Skeletal Muscle Physiology

Skeletal Muscle Physiology

Muscle cells are highly specialized for the conversion of chemical energy to mechanical energy. Specifically, muscle cells use the energy in adenosine triphosphate (ATP) to generate force or do work. Because work can take many forms (such as locomotion, pumping blood, or peristalsis), several types of muscle have evolved. The three basic types of muscle are skeletal muscle, cardiac muscle, and smooth muscle.

Skeletal muscle acts on the skeleton. In limbs, for example, skeletal muscle spans a joint, thereby allowing a lever action. Skeletal muscle is under voluntary control (i.e., controlled by the central nervous system) and plays a key role in numerous activities such as maintenance of posture, locomotion, speech, and respiration. When viewed under the microscope, skeletal muscle exhibits transverse striations (at intervals of 2–3 µm) that result from the highly organized arrangement of actin and myosin molecules within the skeletal muscle cells. Thus skeletal muscle is classified as a striated muscle. The heart is composed of cardiac muscle, and although it is also a striated muscle, it is an involuntary muscle (i.e., controlled by an intrinsic pacemaker and modulated by the autonomic nervous system). Smooth muscle (which lacks the striations evident in skeletal and cardiac muscle) is an involuntary muscle typically found lining hollow organs such as the intestine and blood vessels. In all three muscle types, force is generated by the interaction of actin and myosin molecules, a process that requires transient elevation of intracellular [Ca ⁺⁺ ].

In this chapter, attention is directed at the molecular mechanisms underlying contraction of skeletal muscle. Mechanisms for regulating the force of contraction are also addressed. To put this information into perspective, it is important to first examine the basic organization of skeletal muscle.

Organization of Skeletal Muscle

Fig. 12.1 illustrates skeletal muscles spanning the elbow joint. The muscles are attached to bone on either side of the joint. The point of attachment closest to the spine (proximal) is called the origin, whereas the point of attachment on the far side of the joint (distal) is called the insertion. These points of attachment occur through tendons (connective tissue) at the end of the muscle. Note that the point of insertion is close to the elbow joint, which enables a broad range of motion. Also note that the joint is spanned by a flexor muscle on one side and an extensor muscle on the opposite side of the joint. Thus contraction of the flexor muscle (see the biceps muscle in Fig. 12.1 ) results in a decrease in the angle of the elbow joint (bringing the forearm closer to the shoulder), whereas contraction of the extensor muscle (see the triceps muscle in Fig. 12.1 ) results in the reverse motion (extending the arm).

The basic structure of skeletal muscle is shown in Fig. 12.2 . Each muscle is composed of numerous cells called muscle fibers. A connective tissue layer called the endomysium surrounds each of these fibers. Individual muscle fibers are then grouped together into fascicles, which are surrounded by another connective tissue layer called the perimysium. Within the perimysium are the blood vessels and nerves that supply the individual muscle fibers. The fascicles are joined together to form the muscle. The connective tissue sheath that surrounds the muscle is called the epimysium. At the ends of the muscle, the connective tissue layers come together to form a tendon, which attaches the muscle to the skeleton. The myotendinous junction is a specialized region of the tendon where the ends of the muscle fibers interdigitate with the tendon for the transmission of the force of contraction of the muscle to the tendon to effect movement of the skeleton (discussed later in this section). The tendon and the connective tissue layers are composed mainly of elastin and collagen fibers, and thus they also contribute to passive tension of muscle and prevent damage to the muscle fibers as a result of overstretching or contraction.

Individual skeletal muscle cells are narrow (≈10–80 µm in diameter), but they can be extremely long (up to 25 cm in length). Each skeletal muscle fiber contains bundles of filaments, called myofibrils, running along the axis of the cell. The gross striation pattern of the cell results from a repeating pattern in the myofibrils. Specifically, it is the regular arrangement of the thick and thin filaments within these myofibrils, coupled with the highly organized alignment of adjacent myofibrils, that gives rise to the striated appearance of skeletal muscle. Striations can be observed in intact muscle fibers and in the underlying myofibrils.

A myofibril can be subdivided longitudinally into sarcomeres ( Fig. 12.3 ). The sarcomere is demarcated by two dark lines called Z lines and represents a repeating contractile unit in skeletal muscle. The average length of a sarcomere is 2 µm. On either side of the Z line is a light band (I band) that contains thin filaments composed primarily of the protein actin. The area between two I bands within a sarcomere is the A band, which contains thick filaments composed primarily of the protein myosin. The thin actin filaments extend from the Z line toward the center of the sarcomere and overlap a portion of the thick filaments. The dark area at each end of the A band represents this region of overlap between thick and thin filaments. A light area in the center of the sarcomere is called the H band. This area represents the portion of the A band that contains myosin thick filaments but no thin actin filaments. Thus thin actin filaments extend from the Z line to the edge of the H band and overlap a portion of the thick filament in the A band. A dark line called the M line is evident in the center of the sarcomere and includes proteins that appear to be critical for organization and alignment of the thick filaments in the sarcomere.

As illustrated in Fig. 12.3 B , each myofibril in a muscle fiber is surrounded by sarcoplasmic reticulum (SR). The SR is an intracellular membrane network that plays a critical role in the regulation of intracellular [Ca ⁺⁺ ]. Invaginations of the sarcolemma, called T tubules, pass into the muscle fiber near the ends of the A band (i.e., close to the SR). The SR and the T tubules, however, are distinct membrane systems: The SR is an intracellular network, whereas the T tubules are in contact with the extracellular space. A gap (≈15 nm in width) separates the T tubules from the SR. The portion of the SR nearest the T tubules is called the terminal cisternae, and it is the site of Ca ⁺⁺ release, which is critical for contraction of skeletal muscle (see the section “Excitation-Contraction Coupling”). At the myofibrils, the T tubule is positioned between two terminal cisternae (see Fig. 12.3 B ). The term “triad” refers to this region of the T tubule where it is coupled to two adjacent terminal cisternae, and hence is the site where excitation-contraction coupling is initiated.

The longitudinal portions of the SR are continuous with the terminal cisternae and extend along the length of the sarcomere. This portion of the SR contains a high density of Ca ⁺⁺ pump protein (i.e., SERCA : S arcoplasmic E ndoplasmic R eticulum C a ⁺⁺ - A TPase), which is critical for reaccumulation of Ca ⁺⁺ in the SR and hence for relaxation of the muscle.

The thick and thin filaments are highly organized in the sarcomere of myofibrils. The thin actin filaments extend from the Z line toward the center of the sarcomere, whereas thick myosin filaments are centrally located and overlap a portion of the opposing thin actin filaments. The thick and thin filaments are oriented in such a way that in the region of overlap within the sarcomere, each thick myosin filament is surrounded by a hexagonal array of thin actin filaments (see Fig. 12.3 C ). The Ca ⁺⁺ -dependent interaction of the thick myosin filaments and the thin actin filaments generate the force of contraction after stimulation of the muscle (see the section “Actin-Myosin Interaction: Cross-Bridge Formation”).

The thin filament is formed by aggregation of actin molecules (termed globular actin or G-actin ) into a two-stranded helical filament called filamentous actin or F-actin . The elongated cytoskeletal protein nebulin extends along the length of the thin filament and may participate in regulation of the length of the thin filament. Dimers of the protein tropomyosin extend over the entire actin filament and cover myosin-binding sites on the actin molecules. Each tropomyosin dimer extends across seven actin molecules, with sequential tropomyosin dimers arranged in a head-to-tail configuration. A troponin complex consisting of three subunits (troponin T, troponin I, and troponin C) is present on each tropomyosin dimer and influences the position of the tropomyosin molecule on the actin filament and hence the ability of tropomyosin to inhibit binding of myosin to the actin filament at low cytosolic Ca ⁺⁺ concentrations (see the section “Actin-Myosin Interaction: Cross-Bridge Formation”). Additional proteins associated with the thin filament include tropomodulin, α-actinin, and CapZ protein. Tropomodulin is located at the end of the thin filament, toward the center of the sarcomere, and may participate in setting the length of the thin filament. CapZ protein and α-actinin serve to anchor the thin filament to the Z line.

The thick myosin filaments are tethered to the Z lines by the cytoskeletal protein titin . Titin is a very large protein (molecular weight > 3000 kDa) that extends from the Z line to the center of the sarcomere (see Fig. 12.3 C ), and appears to be important for the organization and alignment of the thick filaments in the sarcomere. Some forms of muscular dystrophy have been attributed to defects in titin (i.e., titinopathies). Additional proteins found in the thick filaments (e.g., myomesin and C protein ) may also participate in the bipolar organization or packing of the thick filament (or both).

The cytoskeleton (including the intermediate filament protein desmin) participates in the highly organized alignment of sarcomeres. Desmin extends from the Z lines of adjacent sarcomeres to the integrin protein complexes on the sarcolemma and thus participates in both the alignment of sarcomeres across muscles and the lateral transmission of force (described later in this section). Defects in desmin have been associated with myofibrillar myopathies.

The force of contraction is transmitted both longitudinally to the tendon (via myotendinous junctions) and laterally to connective tissue adjacent to the muscle fibers (via costameres). The myotendinous junction represents a specialized region where the muscle fiber connects to the tendon ( Fig. 12.4 A,B ). Folding of the sarcolemma at the myotendinous junction results in an interdigitation of the tendon with the end of the muscle fiber, which increases the contact area between the muscle fiber and the connective tissue and hence reduces the force per unit area at the end of the muscle fiber.

Lateral transmission of the force of contraction involves costameres, which link the Z lines of subsarcolemmal sarcomeres to extracellular matrix through a series of proteins (see Fig. 12.4 C ). The lateral transmission of force is thought to stabilize the sarcolemma and to protect it from damage during contraction. Defects in the myotendinous junction and/or costameres (which includes the dystrophin-glycoprotein complex) have been associated with some forms of muscular dystrophy. The myotendinous junction and costameres also contain signaling molecules.

AT THE CELLULAR LEVEL

The muscular dystrophies constitute a group of genetically determined degenerative disorders. Duchenne’s muscular dystrophy (described by G.B. Duchenne in 1861) is the most common of the muscular dystrophies and affects 1 per 3500 boys (3–5 years of age). Severe muscle wasting occurs, and most affected patients are wheelchair bound by the age of 12; many die of respiratory failure in adulthood (30–40 years of age). Duchenne’s muscular dystrophy is an X-linked recessive disease that has been linked to a defect in the dystrophin gene that leads to a deficiency of the dystrophin protein in skeletal muscle, brain, retina, and smooth muscle. Dystrophin is a large (427-kDa) protein that is present at low levels (0.025%) in skeletal muscle. It is localized on the intracellular surface of the sarcolemma in association with several integral membrane glycoproteins (forming a dystrophin-glycoprotein complex; Figs. 12.4 C and 12.5 A ). This dystrophin-glycoprotein complex provides a structural link between the subsarcolemmal cytoskeleton of the muscle cell and the extracellular matrix and appears to stabilize the sarcolemma and hence prevent contraction-induced injury (rupture). The dystrophin-glycoprotein complex may also serve as a scaffold for cell signaling cascades. The enzyme nitric oxide synthase is present in the dystrophin-glycoprotein complex.

Although defects in the dystrophin-glycoprotein complex are involved in many forms of muscular dystrophy, some forms of muscular dystrophy that involve other mechanisms have been identified. Specifically, a defect in sarcolemma repair (attributed to loss/mutation of the protein dysferlin) appears to underlie at least one form of muscular dystrophy ( limb-girdle muscular dystrophy 2B, associated with muscle wasting in the pelvic region). Defects in the protein titin (titinopathies) have been implicated in other forms of muscular dystrophy (e.g., limb-girdle muscular dystrophy 2J and tibial muscular dystrophy ). Mutations in the protease calpain 3 (resulting in loss of protease activity) have also been implicated in some types of muscular dystrophy (e.g., limb-girdle muscular dystrophy 2A), apparently secondary to apoptosis.

Organization of the thick filament is shown in Fig. 12.6 . Each myosin molecule (≈480 kDa) consists of two heavy chains (≈200 kDa) and four light chains (≈20 kDa). The heavy chains are wound together in an α-helical configuration to form a long rod-like segment (which forms the backbone of the thick filament), and an N-terminal globular head (which extends from each myosin heavy chain toward the actin filament).

The globular head of each myosin molecule contains an essential light chain (which is crucial for the ATPase activity of myosin), and a regulatory light chain. The regulatory light chain can be phosphorylated by Ca ⁺⁺ /calmodulin-dependent myosin light chain protein kinase, which can influence the interaction of myosin with actin (see the section “Skeletal Muscle Types”). Thus myosin ATPase activity occurs in the two globular heads of myosin and requires the presence of the “essential” light chain in each globular head.

Myosin filaments form by a tail-to-tail association of myosin molecules, which results in a bipolar arrangement of the thick filament (see Fig. 12.6 A ). The thick filament then extends on either side of the central bare zone by a head-to-tail association of myosin molecules, thus maintaining the filament’s bipolar organization centered on the M line. Such a bipolar arrangement is critical for drawing the Z lines together (i.e., shortening the length of the sarcomere) during contraction.

Control of Skeletal Muscle Activity

Motor Nerves and Motor Units

Skeletal muscle is controlled by the central nervous system. Specifically, each skeletal muscle is innervated by an α motor neuron. The cell bodies of α motor neurons are in the ventral horn of the spinal cord ( Fig. 12.7 ; see also Chapter 9 ). The motor axons exit via the ventral roots and reach the muscle through mixed peripheral nerves. The α motor nerves branch in the muscle, and each branch innervates a single muscle fiber. The specialized cholinergic synapse that forms the neuromuscular junction and the neuromuscular transmission process that generates an action potential in the muscle fiber are described in Chapter 6 .

A motor unit consists of the α motor nerve and all the muscle fibers innervated by the nerve. The motor unit is the functional contractile unit because all the muscle cells within a motor unit contract synchronously when the motor nerve fires. The size of motor units within a muscle varies, depending on the function of the muscle. Activation of motor units with a small number of fibers facilitates fine motor control. Activation of varying numbers of motor units within a muscle is one way in which the tension developed by a muscle can be controlled (see “Recruitment” in the section “Modulation of the Force of Contraction”).

The neuromuscular junction formed by the α motor neuron is called an end plate (see Chapter 6 for details). Acetylcholine released from the α motor neuron at the neuromuscular junction initiates an action potential in the muscle fiber that rapidly spreads along its length. The duration of the action potential in skeletal muscle is less than 5 milliseconds. The short duration of the skeletal muscle action potential allows very rapid contractions of the fiber and provides yet another mechanism by which the force of contraction can be increased. Increasing tension by repetitive stimulation of the muscle is called tetany (see the section “Modulation of the Force of Contraction”).

Excitation-Contraction Coupling

When an action potential is transmitted along the sarcolemma of the muscle fiber and then down the T tubules, Ca ⁺⁺ is released from the terminal cisternae of the SR into the myoplasm ( Fig. 12.8 A ). This Ca ⁺⁺ release causes intracellular [Ca ⁺⁺ ] to rise, which in turn promotes actin-myosin interaction and hence contraction (see Fig. 12.8 B ). The action potential is extremely short-lived (<5 milliseconds). The elevation in intracellular [Ca ⁺⁺ ] begins slightly after the action potential and peaks at approximately 20 milliseconds. This increase in intracellular [Ca ⁺⁺ ] initiates a contraction called a twitch.

The mechanism by which an action potential in a skeletal muscle fiber can induce Ca ⁺⁺ release from the SR involves an interaction of voltage-gated Ca ⁺⁺ channels (Ca _V 1.1) in the T tubules with nearby Ca ⁺⁺ release channels (RYR1) in the terminal cisternae of the SR (see Fig. 12.8 A ). Calcium flux through the voltage-gated Ca ⁺⁺ channels in the T tubule is not needed to induce Ca ⁺⁺ release from the nearby SR. Instead, a depolarization-induced conformational change in the voltage-gated Ca ⁺⁺ channel in the T tubule appears to promote a protein-protein interaction with the nearby SR Ca ⁺⁺ release channel, resulting in Ca ⁺⁺ release from the SR into the cytoplasm. The rise in cytosolic Ca ⁺⁺ then promotes actin-myosin interaction, and hence contraction

The proposed organization of the voltage-gated Ca ⁺⁺ channel and SR Ca ⁺⁺ release channel is shown in Fig. 12.9 A . The voltage-gated Ca ⁺⁺ channel contains five subunits (α _1s , α ₂ , δ, β _1a , γ), with the α _1s subunit acting as a voltage sensor and Ca ⁺⁺ channel. The α _1s subunit is also called Ca _V 1.1. Historically, this voltage-gated Ca ⁺⁺ channel was isolated using the dihydropyridine class of L-type voltage-gated Ca ⁺⁺ channel blockers, so the α _1s subunit (Ca _V 1.1) is also called the dihydropyridine receptor (DHPR).

The SR Ca ⁺⁺ release channel is called the ryanodine receptor (RYR), as it was isolated using the compound ryanodine. The isoform of the ryanodine receptor in skeletal muscle is RYR1. The ryanodine receptor (RYR1) is a large protein (∼480 kDa), that forms a homotetrameric Ca ⁺⁺ channel in the terminal cisternae. Much of the RYR1 extends from the terminal cisternae, across the ∼15 nm gap, to approach the T tubule. Structural analyses confirm a close association of the voltage-gated Ca ⁺⁺ channel in the T tubule and the cytosolic portion of the RYR1 extending from the terminal cisternae (see Fig. 12.9 A ).

Recent studies have shown that the depolarization-induced Ca ⁺⁺ release from the SR characteristic of the excitation-contraction-coupling in skeletal muscle can be reconstituted in an expression system using the following 5 proteins: (1) Ca _V 1.1, (2) the β _1a auxiliary subunit of voltage-gated Ca channel, (3) the adapter protein STAC3, (4) RYR1, and (5) junctiphilin. The proposed interactions among these proteins are depicted in Fig. 12.9 B . Junctiphilin is not shown in Fig. 12.9 , but serves a critical function of promoting formation/maintenance of junctions between the SR and T tubule membrane. Junctiphilin may also participate in localizing this calcium release complex.

The lumen of the terminal cisternae contains the low-affinity Ca ⁺⁺ -binding protein calsequestrin, that allows Ca ⁺⁺ to be “stored” at high concentration and thereby establishes a favorable concentration gradient that facilitates the efflux of Ca ⁺⁺ from the SR into the cytoplasm when the RYR1 opens. The proteins triadin and junctin are also in the terminal cisternae membrane and bind both RYR and calsequestrin; they could anchor calsequestrin near RYR1 and thereby increase Ca ⁺⁺ buffering capacity at the site of Ca ⁺⁺ release. Histidine-rich calcium-binding protein is another low-affinity Ca ⁺⁺ -binding protein in the SR lumen, although it is less abundant than calsequestrin.

There is also evidence for the presence of s tore- o perated C a e ntry ( SOCE ) in skeletal muscle (e.g., via the Orai/Stim1 complex) during tetany. Inhibition of Ca ⁺⁺ influx did not affect excitation-contraction coupling but did reduce maximal tetanic tension at high rates of electrical stimulation, which suggests that there may be some extrusion of intracellular Ca ⁺⁺ during tetany, which is compensated by Ca ⁺⁺ influx to maintain maximal tetanic tension.

AT THE CELLULAR LEVEL

Recent studies indicate that the electromechanical coupling between Ca _V 1.1 and the RYR1 can be accomplished with the following 5 proteins: (1) Ca _V 1.1, (2) β _1a auxiliary subunit of Ca _V 1.1, (3) Stac3, (4) RYR1, and (5) junctiphilin (see Fig. 12.9 B ). It is hypothesized that as the wave of depolarization from an action potential spreads down the T tubule, the Ca _V 1.1 responds to the voltage through a conformational change that opens the underlying RYR1, resulting in Ca ⁺⁺ release from the terminal cisternae of the SR into the muscle cytoplasm, which promotes actin-myosin interaction and hence contraction. The voltage-sensing region of the Ca _V 1.1 involved in intramembranous charge movement is thought to reside in the S ₄ transmembrane segments of Ca _V 1.1, whereas the myoplasmic loop between transmembrane domains II and III in Ca _V 1.1 appears to be important for the interaction between Ca _V 1.1, Stac3, and RYR1. Mutations in Ca _V 1.1, RYR1, and/or Stac3 have been linked to pathologies characterized by altered regulation of intracellular [Ca ⁺⁺ ]. Specifically, mutations in Ca _V 1.1 have been associated with hypokalemic periodic paralysis, and myotonic dystrophy type 1. Malignant hyperthermia susceptibility has been linked to mutations in either Ca _V 1.1 or RYR1. Central core disease involves a defect in the RYR1, discussed later. A mutation in Stac3 is present in the rare congenital disorder Native American myopathy.

Relaxation of skeletal muscle occurs as intracellular Ca ⁺⁺ is resequestered by the SR. Uptake of Ca ⁺⁺ into the SR is due to the action of a Ca ⁺⁺ pump (i.e., Ca ⁺⁺ -ATPase). This pump is not unique to skeletal muscle; it is found in all cells in association with the endoplasmic reticulum. Accordingly, it is named SERCA, which stands for s arcoplasmic e ndoplasmic r eticulum c alcium A TPase. SERCA is the most abundant protein in the SR of skeletal muscle, and it is distributed throughout the longitudinal tubules and the terminal cisternae. It transports two molecules of Ca ⁺⁺ into its lumen for each molecule of ATP hydrolyzed. ^a

a During the transport of Ca ⁺⁺ , SERCA exchanges two Ca ⁺⁺ ions for two H ⁺ ions (i.e., H ⁺ is pumped out of the SR).

Thus the Ca ⁺⁺ transient seen during a twitch contraction (see Fig. 12.8 B ) reflects release of Ca ⁺⁺ from the terminal cisternae via RYR1 and reuptake primarily into the longitudinal portion of the SR by SERCA. The low-affinity Ca ⁺⁺ -binding protein sarcalumenin is present throughout the longitudinal tubules of the SR and nonjunctional regions of the terminal cisternae and is thought to be involved in the transfer of Ca ⁺⁺ from sites of Ca ⁺⁺ uptake in the longitudinal tubules to sites of Ca ⁺⁺ release in the terminal cisternae. Results of studies suggest that sarcalumenin increases Ca ⁺⁺ uptake by SERCA, at least in part by buffering luminal Ca ⁺⁺ near the pump.

The endogenous micropeptides phospholamban, sarcolipin, and myoregulin have been shown to regulate the activity of SERCA by decreasing the Ca ⁺⁺ sensitivity of Ca ⁺⁺ uptake. Protein kinase A–dependent phosphorylation of phospholamban in slow-twitch skeletal muscle has been reported to increase Ca ⁺⁺ transport in the SR, similar to the effect of phospholamban phosphorylation in the heart. Phospholamban and sarcolipin are present in slow-twitch muscle, whereas myoregulin is present in both fast- and slow-twitch muscle.

Actin-Myosin Interaction: Cross-Bridge Formation

As noted, contraction of skeletal muscle requires an increase in intracellular [Ca ⁺⁺ ]. Moreover, the process of contraction is regulated by the thin filament. As shown in Fig. 12.10 , contractile force (i.e., tension) increases in a sigmoidal manner as intracellular [Ca ⁺⁺ ] is elevated above 0.1 µM, with half-maximal force occurring at less than 1 µM Ca ⁺⁺ . The mechanism by which Ca ⁺⁺ promotes this increase in tension is as follows: Ca ⁺⁺ released from the SR binds to troponin C. Once bound with Ca ⁺⁺ , troponin C facilitates movement of the associated tropomyosin molecule toward the cleft of the actin filament. This movement of tropomyosin exposes myosin-binding sites on the actin filament and allows a cross-bridge to form and thereby generate tension (see section “Cross-Bridge Cycling: Sarcomere Shortening”). Troponin C has four Ca ⁺⁺ -binding sites. Two of these sites have high affinity for Ca ⁺⁺ but also bind Mg ⁺⁺ at rest. These sites seem to be involved in controlling and enhancing the interaction between the troponin I and troponin T subunits. The other two binding sites have lower affinity and bind Ca ⁺⁺ as its concentration rises after release from the SR. Binding of myosin to the actin filaments appears to cause a further shift in tropomyosin. Although a given tropomyosin molecule extends over seven actin molecules, it is hypothesized that the strong binding of myosin to actin results in movement of an adjacent tropomyosin molecule, perhaps exposing myosin-binding sites on as many as 14 actin molecules. This ability of one tropomyosin molecule to influence the movement of another may be a consequence of the close proximity of adjacent tropomyosin molecules.

IN THE CLINIC

Genetic diseases that cause disturbances in Ca ⁺⁺ homeostasis in skeletal muscle include malignant hyperthermia, central core disease, and Brody’s disease. Malignant hyperthermia is an autosomal dominant trait that has life-threatening consequences in certain surgical instances. Anesthetics such as halothane or ether and the muscle relaxant succinylcholine can produce uncontrolled release of Ca ⁺⁺ from the SR, thereby resulting in skeletal muscle rigidity, tachycardia, hyperventilation, and hyperthermia. This condition is lethal if not treated immediately (typically by administering dantrolene to block this uncontrolled Ca ⁺⁺ release from the SR). The incidence of malignant hyperthermia susceptibility is approximately 1 per 15,000 children and 1 per 50,000 adults treated with anesthetics. Malignant hyperthermia is the result of a defect in the SR Ca ⁺⁺ release channel (RYR1), which becomes activated in the presence of the aforementioned anesthetics, causes the release of Ca ⁺⁺ into the cytoplasm, and hence prolongs muscle contraction (rigidity). The defect in the RYR1 is not restricted to a single locus. In some cases, malignant hyperthermia has been linked to a defect in the Ca _V 1.1 of the T tubule.

Central core disease is a rare autosomal dominant trait that results in muscle weakness, loss of mitochondria in the core of skeletal muscle fibers, and some disintegration of contractile filaments. It is often closely associated with malignant hyperthermia, and so patients with central core disease are treated as though they are susceptible to malignant hyperthermia in surgical situations. It is hypothesized that central cores devoid of mitochondria represent areas of elevated intracellular Ca ⁺⁺ secondary to a mutation in the RYR. The loss of mitochondria is thought to occur when they take up the elevated Ca ⁺⁺ , which leads to mitochondrial Ca ⁺⁺ overload.

Brody’s disease is characterized by painless muscle cramping and impaired muscle relaxation during exercise. While an affected person runs upstairs, for example, muscles may stiffen and temporarily cannot be used. This relaxation abnormality is seen in muscles of the legs, arms, and eyelid, and the response is worsened in cold weather. Brody’s disease can be either autosomal recessive or autosomal dominant and may involve mutations in up to three genes; however, it is rare (affecting 1 per 10,000,000 births). It appears to result from decreased activity of the SERCA1 Ca ⁺⁺ pump found in fast-twitch skeletal muscle (see the section “Skeletal Muscle Types”). The decreased activity of SERCA1 has been associated with mutations in the gene that encodes SERCA1, although another accessory factor may contribute to the decreased SR Ca ⁺⁺ uptake in the fast-twitch skeletal muscle of individuals with Brody’s disease.

Myotonia congenita is also associated with prolonged muscle contractions (painless cramping) after voluntary contractions, as a result of mutations in the CLCN1 gene, which encodes the chloride voltage-gated channel 1 in skeletal muscle sarcolemma and T tubules. Chloride conductance in the skeletal muscle is important for repolarization and stabilization of the membrane potential, and so the reduced chloride conductance in skeletal muscles of individuals with myotonia congenita results in hyperexcitability of the muscle fiber. Voluntary contraction may therefore be followed by a series of action potentials (afterdepolarizations) in the muscle that result in prolonged contractions (i.e., cramping). Epinephrine (e.g., during stressful situations) often worsens the condition, as shown in myotonic (“fainting”) goats. Muscle stiffness can be relieved by repeated contractions (i.e., the warm-up phenomenon), although the mechanism underlying the warm-up phenomenon is not known. Mutations in the CLCN1 gene in myotonia congenita may be transmitted in either an autosomal recessive manner (as in Becker’s disease, one type of myotonia congenita) or an autosomal dominant manner (as in Thomsen’s disease, the other type of myotonia congenita). The prevalence of myotonia congenita is approximately 1 per 100,000 worldwide; the incidence is higher (≈1 per 10,000) in northern Scandinavia.

Cross-Bridge Cycling: Sarcomere Shortening

Once myosin and actin are bound, ATP-dependent conformational changes in the myosin molecule result in movement of the actin filaments toward the center of the sarcomere. Such movement shortens the length of the sarcomere and thereby contracts the muscle fiber. The mechanism by which myosin produces force and shortens the sarcomere is thought to involve four basic steps that are collectively termed the cross-bridge cycle (labeled a to d in Fig. 12.11 ). In the resting state, myosin is thought to have partially hydrolyzed ATP (state a ). When Ca ⁺⁺ is released from the terminal cisternae of the SR, it binds to troponin C, which in turn promotes movement of tropomyosin on the actin filament in such a way that myosin-binding sites on actin are exposed. This then allows the “energized” myosin head to bind to the underlying actin (state b ). Myosin next undergoes a conformational change termed “ratchet action” that pulls the actin filament toward the center of the sarcomere (state c ). Myosin releases adenosine diphosphate (ADP) and inorganic phosphate during the transition to state c. Binding of ATP to myosin decreases the affinity of myosin for actin, thereby resulting in the release of myosin from the actin filament (state d ). Myosin then partially hydrolyzes the ATP, and part of the energy in the ATP is used to recock the head and return to the resting state.

If intracellular [Ca ⁺⁺ ] is still elevated, myosin undergoes another cross-bridge cycle and produces further contraction of the muscle. The ratchet action of the cross-bridge is capable of moving the thin filament approximately 10 nm. The cycle continues until the SERCA pumps Ca ⁺⁺ back into the SR. As [Ca ⁺⁺ ] falls, Ca ⁺⁺ dissociates from troponin C, and the troponin-tropomyosin complex moves and blocks the myosin-binding sites on the actin filament. If the supply of ATP is exhausted, as occurs with death, the cycle stops in state c with the formation of permanent actin-myosin complexes (i.e., the rigor state). In this state, the muscle is rigid, and the condition is termed rigor mortis.

As already noted, formation of the thick filaments involves the association of myosin molecules in a tail-to-tail configuration to produce a bipolar orientation (see Fig. 12.6 ). Such a bipolar orientation allows myosin to pull the actin filaments toward the center of the sarcomere during the cross-bridge cycle. The myosin molecules are also oriented in a helical array in the thick filament in such a way that cross-bridges extend toward each of the six thin filaments surrounding the thick filament (see Fig. 12.3 C ). These myosin projections/cross-bridges can be seen on electron micrographs of skeletal muscle and appear to extend perpendicular from the thick filaments at rest. In the contracted state, the myosin cross-bridges slant toward the center of the sarcomere, which is consistent with the ratchet action of the myosin head.

The cross-bridge cycling mechanism just described is called the sliding filament theory because the myosin cross-bridge is pulling the actin thin filament toward the center of the sarcomere, which results in an apparent “sliding” of the thin filament past the thick filament. There is, however, uncertainty about how many myosin molecules contribute to the generation of force and whether both myosin heads in a given myosin molecule are involved. It has been calculated that there may be 600 myosin heads per thick filament, with a stoichiometry of 1 myosin head per 1.8 actin molecules. As a result of steric considerations, it is unlikely that all myosin heads can interact with actin, and calculations suggest that even during maximal force generation, only 20% to 40% of the myosin heads bind to actin.

The conversion of chemical energy (i.e., ATP) to mechanical energy by muscle is highly efficient. In isolated muscle preparations, maximum mechanical efficiency (≈65% efficiency) is obtained at a submaximal force of 30% maximal tension. In humans performing steady-state ergometer exercise, mechanical efficiencies range from 40% to 57%.

Skeletal Muscle Types

Skeletal muscle fibers can be classified into two main groups according to the speed of contraction: fast-twitch and slow-twitch muscle fibers. As shown in Fig. 12.12 A , the lateral rectus of the eye contracts very quickly in response to an action potential, reaching peak tension within 8 milliseconds, and then relaxes quickly, which results in a short duration of contraction. The soleus muscle of the leg, in contrast, requires 90 milliseconds to reach peak tension in response to an action potential, and then it relaxes slowly. The gastrocnemius muscle requires an intermediate time to reach peak tension (40 milliseconds) because of the presence of both fast-twitch and slow-twitch muscle fibers in this muscle.

The difference in speed of contraction between fast-twitch and slow-twitch muscles is correlated with myosin ATPase activity (see Fig. 12.12 B ), which in turn reflects the type of myosin present in the muscle fiber. Thus fast-twitch muscle fibers contain myosin isoforms that hydrolyze ATP quickly, whereas slow-twitch muscle fibers contain myosin isoforms that hydrolyze ATP slowly. These two types of myosin isoforms have the same basic structure described previously, with two heavy chains and two pairs of light chains, although they differ in amino acid composition.

It is very difficult to convert a slow-twitch muscle fiber into a fast-twitch fiber, although it can be accomplished by cross-innervation, which involves surgically interconnecting two motor neurons. As shown in Fig. 12.12 B, when the soleus muscle and extensor digitorum longus muscle underwent cross-innervation, so that contraction of the soleus muscle was controlled by the extensor digitorum motor neuron (and vice versa), the speed of the contraction and the myosin ATPase activity of the soleus muscle increased (labeled X-SOL in Fig. 12.12 B ), whereas the extensor digitorum longus exhibited a decrease in shortening velocity and myosin ATPase activity (labeled X-EDL ). Thus the motor innervation of the muscle fiber plays an important role in determining which type of myosin isoform is expressed in the muscle fiber. Further study showed that the intracellular Ca ⁺⁺ concentration in the muscle (secondary to differences in the activity pattern of the motor neuron) was an important determinant of whether the muscle fiber expressed the slow myosin isoform or the fast myosin isoform (see the section “Growth and Development”).

The myosin isoforms expressed in skeletal muscle can be distinguished on the basis of myosin heavy chain composition. Slow-twitch muscle fibers express Type I myosin heavy chain, whereas fast-twitch skeletal muscle fibers could contain Type IIa, Type IIx, or Type IIb myosin heavy chains ( Fig. 12.13 ). The Type IIb myosin isoform is not present in human skeletal muscle, so human skeletal muscle fiber types are classified as Type 1, Type IIa, or Type IIx. The distribution of Type I, Type IIa, and Type IIx myosins in a biopsy of human vastus lateralis muscle is shown in Fig. 12.14 A . Note that a few muscle fibers (denoted with an asterix) contain two types of myosin heavy chain. Endurance training or chronic stimulation promotes the expression of the Type 1 myosin isoform, whereas strength training promotes the expression of the Type II myosin isoform (as depicted in Fig. 12.13 ). Typically, changes in the expression of myosin isoforms follow a progression, wherein

The maximal contraction speeds of human and mouse skeletal muscle fibers expressing Type 1, Type IIa, Type IIx, or Type IIb myosins are shown in Fig. 12.14 B . The human maximal contraction velocities were determined from the Y-intercepts of the force-velocity relationships shown in Fig. 12.14 C , using permeabilized single muscle fibers from biopsies of human vastus lateralis muscle. In both humans and mice, the contraction speeds were consistent with the myosin isoform expressed in the fiber in that:

Type IIb myosin is rarely expressed in humans but is expressed in rodents. The contraction speed of mouse muscle fibers expressing Type 1Ib myosins was the fastest of the four myosin isoforms. Additional characteristics of the Type 1, Type IIa, Type IIx, and Type IIb muscle fibers are shown in Table 12.1 .

All of the muscle fibers innervated by a given α motor neuron typically express the same myosin isoform, so there are slow motor units and two or three types of fast motor units (for human and rodents, respectively) (see Fig. 12.13 ). The α motor neurons innervating Type I muscle fibers have small cell bodies, and are easily excited ( Table 12.2 ). The α motor neurons innervating Type II muscle fibers are larger, with a higher threshold for activation. The data in Fig. 12.14 D are consistent with this recruitment pattern, in that over a 24-hour period the cumulative activation time of rat slow-twitch motor units greatly exceeded that for rat fast-twitch motor units. Type IIb motor units in the rat were rarely recruited. This pattern of recruitment is consistent with the size principle for motor unit recruitment (see Chapter 9 ), wherein motor units with small motor axons are more easily activated than large motor neurons,

Slow-twitch skeletal muscles are also characterized by a high oxidative capacity (see Table 12.1 ), which in combination with the low myosin ATPase activity contribute to the fatigue resistance of slow-twitch muscle fibers. The oxidative capacity of the fast-twitch muscle fiber ranges from relatively high (in muscle fibers expressing Type IIa myosin heavy chain) to low (in muscle fibers expressing Type IIb myosin heavy chains). Muscle fibers expressing a Type IIx myosin heavy chain have a speed of contraction and an oxidative capacity that is intermediate between fiber types IIa and IIb, so in human skeletal muscle (which lacks Type IIb fibers), the Type IIx muscle fibers have a slightly higher contraction speed but lower oxidative capacity than Type I muscle fibers (see Table 12.1 )

Although motor units are generally composed of only one type of muscle fiber (see Fig. 12.13 ), there are conditions that may trigger a change in the type of myosin expressed in a muscle fiber. Chronic conditions such as microgravity (in space flight), denervation, spinal cord injury, and chronic unloading, for example, are associated with severe atrophy and promote the gradual transition from the expression of slow muscle myosin (Type I) in the fiber to the expression of fast muscle myosin (Types IIa and IIx).

An important function of slow motor units is in the maintenance of posture (see Fig. 12.13 ). The low ATPase activity of myosin in slow motor units, coupled with their high oxidative capacity, facilitates the ability of these slow motor units to maintain posture at low energy cost and thus resist fatigue. The smaller diameter of slow muscle fibers, and the higher capillary density in slow muscle, also helps slow muscle resist fatigue.

Fast muscle, in contrast, is recruited for activities that require faster movements, more force, or both (see Fig. 12.13 ). Weightlifting, for example, can require a lot of power for short duration. To meet the demands for more force, additional motor units are recruited. In comparison with slow motor units, the fast motor units typically contain more muscle fibers (see Table 12.2 ). Fast muscle fibers also have a larger diameter than do slow muscle fibers. Thus recruitment of fast motor units can help meet the increased demands of burst activities such as weightlifting. The high myosin ATPase activity in fast muscle fibers and the increase in diffusion distance (resulting from the large diameter of the fast muscle fibers), however, increase the susceptibility of fast muscle fibers to fatigue.

Additional differences between fast and slow muscles include the following:

• 1

  The neuromuscular junction of fast muscle differs from that in slow muscle in terms of acetylcholine vesicle content, the amount of acetylcholine released, the density of nicotinic acetylcholine receptors, the acetylcholine esterase activity, and Na+ channel density, all of which endow the fast muscle with a higher safety factor for initiation of an action potential. During repetitive stimulation, however, the safety factor in fast muscle drops quickly (faster than that seen in slow muscle).

• 2

  The SR is more highly developed in fast muscle than in slow muscle, with higher levels of RYR1, SERCA, luminal Ca++, and a higher Ca _V 1.1/RYR1 ratio, all of which promote the development of a larger, faster intracellular Ca++ transient in fast muscle, which is important for quick, forceful contraction.

In addition to the differences between fast and slow fibers just noted, other muscle proteins are also expressed in a fiber type–specific manner. Such proteins include the three troponin subunits, tropomyosin, and C protein. The differential expression of troponin and tropomyosin isoforms influences the dependency of contraction on Ca ⁺⁺ . Slow fibers begin to develop tension at lower [Ca ⁺⁺ ] than fast fibers do. This difference in sensitivity to Ca ⁺⁺ is related in part to the fact that the troponin C isoform in slow fibers has only a single low-affinity Ca ⁺⁺ -binding site, whereas the troponin C of fast fibers has two low-affinity binding sites. Changes in the dependence of contraction on Ca ⁺⁺ , however, are not restricted to differences in the troponin C isoforms. Differences in troponin T and tropomyosin isoforms are also found. Thus regulation of the dependence of contraction on Ca ⁺⁺ is complex and involves contributions from multiple proteins on the thin filament. Phosphorylation of the regulatory light chain of myosin by Ca ⁺⁺ /calmodulin-dependent myosin light chain kinase, however, can increase Ca ⁺⁺ sensitivity of contraction, particularly in fast muscle fibers (partly because of the reported higher activity of myosin light chain kinase in fast muscle fibers).

Modulation of the Force of Contraction