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Structure and Function of Membrane Systems of Skeletal Muscle Cells

Lee D. Peachey, Clara Franzini‐Armstrong

10.1002/cphy.cp100102

Source: Supplement 27: Handbook of Physiology, Skeletal Muscle

Originally published: 1983

Published online: January 2011

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 Structural Components of Skeletal Muscle Fibers
1.1 Sarcomeres, Striations, and Fibrils
1.2 Membranes
2 Physiological Correlates
2.1 Local Activation Experiments
2.2 Comparison of Slow‐Acting and Fast‐Acting Muscle Fibers
2.3 Relation of Total Surface Area to Fiber Capacitance
2.4 Glycerol‐Shock Experiments
3 Microscopic Methods in Study of Cellular Membrane Structure
3.1 Scanning Electron Microscopy
3.2 High‐Voltage Electron Microscopy of Thick Sections
3.3 Freeze‐Fracture Electron Microscopy
4 T System
4.1 Definition, Development, and Function
4.2 T‐Tubule Networks
4.3 T‐Tubule Shapes
5 Sarcoplasmic Reticulum
5.1 Definition, Development, and Function
5.2 Structural Relationship to Myofibrils and Striations
5.3 Form of Sarcoplasmic Reticulum
5.4 Content of Sarcoplasmic Reticulum
5.5 Calcium Movements
6 Intrinsic Membrane Proteins
6.1 Sarcoplasmic Reticulum
6.2 T Tubules
7 Comparative Structure of Sarcoplasmic Reticulum and T System
7.1 Fibers With One Small Dimension and No T System
7.2 Fibers With Low Speed and Small Quantity of Membranes
7.3 Fibers With High Speed and Large Quantity of Membranes
7.4 Variation in Speed and Quantity of Membranes in Mammalian Fibers
7.5 Invertebrate Fibers
8 T‐System‐Sarcoplasmic Reticulum Couplings
8.1 Basic Form
8.2 Structural Details
8.3 Role of T‐Tubular Calcium Current in Excitation‐ Contraction Coupling
8.4 Functional Mechanism of Coupling Between T Tubules and Sarcoplasmic Reticulum
9 Special Geometry of T System
9.1 Longitudinal T Tubules
9.2 Helicoids
9.3 Structural and Functional Implications
Figure 1. Figure 1.

Bowman 32 was hesitant to accept the fibrillar nature of muscle fibers because the “fibrillae” were so small and thus difficult to separate from one another without “suffering an unnatural mutilation” and because of the difficulties of interpreting optical images of such small objects. He believed, however, that the internal structure of muscle fibers basically is fibrillar. Bowman depicted the fibrils from a macerated ox heart 17 as periodically beaded; this was the basis for his belief that muscle fiber striations are not just on the surface, as van Leeuwenhoek 184 observed earlier, but come from the banded nature of the fibrils inside the fiber. Bowman also depicted a tendency for muscle fibers from a variety of species to disintegrate under certain conditions into transverse disks 21,22,23,24,25,26,27,28,29,30, which led him to challenge van Leeuwenhoek's interpretation of muscle striations as helical. 31, 32, 34: Bowman's view of the sarcolemma in regions where the fiber had broken internally and the fibrillar mass retracted, leaving an empty sarcolemmal tube covering the surface of a muscle fiber. [From Bowman 32.]
Figure 2. Figure 2.

Scanning electron micrograph of portion of a single muscle fiber from frog sartorius muscle. Sarcolemma shows a series of ridges around circumference of fiber. Ridges must be similar to what van Leeuwenhoek observed in his light microscope more than 250 years ago. Satellite cell (S) lies in longitudinal groove in surface of muscle fiber.

Courtesy of R. Mazanet
Figure 3. Figure 3.

High‐voltage electron micrographs of longitudinal sections ∼ 1 μm thick of frog sartorius muscle, lanthanum colloid method. A: low magnification. Surfaces of 2 adjacent muscle fibers pass obliquely through section in this region. Light space between 2 fibers in center of figure shows collagen fibrils. Caveolae in each fiber are filled with dense lanthanum material, making them especially apparent. T tubules also appear dark because of their lanthanum content. 1,000 kV; × 8,400. B: higher magnification and stereo. (Stereoscopic images should be viewed with a simple stereoscope, by diverging the eyes until both images are fused, or by crossing the eyes, in which case front and rear of image will be reversed from description in legends.) Caveolae appear as small, approximately circular structures underlying fibrous connective tissue layer on surface of fiber and plasmalemma and overlying myofibrils. Caveolae groups follow undulations of fiber surface and concentrate in I‐band regions. Arrow, probable connection of T tubule to fiber surface. Total tilt 24°; 1,000 kV; × 12,000.
Figure 4. Figure 4.

A: Emilio Veratti (born in Varese, Italy, Nov. 24, 1872) was a student of medicine at the University of Pavia, where he worked with Camillo Golgi for 5 yr and then spent 1 yr at the University of Bologna, where he obtained his degree (Laurea in Medicina e Chirurgia) in 1896. He was appointed Aiuto of Histology in 1896 and of General Pathology in 1906, both at the University of Pavia. He died in Varese, February 28, 1967, in the same house in which he was born. B: Camillo Golgi, Veratti's professor. Golgi's black‐reaction staining technique, used by Veratti, was recently adapted for use in high‐voltage electron‐microscopic studies of muscle cells 111.
Figure 5. Figure 5.

Phase‐light micrograph of section 1–2 μm. thick prepared using a modification of the Golgi black reaction as used by Veratti. Longitudinal view of long‐sarcomere, tonic‐type muscle fiber from walking leg of crayfish (Astacus fluviatilis). Large regions of extracellular space appear orange. Orange region at lower right, surface of muscle fiber: orange region at upper left, deep infolding or cleft in fiber surface. Smaller clefts enter fiber from surface at right and from large cleft at left. Finer terminations of these smaller clefts tend to lie in a longitudinal position corresponding to Z line of nearby sarcomeres. C, cleft with zigzag profile near micrograph center. Two classes of tubules extend from all fiber surfaces, including those of the clefts. One class is called Z tubulès (Z) because of its sarcomeric location 230. Second class of tubular invaginations from various fiber surfaces is found in 2 sets per sarcomere near ends of A bands (T). Because latter tubules form dyadic associations with sarcoplasmic reticulum (SR), they represent true T tubules of these muscle fibers. Longitudinally oriented tubules (L) connect from individual Z tubules to adjacent T tubules in these muscle fibers, though not as frequently as in some other types of crustacean muscle fibers. × 2,752. (Micrograph by Franzini‐Armstrong, Eastwood, and Peachey.)
Figure 6. Figure 6.

Results of local activation experiments on 3 kinds of muscle fibers by A. F. Huxley and co‐workers 148,150,151,152,244. In actual experiments a single pipette depolarized small patches of surface of an isolated muscle fiber; resulting contractions observed with light microscope. A: frog twitch muscle fibers. Right, contractions obtained only when depolarized patch of membrane is adjacent to Z line of underlying striatum pattern. Left, contraction seen is always symmetric on 2 sides of Z line, even when pipette is displaced to 1 side of Z line. Center, contractions never seen when pipette is over an A band. B: results from local activation experiments on crab muscle fibers 150,152. In these fibers and in lizard fibers, contractions observed only when an area over a region near junction of A bands and I bands was depolarized; resulting contraction always asymmetric and stronger in half I band closer to pipette. Extensive attempts 150 to obtain contractions when a region over Z line and within I band was depolarized gave negative results (center pipette). Extensive longitudinal clefts in sarcolemma of crab fibers; when pipette passes current into these clefts, contractions involving several sarcomeres are seen, regardless of pipette placement with respect to striations. Pipette cannot be considered sealed well enough against fiber surface to restrict depolarization to a small area of membrane. This result shows only that activation can be obtained by depolarization of the membrane within clefts. C: results from local activation experiments on frog slow fibers 244. Fibers behaved quite differently from twitch fibers of same species. Although not all areas of fiber surface led to contraction when depolarized, as was also true with twitch fibers, sensitive spots were found at all levels of the striation pattern in slow fibers. Furthermore, contraction often spread as far longitudinally as radially and thus could involve several sarcomeres. Lack of precise localization of sensitivity and lack of specific spread of activation in the radial direction can be correlated with a similarly imprecise arrangement of T tubules and peripheral couplings in these muscle fibers 222,245.
Figure 7. Figure 7.

T system and SR of frog twitch fiber. Longitudinal axis of fiber is vertical, as drawn. Slightly more than 1 sarcomere length of fiber shown. SR forms a 3–dimensional network of cisternae and tubules around myofibrils, with specific structural differentiation of membrane forms adjacent to specific bands of myofibrillar striations. Two levels of T‐tubular networks shown, adjacent to Z lines within I bands of myofibrils and forming central elements of 3–part structures (triads), which also include 2 SR terminal cisternae (TC). TC of SR connect to SR longitudinal tubules (L) either directly or through SR intermediate cisternae (IC). Near center of A band, longitudinal tubules join fenestrated collar (FC) of SR. [From Peachey 233, by copyright permission of The Rockefeller University Press.]
Figure 8. Figure 8.

Longitudinal section of frog sartorius muscle fiber, seen in thin section in electron microscope. T, T tubules; TC, terminal cisternae; IC, intermediate cisternae; L, longitudinal tubules; FC, fenestrated collar. Glycogen granules (G) intermingled with SR elements between myofibrils. × 23,000.
Figure 9. Figure 9.

Reconstruction of T system of frog twitch muscle fiber made by tracing T tubules stained with peroxidase method in series of high‐voltage electron micrographs of serial transverse sections 0.7 μm thick. T tubules projected into transverse plane of fiber. 1,000 kV; × 1,800. [From Peachey and Eisenberg 240, by copyright permission of the Biophysical Society.]
Figure 10. Figure 10.

Stereoscopic high‐voltage electron micrograph of frog twitch skeletal muscle fiber, Golgi stain. Section 1.5 μm thick. Only T tubules stained in this region. Some artifactual precipitate from stain is found outside T tubules. Nucleus near figure bottom. T‐system networks lie at an oblique angle within thickness of transverse section. Viewed stereoscopically, T tubules do not confine themselves to a flat plane but wander considerably in longitudinal direction of fiber. Total tilt 30°; 1,000 kV; × 6,000. [From Franzini‐Armstrong and Peachey 111; micrograph incorrectly described when originally published.]
Figure 11. Figure 11.

Stereoscopic high‐voltage electron micrograph of frog twitch muscle fiber, Golgi stain. Section 1.5 μm thick. T‐system plane undulates through entire thickness of section within small area represented in micrograph. Total tilt 30°; 1,000 kV; × 5,000.
Figure 12. Figure 12.

Transverse section 0.25–0.5 μm thick of rat white muscle fiber from sternomastoid muscle, Golgi stain. At this thickness, only part of single layer of T system is seen. Clear areas, lipid droplets. 100 kV; × 11,000.
Figure 13. Figure 13.

Transverse section 2.5 μm thick of red muscle fiber from rat sternomastoid muscle, Golgi stain. Two layers of T‐system networks lie within section thickness, resulting in a double image in some regions. Clear areas, lipid droplets, numerous in these red fibers. 1,000 kV; × 9,000.
Figure 14. Figure 14.

Stereoscopic high‐voltage electron micrograph of longitudinal section of tonic, long‐sarcomere muscle fiber from flexor muscle of crayfish leg. Groups of 3 transverse reticula extend transversely across fiber, approximately horizontally. Central of the 3, found at level of Z lines of myofibrils, consists of Z tubules, which do not directly form contacts with SR. Remaining 2 reticula in each sarcomere are T tubules, located near ends of myofibrillar A bands. Occasional longitudinal connections from one network to the next. Section 2 μm thick. Total tilt 14°; 1,000 kV; × 6,000. (Micrograph by Franzini‐Armstrong, Eastwood, and Peachey.)
Figure 15. Figure 15.

Stereoscopic high‐voltage electon micrograph of longitudinal section of phasic muscle fiber from abdominal flexor muscle of crayfish. Only T tubules are seen, forming a network with frequent longitudinal elements and tubules oriented transversely. Section 1 μm thick. Total tilt 30°; 1,000 kV; × 4,000. (Micrograph by Franzini‐Armstrong, Eastwood, and Peachey.)
Figure 16. Figure 16.

Stereoscopic high‐voltage micrograph of transverse section of frog twitch muscle fiber showing 2 forms of T tubules. Flattened ribbonlike tubules predominate, but short segments of round tubules also are seen, especially near nodes in network. Round tubules appear narrower in this transverse view. Section 0.5 μm thick. Total tilt 40°; 1,000 kV; × 10,000.
Figure 17. Figure 17.

Longitudinal section of frog sartorius fiber. Grazing view of SR and T tubules near image center. Terminal cisternae (lateral sacs of triad) face T tubules and contain a dense meshwork of calsequestrin. Intermediate cisternae (arrows) are flat and join lateral sacs to longitudinal tubules. Fenestrated collar opposite sarcomere center. × 45,000.
Figure 18. Figure 18.

Collapsed SR. A: comparison of left, normal SR and right, lumen of SR obliterated. Membranes fuse when portion of SR collapses. B: SR from frog sartorius muscle collapsed with ruthenium red in fixative. Collapsed regions of SR have lost tubular shape, × 43,500. C and D: transverse sections of fibers from frog cutaneous pectoris muscle, frozen rapidly and freeze substituted. Collapse of SR wherever SR membranes have flat shape and SR lumen is obliterated. C, × 42,600; D, × 21,300.

A courtesy of J. Sommer; B from Sommer et al. 307; C and D courtesy of J. Heuser
Figure 19. Figure 19.

Cross sections of frog sartorius fibers at sarcomere center. A: area within A band, except for small area at lower right, in I band. H zone extends vertically in middle of figure. Overlap regions of A band on either side. Transversely cut longitudinal tubules separate myofibrils in overlap region. Fenestrated collar surrounds fibrils opposite H zone, × 31,000. B: most of image occupied by H zone. Pores of fenestrated collar entire SR (arrows) as can be seen at this higher magnification, × 60,000.
Figure 20. Figure 20.

Stereoscopic high‐voltage electron micrograph of Golgi‐stained crab tonic muscle fiber with SR stained. T tubules, stained more densely than SR, show both transversely oriented tubules and flattened disklike regions that form dyad contacts with SR. SR in form of fenestrated cisternae in both A bands and I bands. Section 0.5 μm thick. Total tilt 12°; 1,000 kV; × 6,000. (Micrograph by Franzini‐Armstrong, Eastwood, and Peachey.)
Figure 21. Figure 21.

Golgi‐stained preparation from phasic fiber from crayfish abdominal flexor muscle. Only SR is stained. Numerous flat cisternae of SR match similar regions of T‐tubule network (Fig. 15) with which they form triads and dyads. Fenestrated cisternae of SR less prominent than in tonic fibers. Section 0.5 μm thick. 100 kV; × 15,000. (Micrograph by Franzini‐Armstrong, Eastwood, and Peachey.)
Figure 22. Figure 22.

Freeze‐fractured deep‐etched image of SR in toadfish swimbladder muscle. Between arrows fracture splits interior of lateral sac of triad. Calsequestrin content of cisternae and its connections to SR membrane revealed, × 39,000.

From M. G. Nunzi and C. Franzini‐Armstrong, unpublished observations
Figure 23. Figure 23.

Longitudinal fracture along SR of fish muscle (guppy). Myofilaments barely identifiable in this standard freeze‐fracture image. Membranes very visible (see also Fig. 28). Fracture plane preferentially follows membranes, splitting them into 2 leaflets. Cytoplasmic leaflet (SRC) in SR decorated by uniformly distributed population of small particles of variable size thought to represent aggregates of calcium‐pump proteins. Luminal leaflet (SRL) is smooth. Nonjunctional regions only of T tubules shown; very few particles on either leaflet. × 50,000. [From Franzini‐Armstrong 105.]
Figure 24. Figure 24.

(top left). Detail of cytoplasmic leaflet of SR in frog sartorius fiber. Rotary shadowing allows better visibility of numerous intramembranous particles, × 88,000.
Figure 25. Figure 25.

Transverse section of body musculature of amphioxus. Each flat myofibril (m) forms a muscle fiber, completely covered by plasma membrane (pm). × 26,600. [From Peachey 232, by copyright permission of The Rockefeller University Press.]
Figure 26. Figure 26.

Transverse sections of muscle fiber from superior rectus (extraocular) muscle of domestic cat. Ribbon‐shaped myofibrils and abundant SR arranged in multiple layers between myofibrils, especially in I bands. A, × 20,000; B, × 52,000.
Figure 27. Figure 27.

Longitudinal fracture along fiber from red portion of rat sternomastoid muscle. Location of Z lines (Z) detectable on myofibrils. As in most mammalian fibers, triads (arrows) located at A‐I junction. Extensions of mitochondria (m) encircle myofibrils at I‐band level, immediately adjacent to triads. (See also Fig. 28.) × 25,000.

From R. Mazanet and C. Franzini‐Armstrong, unpublished observations
Figure 28. Figure 28.

Longitudinal thin section of human skeletal muscle showing 2 triads (T) in each sarcomere, large mitochondria (M) in I bands, and large accumulations of glycogen granules (G). × 60,000.

From biopsy material supplied by D. Schotland
Figure 29. Figure 29.

Three views at right angles of junctional feet in triads from toadfish swimbladder. A: grazing view of junction; 2 or 3 parallel rows of feet in tetragonal arrangement, × 36,000. B: section at right angle to long axis of T tubule. Double set of feet covers both flat junctional surfaces of T tubules. × 114,000. C: section approximately parallel to row of feet, showing their periodic arrangement, × 90,000. Feet have different appearances, 2 most frequently illustrated in the literature being a hollow‐cored appearance (arrow, B) and a line parallel to 2 junctional membranes (arrows, C). [C from Franzini‐Armstrong 107.]
Figure 30. Figure 30.

Cytoplasmic leaflets of junctional T‐tubule membrane (A) and plasmalemma at sites of peripheral couplings (B) occupied by similar‐looking large and tall particles. Where most regularly arranged (arrows), particles form groups of 4 separated by twice the distance that separates junctional feet. A: × 74,500; B: × 44,600. [A from Franzini‐Armstrong and Nunzi 109. B from Eastwood, Franzini‐Armstrong, and Peracchia 59.]
Figure 31. Figure 31.

Triad in frog skeletal muscle. Results from both thin sections and freeze‐fracture preparations. [From Franzini‐Armstrong and Nunzi 109.]
Figure 32. Figure 32.

Triads in muscle fibers from frog semitendinosus muscles. A: control untreated with ruthenium red. B and C: muscles treated with ruthenium red after and before glutaraldehyde fixation, respectively. Uptake of ruthenium red by SR and fusion of SR membranes at intermediate cisternae level. A, × 48,000; B, × 94,000; C, × 48,000. [From Howell 142, by copyright permission of the Rockefeller University Press.]
Figure 33. Figure 33.

Stereoscopic high‐voltage electron micrograph of an approximately longitudinal section 3 μm thick of frog sartorius muscle fiber. Lanthanum colloid method used to show T tubules. Several longitudinal connections of T tubules from one network to the next. Left, fiber surface. Total tilt 10°; 1,000 kV; × 10,000.
Figure 34. Figure 34.

Stereoscopic high‐voltage electron micrograph of transverse section of frog muscle fiber, Golgi stain. Region of fiber with helicoid. Almost 2 turns of helicoid within section 5 μm thick. Total tilt 10°; 1,000 kV; × 8,000.
Figure 35. Figure 35.

Stereoscopic high‐voltage electron micrograph of transverse section 3 μm thick from rat white muscle fiber, Golgi stain. Double helicoid with 2 T‐system networks in sarcomere, each completing slightly more than a single turn of the helicoid. Total tilt 16°; 1,000 kV; × 4,000.
Figure 36. Figure 36.

High‐voltage electron micrograph of longitudinal section of frog twitch muscle fiber. Muscle stained with method for SR staining 324, though staining does not show. Displacement of myofibrillar striations occurs, including a vernier displacement between arrows. One more sarcomere between 2 upper arrows than between lower arrows. Section 1 μm thick. 1,000 kV; × 4,000.


Figure 1.

Bowman 32 was hesitant to accept the fibrillar nature of muscle fibers because the “fibrillae” were so small and thus difficult to separate from one another without “suffering an unnatural mutilation” and because of the difficulties of interpreting optical images of such small objects. He believed, however, that the internal structure of muscle fibers basically is fibrillar. Bowman depicted the fibrils from a macerated ox heart 17 as periodically beaded; this was the basis for his belief that muscle fiber striations are not just on the surface, as van Leeuwenhoek 184 observed earlier, but come from the banded nature of the fibrils inside the fiber. Bowman also depicted a tendency for muscle fibers from a variety of species to disintegrate under certain conditions into transverse disks 21,22,23,24,25,26,27,28,29,30, which led him to challenge van Leeuwenhoek's interpretation of muscle striations as helical. 31, 32, 34: Bowman's view of the sarcolemma in regions where the fiber had broken internally and the fibrillar mass retracted, leaving an empty sarcolemmal tube covering the surface of a muscle fiber. [From Bowman 32.]



Figure 2.

Scanning electron micrograph of portion of a single muscle fiber from frog sartorius muscle. Sarcolemma shows a series of ridges around circumference of fiber. Ridges must be similar to what van Leeuwenhoek observed in his light microscope more than 250 years ago. Satellite cell (S) lies in longitudinal groove in surface of muscle fiber.

Courtesy of R. Mazanet


Figure 3.

High‐voltage electron micrographs of longitudinal sections ∼ 1 μm thick of frog sartorius muscle, lanthanum colloid method. A: low magnification. Surfaces of 2 adjacent muscle fibers pass obliquely through section in this region. Light space between 2 fibers in center of figure shows collagen fibrils. Caveolae in each fiber are filled with dense lanthanum material, making them especially apparent. T tubules also appear dark because of their lanthanum content. 1,000 kV; × 8,400. B: higher magnification and stereo. (Stereoscopic images should be viewed with a simple stereoscope, by diverging the eyes until both images are fused, or by crossing the eyes, in which case front and rear of image will be reversed from description in legends.) Caveolae appear as small, approximately circular structures underlying fibrous connective tissue layer on surface of fiber and plasmalemma and overlying myofibrils. Caveolae groups follow undulations of fiber surface and concentrate in I‐band regions. Arrow, probable connection of T tubule to fiber surface. Total tilt 24°; 1,000 kV; × 12,000.



Figure 4.

A: Emilio Veratti (born in Varese, Italy, Nov. 24, 1872) was a student of medicine at the University of Pavia, where he worked with Camillo Golgi for 5 yr and then spent 1 yr at the University of Bologna, where he obtained his degree (Laurea in Medicina e Chirurgia) in 1896. He was appointed Aiuto of Histology in 1896 and of General Pathology in 1906, both at the University of Pavia. He died in Varese, February 28, 1967, in the same house in which he was born. B: Camillo Golgi, Veratti's professor. Golgi's black‐reaction staining technique, used by Veratti, was recently adapted for use in high‐voltage electron‐microscopic studies of muscle cells 111.



Figure 5.

Phase‐light micrograph of section 1–2 μm. thick prepared using a modification of the Golgi black reaction as used by Veratti. Longitudinal view of long‐sarcomere, tonic‐type muscle fiber from walking leg of crayfish (Astacus fluviatilis). Large regions of extracellular space appear orange. Orange region at lower right, surface of muscle fiber: orange region at upper left, deep infolding or cleft in fiber surface. Smaller clefts enter fiber from surface at right and from large cleft at left. Finer terminations of these smaller clefts tend to lie in a longitudinal position corresponding to Z line of nearby sarcomeres. C, cleft with zigzag profile near micrograph center. Two classes of tubules extend from all fiber surfaces, including those of the clefts. One class is called Z tubulès (Z) because of its sarcomeric location 230. Second class of tubular invaginations from various fiber surfaces is found in 2 sets per sarcomere near ends of A bands (T). Because latter tubules form dyadic associations with sarcoplasmic reticulum (SR), they represent true T tubules of these muscle fibers. Longitudinally oriented tubules (L) connect from individual Z tubules to adjacent T tubules in these muscle fibers, though not as frequently as in some other types of crustacean muscle fibers. × 2,752. (Micrograph by Franzini‐Armstrong, Eastwood, and Peachey.)



Figure 6.

Results of local activation experiments on 3 kinds of muscle fibers by A. F. Huxley and co‐workers 148,150,151,152,244. In actual experiments a single pipette depolarized small patches of surface of an isolated muscle fiber; resulting contractions observed with light microscope. A: frog twitch muscle fibers. Right, contractions obtained only when depolarized patch of membrane is adjacent to Z line of underlying striatum pattern. Left, contraction seen is always symmetric on 2 sides of Z line, even when pipette is displaced to 1 side of Z line. Center, contractions never seen when pipette is over an A band. B: results from local activation experiments on crab muscle fibers 150,152. In these fibers and in lizard fibers, contractions observed only when an area over a region near junction of A bands and I bands was depolarized; resulting contraction always asymmetric and stronger in half I band closer to pipette. Extensive attempts 150 to obtain contractions when a region over Z line and within I band was depolarized gave negative results (center pipette). Extensive longitudinal clefts in sarcolemma of crab fibers; when pipette passes current into these clefts, contractions involving several sarcomeres are seen, regardless of pipette placement with respect to striations. Pipette cannot be considered sealed well enough against fiber surface to restrict depolarization to a small area of membrane. This result shows only that activation can be obtained by depolarization of the membrane within clefts. C: results from local activation experiments on frog slow fibers 244. Fibers behaved quite differently from twitch fibers of same species. Although not all areas of fiber surface led to contraction when depolarized, as was also true with twitch fibers, sensitive spots were found at all levels of the striation pattern in slow fibers. Furthermore, contraction often spread as far longitudinally as radially and thus could involve several sarcomeres. Lack of precise localization of sensitivity and lack of specific spread of activation in the radial direction can be correlated with a similarly imprecise arrangement of T tubules and peripheral couplings in these muscle fibers 222,245.



Figure 7.

T system and SR of frog twitch fiber. Longitudinal axis of fiber is vertical, as drawn. Slightly more than 1 sarcomere length of fiber shown. SR forms a 3–dimensional network of cisternae and tubules around myofibrils, with specific structural differentiation of membrane forms adjacent to specific bands of myofibrillar striations. Two levels of T‐tubular networks shown, adjacent to Z lines within I bands of myofibrils and forming central elements of 3–part structures (triads), which also include 2 SR terminal cisternae (TC). TC of SR connect to SR longitudinal tubules (L) either directly or through SR intermediate cisternae (IC). Near center of A band, longitudinal tubules join fenestrated collar (FC) of SR. [From Peachey 233, by copyright permission of The Rockefeller University Press.]



Figure 8.

Longitudinal section of frog sartorius muscle fiber, seen in thin section in electron microscope. T, T tubules; TC, terminal cisternae; IC, intermediate cisternae; L, longitudinal tubules; FC, fenestrated collar. Glycogen granules (G) intermingled with SR elements between myofibrils. × 23,000.



Figure 9.

Reconstruction of T system of frog twitch muscle fiber made by tracing T tubules stained with peroxidase method in series of high‐voltage electron micrographs of serial transverse sections 0.7 μm thick. T tubules projected into transverse plane of fiber. 1,000 kV; × 1,800. [From Peachey and Eisenberg 240, by copyright permission of the Biophysical Society.]



Figure 10.

Stereoscopic high‐voltage electron micrograph of frog twitch skeletal muscle fiber, Golgi stain. Section 1.5 μm thick. Only T tubules stained in this region. Some artifactual precipitate from stain is found outside T tubules. Nucleus near figure bottom. T‐system networks lie at an oblique angle within thickness of transverse section. Viewed stereoscopically, T tubules do not confine themselves to a flat plane but wander considerably in longitudinal direction of fiber. Total tilt 30°; 1,000 kV; × 6,000. [From Franzini‐Armstrong and Peachey 111; micrograph incorrectly described when originally published.]



Figure 11.

Stereoscopic high‐voltage electron micrograph of frog twitch muscle fiber, Golgi stain. Section 1.5 μm thick. T‐system plane undulates through entire thickness of section within small area represented in micrograph. Total tilt 30°; 1,000 kV; × 5,000.



Figure 12.

Transverse section 0.25–0.5 μm thick of rat white muscle fiber from sternomastoid muscle, Golgi stain. At this thickness, only part of single layer of T system is seen. Clear areas, lipid droplets. 100 kV; × 11,000.



Figure 13.

Transverse section 2.5 μm thick of red muscle fiber from rat sternomastoid muscle, Golgi stain. Two layers of T‐system networks lie within section thickness, resulting in a double image in some regions. Clear areas, lipid droplets, numerous in these red fibers. 1,000 kV; × 9,000.



Figure 14.

Stereoscopic high‐voltage electron micrograph of longitudinal section of tonic, long‐sarcomere muscle fiber from flexor muscle of crayfish leg. Groups of 3 transverse reticula extend transversely across fiber, approximately horizontally. Central of the 3, found at level of Z lines of myofibrils, consists of Z tubules, which do not directly form contacts with SR. Remaining 2 reticula in each sarcomere are T tubules, located near ends of myofibrillar A bands. Occasional longitudinal connections from one network to the next. Section 2 μm thick. Total tilt 14°; 1,000 kV; × 6,000. (Micrograph by Franzini‐Armstrong, Eastwood, and Peachey.)



Figure 15.

Stereoscopic high‐voltage electon micrograph of longitudinal section of phasic muscle fiber from abdominal flexor muscle of crayfish. Only T tubules are seen, forming a network with frequent longitudinal elements and tubules oriented transversely. Section 1 μm thick. Total tilt 30°; 1,000 kV; × 4,000. (Micrograph by Franzini‐Armstrong, Eastwood, and Peachey.)



Figure 16.

Stereoscopic high‐voltage micrograph of transverse section of frog twitch muscle fiber showing 2 forms of T tubules. Flattened ribbonlike tubules predominate, but short segments of round tubules also are seen, especially near nodes in network. Round tubules appear narrower in this transverse view. Section 0.5 μm thick. Total tilt 40°; 1,000 kV; × 10,000.



Figure 17.

Longitudinal section of frog sartorius fiber. Grazing view of SR and T tubules near image center. Terminal cisternae (lateral sacs of triad) face T tubules and contain a dense meshwork of calsequestrin. Intermediate cisternae (arrows) are flat and join lateral sacs to longitudinal tubules. Fenestrated collar opposite sarcomere center. × 45,000.



Figure 18.

Collapsed SR. A: comparison of left, normal SR and right, lumen of SR obliterated. Membranes fuse when portion of SR collapses. B: SR from frog sartorius muscle collapsed with ruthenium red in fixative. Collapsed regions of SR have lost tubular shape, × 43,500. C and D: transverse sections of fibers from frog cutaneous pectoris muscle, frozen rapidly and freeze substituted. Collapse of SR wherever SR membranes have flat shape and SR lumen is obliterated. C, × 42,600; D, × 21,300.

A courtesy of J. Sommer; B from Sommer et al. 307; C and D courtesy of J. Heuser


Figure 19.

Cross sections of frog sartorius fibers at sarcomere center. A: area within A band, except for small area at lower right, in I band. H zone extends vertically in middle of figure. Overlap regions of A band on either side. Transversely cut longitudinal tubules separate myofibrils in overlap region. Fenestrated collar surrounds fibrils opposite H zone, × 31,000. B: most of image occupied by H zone. Pores of fenestrated collar entire SR (arrows) as can be seen at this higher magnification, × 60,000.



Figure 20.

Stereoscopic high‐voltage electron micrograph of Golgi‐stained crab tonic muscle fiber with SR stained. T tubules, stained more densely than SR, show both transversely oriented tubules and flattened disklike regions that form dyad contacts with SR. SR in form of fenestrated cisternae in both A bands and I bands. Section 0.5 μm thick. Total tilt 12°; 1,000 kV; × 6,000. (Micrograph by Franzini‐Armstrong, Eastwood, and Peachey.)



Figure 21.

Golgi‐stained preparation from phasic fiber from crayfish abdominal flexor muscle. Only SR is stained. Numerous flat cisternae of SR match similar regions of T‐tubule network (Fig. 15) with which they form triads and dyads. Fenestrated cisternae of SR less prominent than in tonic fibers. Section 0.5 μm thick. 100 kV; × 15,000. (Micrograph by Franzini‐Armstrong, Eastwood, and Peachey.)



Figure 22.

Freeze‐fractured deep‐etched image of SR in toadfish swimbladder muscle. Between arrows fracture splits interior of lateral sac of triad. Calsequestrin content of cisternae and its connections to SR membrane revealed, × 39,000.

From M. G. Nunzi and C. Franzini‐Armstrong, unpublished observations


Figure 23.

Longitudinal fracture along SR of fish muscle (guppy). Myofilaments barely identifiable in this standard freeze‐fracture image. Membranes very visible (see also Fig. 28). Fracture plane preferentially follows membranes, splitting them into 2 leaflets. Cytoplasmic leaflet (SRC) in SR decorated by uniformly distributed population of small particles of variable size thought to represent aggregates of calcium‐pump proteins. Luminal leaflet (SRL) is smooth. Nonjunctional regions only of T tubules shown; very few particles on either leaflet. × 50,000. [From Franzini‐Armstrong 105.]



Figure 24.

(top left). Detail of cytoplasmic leaflet of SR in frog sartorius fiber. Rotary shadowing allows better visibility of numerous intramembranous particles, × 88,000.



Figure 25.

Transverse section of body musculature of amphioxus. Each flat myofibril (m) forms a muscle fiber, completely covered by plasma membrane (pm). × 26,600. [From Peachey 232, by copyright permission of The Rockefeller University Press.]



Figure 26.

Transverse sections of muscle fiber from superior rectus (extraocular) muscle of domestic cat. Ribbon‐shaped myofibrils and abundant SR arranged in multiple layers between myofibrils, especially in I bands. A, × 20,000; B, × 52,000.



Figure 27.

Longitudinal fracture along fiber from red portion of rat sternomastoid muscle. Location of Z lines (Z) detectable on myofibrils. As in most mammalian fibers, triads (arrows) located at A‐I junction. Extensions of mitochondria (m) encircle myofibrils at I‐band level, immediately adjacent to triads. (See also Fig. 28.) × 25,000.

From R. Mazanet and C. Franzini‐Armstrong, unpublished observations


Figure 28.

Longitudinal thin section of human skeletal muscle showing 2 triads (T) in each sarcomere, large mitochondria (M) in I bands, and large accumulations of glycogen granules (G). × 60,000.

From biopsy material supplied by D. Schotland


Figure 29.

Three views at right angles of junctional feet in triads from toadfish swimbladder. A: grazing view of junction; 2 or 3 parallel rows of feet in tetragonal arrangement, × 36,000. B: section at right angle to long axis of T tubule. Double set of feet covers both flat junctional surfaces of T tubules. × 114,000. C: section approximately parallel to row of feet, showing their periodic arrangement, × 90,000. Feet have different appearances, 2 most frequently illustrated in the literature being a hollow‐cored appearance (arrow, B) and a line parallel to 2 junctional membranes (arrows, C). [C from Franzini‐Armstrong 107.]



Figure 30.

Cytoplasmic leaflets of junctional T‐tubule membrane (A) and plasmalemma at sites of peripheral couplings (B) occupied by similar‐looking large and tall particles. Where most regularly arranged (arrows), particles form groups of 4 separated by twice the distance that separates junctional feet. A: × 74,500; B: × 44,600. [A from Franzini‐Armstrong and Nunzi 109. B from Eastwood, Franzini‐Armstrong, and Peracchia 59.]



Figure 31.

Triad in frog skeletal muscle. Results from both thin sections and freeze‐fracture preparations. [From Franzini‐Armstrong and Nunzi 109.]



Figure 32.

Triads in muscle fibers from frog semitendinosus muscles. A: control untreated with ruthenium red. B and C: muscles treated with ruthenium red after and before glutaraldehyde fixation, respectively. Uptake of ruthenium red by SR and fusion of SR membranes at intermediate cisternae level. A, × 48,000; B, × 94,000; C, × 48,000. [From Howell 142, by copyright permission of the Rockefeller University Press.]



Figure 33.

Stereoscopic high‐voltage electron micrograph of an approximately longitudinal section 3 μm thick of frog sartorius muscle fiber. Lanthanum colloid method used to show T tubules. Several longitudinal connections of T tubules from one network to the next. Left, fiber surface. Total tilt 10°; 1,000 kV; × 10,000.



Figure 34.

Stereoscopic high‐voltage electron micrograph of transverse section of frog muscle fiber, Golgi stain. Region of fiber with helicoid. Almost 2 turns of helicoid within section 5 μm thick. Total tilt 10°; 1,000 kV; × 8,000.



Figure 35.

Stereoscopic high‐voltage electron micrograph of transverse section 3 μm thick from rat white muscle fiber, Golgi stain. Double helicoid with 2 T‐system networks in sarcomere, each completing slightly more than a single turn of the helicoid. Total tilt 16°; 1,000 kV; × 4,000.



Figure 36.

High‐voltage electron micrograph of longitudinal section of frog twitch muscle fiber. Muscle stained with method for SR staining 324, though staining does not show. Displacement of myofibrillar striations occurs, including a vernier displacement between arrows. One more sarcomere between 2 upper arrows than between lower arrows. Section 1 μm thick. 1,000 kV; × 4,000.

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Article Title: Structure and Function of Membrane Systems of Skeletal Muscle Cells
 

How to Cite

Lee D. Peachey, Clara Franzini‐Armstrong. Structure and Function of Membrane Systems of Skeletal Muscle Cells. Compr Physiol 2011, Supplement 27: Handbook of Physiology, Skeletal Muscle: 23-71. First published in print 1983. doi: 10.1002/cphy.cp100102