bioelectricity || skeletal muscle

11SKELETAL MUSCLE

The goal of the material in this chapter is to provide a very brief introduction to skeletal muscle,its structure, and its electrophysiological and contractile properties.1

11.1. MUSCLE STRUCTURE

A whole muscle can be divided into separate bundles. Each bundle contains many individualfibers. The fiber is the basic (smallest) functional unit (it constitutes a single cell). It is boundedby a plasma membrane and a thin sheet of connective tissue, the endomysium. The bundles arealso surrounded by a connective tissue sheet, the perimysium, which delineates specific fascicles.The whole muscle is encased in its connective tissue sheet, namely, the epimysium.

Most skeletal muscles begin and end in tendons. Muscle fibers lie parallel to each other, sothe force of contraction contributed by each is additive. The general features noted above are illus-trated in Figure 11.1. In this chapter attention will be primarily directed to the electromechanicalproperties of the single muscle fiber.

Each muscle fiber is made up of many fibrils, each of which, in turn, is divisible into indi-vidual filaments. The filaments are composed of contractile proteins, essentially myosin, actin,tropomyosin, and troponin.

Mature fibers may be as long as the muscle of which they are a part (tens of centimeters); theyvary in diameter from 10 to 100 μm. As noted above, each fiber contains myofibrils, which areproteins and which lie in the cytoplasm. The cytoplasm also contains mitochondria, the SR and Tsystems, plus glycogen granules. When examined under light microscopy (LM), the myofilamentsshow characteristic cross-striations (banding), which are in register in all myofilaments (seeFigure 11.1.) It is the latter property from which skeletal muscle derives the alternate name ofstriated (muscle).

The overall physical features of a muscle fiber are shown in Figure 11.2. This shows thedense packing of myofibrils, the transverse tubular system (TTS), and the sarcoplasm reticulum

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342 CH. 11: SKELETAL MUSCLE

Figure 11.1. Structure of a Whole Muscle and Its Components. The cross-striations arevisible under light microscopy. From Keynes RD, Aidley DL. 1981. Nerve and mus-cle. Cambridge: Cambridge UP. Based on Schmidt-Nielsen K. 1979. Animal physiology.Cambridge: Cambridge UP. Reprinted with the permission of Cambridge University Press.

(SR). Both the bounding membrane and the TTS membrane are excitable and play an importantpart in the process whereby contraction is initiated.

11.2. MUSCLE CONTRACTION

Each mammalian muscle fiber is contacted by a single nerve terminal. The muscle fiber isknown as a twitch fiber, since the response to a single nerve stimulus is a twitch. The time toreach the peak of a typical twitch contraction is around 200 msec, while recovery requires anadditional 600 msec.

In normal activity a muscle will shorten as it develops force (tension). However, experimentsare often carried out under conditions of constant muscle length (isometric) as well as underconditions of constant muscle load (isotonic). To study behavior under isometric conditions, atransducer is needed that converts force into an electrical signal while itself undergoing very littledeflection.

If a second stimulus is applied before the effect of the previous twitch has ended, then the sec-ond (twitch) response will build on the residual of the first and summation results. Correspondingto a long inter-stimulus interval, a “bumpy” response is seen. For increasing stimulus frequency,a value will be reached where the bumps disappear and a smooth buildup to a maximum steadylevel results, as illustrated in Figure 11.3. The frequency is known as the fusion frequency, andthe muscle is said to be in tetanus. The peak twitch tension to the maximum tetanus tension isthe twitch/tetanus ratio, which is about 0.2 for mammalian muscle.

BIOELECTRICITY: A QUANTITATIVE APPROACH 343

Figure 11.2. Magnified View of the Structure of a Single Muscle Fiber, with a cutawayview of the myofibrillar structure. Each fibril is surrounded by a sarcoplasmic reticulum(SR) and by the transverse tubules system (TTS), which opens to the exterior of the fiber.From Krstic RV. 1970. Ultrastructure of the mammalian cell. Berlin: Springer-Verlag,with permission.


Figure 11.3. Tension versus Time for a Single Stimulus (twitch response) and for a trainof stimuli of increasing frequency b, c, d. From Keynes RD, Aidley DJ. 1981. Nerveand muscle. Cambridge: Cambridge UP. Reprinted with the permission of CambridgeUniversity Press.

Mammalian muscle can be classified into fast glycolytic or type II fibers, and slow oxidativeor type I muscle.2 Fast (white) fibers contract and relax much more rapidly than slow (red) ones.The former are found where rapid movement is encountered (e.g., muscles involved in fast runningand jumping), while the slow muscle is more involved in, for example, long-distance running orpostural movement.

The characteristics of the fast muscle include (1) larger diameter fibers, (2) greater developedtension, (3) mainly dependent on glycolytic and less on oxidative metabolism, (4) contractionsof short duration, and (5) muscle fatigues rapidly and recovers slowly.

Distinguishing the slow muscle is (1) a smaller diameter fiber, (2) lower tension, (3) primarilyoxidative metabolism (hence more extensive vasculature and mitochondria), (4) long-durationtwitch, and (5) fatigues slowly and recovers quickly. All muscles are actually some combinationof the fast and slow muscle, each having their own particular characteristics.

The length–tension relation of skeletal muscle is illustrated in Figure 11.4. Under isometricconditions, the total active (tetanus) tension depends on the (fixed) length of the fiber accordingto the plotted data.

A passive tension is required to extend the muscle beyond its resting length (mainly becauseof the need to stretch the connective tissue associated with the muscle). The passive tension ismeasured on the muscle in the absence of stimulation.

The difference between the total active tension and the passive tension is a measure of thecontractile force derived from stimulation and is called the active increment. The latter quantityreaches a maximum at the resting length and is lower for either greater or lesser lengths. Anexplanation of this behavior is given in a subsequent section.


Figure 11.4. Length–Tension Relationship for a Skeletal Muscle under Isometric Condi-tions. From Keynes RD, Aidley DJ. 1981. Nerve and muscle. Cambridge: Cambridge UP.Reprinted with the permission of Cambridge University Press.

11.2.1. Structure of the Myofibril

Each fiber contains a large number of cylindrical (protein) constituents called myofibrils.The banded structure seen for the fiber as a whole is, in fact, a consequence of the Saffie bandingand alignment of the individual fibrils. The banding corresponds to the structure of the proteincomponents of the myofibril, namely, the thick and thin filaments.

The thick filaments are around 11 nanometers in diameter, while the thin filaments are around5 nm in diameter.

The arrangement of these filaments is shown in Figure 11.5a, where it is seen that in the cross-section they are interdigitated in a hexagonal array, while along the axis they lie in a recurringpattern of overlapping and non overlapping regions. When viewed lengthwise, the banding effectarises from the relative amounts of transmitted light permitted by the thick and thin filaments.

In Figure 11.6 we show both the structural organization of the thin and thick filaments andthe associated banding that would be observed in the LM. The two main bands are the dark Aband and the lighter I band. The bands alternate regularly along the myofibril. In the middle ofthe I band is the Z line (dark line), while the middle of the A band has a lighter region, the H zone.The H zone is bisected by a darker M line surrounded by a lighter region, the L zone (not alwaysseen). The repeating unit (Z–Z distance) is the sarcomere.

These characteristic bands of different light intensity derive from the underlying thin andthick filament structure, the major elements of which can be recognized in Figure 11.6.

The dark A band arises from the overlapping thin and thick filaments, while the lighter Hzone reflects the presence of thick filaments alone.

The M line and L zone derive from the structural details of the thick filament at its center,the M line from crosslinks at the center.


Figure 11.5. Axial and Cross-Sectional View of a Portion of the Array of Thin and ThickFilaments that constitutes a single fibril. The cross-section at (a) registers the presence ofboth thin and thick filaments, while that at (b) thick filaments only, and at (c) thin filamentsonly. From Aidley DJ. 1978. The physiology of excitable cells. Cambridge: CambridgeUP. Reprinted with the permission of Cambridge University Press.

The L zone is due to the absence of projections on either side of the thick filament (to bedescribed later); the L zone is around 0.15 μm in width.

The Z line reflects the interconnection of the I filaments from the region to its left and itsright.

The above letters are derived from the German and reflect certain properties of theirdesignated regions. They are A = anisotropic (polarizes light), I = isotropic, Z = zwis-chenscheibe, H = Henrens disc, and M = mittlemembrane

The thick filament is made up of myosin, a complex protein. Trypsin splits it into lightmeromyosin (LMM) and heavy meromyosin (HMM). The latter has a short tail and two globularheads; it has an ATPase behavior (i.e., it hydrolyzes ATP into ADP + P with the release of largeamounts of energy). The light meromyosin is rod-like and does not split ATP.

The thin filament is actin, which is also a protein. There are two forms, but neither hasATPase behavior. (The important ATPase activity is actually confined only to the globular subfragments.) The LMM and the tail of the HMM are composed of two α-helices that coil aroundeach other. When combined in a solution, the actin and myosin form a complex called actomyosin(a quite viscous material). A description of the myosin structure is given in Figure 11.7.

Glycerol-extracted fibers are prepared by soaking muscle fibers in 50% glycerol for severalweeks, a process that removes most sarcoplasmic material except for the contractile elements.The fibers are found to be in rigor (they are stiff and resist contraction, a result of the formation


Figure 11.6. Myofibrillar Structure and Associated Pattern Seen in a Light Microscope.The banding nomenclature is given. The observed pattern of light intensity in (a) can beexplained by the underlying structure shown in (b). From Keynes RD, Aidley J. 1981.Nerve and muscle. Cambridge University Press. (a) is based on a photograph by Dr. HEHuxley. Reprinted with the permission of Cambridge University Press.

Figure 11.7. Different Components of the Myosin Molecule. Proteolytic enzymes cleavethe molecule into heavy meromyosin (HMM) and light meromyosin (LMM). The HMMcomprises a short segment of the α-helical rod (S2) and the two globular heads (S1), towhich the light chains are attached. The globular heads form the cross-bridges. Reprintedby permission from McComas AJ. 1996. Skeletal muscle, Champaign, IL: Human Kinetics.Based on Vibert P, Cohen C. 1988. Domains, motions, and regulation in the myosin head.J Muscle Res Cell Motility 9:296–305, and Rayment I, et al. 1993. Structure of the actin–myosin complex and its implications for muscle contraction. Science 261:58–65.

of cross-bridges between the actin and myosin). If ATP and magnesium are added, the fibersbecome readily extensible due to the breakage of crosslinks by the ATP. If Ca++ is also added,then contraction takes place.

In the transverse plane, the relative positions of the thin and thick filaments in a region ofoverlap is as illustrated in Figure 11.8. One notes that each thick filament is surrounded by sixthin filaments, while each thin filament is surrounded by three thick ones. Hence, there are twiceas many thin as thick filaments.


Figure 11.8. Transverse Plane View of the Thin and Thick Filament Structure in an axialplane in which they overlap (see Figure 11.6).

In the ultrastructural studies of the myosin (thick) filament, one finds the occurrence ofprojection pairs at a regular interval of 14.3 nm; however, successive pairs are found to be rotatedby 120◦. Consequently, when established, cross-bridges are then 14.3 nm apart, while an identicalrepetition occurs every 43 nm. An illustration of this is given in Figure 11.9a.

One can derive the thick filament structure from an aggregation of myosin molecules, asillustrated in Figure 11.10. Each projection is identified as a globular head pair of the myosinmolecule. Note the necessarily projection-free region in the center, which is the explanation forthe observed L zone. Note also the reversed orientation of molecules on either side of the center.

11.3. SLIDING FILAMENT THEORY

The idea that muscular contraction is a consequence of the contraction of protein unitspatterned after that of a helical spring had to be abandoned when measurements revealed that theA band does not change length during contraction or lengthening.

In fact, in frog muscle, as the sarcomere length is varied from 2.2 to 3.8 μm, the I filamentsremain essentially at 2.05 μm in length and the A filaments at around 1.6 μm. (The Z line is≈ 0.05 μm wide, and each side of the I filament has a length of 1.0 μm, to account for the totalof 2.05 μm.)

As a consequence of the above, the sliding-filament model was advanced. According to thisidea, contraction involves the relative movement of the thin and thick filaments, as illustratedin Figure 11.11, where contraction yields a reduced sarcomere length while the filaments areunchanged in length.


Figure 11.9. Models of the Structure of the Thick and Thin Filaments: (a) myosin; (b) F-actin; (c) thin filament. In (a) the two globular heads of myosin, which split ATP, are shown(a more detailed view is given in Figure 11.11). From Keynes RD, Aidley DJ. 1981. Nerveand muscle. Cambridge: Cambridge UP. Reprinted with the permission of CambridgeUniversity Press. Based on Offer G. 1978. The molecular basis of muscular contraction.In Companion to biochemistry, Ed AT Bull et al. London: Longman; Huxley HE, BrownW. 1967. The low angle x-ray diagram of vertebrate striated muscle and its behavior duringcontraction and rigor. J Mol Biol 30:383–434; and Huxley HE. 1972. Molecular basisof contraction in cross-striated muscles. In Structure and function of muscle, 2nd ed., pp.301–387. Ed GH Bourne. New York: Academic Press.

Figure 11.10. Huxley’s Suggestion as to How Myosin Molecules Aggregate to Form aThick Filament. See also Figure 11.2 for details of myosin structure. From Huxley HE.1971. The structural basis of molecular contraction. Proc R Soc 178:131–149. Redrawnin Aidley DJ. 1978. The physiology of excitable cells. Cambridge: Cambridge UP.

The sliding itself is thought to be produced by reactions between the projections on themyosin filaments and active sites on the thin filament. Each projection first attaches itself tothe actin filament to form a cross-bridge, then pulls on it, causing the sliding of the actin, thenreleases it, and finally moves to attach to another site which is further along the thin filament.

The sliding filament theory is generally (though not universally) accepted. Accordingly, oneexpects isometric tension to depend on the degree of overlap in the thin and thick filaments. This


Figure 11.11. This figure illustrates the sliding-filament model: (a) the muscle is elongated;(b) the muscle is contracted. In each case the lengths of the thick and thin filaments areunchanged.

Figure 11.12. Isometric Tension of a Frog Muscle Fiber, measured as a percentage ofits maximum value at different sarcomere lengths. The numbers 1–6 refer to the myofil-ament positions illustrated in Figure 11.13. Note that the general shape is anticipated inFigure 11.13. From Gordon AM, Huxley AF, Julian FJ. 1966. The variation in isometrictension with sarcomere length in vertebrate muscle fibers. J Physiol 184:170–192. Re-drawn by Aidley DJ. 1978. The physiology of excitable cells. Cambridge: CambridgeUP.

result is supported by the study illustrated in Figures 11.12 and 11.13 and can be understood inthe following discussion.

Stage 1 (in Figures 11.12 and 11.13) refers to full extension of the myofibril. Using thedimensions given above for the thin and thick filament lengths, the sarcomere length is 2.05 +


Figure 11.13. Myofilament Arrangements at Different Lengths. The numbers are thepositions corresponding to the curve given in Figure 11.12. a = thick filament length (1.6μm); b = thin filament length including z line (2.05 μm); c = thick filament region baseof projections (0.15 μm); and z = z line width (0.05 μm). From Gordon AM, HuxleyAF, Julian FJ. 1966. The variation in isometric tension with sarcomere length in vertebratemuscle fibers. J Physiol 184:170–192. Redrawn in Aidley DJ. 1978. The physiology ofexcitable cells. Cambridge: Cambridge UP.

1.60 = 3.65 μm, which is the sum of the length of the thin plus thick filament. There can be nocross-bridges and the observed zero tension is explained on this account.

As the myofibril shortens so that the sarcomere diminishes from 3.6 to 2.2–2.25 μm (stage 2),the number of cross-bridges increases linearly with decreasing length. Therefore, the isometrictension should show a similar increase. In fact, such an increase in tension with decreased lengthis seen in Figure 11.12. This linear behavior ends at stage 2, when the Z–Z distance equals 2.05μm plus the L zone width (≈ 0.15 μm), or 2.20 μm.

With further shortening, the number of cross-bridges remains unchanged and a plateau intension is both expected and observed. Stage 3 is reached when the thin filaments touch. Thesarcomere equals the length of the thin filament, namely, 2.05 μm at this point.

From stage 3 to stage 4 one anticipates some internal resistance to shortening to develop,since actin filaments now overlap.

Beyond stage 4 this overlap not only constitutes a “frictional” impediment, but it also in-terferes with cross-bridge formation. When stage 5 (1.65 μm) is reached, the myosin filamentshit the Z line and a further increase in resistance is associated with the deformation that resultsbeyond this point.


Figure 11.14. Interaction of Actin and Myosin on a Molecular Level. From Huxley HE.1975. The structural basis for contraction and regulation in skeletal muscle. In Molecularbasis of motility. Ed LMG Heilmeyer et al. Berlin: Springer.

The curve in Figure 11.12 shows a break point at stage 5 and a rapid decrease in tension withfurther shortening. Zero tension is reached at a sarcomere length of 1.3 μm, which designatesstage 6.

The actin structure is described in Figure 11.9b,c and in Figure 11.14 as a double helixinvolving chains of monomers. The thin filament is made up of actin, troponin, and tropomyosin,as shown in Figure 11.9c. The thick filament is shown in Figure 11.14 as containing an S2filament subunit and the S1 (globular head) subunits. The S1 subunits can rotate about their pointof attachment with S2. Together, S1 and S2 make up the heavy meromyosin (HMM) portionof the myosin molecule; the remainder of the molecule is filamentary and constitutes the lightmeromyosin (LMM) (see Figure 11.7).

Sliding is accomplished by the rotation of S1 about S2, as noted earlier. In the upper portionof Figure 11.14, the left-hand cross-bridge has just attached while the S1 subunit of the right-handone has nearly completed its rotation. The lower diagram, which illustrates conditions a momentlater, shows the S1 subunit on the left-hand cross-bridge having rotated to cause the actin filamentto slide leftward; the right-hand cross-bridge is now separated.


There are two S1 cross-bridges for each myosin molecule, and each cross-bridge is relativelyindependent of the other, though each behaves as described here.

The biochemical events associated with these mechanical events can be described accordingto the following sequence:

1. Myosin is released from a cross-bridge with actin. This results from the action of ATPwith which the myosin combines. That is,

AM + ATP→ A + M ·ATP

where A ≡ actin and M ≡ myosin).

2. ATP is split into ADP + P, while the myosin (S2) repositions for reattachment with thethin filament. The products remain attached to the myosin, which now has a high affinityfor actin.

3. Myosin cross-bridges attach to a new actin monomer.

4. This results in products being released and the energy so derived utilized as the powerstroke (rotation of S2 and linear movement of actin). At this point, return to step 1.

While actin will react with pure myosin so as to split ATP in the absence of calcium ions,when tropomyosin and troponin are also present, calcium ions are required. In the case of muscle,the tropomyosin and troponin are, in fact, always present and appear to exert a regulatory (control)role.

11.4. EXCITATION–CONTRACTION

The details of the process, starting with propagation of an action potential along a musclefiber and ending with contraction of the target muscle, can now be examined. The possibility thatthe influx of calcium ions, associated with the membrane depolarization, is the primary initiatorof the contractile mechanism has to be discarded since only about a 0.2 picomole Ca++/cm2

influx is observed (frog sartorius). This amount corresponds to an increase in internal calciumion concentration of only 0.08 μmole (assuming a fiber diameter of 50 μm).

To better understand contemporary ideas, one must include the presence of the sarcoplasmicreticulum (SR) and the transverse tubular system (TTS). The T-system lies transverse to the fiberaxis and consists of tubules that are open to the extracellular space and form a meshwork shapedsomewhat as the spokes of a wheel (described in Figure 11.2).

The TTSs are located at the Z lines of frog muscle and the A–I boundary in most otherstriated muscle. The SR is in close proximity to the T-system but extends in the axial direction,mainly. It constitutes a network of vesicular elements surrounding the myofibrils. It is not directlyconnected to the TTS and is otherwise isolated from extracellular space.

Excitation propagating along the surface membrane of the muscle fiber passes the outsideopening of each of the many T-tubules. It is believed that this excitation can propagate inward,


that the membranes defining the T-tubules are excitable in the usual way. The inward speed ofconduction has, in fact, been measured and is about 7 cm/sec (in a fiber 100 μm in diameter alatency of 0.7 msec from outside to inside would consequently be observed).

The SR, while not continuous with the TTS, in places, is in close proximity via a structurecalled “feet.” The SR sequesters Ca++ (which is pumped into the SR vesicles by an ATP-drivencalcium pump). This sequestration can reduce the calcium ion concentration in the muscle to apoint below that necessary for contraction (i.e., it results in the relaxation of the muscle).

Then activation results from the action potential propagating throughout the TTS, which inturn results in a movement of ions to open the calcium channels in the SR membrane. This resultsin a release of Ca++ from the SR into the myoplasm. The consequent contractile process thenarises as described earlier.

We assumed in the above that in the presence of tropomyosin and troponin Ca++ is requiredfor ATP to be split. The tropomyosin and troponin appear, in fact, to be a structural component ofthe thin filament, as described in Figure 11.9c. The tropomyosin in the resting muscle is positionedto prevent the myosin heads combining with the actin monomers, but it can be moved out of theway by a conformational change in the troponin complex when calcium binds to troponin C.

11.5. NOTES

1. The interested reader can find further information in [1, 2, 3]. These works were the primary sources for the materialof this chapter.

2. A continuum of fiber types actually exists and this classification describes those at each end of this “spectrum.” Otherclassification schemes have been proposed, but this choice identifies the basic differences that are found.

11.6. REFERENCES

1. Junge D. 1981. Nerve and muscle excitation. Sunderland, MA: Sinauer Associates.

2. Katz B. 1966. Nerve, muscle and synapse. New York: McGraw-Hill.

3. Kenes RD, Aidley DJ. 1981. Nerve and muscle. Cambridge: Cambridge UP.

Additional ReferencesAidley DJ. 1978. The physiology of excitable cells. Cambridge: Cambridge UP.

Stein RB. 1980. Nerve and muscle. New York: Plenum Press.

Kenes RD, Aidley DJ. 1991. Nerve and muscle, 2nd ed. Cambridge: Cambridge UP.

bioelectricity || skeletal muscle

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