circle represent number of boutons provided by single Ia‐fiber to each motoneuron.
Figure 41. Figure 41.

Scatter diagram of relationship between rise time and latency for single‐fiber excitatory postsynaptic potentials (EPSPs) in 78 medial gastrocnemius (MG) and 23 lateral gastrocnemius and soleus (LG‐SOL) motoneurons. MG Ia‐afferent fibers throughout. Dashed vertical line at 0.27 ms denotes minimum latency recorded in heteronymous LG‐SOL motoneurons. Inset shows intracellular (A), extracellular (C) records from a motoneuron with spike‐triggered averaging. Note reversal of EPSP, but not diphasic prespike following withdrawal of electrode. Calibration pulses 50 μ V, 1 ms. B is same as A with both scales expanded by 4. Latency was measured from negative (downward going) peak of prespike to onset of EPSP.

From Munson and Sypert 263
Figure 42. Figure 42.

Comparison of synaptic delay associated with single‐fiber excitatory postsynaptic potentials (EPSPs) produced by group Ia and group II fibers in cat. Left, trace A represents potential recorded inside a medial gastrocnemius (MG) motoneuron with spike‐triggered averaging from a group II afferent fiber. The large upward potential is EPSP, which is preceded by a brief diphasic potential considered to arise from activity in presynaptic terminals of group II fiber. Trace B is record obtained by spike‐triggered averaging from a position just outside motoneuron. The early potential is not inverted, indicating that it is produced outside motoneuron and unlike EPSP, which reverses, showing that it is generated across the membrane. Trace C is averaged record recorded from group II fiber in dorsal rootlet. Interval from dorsal root spike to presynaptic spike is conduction time (CT) and remaining time to EPSP onset is synaptic delay (SD). Right, histograms of conduction time and synaptic delay for group II fibers (top) and group Ia‐fibers (bottom). Note similarities in synaptic delay for both types of fiber and trend for shorter conduction time in (the larger) group‐Ia fibers.

From Stauffer et al. 328
Figure 43. Figure 43.

Inhibitory postsynaptic potential (IPSP) in a cat motoneuron analyzed by spike‐triggered averaging. A: top trace is intracellular record from a posterior biceps or semitendinosus motoneuron. Bottom trace is extracellular spike activity from an interneuron satisfying the criteria required for a Ia‐interneuron. Interneuron is excited by iontophoretically applied glutamate and discharges rhythmically. Each spike is associated with an IPSP (overlaid by row of dots). B: averages of 512 sweeps triġgered by interneuronal spike from motoneuron (above) and interneuron (below). Trigger level was set so that averaging began at beginning of interneuronal spike.

From Jankowska and Roberts 189
Figure 44. Figure 44.

Influence of entry level of Ia‐fiber of cat on amplitude of postsynaptic population potentials (PSPPs) its impulses elicit.

Data are from a single experiment. Schematic drawings in top of figure illustrate recording arrangements on ventral roots (VR); CV represents conduction velocity. Arrow points at spinal segment that the afferent fiber enters. The PSPPs recorded from caudal part of 7th lumbar ventral root (L7VR) and 1st sacral ventral root 1 and 1st sacral (S1 VR) 2 are reproduced below. A: Ia‐fiber arising in medial gastrocnemius (MG) enters through S1 dorsal root. B: Ia‐fiber from MG enters spinal segment L7. C: Ia‐fiber from lateral gastrocnemius (LG) enters spinal segment L7. Each PSPP was averaged 4,096 sweeps. From Lüscher, Ruenzel, and Henneman 246
Figure 45. Figure 45.

Three‐dimensional graph illustrating relationship between amplitude of postsynaptic population potentials (PSPPs) and conduction velocity and spinal entry point of afferent impulses evoking them. Amplitudes were averaged over 4,096 sweeps and were recorded from 1st sacral ventral root. Data were derived from a single experiment. All afferent fibers had origin in medial gastrocnemius muscle.

From Lüscher, Ruenzel, and Henneman 246
Figure 46. Figure 46.

Maximal firing rates of 4 plantaris motoneurons of cat with approximately equal critical firing levels recorded simultaneously during first 940 ms of their response to electrical stimulation of plantaris nerve. Stimulus parameters adjusted to produce maximal firing rates. Bottom unit fires significantly slower than top 3 units, giving impression of having been sampled from a different population than the others.

From Harris and Henneman 145
Figure 47. Figure 47.

Firing rates (averaged over first second) of 2 pairs of units of cat with similar critical firing levels, compared simultaneously. Data obtained by applying a wide range of stimulus intensities and frequencies to plantaris nerve. A: for each pair of units firing rate (FR) of 1 unit is plotted versus FR of other unit. Dashed lines indicate linear regression. B: ratio of FRs of each pair plotted as a function of the rate of 1 of the units expressed as a percentage of its maximum rate. Bottom pair (open circles) had very similar FRs (ratio ≃ 1.0); top pair (closed circles) had widely differing FRs (ratio > 2.0). Linear regression lines with slopes near zero indicate ratio of FRs remained constant for all these inputs.

From Harris and Henneman 145
Figure 48. Figure 48.

Examples of aggregate excitatory postsynaptic potentials (EPSPs) recorded from 3 medial gastrocnemius motoneurons of cat to illustrate effects of posttetanic potentiation on cells of different input resistance (IR) and size. Bottom traces illustrate that average size of aggregate EPSPs before potentiation varied directly with IR of motoneuron. Top traces are averages of 16 EPSPs following tetanus. Percent potentiations in A–C were 123, 92, and 11, respectively.

From Lüscher, Ruenzel, and Henneman 245
Figure 49. Figure 49.

Relation between input resistance and amplitude of aggregate excitatory postsynaptic potentials (EPSPs) before (A) and during (B) posttetanic potentiation in cat. Circles, medial gastrocnemius motoneurons; triangles, semitendinosus motoneurons.

From Lüscher, Ruenzel, and Henneman 245
Figure 50. Figure 50.

Plot of changes in rise times of excitatory postsynaptic potentials (EPSPs) versus changes in their half‐widths following a tetanus to cat muscle nerve at 500/s for 10 s. Arrows indicate directions of change.

From Lüscher, Ruenzel, and Henneman 245
Figure 51. Figure 51.

Some features of a motoneuron pool. Heights of vertical lines represent sizes of motoneurons. Their spacing indicates relative numbers of cells of different sizes. Horizontal line below base line shows combination of motoneurons firing in response to a given input. Largest cell discharged denoted by X.

From Henneman 361. Organization of the motoneuron pool: the size principle. In: Medical Physiology (14th ed.), edited by V. B. Mountcastle. St. Louis, MO: The C. V. Mosby Co., 1980
Figure 52. Figure 52.

Concomitant changes in mean firing rates of units i and j (fi and fj) during slow volitional changes in an isometric contraction in human. The fi‐fj relationship of each pair is approximated by 1 or 2 straight (dashed) lines. The same i unit was used in pairs 5–8.

From Monster and Chan 261


Figure 1.

Reconstructions of medial gastrocnemius (MG) and soleus (SOL) nuclei from serial sagittal sections in cat. Right, dorsal view of spinal cord outline (white matter–pia boundary in heavy lines) on which are superimposed positions (dots) of MG (left hemicord) and SOL motoneuron cell bodies (right hemicord). Boundaries between L6, L7, and S1 segments (identified by dorsal root entry zones), are indicated by horizontal heavy lines; midline denoted by vertical dashed line. Dashed lines across the cord denote levels (labeled A–E) at which reconstructions of cross sections were made. Left, reconstructions of cross sections at levels A–E showing white matter–pia boundary in light lines and gray‐white matter boundary in heavy lines. Dashed lines at most lateral parts of cross sections indicate estimated outline; the most lateral parts were lost in sectioning. All diagrams drawn on same scale. Neurons indicated on each cross‐section diagram are cells located within 300 μm rostral and caudal to that level.

From Burke et al. 47


Figure 2.

Distribution of motoneuron sizes in the 7th cervical segment of 6 normal rhesus monkeys.

Adapted from Hodes et al. 161


Figure 3.

A: diameters of sciatic α‐motor axons of cat at initial segment (IS) are plotted against mean diameters of their parent cell bodies (n = 68). Linear correlation coefficient (r) = +0.522 (P < 0.001). B: mean diameters of sciatic α‐motor axons in white matter are plotted against mean diameters of their parent cell bodies (n = 42). Linear correlation coefficient (r) = +0.516 (P < 0.001).

From Cullheim 80


Figure 4.

Axon conduction velocities in cat are plotted in A against size of parent cell bodies (n = 24), in B against axon diameters at initial segment IS (n = 24), and in C against axon diameters in white matter (n = 33). In A the linear correlation coefficient (r) = +0.673 (P < 0.001); in B, r = +0.869 (P < 0.001); and in C, r = +0.804 (P < 0.001).

From Cullheim 80


Figure 5.

Relation between axonal diameters in spinal cord white matter and diameters of initial segments (IS) for 39 sciatic α‐motor axons in cat. Linear correlation coefficient (r) = +0.698 (P < 0.0001).

From Cullheim and Kellerth 82


Figure 6.

Relation between maximum tetanic tension of individual motor units in cat soleus muscle and conduction velocity of their axons in 3 different experiments.

From McPhedran, Wuerker, and Henneman 253


Figure 7.

Relation between logarithm of tetanic tensions developed by motor units and the conduction velocities of their motor axons in 4 leg muscles of the cat. Heavy dots on vertical bars indicate mean value of tension ± SD. For peroneus longus and tibialis anterior, intervals corresponding to conduction velocities under 85 m/s contained too few motor units to allow valid calculation of mean tension. Range of tensions developed by these motor units are represented by a vertical bar. Open circle represents tension of a single unit. For soleus, 4 motor units innervated by axons, either faster than 90 m/s or slower than 60 m/s. are not represented.

From Jami and Petit 185


Figure 8.

Distribution of maximal tetanic tensions of motor units of cat's soleus (A) and medial gastrocnemius (B).

A, from McPhedran, Wuerker, and Henneman 253; B from Wuerker, McPhedran, and Henneman 353


Figure 9.

Smoothed version of histogram in Figure 8B, illustrating distribution of some of the properties of motor units in typical limb muscles. Height of columns indicates relative numbers of motor units with the properties listed below.

From Henneman 154. Skeletal muscle: the servant of the nervous system. In: Medical Physiology (14th ed.), edited by V. B. Mountcastle. St. Louis, MO: The C. V. Mosby Co., 1980


Figure 10.

Reflex discharges recorded from a filament of 7th lumbar ventral root in response to stimulation of ipsilateral sciatic nerve of cat. Numbers at right of each tracing indicate relative intensity of stimulation. Initial deflection on left of each tracing is the early discharge referred to in text.

From Henneman 153


Figure 11.

Stretch‐evoked responses of 2 α‐motoneurons recorded from a filament of 7th lumbar ventral root of cat. Amount of tension applied and developed reflexly is indicated by separation of 2 top beams in each frame. Several seconds elapsed between successive frames while muscle was stretched 1,2,3,4,5 and released 6,7,8,9.

From Henneman et al. 159


Figure 12.

Stretch‐evoked responses of 5 α‐motoneurons recorded from a filament of the 1st sacral ventral root of cat. Numbers above action potentials indicate rank of units according to size.

From Henneman et al. 159


Figure 13.

Frequency distribution of thresholds of tonic responses to stretch in 3 experiments on cats. Abscissa, threshold tension of deefferented triceps muscle. Ordinate, number of units whose thresholds fell between values indicated on abscissa.

From Henneman et al. 159


Figure 14.

A: orderly recruitment in cat of 3 triceps motoneurons of different sizes in response to increasing degrees of stretch. Tension produced by stretch of triceps muscle indicated by separation of top traces in each frame. B: orderly inhibition of same 3 units during a constant stretch of 4 kg. Records 1–4 on left show control responses of stretch before, between and after 3 inhibitory stimulations. Records on right were obtained during 100/s stimulation of ipsilateral deep peroneal nerve at 3 intensities. Largest unit was silenced first (line 1), intermediate unit next (line 2), and smallest unit last (line 3). Note lasting effects produced by brief inhibitions, i.e., failure to recover to previous levels in lines 2 and 4.

From Henneman et al. 160


Figure 15.

Graphic representation of discharge frequency of 3 motoneurons of different sizes (A–C) in triceps surae of cat, and of contractile tension developed by the muscle itself (D) in response to varying degrees of excitation and inhibition: x‐axis, intensity of inhibitory stimulation applied to ipsilateral deep peroneal nerve; y‐axis, frequency of discharge in A–C and contractile tension in D; z‐axis, stretch of triceps surae, in millimeters. Data for all 4 graphs obtained simultaneously. Cells were subjected simultaneously to various mixtures of excitatory (z‐axis) and inhibitory (x‐axis) stimuli. Each plotted point represents mean of 2 successive determinations. Inhibitory stimuli were 100/s shocks applied for 0.9‐s periods at 4‐s intervals. Note that unit A (smallest) fired spontaneously without stretch, unit B began to discharge between 0 and 5 mm of stretch, and unit C (the largest) between 5 and 10 mm.

From Henneman et al. 160


Figure 16.

Scheme of experiment used to measure maximal monosynaptic discharge of a motoneuron pool. DR, dorsal root; VR, ventral root.

From Clamann, Henneman, et al. 59


Figure 17.

Pairs of responses in cat recorded as in Figure 16 from proximal and distal halves of 1st sacral ventral root in response to single‐shock stimulation of combined nerves to medial and lateral gastrocnemius (MG and LG). Left and right deflections of each pair are antidromic and reflex responses, respectively, displayed on superimposed traces of a 2‐beam oscilloscope. First 10 pairs in top row recorded before 12‐s tetanus (500 shocks/s) to MG and LG nerves. Subsequent pairs recorded at 2‐s intervals after tetanus.

From Clamann, Henneman, et al. 59


Figure 18.

Simultaneous recordings in cat of a series of monosynaptic reflexes of triceps surae pool (top) and a single triceps motoneuron (bottom) showing critical firing level of the latter. Top trace, time integrals of monosynaptic reflexes recorded at 1/s from proximal half of 1st sacral ventral root. Height of each vertical line measures size of population response. Bottom trace, monosynaptic reflexes of a single triceps motoneuron recorded from a small filament of 7th lumbar ventral root. Records were made during decline in posttetanic potentiation following a brief train of conditioning shocks that were applied to the combined medial gastrocnemius and lateral gastrocnemius and soleus nerves at 500/s.

From Henneman et al. 155


Figure 19.

Critical firing levels of 3 triceps motoneurons in cat. Single unit in bottom left had a sharp threshold at 39%. The 2 units in bottom right tracing had thresholds of 60% and 33%. Plateau in top left record indicates 100% discharge level of triceps pool.

From Henneman et al. 155


Figure 20.

Susceptibilities of 2 pairs of cat plantaris motoneurons with known critical firing levels (CFLs) to repetitive discharge in cat. Electrical stimulation of plantaris nerve at 300 stimuli/s in both cases. Stimulus intensity (i.e., voltage) indicated by level of trace a with reference to base line d. Responses of plantaris units recorded on traces b and c. CFLs indicated at right end of these traces. Time marks at 1‐s intervals for both top and bottom records.

From Henneman et al. 155


Figure 21.

Differences in susceptibility to repetitive firing of 2 cat plantaris motoneurons whose critical firing levels were 12.0% (top) and 15.5% (bottom). A: shocks applied to plantaris nerve were supramaximal for all group Ia‐fibers. Frequency was switched from 20 shocks/s initially to 30 shocks/s at 1st arrow, and to 40 shocks/s at 2nd arrow. B: frequency of stimulation was 500 shocks/s throughout and intensity was increased gradually as trace moved from left to right.

From Henneman et al. 155


Figure 22.

Relation between axon size and critical firing level (rank order) of 21 cat plantaris motoneurons isolated in a single experiment.

From Clamann and Henneman 60


Figure 23.

Critical firing level (CFL) of a cat plantaris motoneuron with and without inhibitory input from lateral popliteal nerve. Top, time integrals of monosynaptic reflexes of plantaris population elicited at 2‐s intervals during declining phase of posttetanic potentiation. Bottom, simultaneous responses of a plantaris unit with a critical firing level of 61%. Alternate reflexes preceded by a 200‐ms train of pulses to lateral popliteal nerve. Horizontal line indicating critical firing level drawn as described in text.

From Clamann, Gillies, and Henneman 58


Figure 24.

Effects of recurrent inhibition (A) and lateral popliteal inhibition (B) in cat on monosynaptic reflexes (traces 1, 2) and repetitive firing (traces 3, 4) of a pair of plantaris motoneurons with critical firing levels of 18.6% (top) and 18.4% (bottom). A: in traces 1, 2 the underlined reflexes were preceded and accompanied by a 100‐ms train of shocks (100/s) to proximal portion of 7th lumbar ventral root. Antidromic volleys set up by stimulation in A1, A2 resulted in recurrent inhibition mediated by Renshaw cells. Of the 18 reflexes in trace 1, 9 were inhibited by this input; in trace 2 only 1 reflex was silenced. In A3, A4 the same pair of units is seen responding rhythmically to repetitive stimulation of the plantaris nerve. When continuous recurrent inhibition was begun, indicated by thick base line in A4, top unit ceased firing and bottom unit slowed to about one‐half original rate of discharge. B: same pair of units was subjected to inhibitory input elicited by stimulating lateral popliteal nerve at 100/s. In B1, B2 the underlined monosynaptic reflexes were somewhat more susceptible to this inhibition than to the recurrent inhibition. Of the 16 reflexes receiving this inhibition, 15 in trace 1 and 4 in trace 2 were silenced. In B3, B4 the same pair of units firing rhythmically in response to 100/s stimulation of the plantaris nerve was subjected to an inhibitory input whose intensity was gradually increased as the traces moved across the oscilloscope. The top unit was finally suppressed by this input whereas the bottom unit was only slowed in rate.

Clamann, Gillies, and Henneman 58


Figure 25.

Records illustrating quantitative relationships between intensity of excitatory stimulus causing 2 cat motoneurons with critical firing levels of 18.8% and 12.7% to discharge repetitively and intensity of inhibitory stimulus required to silence them. Stimulus strength of excitatory input (100/s stimulation of a plantaris nerve), indicated by level of line E with reference to base line B, was constant during each test but varied in different tests. Stimulus strength of inhibitory input (100/s stimulation of lateral popliteal nerve), indicated by line I, was increased as trace moved across screen. Intensity of inhibition required to silence each unit was measured at time of their last discharges, indicated by dashed lines. Time signals at top of record represent 1‐s intervals.

From Clamann, Gillies, and Henneman 58


Figure 26.

Graph showing linear relation between strength of excitatory stimuli causing each of 2 cat motoneurons to discharge repetitively and strength of inhibitory stimuli required to silence them. Data obtained as illustrated in Figure 25. Data for lower unit, with critical firing level of 12.7%, are plotted with filled circles and solid line; data for upper unit, with a critical firing level of 18.8%, are represented by open circles and dashed line.

From Clamann, Gillies, and Henneman 58


Figure 27.

Number of human motor units having the twitch tensions indicated. A: linear scale. B: logarithmic scale. Distributions are similar for all 3 subjects. The computed best‐fitting line on the semilog plot (B) indicates an approximately exponential relation between number of motor units and twitch tension.

From Milner‐Brown et al. 258


Figure 28.

A: firing pattern of a high‐threshold unit recorded from human extensor indicis during isometric contractions with different rates of rise of tension (A–D). Each registration shows (top to bottom) the spike record, isometric force, and tracking signal which was visually displayed to the subject. Time scale in bottom 2 records is one‐half that of top 2 records. During increasingly faster contractions, unit starts firing at successively lower force levels. B: dependence of time between beginning of muscle contraction and firing onset of 3 motor units on rate of rise of tension. Double logarithmic scale. Faster contraction causes earlier recruitment. C: change of threshold force of recruitment (ordinate) associated with variation of rate of rise of isometric tension. Recordings from 7 motor units of extensor indicis of 3 subjects (▪, ▴ •). D: regression line of change of threshold force of recruitment on rate of rise of isometric tension (y1 = −0.067x + 198.66) calculated from 12 motor units of extensor indicis. The increase of muscle force that occurs within the mean contraction time of the muscle is shown for comparison (y2 = 0.060x).

From Büdingen and Freund 30


Figure 29.

Records of EMG activity and force outputs of medial gastrocnemius (top 2 traces) and soleus (bottom 2 traces) muscles of cat, together with simultaneous record of activity of a small number of medial gastrocnemius (MG) motor axons in a natural filament of MG muscle nerve (3rd trace). During MG vibration (160 Hz, 90 μm) activity in whole MG increased and that in soleus (SOL) showed a smaller increase. Pinching the skin over the lateral ankle with toothed forceps produced 2 bursts of MG activity, with recruitment of large amplitude motor axons in the MG filament, but caused quite complete suppression of SOL activity. Isolated activity of 5 identifiable MG motoneuron axons is shown in A–E. Note slowing of unit A discharge during each burst of activity in the higher threshold units B–E.

From Kanda et al. 192


Figure 30.

Simultaneous recording of one continuously firing long‐interval motor unit and one intermittently firing short‐interval motor unit in human. A: increasing contraction. B: prolonged maximum effort. C, D: rapid twitches. Time bar, 100 ms.

From Grimby and Hannerz 139


Figure 31.

Properties of a small (A), medium (B), and large (C) motor neuron of lobster innervating the main power stroke muscle. 1, Soma diameters; 2, axonal conduction velocities; 3, amplitudes of action potentials recorded with 2 extracellular electrodes at different positions on the motor nerve; 4, amplitudes of excitatory junctional potentials (EJPs) recorded from a single muscle fiber (top trace in each record); 5, adaptation to a maintained intracellular depolarizing current; and 6, facilitation properties of extracellular EJPs (top trace in each record) during 50‐Hz stimulation. Note antifacilitation produced by the largest motoneuron. Time marks in row 5 (lowest trace), 0.1 ms.

From Davis 85


Figure 32.

Morphology of Ia‐collaterals to motoneurons and arrangements of boutons on dendrites. A: sagittal section through lumbosacral cord of 40‐day‐old kitten (section shown at right). Primary fibers in dorsal columns (a) send ventrally directed branches to ventral horn (b) to terminate in region of motoneurons (c and d) that have sagitally running dendrites. Primary afferent branches also give off branches to dorsal horn and intermediate zone; these are described in more detail in the original illustration of Scheibel and Scheibel 310 from which this figure is modified. B: scale drawing illustrating morphology of collaterals of a single triceps surae Ia‐fiber as revealed by horseradish peroxidase (HRP) filling. This 3‐dimensional representation provides both a cross‐sectional (transverse) and sagittal perspective. The Ia‐fiber in the dorsal columns is drawn as a thick black line along with the dorsal root ganglion cell and dorsal root portion of the cell. Four collaterals emanating from the dorsal column region are illustrated, each one coursing through the dorsal horn (in a ventrallateral and rostral direction) to terminate in the cylindrical motoneuron region (lamina IX). Other terminals are given off by the same fiber in laminae VI and VII. C: morphology of a single soleus Ia‐fiber in the dorsal columns with 9 branches given off to ventral horn; HRP technique. Two motoneurons—soleus (SOL), which innervates type S motor unit, and lateral gastrocnemius (LG), which innervates a type F, fatigue‐resistant motor unit (R)—were also filled with HRP. Circles and stippling denote somata and maximum dendritic extents of these neurons. Note that at most only 2 of 9 Ia‐fiber branches have access to each of these motoneurons. Also ventrally directed collaterals of this fiber do not exhibit rostral component shown by Brown and Fyffe (Fig. 32B). Top and bottom solid lines denote dorsal and ventral borders of cord. Dotted lines represent dorsal and ventral limits of ventral horn. D: origins of primary collaterals of MG, LG‐S, and posterior tibial (PT) Ia‐fibers. E: schematic drawing of disposition of Ia‐fiber boutons on α‐motoneurons. In each case, both Ia‐fiber and motoneuron filled with HRP. Note in SOL motoneuron that bouton A is on proximal dendrites in contrast to B‐F, which are on distal dendrites. Terminals on LG‐type FR motoneuron are of en passant variety (triplet) and of terminal variety.

A, adapted from Scheibel and Scheibel 310; B and D, from Brown and Fyffe 26; C and E, from Burke et al. 51


Figure 33.

A: arrangement for spike‐triggered averaging. Muscle‐stretch activated group Ia impulse recorded from dorsal rootlet (a) is used to trigger an averaging computer, using a pulse‐height analyzer to select impulses of constant height produced by a single fiber. Each trigger pulse causes averager to process the next several milliseconds of signal recorded from the microelectrode impaling the motoneuron. Motoneuron is identified by antidromic invasion following muscle nerve stimulation (b). If Ia‐fiber acting as source of trigger signal sends terminals to motoneuron, then an EPSP is averaged. B: examples of single 20‐ms records (sweeps) from a medial gastrocnemius (MG) motoneuron triggered by an MG group Ia impulse. The EPSP is seen at start of sweep superimposed on highly variable synaptic activity. Note variability in amplitude of Ia‐evoked EPSP. C–F: 4, 16, 64, and 256 sweeps are averaged, respectively. Note improvement in resolution of EPSP as number of sweeps averaged is increased. Only potentials synchronized with the MG afferent are preserved. Others are asynchronous with respect to the trigger signal and so are averaged out. G, H: dorsal root recording (G) and simultaneous intracellular recording from motoneuron (H) at slower sweep speed. Note 2 Ia impulses in G and synaptic noise in H. Calibration, 500 μV for B–F, and H; 0.7 ms for B–F; 5 ms for H.

A, adapted from Mendell and Henneman 255


Figure 34.

Amplitude histograms of single‐fiber EPSPs in homonymous and heteronymous motoneurons. A: histogram obtained using electrical stimulation method. B: histogram with spike‐triggered averaging method. Arrows represent values of less than 1% of N. Afferents and motoneurons are mainly from triceps surae and semitendinosus muscle groups in A and exclusively so in B.

A, from Jack et al. 82; B, from data of Mendell and Henneman 255, Nelson and Mendell 269,270, Scott and Mendell 315


Figure 35.

Plot of mean amplitudes of postsynaptic population potentials (PSPPs) elicited by afferent impulses with mean condition velocities shown on abscissa. The PSPPs were evoked by impulses in single group Ia and II fibers of cat medial gastrocnemius muscle, using spike‐triggered averaging. Responses recorded from 1st sacral ventral root and lower part of 7th lumbar ventral root. The 153 data points were grouped in pentads of conduction velocity and mean and standard deviations of PSPPs for each group are shown. Mean data points fell into 2 groups, each of which was fitted with a straight line by method of least squares, using point at 72.5 m/s for both calculations.

From Lüscher, Henneman, et al. 244


Figure 36.

Dependence of amplitude of single‐fiber EPSPs on size of motoneuron in cat. Top, relationship between EPSP amplitude (obtained by spike‐triggered averaging) and motor axon conduction velocity determined for 4 separate group Ia afferent fibers. Axonal conduction velocity determined by antidromic stimulation. Medial gastrocnemius (MG) Ia‐fibers and motoneurons in each case. Note inverse relationship. Bottom, each point represents median value of amplitude distribution of a presumed single‐fiber EPSP evoked in a motoneuron during stretch of triceps surae. The EPSPs are not restricted to those generated by Ia‐fibers. Many of these EPSPs are probably produced by interneurons. It is assumed that variation of these EPSPs with motoneuron input resistance as shown would also occur in the subpopulation of Ia‐evoked EPSPs. Small cells with large input resistance 203 generate larger EPSPs. ○, Gastrocnemius motoneurons supplying type F (rapidly contracting) motor units; ⊕, MG motoneurons supplying type S (slowly contracting) motor units; and •, soleus motoneurons.

Top from Mendell and Henneman 255; bottom from Burke 36


Figure 37.

A: EPSP shapes computed for input restricted to specific regions of the dendritic tree of cat. All EPSPs assumed to be recorded from compartment 1 (the soma) of a 10‐compartment motoneuron. Compartments 2–10 represent dendritic regions of equal electrotonic length but increasing distance from the soma. Top, time course of the conductance transient generating EPSPs is given by dotted line; a is EPSP expected for a transient delivered to all compartments simultaneously. Bottom, b–d are EPSPs expected for transients delivered to compartments 1, 4, and 8, respectively. Ordinate is in arbitrary units; to produce EPSPs with equal peak amplitudes as shown, intensity of conductance changes for d > c > b > a. Note differences in time course of EPSPs, and for b–d, note differences in time of their onset. B: end plate potentials at different locations in same muscle fiber (frog sartorius in vitro). Microelectrode was inserted sequentially into curarized fiber and end plate potential in response to motor nerve stimulation was measured. Numbers at left of each trace represent distance in millimeters from end plate focus. S is the stimulus artifact. Time is in ms.

A, from Rall 287; B, from Fatt and Katz 112


Figure 38.

A: single‐fiber excitatory postsynaptic potentials (EPSPs) obtained using spike‐triggered averaging produced by the same medial gastrocnemius (MG) Ia‐afferent fiber in 6 different MG motoneurons in cat. Note different shapes of the 6 EPSPs. Calibration pulse at the end of each sweep is 200 μV. B: single‐fiber EPSPs (by spike‐triggered averaging) produced in the same LG motoneuron by an LG Ia‐fiber (top) and a soleus Ia‐fiber (bottom). Calibration pulse is 100 μV, 1 ms. Note differences in shape of these EPSPs. C: example of a single fiber EPSP (MG Ia‐fiber and MG motoneuron) with compound shape (spike‐triggered averaging). Calibration is 1 ms, 50 μV.

A, from Mendell and Henneman 255; B, from Scott and Mendell 315; C, from Mendell and Henneman 255


Figure 39.

A: plot of ratio of excitatory postsynaptic potential (EPSP) amplitudes against ratio of their rise times (RT2/RT1). Each point was obtained by consideration of 2 averaged single‐fiber EPSPs (spike‐triggered averaging) evoked by different Ia‐fibers in the same motoneuron. Medial gastrocnemius (MG) Ia‐fibers and MG motoneurons throughout. By definition RT1 < RT2 so that RT2/RT1> 1. Furthermore , the ratio of the amplitudes, is defined so that amplitude of faster rising EPSP is in numerator. Majority of points have suggesting a tendency for faster rising EPSPs to be larger than slower rising ones. Correlation between and RT2/RT1 is very weak. B: plot of single‐fiber EPSP amplitude (obtained by electrical stimulation) against distance (in units of space constant δ) between the terminals and the electrode (assumed to be in soma). The EPSPs are chiefly from hamstring and triceps surae motoneurons with both homonymous and heteronymous combinations.

A, from Mendell and Weiner 256; B, from Iansek and Redman 176


Figure 40.

Model of projection of single Ia‐fiber to homonymous and heteronymous motoneurons (circles). Systematic differences in mean number of boutons provided by single Ia‐fibers to homonymous and heteronymous motoneurons would lead to differences in projection frequency and excitatory postsynaptic potential (EPSP) amplitude in these 2 types of projections. Mean bouton number was chosen using estimates of Iles 177 as a guide. Numbers below each circle represent number of boutons provided by single Ia‐fiber to each motoneuron.



Figure 41.

Scatter diagram of relationship between rise time and latency for single‐fiber excitatory postsynaptic potentials (EPSPs) in 78 medial gastrocnemius (MG) and 23 lateral gastrocnemius and soleus (LG‐SOL) motoneurons. MG Ia‐afferent fibers throughout. Dashed vertical line at 0.27 ms denotes minimum latency recorded in heteronymous LG‐SOL motoneurons. Inset shows intracellular (A), extracellular (C) records from a motoneuron with spike‐triggered averaging. Note reversal of EPSP, but not diphasic prespike following withdrawal of electrode. Calibration pulses 50 μ V, 1 ms. B is same as A with both scales expanded by 4. Latency was measured from negative (downward going) peak of prespike to onset of EPSP.

From Munson and Sypert 263


Figure 42.

Comparison of synaptic delay associated with single‐fiber excitatory postsynaptic potentials (EPSPs) produced by group Ia and group II fibers in cat. Left, trace A represents potential recorded inside a medial gastrocnemius (MG) motoneuron with spike‐triggered averaging from a group II afferent fiber. The large upward potential is EPSP, which is preceded by a brief diphasic potential considered to arise from activity in presynaptic terminals of group II fiber. Trace B is record obtained by spike‐triggered averaging from a position just outside motoneuron. The early potential is not inverted, indicating that it is produced outside motoneuron and unlike EPSP, which reverses, showing that it is generated across the membrane. Trace C is averaged record recorded from group II fiber in dorsal rootlet. Interval from dorsal root spike to presynaptic spike is conduction time (CT) and remaining time to EPSP onset is synaptic delay (SD). Right, histograms of conduction time and synaptic delay for group II fibers (top) and group Ia‐fibers (bottom). Note similarities in synaptic delay for both types of fiber and trend for shorter conduction time in (the larger) group‐Ia fibers.

From Stauffer et al. 328


Figure 43.

Inhibitory postsynaptic potential (IPSP) in a cat motoneuron analyzed by spike‐triggered averaging. A: top trace is intracellular record from a posterior biceps or semitendinosus motoneuron. Bottom trace is extracellular spike activity from an interneuron satisfying the criteria required for a Ia‐interneuron. Interneuron is excited by iontophoretically applied glutamate and discharges rhythmically. Each spike is associated with an IPSP (overlaid by row of dots). B: averages of 512 sweeps triġgered by interneuronal spike from motoneuron (above) and interneuron (below). Trigger level was set so that averaging began at beginning of interneuronal spike.

From Jankowska and Roberts 189


Figure 44.

Influence of entry level of Ia‐fiber of cat on amplitude of postsynaptic population potentials (PSPPs) its impulses elicit.

Data are from a single experiment. Schematic drawings in top of figure illustrate recording arrangements on ventral roots (VR); CV represents conduction velocity. Arrow points at spinal segment that the afferent fiber enters. The PSPPs recorded from caudal part of 7th lumbar ventral root (L7VR) and 1st sacral ventral root 1 and 1st sacral (S1 VR) 2 are reproduced below. A: Ia‐fiber arising in medial gastrocnemius (MG) enters through S1 dorsal root. B: Ia‐fiber from MG enters spinal segment L7. C: Ia‐fiber from lateral gastrocnemius (LG) enters spinal segment L7. Each PSPP was averaged 4,096 sweeps. From Lüscher, Ruenzel, and Henneman 246


Figure 45.

Three‐dimensional graph illustrating relationship between amplitude of postsynaptic population potentials (PSPPs) and conduction velocity and spinal entry point of afferent impulses evoking them. Amplitudes were averaged over 4,096 sweeps and were recorded from 1st sacral ventral root. Data were derived from a single experiment. All afferent fibers had origin in medial gastrocnemius muscle.

From Lüscher, Ruenzel, and Henneman 246


Figure 46.

Maximal firing rates of 4 plantaris motoneurons of cat with approximately equal critical firing levels recorded simultaneously during first 940 ms of their response to electrical stimulation of plantaris nerve. Stimulus parameters adjusted to produce maximal firing rates. Bottom unit fires significantly slower than top 3 units, giving impression of having been sampled from a different population than the others.

From Harris and Henneman 145


Figure 47.

Firing rates (averaged over first second) of 2 pairs of units of cat with similar critical firing levels, compared simultaneously. Data obtained by applying a wide range of stimulus intensities and frequencies to plantaris nerve. A: for each pair of units firing rate (FR) of 1 unit is plotted versus FR of other unit. Dashed lines indicate linear regression. B: ratio of FRs of each pair plotted as a function of the rate of 1 of the units expressed as a percentage of its maximum rate. Bottom pair (open circles) had very similar FRs (ratio ≃ 1.0); top pair (closed circles) had widely differing FRs (ratio > 2.0). Linear regression lines with slopes near zero indicate ratio of FRs remained constant for all these inputs.

From Harris and Henneman 145


Figure 48.

Examples of aggregate excitatory postsynaptic potentials (EPSPs) recorded from 3 medial gastrocnemius motoneurons of cat to illustrate effects of posttetanic potentiation on cells of different input resistance (IR) and size. Bottom traces illustrate that average size of aggregate EPSPs before potentiation varied directly with IR of motoneuron. Top traces are averages of 16 EPSPs following tetanus. Percent potentiations in A–C were 123, 92, and 11, respectively.

From Lüscher, Ruenzel, and Henneman 245


Figure 49.

Relation between input resistance and amplitude of aggregate excitatory postsynaptic potentials (EPSPs) before (A) and during (B) posttetanic potentiation in cat. Circles, medial gastrocnemius motoneurons; triangles, semitendinosus motoneurons.

From Lüscher, Ruenzel, and Henneman 245


Figure 50.

Plot of changes in rise times of excitatory postsynaptic potentials (EPSPs) versus changes in their half‐widths following a tetanus to cat muscle nerve at 500/s for 10 s. Arrows indicate directions of change.

From Lüscher, Ruenzel, and Henneman 245


Figure 51.

Some features of a motoneuron pool. Heights of vertical lines represent sizes of motoneurons. Their spacing indicates relative numbers of cells of different sizes. Horizontal line below base line shows combination of motoneurons firing in response to a given input. Largest cell discharged denoted by X.

From Henneman 361. Organization of the motoneuron pool: the size principle. In: Medical Physiology (14th ed.), edited by V. B. Mountcastle. St. Louis, MO: The C. V. Mosby Co., 1980


Figure 52.

Concomitant changes in mean firing rates of units i and j (fi and fj) during slow volitional changes in an isometric contraction in human. The fi‐fj relationship of each pair is approximated by 1 or 2 straight (dashed) lines. The same i unit was used in pairs 5–8.

From Monster and Chan 261
References
 1. Abrahams, V. C., R. Richmond, and P. K. Rose. Absence of monosynaptic reflex in dorsal neck muscles of the cat. Brain Res. 92: 130–131, 1975.
 2. Abzug, C., M. Maeda, B. W. Peterson, and V. J. Wilson. Branching of individual lateral vestibulospinal axons at different spinal cord levels. Brain Res. 56: 327–330, 1973.
 3. Adrian, E. D., and D. W. Bronk. The discharge of impulses in motor nerve fibers. II. The frequency of discharge in reflex and voluntary contractions. J. Physiol. London 67: 119–151, 1929.
 4. Anderson, M. E. Segmental reflex inputs to motoneurons innervating dorsal neck musculature in the cat. Exp. Brain Res. 28: 175–187, 1977.
 5. Anzenbacher, H., and W. Zenker. Über die Grössenbeziehung der Muskelfasern zu ihren motorischen Endplatten und Nerven. Z. Zellforsch. Mikrosk. Anat. 60: 860–871, 1963.
 6. Appelberg, B., and F. Emonet‐Dénand. Motor units of the first superficial lumbrical muscle of the cat. J. Neurophysiol. 30: 154–160, 1967.
 7. Appenteng, K., M. J. O'Donovan, G. Somjen, J. A. Stephens, and A. Taylor. The projection of jaw elevator muscle spindle afferents to fifth nerve motoneurons in the cat. J. Physiol. London 279: 409–423, 1978.
 8. Araki, T., K. Endo, Y. Kawai, K. Ito, and Y. Shigenaga. Supraspinal control of slow and fast spinal motoneurons of the cat. In: Progress in Brain Research. Understanding the Stretch Reflex, edited by S. Homma. Amsterdam: Elsevier, 1976, vol. 44, p. 413–432.
 9. Baginsky, R. G. Voluntary control of motor unit activity by visual and aural feed‐back. Electroencephalogr. Clin. Neurophysiol. 27: 724–725, 1969.
 10. Bagust, J. Relationships between motor nerve conduction velocities and motor unit contraction characteristics in a slow twitch muscle of the cat. J. Physiol. London 238: 269–278, 1974.
 11. Bagust, J., S. Knott, D. M. Lewis, J. C. Luck, and R. A. Westerman. Isometric contractions of motor units in a fast twitch muscle of the cat. J. Physiol. London 231: 87–104, 1973.
 12. Barrett, J. N. Motoneuron dendrites: role in synaptic integration. Federation Proc. 34: 1398–1407, 1975.
 13. Barrett, J. N., and W. E. Crill. Specific membrane resistivity of dye‐injected cat motoneuron. Brain Res. 28: 556–561, 1971.
 14. Barrett, J. N., and W. E. Crill. Specific membrane properties of cat motoneurons. J. Physiol. London 239: 301–324, 1974.
 15. Barrett, J. N., and W. E. Crill. Influence of dendritic location and membrane properties on the effectiveness of synapses in cat motoneurons. J. Physiol. London 239: 325–345, 1974.
 16. Barrett, E. F., and C. F. Stevens. Quantal independence and uniformity of presynaptic release kinetics at the frog neuromuscular junction. J. Physiol. London 227: 665–689, 1972.
 17. Basmajian, J. V. Control and training of individual motor units. Science 141: 440, 1963.
 18. Basmajian, J. V. Control of individual motor units. Am. J. Phys. Med. 46: 480, 1967.
 19. Basmajian, J. V. Electromyography comes of age. Science 176: 603, 1972.
 20. Bessou, P., F. Emonet‐Dénand, and Y. Laporte. Relation entre la vitesse de conduction des fibres nerveuses motrices et le temps de contraction de leurs unités motrices. C. R. Acad. Sci. 256: 5625–5627, 1963.
 21. Bigland, B., and O. C. J. Lippold. Motor unit activity in the voluntary contraction of human muscle. J. Physiol. London 125: 322–335, 1954.
 22. Blankenship, J. E., and M. Kuno. Analysis of spontaneous subthreshold activity in spinal motoneurons of the cat. J. Neurophysiol. 31: 195–209, 1968.
 23. Borg, J., L. Grimby, and J. Hannerz. Axonal conduction velocity and voluntary discharge properties of individual short toe extensor motor units in man. J. Physiol. London 277: 143–152, 1978.
 24. Bracchi, F., M. Decandia, and T. Gualtierotti. Frequency stabilization in the motor centers of spinal cord and caudal brain stem. Am. J. Physiol. 210: 1170–1177, 1966.
 25. Brock, L. G., J. S. Coombs, and J. C. Eccles. The recording of potentials from motoneurons with an intracellular electrode. J. Physiol. London 117: 431–460, 1952.
 26. Brown, A. G., and R. E. W. Fyffe. The morphology of group Ia afferent fiber collaterals in the spinal cord of the cat. J. Physiol. London 274: 111–127, 1978.
 27. Brown, A. G., and R. E. W. Fyffe. The morphology of Group Ib muscle afferent fiber collaterals. J. Physiol. London 277: 44–45, 1978.
 28. Brown, M. C., J. K. S. Jansen, and D. Van Essen. Polyneuronal innervation of skeletal muscle in newborn rats and its elimination during maturation. J. Physiol. London 261: 387–422, 1976.
 29. Buchthal, F., and H. Schmalbruch. Contraction times and fibre types in intact human muscle. Acta Physiol. Scand. 79: 435–452, 1970.
 30. Büdingen, H. J., and H. J. Freund. The relationship between the rate of rise of isometric tension and motor unit recruitment in a human forearm muscle. Pfluegers Arch. 362: 61–67, 1976.
 31. Buller, A. J., J. C. Eccles, and R. M. Eccles. Interactions between motoneurons and muscles in respect of the characteristic speeds of their responses. J. Physiol. London 150: 417–439, 1960.
 32. Bullock, T. H. Comparative aspects of some biological transducers. Federation Proc. 12: 666–672, 1953.
 33. Bullock, T. H., and G. A. Horridge. Structure and Function in the Nervous Systems of Invertebrates. San Francisco: Freeman, 1965.
 34. Burke, R. E. Composite nature of the monosynaptic excitatory postsynaptic potential. J. Neurophysiol. 30: 1114–1137, 1967.
 35. Burke, R. E. Motor unit types of cat triceps surae muscle. J. Physiol. London 193: 141–160, 1967.
 36. Burke, R. E. Group Ia synaptic input to fast and slow twitch motor units of cat triceps surae. J. Physiol. London 196: 605–630, 1968.
 37. Burke, R. E. On the central nervous control of fast and slow twitch motor units. In: New Developments in Electromyography and Clinical Neurophysiology. Human Reflexes, Pathophysiology of Motor Systems, Methodology of Human Reflexes, edited by J. E. Desmedt. Basel: Karger, 1973, vol. 3, p. 69–94.
 38. Burke, R. E., L. Fedina, and A. Lundberg. Spatial synaptic distribution of recurrent and group Ia inhibitory systems in cat spinal motoneurons. J. Physiol. London 214: 305–326, 1971.
 39. Burke, R. E., E. Jankowska, and G. ten eBruggencate. A comparison of peripheral and rubrospinal synaptic input to slow and fast twitch motor units of triceps surae. J. Physiol. London 207: 709–732, 1970.
 40. Burke, R. E., D. N. Levine, M. Salcman, and P. Tsairis. Motor units in cat soleus muscle: physiological, histochemical and morphological characteristics. J. Physiol. London 238: 503–514, 1974.
 41. Burke, R. E., D. N. Levine, P. Tsairis, and F. E. Zajac III. Physiological types and histochemical profiles in motor units of the cat gastrocnemius. J. Physiol. London 234: 723–748, 1973.
 42. Burke, R. E., and P. G. Nelson. Synaptic activity in motoneurons during natural stimulation of muscle spindles. Science 151: 1088–1091, 1966.
 43. Burke, R. E., and P. G. Nelson. Accommodation to current ramps in motoneurons of fast and slow twitch motor units. Int. J. Neurosci. 1: 347–356, 1971.
 44. Burke, R. E., and P. Rudomin. Spinal neurons and synapses. In: Handbook of Physiology. The Nervous System. Cellular Biology of Neurons, edited by E. Kandel. Bethesda, MD: Am. Physiol. Soc., 1977, sect. 1, vol. I, pt. 2, chapt. 24, p. 877–944.
 45. Burke, R. E., W. Z. Rymer, and J. V. Walsh, Jr. Functional specialization in the motor unit population of cat medial gastrocnemius muscle. In: Control of Posture and Locomotion, edited by R. B. Stein, K. B. Pearson, R. S. Smith, and J. B. Redford. New York: Plenum, 1974. (Adv. Behav. Biol. Ser., vol. 7.).
 46. Burke, R. E., W. Z. Rymer, and J. V. Walsh, Jr. Relative strength of synaptic input from short‐latency pathways to motor units of defined type in cat medial gastrocnemius. J. Neurophysiol. 39: 447–458, 1976.
 47. Burke, R. E., P. L. Strick, K. Kanda, C. C. Kim, and B. Walmsley. Anatomy of medial gastrocnemius and soleus motor nuclei in cat spinal cord. J. Neurophysiol. 40: 667–680, 1977.
 48. Burke, R. E., and G. ten eBruggencate. Electrotonic characteristics of alpha motoneurons of varying size. J. Physiol. London 212: 1–20, 1971.
 49. Burke, R. E., and P. Tsairis. Anatomy and innervation ratios in motor units of cat gastrocnemius. J. Physiol. London 234: 749–765, 1973.
 50. Burke, R. E., and P. Tsairis. The correlation of physiological properties with histochemical characteristics in single muscle units. Ann. NY Acad. Sci. 228: 145–159, 1974.
 51. Burke, R. E., B. Walmsley, and J. A. Hodgson. Structural‐functional relations in monosynaptic action on spinal motoneurons. In: Integration in the Nervous System, edited by H. Asanuma and V. J. Wilson. Toyko: Igaku‐Shoin, 1979, p. 27–45.
 52. Bush, B. M. H. Proprioceptive reflexes in the legs of Carcinus maenas (L.). J. Exp. Biol. 39: 89, 1962.
 53. Cangiano, E., W. A. Cook, and O. Pompeiano. Primary afferent depolarization in the lumbar cord evoked from the fastigial nucleus. Arch. Ital. Biol. 107: 321–340, 1969.
 54. Carpenter, D., I. Engberg, and A. Lundberg. Primary afferent depolarization evoked from the brain stem and the cerebellum. Arch. Ital. Biol. 104: 73–85, 1966.
 55. Chapple, W. D. Motoneurons innervating the dorsal superficial muscles of the hermit crab, Pagurus pollicarus, and their reflex asymmetry. J. Comp. Physiol. 121: 413–431, 1977.
 56. Chin, N. K., M. Cope, and M. Pang. Number and distribution of spindle capsules in seven hindlimb muscles of the cat. In: Symposium on Muscle Receptors, edited by D. Barker. Hong Kong: Hong Kong Univ. Press, 1962, p. 241–248.
 57. Clamann, H. P. Activity of single motor units during isometric tension. Neurology 20: 254–260, 1970.
 58. Clamann, H. P., J. D. Gillies, and E. Henneman. Effects of inhibitory inputs on critical firing level and rank order of motoneurons. J. Neurophysiol. 37: 1350–1360, 1974.
 59. Clamann, H. P., J. D. Gillies, R. D. Skinner, and E. Henneman. Quantitative measures of output of a motoneuron pool during monosynaptic reflexes. J. Neurophysiol. 37: 1328–1337, 1974.
 60. Clamann, H. P., and E. Henneman. Electrical measurement of axon diameter and its use in relating motoneuron size to critical firing level. J. Neurophysiol. 39: 844–851, 1976.
 61. Clamann, H. P., and C. G. Kukulka. Evidence for selective inputs to a motoneuron pool. Neurosci. Abst. 2: 539, 1976.
 62. Clamann, H. P., and C. G. Kukulka. The relation between size of motoneurons and their position in the cat spinal cord. J. Morphol. 153: 461–466, 1977.
 63. Clamann, H. P., and C. G. Kukulka. Reversals of recruitment order in medial gastrocnemius produced by stimulation of Deiters' nucleus. Neurosci. Abst. 3: 269, 1977.
 64. Clark, D. A. Muscle counts of motor units: a study in innervation ratios. Am. J. Physiol. 96: 296–304, 1931.
 65. Clough, J. F. M., D. Kernell and C. G. Phillips. The distribution of monosynaptic excitation from the pyramidal tract and from primary spindle afferents to motoneurons of the baboon's hand and forearm. J. Physiol. London 198: 145–166, 1968.
 66. Coërs, C. Les variations structurelles normales et pathologiques de la jonction neuromusculaire. Acta Neurol. Psychiatr. Belg. 55: 741–866, 1955.
 67. Cohen, L. A. Localization of stretch reflex. J. Neurophysiol. 16: 272–285, 1953.
 68. Cohen, L. A. Organization of stretch reflex into two types of direct spinal arcs. J. Neurophysiol. 17: 443–453, 1954.
 69. Cole, W. V. Structural variations of nerve endings in the striated muscle of rat. J. Comp. Neurol. 108: 445–464, 1957.
 70. Collatos, T. C., A. Niechaj, S. J. Nelson, and L. M. Mendell. Fluctuations in time of onset of Ia‐motoneuron EPSPs in the cat. Brain Res. 160: 514–518, 1979.
 71. Conradi, S. Ultrastructure and distribution of neuronal and glial elements on the motoneuron surface in the lumbosacral spinal cord of the adult cat. Acta Physiol. Scand. Suppl. 332: 5–48, 1969.
 72. Conradi, S. Ultrastructure of dorsal root boutons on lumbosacral motoneurons of the adult cat. Acta Physiol. Scand. Suppl. 332: 85–115, 1969.
 73. Conradi, S. Functional anatomy of the anterior horn motor neuron. In: The Peripheral Nerve, edited by D. H. Landon. London: Chapman Hall, 1976, p. 279–329.
 74. Conradi, S., and L. O. Ronnevi. Spontaneous elimination of synapses on cat spinal motoneurons after birth: do half of the synapses on the cell bodies disappear? Brain Res. 92: 505–510, 1975.
 75. Coombs, J. S., D. R. Curtis, and J. C. Eccles. The generation of impulses in motoneurons. J. Physiol. London 139: 232–249, 1957.
 76. Corda, M., G. Eklund, and C. V. Euler. External intercostal and phrenic alpha motor responses to changes in respiratory load. Acta Physiol. Scand. 63: 391–400, 1965.
 77. Cowan, W. M. The development of the brain. Sci. Am. 241: 113–133, 1979.
 78. Creed, R. S., D. Denny‐Brown, J. C. Eccles, E. G. T. Liddell, and C. S. Sherrington. Reflex Activity of the Spinal Cord. London: Oxford Univ. Press, 1932.
 79. Creed, R. S., and C. Sherrington. Observations on concurrent contraction of flexor muscles in the flexion reflex. Proc. R. Soc. London Ser. B 100: 258–267, 1926.
 80. Cullheim, S. Relations between cell body size, axon diameter and axon conduction velocity of cat sciatic alpha‐motoneurons stained with horseradish peroxidase. Neurosci. Letters 8: 17–20, 1978.
 81. Cullheim, S., and J. O. Kellerth. A morphological study of the axons and recurrent axon collaterals of cat alpha‐motoneurons supplying different functional types of muscle unit. J. Physiol. London 281: 301–313, 1978.
 82. Cullheim, S., and J. O. Kellerth. A morphological study of the axons and recurrent axon collaterals of cat sciatic α‐motoneurons after intracellular staining with horseradish peroxidase. J. Comp. Neurol. 178: 537–558, 1978.
 83. Cullheim, S., J. O. Kellerth, and S. Conradi. Evidence for direct synaptic interconnections between cat spinal α‐motoneurons via the recurrent axon collaterals: a morphological study using intracellular injection of horseradish peroxidase. Brain Res. 132: 1–10, 1977.
 84. Czéh, G., R. Gallego, N. Kudo, and M. Kuno. Evidence for the maintenance of motoneuron properties by muscle activity. J. Physiol. London 281: 239–252, 1978.
 85. Davis, W. J. Functional significance of motoneuron size and soma position in swimmeret system of the lobster. J. Neurophysiol. 34: 274–288, 1971.
 86. Deiters, O. Untersuchung über Gehirn and Rückenmark des Menschen und der Säugethiere. Braunschweig, 1865.
 87. Dekker, J. J., D. G. Lawrence, and H. G. J. M. Kuypers. The location of longitudinally running dendrites in the ventral horn of the cat spinal cord. Brain Res. 51: 319–325, 1973.
 88. Del Castillo, J., and B. Katz. Quantal components of the end plate potential. J. Physiol. London 124: 560–573, 1954.
 89. Denny‐Brown, D. Nature of postural reflexes, Proc. R. Soc. London Ser. B 104: 252–301, 1929.
 90. Desmedt, J. E., and E. Godaux. Ballistic contractions in man: characteristic recruitment pattern of single motor units of the tibialis anterior muscle. J. Physiol. London 264: 673–693, 1977.
 91. Desmedt, J. E., and E. Godaux. Critical evaluation of the size principle of human motoneuron recruitment in ballistic movements and in vibration‐induced inhibition or potentiation. Trans. Am. Neurol. Assoc. 102: 104–108, 1977.
 92. Desmedt, J. E., and E. Godaux. Fast motor units are not preferentially activated in rapid voluntary contractions in man. Nature London 267: 717–719, 1977.
 93. Desmedt, J. E., and E. Godaux. Ballistic skilled movements: load compensation and patterning of the motor commands. In: Progress in Clinical Neurophysiology. Cerebral Motor Control in Man: Long Loop Mechanisms, edited by J. E. Desmedt. Basel: Karger, 1978, vol. 4, p. 21–55.
 94. Desmedt, J. E., and E. Godaux. Ballistic contractions in fast or slow human muscles: discharge patterns of single motor units. J. Physiol. London 285: 185–196, 1978.
 95. Desmedt, J. E., and E. Godaux. Mechanism of the vibration paradox: excitatory and inhibitory effects of tendon vibration on single soleus muscle motor units in man. J. Physiol. London 285: 197–207, 1978.
 96. Eccles, J. C. The Physiology of Synapses. New York: Academic, 1964.
 97. Eccles, J. C., R. M. Eccles, A. Iggo, and M. Ito. Distribution of recurrent inhibition among motoneurons. J. Physiol. London 159: 479–499, 1961.
 98. Eccles, J. C., R. M. Eccles, A. Iggo, and A. Lundberg. Electrophysiological studies on gamma motoneurons. Acta Physiol. Scand. 50: 32–40, 1960.
 99. Eccles, J. C., R. M. Eccles, and A. Lundberg. The convergence of monosynaptic excitatory afferents onto many different species of alpha motoneurons. J. Physiol. London 137: 22–50, 1957.
 100. Eccles, J. C., R. M. Eccles, and A. Lundberg. Synaptic actions on motoneurons caused by impulses in Golgi tendon organ afferents. J. Physiol. London 138: 227–252, 1957.
 101. Eccles, J. C., R. M. Eccles, and F. Magni. Central inhibitory action attributable to presynaptic depolarization produced by muscle afferent volleys. J. Physiol. London 159: 147–166, 1961.
 102. Eccles, J. C., and C. S. Sherrington. Numbers and contraction values of individual motor‐units examined in some muscles of the limb. Proc. R. Soc. Lond. Ser. B 106: 326, 1939.
 103. Eccles, R. M., and A. Lundberg. Integrative pattern of Ia synaptic actions of motoneurons of hip and knee muscles. J. Physiol. London 144: 271–298, 1958.
 104. Eccles, R. M., and A. Lundberg. Synaptic actions in motoneurons by afferents which may evoke the flexion reflex. Arch. Ital. Biol. 97: 199–221, 1959.
 105. Edwards, F. R., S. J. Redman, and B. Walmsley. Statistical fluctuations in charge transfer at Ia synapses on spinal motoneurons. J. Physiol. London 259: 665–688, 1976.
 106. Edwards, F. R., S. J. Redman, and B. Walmsley. Non‐quantal fluctuations and transmission failures in charge transfer at Ia synapses on spinal motoneurons. J. Physiol. London 259: 689–704, 1976.
 107. Edwards, F. R., S. J. Redman, and B. Walmsley. The effect of polarizing currents on unitary Ia excitatory postsynaptic potentials evoked in spinal motoneurons. J. Physiol. London 259: 705–723, 1976.
 108. Emonet‐Dénand, F., L. Jami, and Y. Laporte. Skeleto‐fusimotor axons in hind‐limb muscles of the cat. J. Physiol. London 249: 153–166, 1975.
 109. Erlanger, J., and H. S. Gasser. Electrical Signs of Nervous Activity. Philadelphia: Univ. Pennsylvania Press, 1937.
 110. Faden, J. S., and F. F. Zajac. Direct comparison of recruitment order with neural and muscular properties of motor units. Soc. Neurosci. Abstr. 3: 271, 1977.
 111. Fatt, P. Sequence of events in synaptic activation of a motoneurone. J. Neurophysiol. 20: 61–80, 1957.
 112. Fatt, P., and B. Katz. An analysis of the end‐plate potential recorded with an intracellular electrode. J. Physiol. London 115: 320–370, 1951.
 113. Fedde, M. R., P. D. de Wet, and R. L. Kitchell. Motor unit recruitment pattern and tonic activity in respiratory muscles of Gallus domesticus. J. Neurophysiol. 32: 995–1004, 1969.
 114. Frank, K., and M. G. F. Fuortes. Accommodation of spinal motoneurons of cats. Arch. Ital. Biol. 98: 165–170, 1960.
 115. Freund, H.‐J., H. J. Büdingen, and V. Dietz. Activity of single motor units from human forearm muscles during voluntary isometric contractions. J. Neurophysiol. 38: 933–946, 1975.
 116. Fu, T. C., H. Hultborn, R. Larsson, and A. Lundberg. Reciprocal inhibition during the tonic stretch reflex in the decerebrate cat. J. Physiol. London 284: 345–369, 1978.
 117. Fu, T. C., and E. D. Schomburg. Electrophysiological investigation of the projection of secondary muscle spindle afferents in the cat spinal cord. Acta Physiol. Scand. 91: 314–329, 1974.
 118. Fuortes, M. G. F., K. Frank, and M. C. Becker. Steps in the production of motoneuron spikes. J. Gen. Physiol. 40: 732–752, 1957.
 119. Garnett, R., and J. A. Stephens. Changes in the recruitment threshold of motor units in human first dorsal interosseous muscle produced by skin stimulation. J. Physiol. London 282: 13–14, 1978.
 120. Gasser, H. The classification of nerve fibers. Ohio J. Sci. 41: 145, 1941.
 121. Gelfan, S. Neuron and synapse populations in the spinal cord: indication of role in total integration. Nature London 198: 162, 1963.
 122. Gelfan, S., and A. G. Rapisarda. Synaptic density on spinal neurons of normal dogs and dogs with experimental hindlimb rigidity. J. Comp. Neurol. 123: 73–95, 1964.
 123. Gibbs, C. L., and W. R. Gibson. Energy production of rat soleus muscle. Am. J. Physiol. 223: 864–871, 1972.
 124. Gogan, P., J. P. Geuritaud, G. Horchelle‐Bossavit, and S. Tyc‐Dumont. Direct excitatory interactions between spinal motoneurons of the cat. J. Physiol. London 272: 755–767, 1977.
 125. Goldberg, L. J., and B. Derfler. Relationship among recruitment order, spike amplitude, and twitch tension of single motor units in human masseter muscle. J. Neurophysiol. 40: 879–890, 1977.
 126. Goldspink, D. F. The influence of immobilization and stretch on protein turnover of rat skeletal muscle. J. Physiol. London 264: 267–282, 1977.
 127. Goldspink, D. F. The influence of activity on muscle size and protein turnover. J. Physiol. London 264: 283–296, 1977.
 128. Granit, R. Neuromuscular interaction in postural tone of the cat's isometric soleus muscle. J. Physiol. London 143: 387–402, 1958.
 129. Granit, R., H. D. Henatsch, and G. Steg. Tonic and phasic ventral horn cells differentiated by post‐tetanic potentiation in cat extensors. Acta Physiol. Scand. 37: 114–126, 1956.
 130. Granit, R., D. Kernell, and G. K. Shortess. Quantitative aspects of repetitive firing of mammalian motoneurons, caused by injected currents. J. Physiol. London 168: 911–931, 1963.
 131. Granit, R., J. E. Pascoe, and G. Steg. The behaviour of tonic alpha and gamma motoneurons during stimulation of recurrent collaterals. J. Physiol. London 138: 381–400, 1957.
 132. Grillner, S. Muscle stiffness and motor control—forces in the ankle during locomotion and standing. In: Motor Control, edited by A. A. Gydikov, N. T. Tankov, and D. S. Kosarov. New York: Plenum, 1973, p. 195–215. (Proc. Int. Symp. Motor Control “Zlatni Pyassatsi,” 2nd, near Varni, Bulgaria, 1973.).
 133. Grillner, S., T. Hongo, and S. Lund. The vestibulospinal tract. Effects on alpha‐motoneurons in the lumbosacral spinal cord in the cat. Exp. Brain Res. 10: 94–120, 1970.
 134. Grillner, S., T. Hongo, and S. Lund. Convergent effects on alpha motoneurons from the vestibulospinal tract and a pathway descending in the medial longitudinal fasciculus. Exp. Brain Res. 12: 457–479, 1971.
 135. Grillner, S., and M. Udo. Recruitment in the tonic stretch reflex. Acta Physiol. Scand. 81: 571–573, 1971.
 136. Grimby, L., and J. Hannerz. Recruitment order of motor units in voluntary contraction: changes induced by proprioceptive afferent activity. J. Neurol. Neurosurg. Psychiatry 31: 565–573, 1968.
 137. Grimby, L., and J. Hannerz. Differences in recruitment order and discharge pattern of motor units in the early and late flexion reflex components in man. Acta Physiol. Scand. 90: 555–564, 1974.
 138. Grimby, L., and J. Hannerz. Disturbances in voluntary recruitment order of low and high frequency motor units on blockade of proprioceptive afferent activity. Acta Physiol. Scand. 96: 207–216, 1976.
 139. Grimby, L., and J. Hannerz. Firing rate and recruitment order of toe extensor motor units in different modes of voluntary contraction. J. Physiol. London 264: 865–879, 1977.
 140. Grinell, A. D. A study of the interaction between motoneurons in the frog spinal cord. J. Physiol. London 182: 612–648, 1966.
 141. Grossman, Y., M. E. Spria, and I. Parnas. Differential flow of information into branches of a single axon. Brain Res. 64: 379–386, 1973.
 142. Hamburger, V. Regression vs. peripheral control of differentiation in motor hypoplasia. Am. J. Anat. 102: 365–410, 1958.
 143. Hamburger, V. Cell death in the development of the lateral motor column of the chick embryo. J. Comp. Neurol. 160: 535–546, 1975.
 144. Harris, C. The morphology of the myoneural junction as influenced by neurotoxic drugs. Am. J. Pathol. 30: 501–519, 1954.
 145. Harris, D. A., and E. Henneman. Identification of two species of alpha motoneurons in cat's plantaris pool. J. Neurophysiol. 40: 16–25, 1977.
 146. Harris, D. A., and E. Henneman. Different species of alpha motoneurons in same pool: further evidence from effects of inhibition on their firing rates. J. Neurophysiol. 42: 927–935, 1979.
 147. Harrison, V. F., and O. A. Mortenson. Identification and voluntary control of single motor unit activity in the tibialis anterior muscle. Anat. Rec. 144: 109, 1962.
 148. Harvey, R. J., and P. B. C. Matthews. The response of deefferented muscle spindle endings in the cat's soleus to slow extension of the muscle. J. Physiol. London 157: 370–392, 1961.
 149. Hatze, H. The relative contribution of motor unit recruitment and rate coding to the production of static isometric muscle force. Biol. Cybernetics 27: 21–25, 1977.
 150. Hatze, H. A teleological explanation of Weber's Law and the motor unit size law. Bull. Math. Biol. 41: 407–425, 1979.
 151. Hatze, H., and J. D. Buys. Energy‐optimal controls in the mammalian neuromuscular system. Biol. Cybernetics 27: 9–20, 1977.
 152. Henatsch, H. D., and F. J. Schulte. Reflexerregung und Eigenhemmung tonischer und phossicher Alpha‐motoneurone wahrend chemischer Dauererregung der Muskelspindeln. Pfluegers Arch. 268: 134–147, 1958.
 153. Henneman, E. Relation between size of neurons and their susceptibility to discharge. Science 126: 1345–1347, 1957.
 154. Henneman, E. Skeletal muscle: the servant of the nervous system. In: Medical Physiology (14th ed.), edited by V. B. Mountcastle. St. Louis: Mosby, 1980, p. 674–702.
 155. Henneman, E. Organization of the motoneuron pool: the size principle. In: Medical Physiology (14th ed.), edited by V. B. Mountcastle. St. Louis: Mosby, 1980, p. 718–741.
 156. Henneman, E., H. P. Clamann, J. D. Gillies, and R. D. Skinner. Rank‐order of motoneurons within a pool: law of combination. J. Neurophysiol. 37: 1338, 1974.
 157. Henneman, E., H.‐R. Lüscher, and P. Ruenzel. Terminal distribution of Ia and group II spindle afferent fibers in the spinal cord as revealed by postsynaptic population potentials recorded from two adjacent spinal segments. Neurosci. Abstr. 5: 724, 1979.
 158. Henneman, E., and C. B. Olson. Relations between structure and function in the design of skeletal muscles. J. Neurophysiol. 28: 581–598, 1965.
 159. Henneman, E., B. T. Shahani, and R. R. Young. Voluntary control of human motor units. In: The Motor System: Neurophysiology and Muscle Mechanisms, edited by M. Shahani. Amsterdam: Elsevier, 1976, p. 73–78.
 160. Henneman, E., G. Somjen, and D. O. Carpenter. Functional significance of cell size in spinal motoneurons. J. Neurophysiol. 28: 560–580, 1965.
 161. Henneman, E., G. Somjen, and D. O. Carpenter. Excitability and inhibitibility of motoneurons of different sizes. J. Neurophysiol. 28: 599–620, 1965.
 162. Hodes, R., S. M. Peacock, Jr., and D. Bodian. Selective destruction of large motoneurons by poliomyelitis virus. II. Size of motoneurons in the spinal cord of rhesus monkeys. J. Neuropathol. Exp. Neurol. 8: 400–410, 1949.
 163. Hollyday, M., and V. Hamburger. Reduction of the naturally occurring motor neuron loss by enlargement of the periphery. J. Comp. Neurol. 170: 311–320, 1976.
 164. Hongo, T., N. Ishizuka, H. Mannen, and S. Sasaki. Axonal trajectory of single group Ia and Ib fibers in the cat spinal cord. Neurosci. Letters 8: 321–328, 1978.
 165. Hongo, T., E. Jankowska, and A. Lundberg. The rubrospinal tract. I. Effects on alpha motoneurons innervating hindlimb muscles in cats. Exp. Brain Res. 7: 344–364, 1969.
 166. Horridge, G. A., and M. Burrows. Tonic and phasic systems in parallel in the eyecup responses of the crab Carcinus. J. Exp. Biol. 49: 269, 1968.
 167. Houk, J., and E. Henneman. Responses of Golgi tendon organs to active contractions of the soleus muscle of the cat. J. Neurophysiol. 30: 466–481, 1967.
 168. Howland, B., J. Y. Lettvin, W. S. McCulloch, W. Pitts, and P. D. Wall. Reflex inhibition by dorsal root interaction. J. Neurophysiol. 18: 1–17, 1955.
 169. Hoyle, G. Exploration of neuronal mechanisms underlying behaviour in insects. In: Neural Theory and Modeling: Proceedings, edited by R. F. Reiss. Stanford, CA: Stanford Univ. Press, 1964, p. 346–376. (Ojai Symp., 1962.).
 170. Hughes, G. M., and C. M. Ballintijn. Electromyography of the respiratory muscles and gill water flow in the dragonet. J. Exp. Biol. 49: 583, 1968.
 171. Huizar, P., N. Kudo, M. Kuno, and Y. Miyata. Reaction of intact spinal motoneurons to partial denervation of the muscle. J. Physiol. London 265: 175–191, 1977.
 172. Hultborn, H. Transmission in the pathway of reciprocal Ia inhibition to motoneurons and its control during the tonic stretch reflex. In: Progress in Brain Research. Understanding the Stretch Reflex, edited by S. Homma. Amsterdam: Elsevier, 1976, vol. 44, p. 235–255.
 173. Hultborn, H., E. Jankowska, and S. Lindstrom. Recurrent inhibition from motor axon collaterals of transmission in the Ia inhibitory pathway to motoneurons. J. Physiol. London 215: 591–612, 1971.
 174. Hunt, C. C. Monosynaptic reflex response of spinal motoneurons to graded afferent stimulation. J. Gen. Physiol. 38: 813–832, 1955.
 175. Hursh, J. B. Conduction velocity and diameter of nerve fibers. Am. J. Physiol. 127: 131–139, 1939.
 176. Iansek, R., and S. J. Redman. An analysis of the cable properties of spinal motoneurons using a brief intracellular current pulse. J. Physiol. London 234: 613–636, 1973.
 177. Iansek, R., and S. J. Redman. The amplitude, time course and charge of unitary excitatory post‐synaptic potentials evoked in spinal motoneuron dendrites. J. Physiol. London 234: 665–688, 1973.
 178. Iles, J. F. Central terminations of muscle afferents on motoneurons in the cat spinal cord. J. Physiol. London 262: 91–117, 1976.
 179. Iles, J. F. The speed of passive dendritic conduction of synaptic potentials in a model motoneuron. Proc. R. Soc. London Ser. B 197: 225–229, 1977.
 180. Illis, L. Spinal cord synapses in the cat: the normal appearances by the light microscope. Brain 87: 543–554, 1964.
 181. Illis, L. The relative densities of monosynaptic pathways to the cell bodies and dendrites of the cat ventral horn. J. Neurol. Sci. 4: 259–270, 1967.
 182. Jack, J. J. B., S. Miller, R. Porter, and S. J. Redman. The distribution of group Ia synapses on lumbrosacral spinal motoneurons in the cat. In: Excitatory Synaptic Mechanisms, edited by P. Anderson and J. K. S. Jansen. Oslo: Universitets‐forlaget, 1970, p. 199–205.
 183. Jack, J. J. B., S. Miller, R. Porter, and S. J. Redman. The time course of minimal excitatory post‐synaptic potentials evoked in spinal motoneuron by group Ia afferent fibers. J. Physiol. London 215: 353–380, 1971.
 184. Jack, J. J. B., D. Noble, and R. W. Tsien. Electric Current Flow in Excitable Cells. Oxford: Clarendon, 1975, p. 502.
 185. Jack, J. B., and R. Porter. Rapidly and slowly rising components of monosynaptic excitatory postsynaptic potentials in spinal motoneurons. J. Physiol. London 186: 106–107, 1966.
 186. Jami, L., and J. Petit. Correlation between axonal conduction velocity and tetanic tension of motor units in four muscles of the cat hind limb. Brain Res. 96: 114–118, 1975.
 187. Jankowska, E., and S. Lindstrom. Morphological identification of Renshaw cells. Acta Physiol. Scand. 81: 428–430, 1971.
 188. Jankowska, E., Y. Padel, and R. Tanaka. Projections of pyramidal tract cells to alpha motoneurons innervating hind limb muscles in the monkey. J. Physiol. London 249: 637–667, 1975.
 189. Jankowska, E., and W. J. Roberts. An electrophysiological demonstration of the axonal projections of single spinal interneurons in the cat. J. Physiol. London 222: 597–622, 1972.
 190. Jankowska, E., and W. J. Roberts. Synaptic actions of single interneurons mediating reciprocal Ia inhibition of motoneurons. J. Physiol. London 222: 623–642, 1972.
 191. Jankowska, E., and D. O. Smith. Antidromic activation of Renshaw cells and their axonal projections. Acta Physiol. Scand. 88: 198–214, 1973.
 192. Kaiser, E. I., and I. Petersen. Muscle action potentials studied by frequency analysis and duration measurements. Acta Neurol. Scand. 41: (Suppl. 13) 213–236, 1965.
 193. Kanda, K., R. E. Burke, and B. Walmsley. Differential control of fast and slow twitch motor units in the decerebrate cat. Exp. Brain Res. 29: 57–74, 1977.
 194. Kato, M., and K. Fukushima. Effect of differential blocking of motor axons on antidromic activation of Renshaw cells. Exp. Brain Res. 20: 135–143, 1974.
 195. Kato, M., and J. Tanji. Volitionally controlled single motor units in human finger muscles. Brain Res. 40: 345–357, 1972.
 196. Katz, B. The transmission of impulses from nerve to muscle, and the subcellular unit of synaptic action. Proc. R. Soc. London Ser. B 155: 455–477, 1962.
 197. Katz, B., and R. Miledi. A study of spontaneous miniature potentials in spinal motoneurons. J. Physiol. London 168: 389–422, 1963.
 198. Katz, B., and R. Miledi. The measurement of synaptic delay and the time course of acetylcholine released at the neuromuscular junction. Proc. R. Soc. London Ser. B 161: 483–495, 1965.
 199. Katz, B., and R. Miledi. A study of synaptic transmission in the absence of nerve impulses. J. Physiol. London 192: 407–436, 1967.
 200. Katz, B., and S. Thesleff. On the factors which determine the amplitude of the miniature end plate potential. J. Physiol. London 137: 267–278, 1957.
 201. Kernell, D. The adaptation and the relation between discharge frequency and current strength of cat lumbosacral motoneurons stimulated by long‐lasting injected currents. Acta Physiol. Scand. 65: 65–73, 1965.
 202. Kernell, D. High‐frequency repetitive firing of cat lumbosacral motoneurons stimulated by long‐lasting injected currents. Acta Physiol. Scand. 65: 74–86, 1965.
 203. Kernell, D. The limits of firing frequency in cat lumbosacral motoneurons possessing different time course of afterhyper‐polarization. Acta Physiol. Scand. 65: 87–100, 1965.
 204. Kernell, D. Input resistance, electrical excitability, and size of ventral horn cells in cat spinal cord. Science 152: 1637–1640, 1966.
 205. Kernell, D. Recruitment, rate modulation and the tonic strength reflex. Progress in Brain Research. Understanding the Stretch Reflex, edited by S. Homma. Amsterdam: Elsevier, 1976, vol. 44, p. 257–266.
 206. Kernell, D., and H. Sjoholm. Repetitive impulse firing: comparisons between neuron models based on “voltage clamp equations” and spinal motoneurons. Acta Physiol. Scand. 87: 40–56, 1973.
 207. Kernell, D., and H. Sjöholm. Recruitment and firing rate modulation of motor unit tension in a small muscle of the cat's foot. Brain Res. 98: 57–72, 1975.
 208. Kirkwood, P. A., and T. A. Sears. Monosynaptic excitation of motoneurons from secondary endings of muscle spindles. Nature London 252: 243–244, 1974.
 209. Kirkwood, P. A., and T. A. Sears. Monosynaptic excitation of motoneurons from muscle spindle secondary endings of intercostal and triceps surae muscles in the cat. J. Physiol. London 245: 64–66, 1975.
 210. Kirkwood, P. A., and T. A. Sears. The synaptic connexions to intercostal motoneurons as revealed by the average common excitation potential. J. Physiol. London 275: 103–134, 1978.
 211. Knott, S., D. M. Lerois, and J. C. Luck. Motor unit area in a cat limb muscle. Exp. Neurol. 30: 475–483, 1971.
 212. Knox, C. K. Cross‐correlation functions for a neuronal model. Biophys. J. 14: 567–582, 1974.
 213. Koziol, J. A., and H. C. Tuckwell. Analysis and estimation of synaptic densities and their spatial variation on the motoneuron surface. Brain Res. 150: 617–624, 1978.
 214. Krnjević, K., and R. J. Miledi. Failure of neuromuscular propagation in rats. J. Physiol. London 140: 440–461, 1958.
 215. Kuno, M. Excitability following antidromic activation in spinal motoneurons supplying red muscles. J. Physiol. London 149: 374–393, 1959.
 216. Kuno, M. Quantal components of excitatory synaptic potentials in spinal motoneurons. J. Physiol. London 175: 81–99, 1964.
 217. Kuno, M. Mechanism of facilitation and depression of the excitatory synaptic potential in spinal motoneurons. J. Physiol. London 175: 100–112, 1964.
 218. Kuno, M. Quantum aspects of central and ganglionic synaptic transmission in vertebrates. Physiol. Rev. 51: 647–678, 1971.
 219. Kuno, M., and R. Llinas. Enhancement of synaptic transmission by dendritic potentials in chromatolyzed motoneurons of the cat. J. Physiol. London 210: 807–821, 1970.
 220. Kuno, M., and J. T. Miyahara. Non‐linear summation of unit synaptic potentials in spinal motoneurons of the cat. J. Physiol. London 201: 465–477, 1969.
 221. Kuno, M., and J. T. Miyahara. Analysis of synaptic efficacy in spinal motoneurons from “quantum” aspects. J. Physiol. London 201: 479–493, 1969.
 222. Kuno, M., Y. Miyata, and E. J. Muñnoz‐Martinez. Differential reactions of fast and slow alpha motoneurons to axotomy. J. Physiol. London 240: 725–739, 1974.
 223. Kuno, M., E. J. Muñoz‐Martinez, and M. Randic. Synaptic action in Clarke's column neurons in relation to afferent terminal size. J. Physiol. London 228: 343–360, 1973.
 224. Kuno, M., S. A. Turkanis, and J. N. Weakly. Correlation between nerve terminal size and transmitter release at the neuromuscular junction of the frog. J. Physiol. London 213: 545–566, 1971.
 225. Kuno, M., and J. N. Weakly. Quantal components of the inhibitory synaptic potential in spinal motoneurons of the cat. J. Physiol. London 224: 287–303, 1972.
 226. Lamb, A. The projection patterns of the ventral horn to the hind limb during development. Dev. Biol. 54: 82–99, 1976.
 227. Lamb, A. H. Neuronal death in the development of the somatotopic projections of the ventral horn in Xenopus. Brain Res. 134: 145–150, 1977.
 228. Landmesser, L. The development of motor projection patterns in the chick hind limb. J. Physiol. London 284: 391–414, 1978.
 229. Landmesser, L., and D. G. Morris. The development of functional innervation in the hind limb of the chick embryo. J. Physiol. London 249: 301–326, 1975.
 230. Laporte, Y., and D. Lloyd. Nature and significance of the reflex connections established by large afferent fibers of muscular origin. Am. J. Physiol. 169: 609–621, 1952.
 231. La eVail, J. H., and M. M. La eVail. The retrograde intra‐axonal transport of horseradish peroxidase in the chick visual system: a light and electron microscopic study. J. Comp. Neurol. 157: 303–358, 1974.
 232. Lennerstrand, G. Position and velocity sensitivity of muscle spindles in the cat. I. Primary and secondary endings deprived of fusimotor activation. Acta Physiol. Scand. 73: 281–299, 1968.
 233. Letbetter, W. D., W. D. Willis, and W. M. Thompson. Monosynaptic excitation of motoneurons by single dorsal root ganglion cells. Federation Proc. 27: 749, 1968.
 234. Liddell, E. G. T., and C. S. Sherrington. Reflexes in response to stretch (myotatic reflexes). Proc. R. Soc. London Ser. B 96: 212–242, 1924.
 235. Liley, A. W. The effects of presynaptic polarization on the spontaneous activity of the mammalian neuromuscular junction. J. Physiol. London 134: 427–443, 1956.
 236. Lloyd, D. P. C. Reflex action in relation to pattern and peripheral source of afferent stimulation. J. Neurophysiol. 6: 111–119, 1943.
 237. Lloyd, D. P. C. Integrative pattern of excitation and inhibition in two‐neuron reflex arcs. J. Neurophysiol. 9: 439–444, 1946.
 238. Lloyd, D. P. C. Post‐tetanic potentiation of response in monosynaptic reflex pathways of the spinal cord. J. Gen. Physiol. 33: 147–170, 1949.
 239. Loeb, G. Decreased conduction velocity in the proximal projections of myelinated dorsal root ganglion cells in the cat. Brain Res. 103: 381–385, 1976.
 240. Lorente de Nó, R. Limits of variation of the synaptic delay of motoneurons. J. Neurophysiol. 1: 187–194, 1938.
 241. Lucas, M. E., and W. D. Willis. Identification of muscle afferents which activate interneurons in the intermediate nucleus. J. Neurophysiol. 37: 282–293, 1974.
 242. Lund, S., and O. Pompeiano. Monosynaptic excitation of alpha motoneurons from supraspinal structures in the cat. Acta Physiol. Scand. 73: 1–21, 1968.
 243. Lundberg, A., K. Malmgren, and E. D. Schomburg. Role of joint afferents in motor control exemplified by effects on reflex pathways from Ib afferents. J. Physiol. London 284: 327–343, 1978.
 244. Lundberg, A., U. Norsell, and P. Voorhoeve. Pyramidal effects on lumbo‐sacral interneurons activated by somatic afferents. Acta Physiol. Scand. 56: 220–229, 1962.
 245. Lüscher, H.‐R., P. Ruenzel, E. Fetz, and E. Henneman. Postsynaptic population potentials recorded from ventral roots perfused with isotonic sucrose: connections of groups Ia and II spindle afferent fibers with large populations of motoneurons. J. Neurophysiol. 42: 1146–1164, 1979.
 246. Lüscher, H.‐R., P. Ruenzel, and E. Henneman. How the size of motoneurons determines their susceptibility to discharge. Nature London 282: 859–861, 1979.
 247. Lüscher, H.‐R., P. Ruenzel, and E. Henneman. Topographic distribution of terminals Ia and Group II fibers in the spinal cord, as revealed by post‐synaptic population potentials. J. Neurophysiol. 43: 968–985, 1980.
 248. Lux, H. D., P. Schubert, and G. W. Kreutzberg. Direct matching of morphological and electrophysiological data in cat spinal motoneurons. In: Excitatory Synaptic Mechanisms, edited by P. Anderson and J. K. S. Jansen. Oslo: Universitetsforlaget, 1970, p. 189–198.
 249. Magherini, P. C., W. Precht, and P. C. Schwindt. Evidence for electrotonic coupling between frog motoneurons in the in situ spinal cord. J. Neurophysiol. 39: 474–483, 1976.
 250. Matthews, P. B. C. The dependence of tension upon extension in the stretch reflex of the soleus muscle of the decerebrate cat. J. Physiol. London 147: 521–546, 1959.
 251. Matthews, P. B. C. The response of de‐efferented muscle spindle receptors to stretching at different velocities. J. Physiol. London 168: 660–678, 1963.
 252. Matthews, M. A., W. D. Willis, and V. Williams. Dendrite bundles in lamina IX of cat spinal cord: a possible source for electrical interaction between motoneurons. Anat. Rec. 171: 313–328, 1971.
 253. McLaughlin, B. J. Dorsal root projections to the motor nuclei in the cat spinal cord. J. Comp. Neurol. 144: 461–474, 1972.
 254. McPhedran, A. M., R. B. Wuerker, and E. Henneman. Properties of motor units in a homogeneous red muscle (soleus) of the cat. J. Neurophysiol. 28: 71–84, 1965.
 255. Mendell, L. M., and E. Henneman. Terminals of single Ia fibers: distribution within a pool of 300 homonymous motor neurons. Science 160: 96–98, 1968.
 256. Mendell, L. M., and E. Henneman. Terminals of single Ia fibers: location, density, and distribution within a pool of 300 homonymous motoneurons. J. Neurophysiol. 34: 171–187, 1971.
 257. Mendell, L. M., and R. Weiner. Analysis of pairs of individual Ia‐EPSPs in single motoneurons. J. Physiol. London 255: 81–104, 1976.
 258. Mill, P. J. Neural patterns associated with ventilatory movements in dragonfly larvae. J. Exp. Biol. 52: 67, 1970.
 259. Milner‐Brown, H. S., R. B. Stein, and R. Yemm. The orderly recruitment of human motor units during voluntary isometric contractions. J. Physiol. London 230: 359–370, 1973.
 260. Milner‐Brown, H. S., R. B. Stein, and R. Yemm. Changes in firing rate of human motor units during linearly changing voluntary contractions. J. Physiol. London 230: 371–390, 1973.
 261. Mishelevich, D. J. Repetitive firing to current in cat motoneurons as a function of muscle unit twitch type. Exp. Neurol. 25: 401–409, 1969.
 262. Monster, A. W., and H. Chan. Isometric force production by motor units of extensor digitorum communis muscle in man. J. Neurophysiol. 40: 1432–1443, 1977.
 263. Mosher, C. G., R. L. Gerlach, and D. G. Stuart. Soleus and anterior tibial motor units of the cat. Brain Res. 44: 1–11, 1972.
 264. Munson, J. B., and G. W. Sypert. Latency‐rise time relationship in unitary postsynaptic potentials. Brain Res. 151: 404–408, 1978.
 265. Munson, J. B., and G. W. Sypert. Properties of central Ia afferent fibers projecting to motoneurons. Neurosci. Abstr. 4: 569, 1978.
 266. Munson, J. B., and G. W. Sypert. Properties of single central Ia fibers projection to motoneurons. J. Physiol. London 296: 315–328, 1979.
 267. Munson, J. B., and G. W. Sypert. Properties of single fiber excitatory post‐synaptic potentials in triceps surae motoneurons. J. Physiol. London 296: 329–342, 1979.
 268. Nelson, P. G. Interaction between spinal motoneurons of the cat. J. Neurophysiol. 29: 275–287, 1966.
 269. Nelson, S. G., T. C. Collatos, A. Niechaj, and L. M. Mendell. Immediate increase in Ia‐motoneuron synaptic transmission caudal to spinal cord transection. J. Neurophysiol. 42: 655–664, 1979.
 270. Nelson, S. G., and L. M. Mendell. Projection of single knee flexor Ia fibers to homonymous and heteronymous motoneurons. J. Neurophysiol. 41: 778–787, 1978.
 271. Nelson, S. G., and L. M. Mendell. Enhancement in Ia‐motoneuron synaptic transmission caudal to chronic spinal cord transection. J. Neurophysiol. 42: 642–654, 1979.
 272. Nystrom, B. Postnatal development of motor nerve terminals in “slow‐red” and “fast‐white” cat muscles. Acta Neurol. Scand. 44: 363–383, 1968.
 273. Olson, C. B., D. O. Carpenter, and E. Henneman. Orderly recruitment of muscle action potentials: motor unit threshold and EMG amplitude. Arch. Neurol. Chicago 19: 591–597, 1968.
 274. Olson, C. B., and C. P. Swett, Jr. A functional and histo‐chemical characterization of motor units in a heterogeneous muscle (flexor digitorum longus) of the cat. J. Comp. Neurol. 128: 475–498, 1966.
 275. Padykula, H. A., and G. F. Gauthier. The ultrastructure of the neuromuscular junctions of mammalian red, white and intermediate skeletal muscle fibers. J. Cell Biol. 46: 27–41, 1970.
 276. Parnas, I. Differential block at high frequency of branches of a single axon innervating two muscles. J. Neurophysiol. 35: 903–914, 1972.
 277. Pascoe, J. E. The responses of anterior horn cells to applied currents. Acta Physiol. Scand. 42: (Suppl. 145) 112–113, 1957.
 278. Pearson, K. G., and J. F. Iles. Discharge patterns of coxal levator and depressor motoneurons of the cockroach, Periplaneta americana. J. Exp. Biol. 52: 139, 1970.
 279. Pearson, K. G., R. B. Stein, and S. K. Malhotra. Properties of action potentials from insect motor nerve fibers. J. Exp. Biol. 53: 299–316, 1970.
 280. Penny, J. E., J. R. Kukums, M. J. Eadie, and J. H. Tyrer. Quantitative oxidative enzyme histochemistry of the spinal cord. Part 1. Distribution of enzyme activity in anterior horn cells. J. Neurol. Sci. 26: 179–185, 1975.
 281. Peterson, R. P. Cell size and rate of protein synthesis in ventral horn neurons. Science 153: 1413–1414, 1966.
 282. Pette, D., M. Klingensberg, and Th. Bücher. Comparable and specific proportions in the mitochondrial enzyme activity pattern. Biochem. Biophys. Res. Commun. 7: 425–429, 1962.
 283. Porter, R., and J. Hore. Time course of minimal corticomotoneuronal excitatory postsynaptic potentials in lumbar motoneurons of the monkey. J. Neurophysiol. 32: 443–451, 1969.
 284. Proske, U., and P. M. E. Waite. Properties of types of motor units in the medial gastrocnemius muscle of the cat. Brain Res. 67: 89–101, 1974.
 285. Rack, P. M. H., and D. R. Westbury. The effects of length and stimulus rate on tension in the isometric cat soleus muscle. J. Physiol. London 204: 443–460, 1969.
 286. Rall, W. Branching dendritic trees and motoneuron membrane resistivity. Exp. Neurol. 1: 491–527, 1959.
 287. Rall, W. Theoretical significance of dendrites for neuronal input‐output relations. In: Neuronal Theory and Modeling: Proceedings, edited by R. Reiss. Stanford, CA: Stanford Univ. Press, 1964, p. 73–97. (Ojai Symp., 1962.).
 288. Rall, W. Distinguishing theoretical synaptic potentials computed for different soma‐dendritic distributions of synaptic input. J. Neurophysiol. 30: 1138–1168, 1967.
 289. Rall, W. Core conductor theory and cable properties of neurons. In: Handbook of Physiology. The Nervous System. Cellular Biology of Neurons, edited by E. Kandel. Bethesda, MD: Am. Physiol. Soc., 1977, sect. 1, vol. I, pt. 1, chapt. 3, p. 39–97.
 290. Rall, W., R. E. Burke, T. G. Smith, P. G. Nelson, and K. Frank. Dendritic location of synapses and possible mechanisms for the monosynaptic EPSP in motoneurons. J. Neurophysiol. 30: 1169–1193, 1967.
 291. Rall, W., and J. Rinzel. Branch input resistance and steady attenuation for input to one branch of a dendritic neuron model. Biophys. J. 13: 648–688, 1973.
 292. Ramón y Cajal, S. Histologie du Système de l'Homme et des Vertèbres. Paris: Maloine, 1909.
 293. Rapaport, S., A. Susswein, Y. Uchino, and V. J. Wilson. Synaptic actions of individual vertibular neurons on cat neck motoneurons. J. Physiol. London 272: 367–382, 1977.
 294. Redfern, P. A. Neuromuscular transmission in newborn rats. J. Physiol. London 209: 701–709, 1970.
 295. Redman, S. J. A quantitative approach to the integrative function of dendrites. In: International Review of Physiology: Neurophysiology II, edited by R. Porter. Baltimore: Univ. Park, 1976, vol. 10, p. 1–36.
 296. Redman, S. J. Junctional mechanisms at group Ia synapses. Prog. Neurobiol. NY 12: 33–83, 1979.
 297. Renshaw, B. Activity in the simplest spinal reflex pathways. J. Neurophysiol. 3: 373–387, 1940.
 298. Renshaw, B. Influence of discharge of motoneurons upon excitation of neighboring motoneurons. J. Neurophysiol. 4: 167–183, 1941.
 299. Renshaw, B. Central effects of centripetal impulses in axons of spinal ventral roots. J. Neurophysiol. 9: 191–204, 1946.
 300. Richmond, F. J. R., and V. C. Abrahams. Morphology and enzyme histochemistry of dorsal muscles of the cat neck. J. Neurophysiol. 38: 1312–1321, 1975.
 301. Rinzel, J., and W. Rall. Transient response in a dendritic neuron model for current injected at one branch. Biophys. J. 14: 759–790, 1974.
 302. Roeder, K. D., L. Tozian, and E. A. Weiant. Endogenous nerve activity and behavior in the mantis and cockroach. J. Insect Physiol. 4: 45, 1960.
 303. Romanes, G. J. The motor cell columns of the lumbo‐sacral spinal cord of the cat. J. Comp. Neurol. 94: 313–363, 1951.
 304. Ronnevi, L. O., and S. Conradi. Ultrastructural evidence for spontaneous elimination of synaptic terminals on spinal motor neurons in the kitten. Brain Res. 80: 335–339, 1974.
 305. Rowell, C. H. F., and J.M. McKay. An acridid auditory interneuron. I. Functional connexions and response to single sounds. J. Exp. Biol. 51: 231, 1969.
 306. Rudomín, P., R. E. Burke, R. Núñez, J. Madrid, and H. Dutton. Control by presynaptic correlation: a mechanism affecting information transmission from Ia fibers to motoneurons. J. Neurophysiol. 38: 267–284, 1975.
 307. Ryall, R. W. Renshaw cell mediated inhibition of Renshaw cells: patterns of excitation and inhibition from impulses in motor axon collaterals. J. Neurophysiol. 33: 257–270, 1970.
 308. Ryall, R. W., M. F. Piercey, C. Polosa, and J. Goldfarb. Excitation of Renshaw cells in relation to orthodromic and antidromic excitation of motoneurons. J. Neurophysiol. 35: 137–148, 1972.
 309. Salmons, S., and F. A. Sreter. Significance of impulse activity in the transformation of skeletal muscle type. Nature London 263: 30–34, 1976.
 310. Scheibel, M. E., and A. B. Scheibel. Spinal motoneurons, interneurons and Renshaw cells. A Golgi study. Arch. Ital. Biol. 104: 328–353, 1966.
 311. Scheibel, M. E., and A. B. Scheibel. Terminal patterns in cat spinal cord. III. Primary afferent collaterals. Brain Res. 13: 417–443, 1969.
 312. Scheibel, M. E., and A. B. Scheibel. Organization of spinal motoneuron dendrites in bundles. Exp. Neurol. 28: 106–112, 1970.
 313. Scheibel, M. E., and A. B. Scheibel. Developmental relationship between spinal motor neuron dendrite bundles and patterned activity in the forelimb of cats. Exp. Neurol. 30: 367–373, 1971.
 314. Scheibel, M. E., and A. B. Scheibel. Inhibition and the Renshaw cell: a structural critique. Brain Behav. Evol. 4: 53–93, 1971.
 315. Schmidt, R. F. Presynaptic inhibition in the vertebrate nervous system. Ergeb. Physiol. Biol. Chem. Exp. Pharmakol. 63: 20–101, 1971.
 316. Scott, J. G., and L. M. Mendell. Individual EPSPs produced by single triceps surae Ia afferent fibers in homonymous and heteronymous motoneurons. J. Neurophysiol. 39: 679–692, 1976.
 317. Sears, T. A. Some properties and reflex connections of respiratory motoneurons of the cat's thoracic spinal cord. J. Physiol. London 175: 386–403, 1964.
 318. Sears, T. A., and D. Stagg. Short‐term synchronization of intercostal motoneuron activity. J. Physiol. London 263: 357–381, 1976.
 319. Shapovalov, A. I., and N. R. Gurevich. Monosynaptic and disynaptic reticulospinal actions on lumbar motoneurons of the cat. Brain Res. 21: 249–263, 1970.
 320. Shapovalov, A. I., O. A. Karamyan, G. G. Kurchavyi, and Z. A. Repina. Synaptic actions evoked from the red nucleus on the spinal alpha‐motoneurons in the Rhesus monkey. Brain Res. 32: 325–348, 1971.
 321. Shapovalov, A. I., and G. G. Kurchavyi. Effects of transmembrane polarization and TEA injection on monosynaptic actions from motor cortex, red nucleus and group Ia afferents on lumbar motoneurons in the monkey. Brain Res. 82: 49–67, 1974.
 322. Sherrington, C. S. The Integrative Action of the Nervous System. New Haven, CT: Yale Univ. Press, 1906.
 323. Sherrington, C. S. Flexion‐reflex of the limb, crossed extension‐reflex and reflex stepping and standing. J. Physiol. London 40: 28–121, 1910.
 324. Shinoda, Y., A. P. Arnold, and H. Asanuma. Spinal branching of corticospinal axons in the cat. Exp. Brain Res. 26: 215–234, 1976.
 325. Snow, P. J., P. K. Rose, and A. G. Brown. Tracing axons and axon collaterals of spinal neurons using intracellular injection of horseradish peroxidase. Science 191: 312–313, 1976.
 326. Somjen, G., D. O. Carpenter, and E. Henneman. Responses of motoneurons of different sizes to graded stimulation of supraspinal centers of the brain. J. Neurophysiol. 28: 958–965, 1965.
 327. Sperry, R. W. Chemoaffinity in the orderly growth of nerve fiber patterns and connections. Proc. Natl. Acad. Sci. USA. 50: 703–710, 1951.
 328. Sprague, J. M. The distribution of dorsal root fibers on motor cells in lumbosacral spinal cord of the cat and the site of excitatory and inhibitory terminals in monosynaptic pathways. Proc. R. Soc. London Ser. B 149: 534–556, 1958.
 329. Stauffer, E. K., D. G. D. Watt, A. Taylor, R. M. Reinking, and D. G. Stuart. Analysis of muscle receptor connections by spike triggered averaging. 2. Spindle group II afferents. J. Neurophysiol. 39: 1393–1402, 1976.
 330. Stein, R. B., and R. Bertoldi. The size principle: a synthesis of neurophysiological data. In: Progress in Clinical Neurophysiology. Recruitment Patterns of Motor Units and the Gradation of Muscle Force, edited by J. E. Desmedt. Basel: Karger, In press.
 331. Stephens, J. A., R. Garnett, and N. P. Buller. Reversal of recruitment order of single motor units produced by cutaneous stimulation during voluntary muscle contraction in man. Nature London 272: 362–364, 1978.
 332. Sterling, P., and H. G. J. M. Kuypers. Simultaneous demonstration of normal boutons and degenerating nerve fibers and their terminals in the spinal cord. J. Anat. 100: 723–732, 1966.
 333. Sterling, P., and H. G. J. M. Kuypers. Anatomical organization of the brachial spinal cord of the cat. II. The motoneuron plexus. Brain Res. 4: 16–32, 1967.
 334. Szekely, G., G. Czeh, and G. Voros. The activity pattern of limb muscles in freely moving normal and deafferented newts. Exp. Brain Res. 9: 53, 1969.
 335. Szentagothai, J. The anatomical basis of synaptic transmission of excitation and inhibition of motoneurons. Acta Morphol. Acad. Sci. Hung. 8: 287–309, 1958.
 336. Tanji, J., and M. Kato. Recruitment of motor units in voluntary contraction of a finger muscle in man. Exp. Neurol. 40: 759–770, 1973.
 337. Teig, E. Tension and contraction times of motor units of the middle ear muscles in the cat. Acta Physiol. Scand. 84: 11–21, 1972.
 338. Thomas, J. S., E. M. Schmidt, and F. T. Hambrecht. Facility of motor unit control during tasks defined directly in terms of unit behaviors. Exp. Neurol. 59: 384–395, 1978.
 339. Traub, R. D. Motoneurons of different geometry and the size principle. Biol. Cybernetics 25: 163–176, 1977.
 340. Van Buren, J. M., and K. Frank. Correlation between the morphology and potential field of a spinal motor nucleus in the cat. Electroencephalogr. Clin. Neurophysiol. 19: 112–126, 1965.
 341. Van De Graaff, K. M., E. C. Frederick, R. G. Williamson, and G. E. Goslow, Jr. Motor units and fiber types of primary ankle extensors of the skunk (Mephitis mephitis). J. Neurophysiol. 40: 1424–1431, 1977.
 342. Wagman, I. H., D. S. Pierce, and R. E. Burges. Proprioceptive influence in volitional control of individual motor units. Nature London 207: 957–958, 1965.
 343. Walcott, B., and M. Burrows. The ultrastructure and physiology of the abdominal airguide retractor muscles in the giant water bug, Lethocerus. J. Insect Physiol. 15: 1865, 1969.
 344. Wani, A. M., and S. K. Guha. A model for gradation of tension‐recruitment and rate coding. Med. Biol. Eng. 13: 870–874, 1975.
 345. Warmoltz, J. R., and W. K. Engel. Open‐biopsy electromyography. I. Correlation of motor unit behavior with histo‐chemical muscle fiber type in human limb muscle. Arch. Neurol. 27: 512–517, 1972.
 346. Watt, D. G. D., E. K. Stauffer, A. Taylor, R. M. Reinking, and D. G. Stuart. Analysis of muscle receptor connections by spike‐triggered averaging. 1. Spindle primary and tendon organ afferents. J. Neurophysiol. 39: 1375–1392, 1976.
 347. Wendt, I. R., and C. L. Gibbs. Energy production of rat extensor digitorum longus muscle. Am. J. Physiol. 224: 1081–1086, 1973.
 348. Willis, W. D., R. Núñez, and P. Rudomín. Excitability changes of terminal arborizations of single Ia and Ib afferent fibers produced by muscle and cutaneous conditioning volleys. J. Neurophysiol. 39: 1150–1159, 1976.
 349. Willis, W. D., and J. C. Willis. Properties of interneurons in the ventral spinal cord. Arch. Ital. Biol. 104: 354–386, 1966.
 350. Wilson, V. J., and M. Yoshida. Comparison of effects of stimulation of Deiters' nucleus and medial longitudinal fasciculus on neck, forelimb, and hindlimb motoneurons. J. Neurophysiol. 32: 743–758, 1969.
 351. Wilson, V. J., and M. Yoshida. Monosynaptic inhibition of neck motoneurons by the medial vestibular nucleus. Exp. Brain Res. 9: 365–380, 1969.
 352. Windhorst, V. Cross correlations between discharge patterns of primary muscle spindle endings in active triceps‐surae muscles of the cat. Neurosci. Lett. 5: 63–67, 1977.
 353. Wuerker, R. B., and E. Henneman. Reflex regulation of primary (annulospiral) stretch receptors via gamma motoneurons in the cat. J. Neurophysiol. 26: 539–550, 1963.
 354. Wuerker, R. B., A. M. McPhedran, and E. Henneman. Properties of motor units in a heterogeneous pale muscle (M. gastrocnemius) of the cat. J. Neurophysiol. 28: 85–99, 1965.
 355. Wyckoff, R. W. G., and J. Z. Young. The motor neuron surface. Proc. R. Soc. London Ser. B 144: 440–450, 1955.
 356. Wyman, R. J. Somatotopic connectivity or species recognition connectivity? In: Control of Posture and Locomotion, edited by R. B. Stein, K. B. Pearson, R. S. Smith, and J. B. Redford. New York: Plenum, 1974, p. 45–53. (Adv. Behav. Biol. Ser., vol. 7.).
 357. Wyman, R. J., I. Waldron, and G. W. Wachtel. Lack of fixed order of recruitment in cat motoneuron pools. Exp. Brain Res. 20: 101–114, 1974.
 358. Yatsuki, Y., S. Yoshino, and J. Chen. Action current of the single lateral line nerve of the fish. II. On the discharge due to stimulation. Jpn. J. Physiol. 1: 179–194, 1951.
 359. Yellin, H. Neural regulation of enzymes in muscle fibers of red and white muscle. Exp. Neurol. 19: 92–103, 1967.
 360. Yemm, R. The orderly recruitment of motor units of the masseter and temporal muscles during voluntary isometric contraction in man. J. Physiol. London 265: 163–174, 1977.
 361. Zucker, R. S. Theoretical implications of the size principle of motoneuron recruitment. J. Theor. Biol. 38: 587–596, 1973.

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Article Title: Functional Organization of Motoneuron Pool and its Inputs
 

How to Cite

Elwood Henneman, Lorne M. Mendell. Functional Organization of Motoneuron Pool and its Inputs. Compr Physiol 2011, Supplement 2: Handbook of Physiology, The Nervous System, Motor Control: 423-507. First published in print 1981. doi: 10.1002/cphy.cp010211