Classification of Muscle

Muscle cells (fibres) are also known as myocytes (the prefixes myo- and sarco- are frequently used when naming structures associated with muscle). They differentiate along one of three main pathways to form skeletal, cardiac or smooth muscle. Both skeletal muscle and cardiac muscle are called striated muscle, because their myosin and actin filaments are organized into regular, repeating elements that give the cells a finely cross-striated appearance when they are viewed microscopically. Smooth muscle, in contrast, lacks such repeating elements and thus has no striations.

Other contractile cells, including myofibroblasts and myoepithelial cells, are different in character and origin. They contain smooth muscle–like contractile proteins and are found singly or in small groups.

Skeletal Muscle

Shape and Fibre Architecture

It is possible to classify muscles based on their general shape and the predominant orientation of their fibres relative to the direction of pull (Fig. 23.1). Muscles with fibres that are largely parallel to the line of pull vary in form from flat, short and quadrilateral (e.g. thyrohyoid) to long and strap-like (e.g. sternohyoid, sartorius). In such muscles, individual fibres may run for the entire length of the muscle or over shorter segments when there are transverse, tendinous intersections at intervals (e.g. rectus abdominis). In a fusiform muscle, the fibres may be close to parallel in the belly but converge to a tendon at one or both ends. Where fibres are oblique to the line of pull, muscles may be triangular (e.g. temporalis, adductor longus) or pennate (feather-like) in construction. The latter vary in complexity (see Fig. 23.1) from unipennate (e.g. flexor pollicis longus) and bipennate (e.g. rectus femoris, dorsal interossei) to multipennate (e.g. deltoid). In some muscles, the fibres pass obliquely between deep and superficial aponeuroses, in a type of unipennate form (e.g. soleus). In other sites, muscle fibres start from the walls of osteofascial compartments and converge obliquely on a central tendon in circumpennate fashion (e.g. tibialis anterior). Some muscles have a spiral or twisted arrangement (e.g. sternocostal fibres of pectoralis major and latissimus dorsi, which undergo a 180° twist between their median and lateral attachments). Others spiral around a bone (e.g. supinator, which winds obliquely around the proximal radial shaft) or contain two or more planes of fibres arranged in different directions, a type of spiral sometimes referred to as cruciate; sternocleidomastoid, masseter and adductor magnus are all partially spiral and cruciate. Many muscles display more than one of these major types of arrangement and show regional variations that correspond to contrasting and, in some cases, independent actions.

Fig. 23.1 Morphological types of muscle based on their general form and fascicular architecture.

These terms are often used in combination—for example, flexor digitorum longus (long flexor of the digits) and latissimus dorsi (broadest muscle of the back). The names given to individual muscles or muscle groups are often oversimplified, and terms denoting action emphasize only one of a number of usual actions. A given muscle may play different roles in different movements, and these roles may change if the movements are assisted or opposed by gravity. The functional roles implied by names should therefore be interpreted with caution.

Microstructure

The cellular units of skeletal muscle are the muscle fibres (Fig. 23.2). These long, cylindrical structures tend to be consistent in size within a given muscle, but in different muscles they may range from 10 to 100 µm in diameter and from millimetres to many centimetres in length. Some typical skeletal muscle fibres are seen in longitudinal section in Figure 23.3. Their staining characteristics are dominated by the contractile apparatus, which constitutes much of the cytoplasm or sarcoplasm. The contractile proteins are organized into cylindrical myofibrils that are too tightly packed to be visible by routine light microscopy. Of greater significance are transverse striations, which are the result of alignment across the fibre of repeating elements, the sarcomeres, within neighbouring myofibrils. These cross-striations are usually evident in sections stained conventionally, but they may be demonstrated more effectively using special stains (Fig. 23.4).

Fig. 23.2 Levels of organization within a skeletal muscle, from whole muscle to fasciculi, single fibres, myofibrils and myofilaments.

Fig. 23.3 Skeletal muscle fibres in longitudinal section. Note the numerous peripherally placed nuclei in these extremely elongated, unbranched syncytial cells and the faint transverse striations in their cytoplasm. The central fibre is sectioned in part through its periphery, close to the sarcolemma, so several nuclei appear to be lying centrally.

(By permission from Young, B., Heath, J.W., 2000. Wheater’s Functional Histology. Churchill Livingstone, Edinburgh.)

Fig. 23.4 Skeletal muscle fibres in longitudinal section. Cross-striations reflect the sarcomeric organization of actin and myosin within the myofibrils and the alignment of myofibrils in register within the cytoplasm. The nuclei between fibres are those of endomysial connective tissue and capillaries, and some may be of satellite cells (phosphotungstic acid–stained preparation).

(Photograph by Sarah-Jane Smith.)

Under polarized light, the striations are even more striking and are seen as a pattern of alternating dark and light bands. The darker bands are birefringent, rotating the plane of polarized light strongly, and are known as anisotropic or A-bands; the lighter bands rotate the plane of polarized light to a negligible degree and are known as isotropic or I-bands. The structures responsible for this appearance are described more readily at the ultrastructural level.

The multiple nuclei are oval and are located at the periphery of the fibres, under the plasma membrane or sarcolemma. They are especially numerous in the region of the neuromuscular junction. The nuclei are moderately euchromatic and usually have one or more nucleoli. They occupy a thin, transparent rim of sarcoplasm between the myofibrils and the sarcolemma and are seen most clearly in transverse sections (Fig. 23.5). Other nuclei belonging to vascular endothelial cells, Schwann cells, fibroblasts, and so forth may be present in the spaces between the fibres, where blood vessels and nerve fibres travel through layers of fine connective tissue, the endomysium. Nuclei of satellite cells lie between the sarcolemma and the surrounding basal lamina.

Fig. 23.5 Transverse cryostat section of adult human skeletal muscle. Note the tight packing of the fibres and the peripheral location of the dark-stained nuclei.

(Photograph by Stanley Salmons, from a specimen provided by Tim Helliwell, Department of Pathology, University of Liverpool.)

In transverse section, the profiles of the fibres are usually polygonal (see Fig. 23.5). Some muscles, such as the extrinsic muscles of the larynx, tend to be less tightly packed. In such situations, as well as in conditions of generalized wasting or muscle damage, the fibres may adopt a more rounded profile; however, in some normal muscles, such as those that close the jaw, the fibres are closely packed but have rounded profiles. The sarcoplasm often has a stippled appearance because the transversely sectioned myofibrils are resolved as dots.

Skeletal muscle fibres are large (with a few exceptions, such as the laryngeal muscles), and unless electron micrographs are taken at very low magnification, they seldom show more than part of the interior of a fibre (Fig. 23.6A). Myofibrils are the dominant ultrastructural feature of such micrographs. They are cylindrical structures approximately 1 µm in diameter, which appear as ribbons in longitudinal section. Thin, very densely stained transverse lines, which correspond to discs in the parent cylindrical structure, appear at regular intervals along these ribbons. They are called Z-lines or, more properly, Z-discs (Zwischenscheiben, or ‘between discs’). They divide the myofibril into a linear series of identical contractile units called sarcomeres, each of which is approximately 2.2 µm long in resting muscle.

Fig. 23.6 A, Low-power electron micrograph of parts of two skeletal muscle fibres in longitudinal section. B, High-power electron micrograph of frog skeletal muscle in longitudinal section, showing a triad, thick and thin filaments and cross-bridges in the spaces between them. Note the overlap of thin filaments within the Z-disc at the top of the micrograph. (The human triad lies at the junction between the A-bands and I-bands, not at the Z-disc as shown here.)

(Photographs by Brenda Russell, Department of Physiology and Biophysics, University of Illinois at Chicago.)

At higher power, sarcomeres are seen to consist of two types of filament—thick and thin—organized into regular arrays (Fig. 23.6B). The thick filaments, which are approximately 15 nm in diameter, are composed mainly of myosin. The thin filaments, which are 8 nm in diameter, are composed mainly of actin. The arrays of thick and thin filaments form a partially overlapping structure in which the electron density varies according to the amount of protein present. The A-band consists of the thick filaments, together with lengths of thin filaments that interdigitate with, and thus overlap, the thick filaments at either end (see Fig. 23.6B; Fig. 23.7). The central, paler region of the A-band, into which the thin filaments have not penetrated, is called the H zone (helle, or ‘light’). At their centres, the thick filaments are linked together transversely by material that constitutes the M line (Mittelscheibe, or ‘middle (of) disc’), which is visible in most muscles.

Fig. 23.7 Sarcomeric structures. The drawings below the electron micrograph (of two myofibrils sectioned longitudinally) indicate the corresponding arrangements of thick and thin filaments. Relaxed and contracted states are shown to illustrate the changes that occur during shortening. Insets at the top show the electron micrographic appearance of transverse sections through the sarcomere at the levels shown. Note that the packing geometry of the thin filaments changes from a square array at the Z-disc to a hexagonal array where they interdigitate with thick filaments in the A-band.

(Photographs by Brenda Russell, Department of Physiology and Biophysics, University of Illinois at Chicago; artwork by Lesley Skeates.)

The I-band consists of the adjacent portions of two neighbouring sarcomeres in which the thin filaments are not overlapped by thick filaments. It is bisected by the Z-disc, into which the thin filaments of adjacent sarcomeres are anchored. In addition to the thick and thin filaments, there is a third type of filament composed of the elastic protein titin. The high degree of organization of the filament arrays is equally evident in electron micrographs of transverse sections (see Fig. 23.7; Fig. 23.8). The thick myosin filaments form a hexagonal lattice; in the regions where they overlap with the thin filaments, each myosin filament is surrounded by six actin filaments at the trigonal points of the lattice. In the I-band, the thin filament pattern changes from hexagonal to square as the filaments approach the Z-disc, where they are incorporated into a square lattice structure.

Fig. 23.8 Electron micrograph of skeletal muscle in transverse section, showing parts of two muscle fibres. Part of a capillary (C) is seen in transverse section in the endomysial space (right). The variation in the appearance of myofibrils in cross-section is explained in Figure 23-7.

(Photograph by Brenda Russell, Department of Physiology and Biophysics, University of Illinois at Chicago.)

The banded appearance of individual myofibrils is thus attributable to the regular alternation of the thick and thin filament arrays. However, myofibrils are at the limit of resolution of light microscopy; the fact that cross-striations are also visible at that level is the result of alignment in register of the bands in adjacent myofibrils across the breadth of the whole muscle fibre. In suitably stained, relaxed material, the A-bands, I-bands and H zones are quite distinct, but the Z-discs, which are such a prominent feature of electron micrographs, are thin and much less conspicuous under the light microscope, and M lines cannot be seen at all.

Muscle Proteins

Myosin, the protein of the thick filament, is the most abundant contractile protein (60% of the total myofibrillar protein). The thick filaments of skeletal (and cardiac) muscle are 1.5 µm long. Actin is the next most abundant contractile protein (20% of the total myofibrillar protein). In its filamentous form (F-actin), it is the principal protein of the thin filaments; the other components, the regulatory proteins tropomyosin and troponin, play a major part in the control of contraction.

The third type of long sarcomeric filament, which connects the thick filaments to the Z-disc, is formed by the giant protein titin, with a molecular mass in the millions. Single titin molecules span the half-sarcomere between the M lines and the Z-discs, into which they are inserted, with a bound portion in the A-band and an elastic portion in the I-band. In the A-band, titin is attached to thick filaments as far as the M line. Its physical properties endow the myosin filaments with elastic recoil after stretching.

A number of proteins that are neither contractile nor regulatory are responsible for the structural integrity of the myofibrils, particularly their regular internal arrangement. A component of the Z-disc, α-actinin, is a rod-shaped molecule that anchors the plus-ends of actin filaments from adjacent sarcomeres to the Z-disc. Nebulin inserts into the Z-disc, associated with the thin filaments, and regulates the length of actin filaments. An intermediate filament protein characteristic of muscle, desmin, encircles the myofibrils at the Z-disc and, with the linking molecule plectrin, forms a meshwork that connects myofibrils together within the muscle fibre. Myomesin holds myosin filaments in their regular lattice arrangement in the region of the M line. Dystrophin is confined to the periphery of the muscle fibre, close to the cytoplasmic face of the sarcolemma. It binds to actin intracellularly and is also associated with a large oligomeric complex of glycoproteins that spans the membrane and links specifically with merosin, the laminin isoform of the muscle basal lamina. This stabilizes the muscle fibre and transmits forces generated internally on contraction to the extracellular matrix.

Dystrophin is the product of the gene affected in Duchenne’s muscular dystrophy, a fatal disorder that develops when mutation of the gene leads to the absence of this protein. A milder form of the disease, Becker’s muscular dystrophy, is associated with reduced size or abundance of dystrophin. Female carriers (heterozygous for the mutant gene) of Duchenne’s muscular dystrophy may also have mild symptoms of muscle weakness. At approximately 2500 kb, the gene is one of the largest yet discovered, which may account for the high mutation rate of Duchenne’s muscular dystrophy (approximately 35% of cases are new mutations).

CASE 1 Duchenne’s Muscular Dystrophy

A 4-year-old boy presents with difficulty rising from the floor. His parents report that he has always walked on his toes and has had trouble climbing stairs and running, but now he must get on all fours, push himself into a squat and then push off his legs with his hands to stand (Gowers’ sign).

On examination, he has large, firm calves (Fig. 23.9). His gait is waddling, with a broad base and marked lumbar lordosis. He has proximal lower extremity weakness, significant neck flexor weakness and Gowers’ sign when rising from the floor. The remainder of the examination is normal.

Fig. 23.9 Duchenne’s muscular dystrophy. Although the muscles of both calves are enlarged, they are weak (pseudohypertrophy rather than true muscular hypertrophy with enhanced strength).

Discussion: Duchenne’s muscular dystrophy is an X-linked recessive disorder caused by a mutation in the gene located on the short arm of the X chromosome (Xp21) that produces dystrophin. Mutation results in the absence of this protein in skeletal muscle fibres (as well as in heart, brain and smooth muscle). The dystrophin gene is very large, and multiple gene mutations can result in the defect. Some mutations result in incomplete loss of dystrophin and milder clinical disease (i.e. Becker’s muscular dystrophy). Within muscle, dystrophin is found in the periphery of the fibres, close to the cytoplasmic face of the sarcolemma. It is part of a large complex of glycoproteins that spans the muscle membrane, binding intracellular actin with merosin in the muscle basal lamina, thus stabilizing the sarcolemmal membrane and allowing the transmission of forces of contraction and relaxation from internally to the extracellular matrix. The exact function of dystrophin is not fully understood, but it most likely acts as a stabilizer in skeletal muscle. Without dystrophin, the muscle membrane may be more susceptible to disruption, with destruction of muscle fibres and increasing fibrosis.

CASE 2 Myotonic Dystrophy

A 34-year-old man is referred for shortness of breath with exercise. Past medical history is significant for bilateral cataract removal at age 31, diabetes mellitus of 2 years’ duration, recurrent pneumonia and testicular atrophy. His father and paternal uncle both experienced sudden cardiac death in their 50s; although neither had significant muscle weakness, his father had cataracts removed in his 30s. The patient’s younger sister has had recurrent respiratory problems.

Examination demonstrates facial weakness (diplegia), with mild bilateral eyelid ptosis and marked frontal balding. He has bilateral distal upper extremity weakness and both proximal and distal lower extremity weakness. Delayed relaxation of grip is striking (myotonia), and percussion of the thenar eminence documents delayed relaxation. Testicles are reduced in size. Creatine kinase is mildly elevated.

Discussion: This man has typical myotonic dystrophy, an autosomal dominant disorder with variable expression. There are two types of myotonic dystrophy: DM1 and DM2. DM1 usually appears earlier, with more severe weakness. Many patients do not present until early adulthood, but when the mother has the disease, infants may exhibit hypotonia and poor feeding at birth. Anticipation is seen in DM1 from one generation to the next. Pathophysiologically, the disease is due to expansion of a trinucleotide (CTG trinucleotide repeat). The myotonic phenomenon exhibited on both physical examination and electrophysiological testing is due to repetitive spontaneous and contraction-induced muscle fibre activation. The diseased muscle fibre has increased expression of calcium-activated potassium channels, but it is not clear how this results in the repetitive muscle fibre activation to which the rubric ‘myotonia’ is applied. Chloride conductance may also be affected. Muscle biopsy shows a mixture of findings, including muscle fibre atrophy and hypertrophy, some necrotic fibres, fibrosis and adipose deposition, with rare eosinophilic cytoplasmic inclusions. Ring and split fibres can also be seen. Ringbinden (aberrant muscle fibres encircling normally oriented fibres) may be found.

It is noteworthy that whereas muscular involvement is proximal in the great majority of muscular dystrophies, in myotonic dystrophy a distal distribution is characteristic.

Other Sarcoplasmic Structures

Although myofibrils are the dominant ultrastructural feature of skeletal muscle, the fibres contain other organelles essential for cellular function, such as ribosomes, Golgi apparatus and mitochondria. Most of them are located around the nuclei, between the myofibrils and the sarcolemma and, to a lesser extent, between the myofibrils. Mitochondria, lipid droplets and glycogen provide the metabolic support needed by active muscle. The mitochondria are elongated, and their cristae are closely packed. Their profiles are usually seen in longitudinal orientation between the myofibrils (see Fig. 23.6A). The number of mitochondria in an adult muscle fibre is not fixed, but it can increase or decrease quite readily in response to sustained changes in activity. Spherical lipid droplets, approximately 0.25 µm in diameter, are distributed uniformly throughout the sarcoplasm between myofibrils. They represent a rich source of energy that can be tapped only by oxidative metabolic pathways; they are therefore more common in fibres that have a high mitochondrial content and good capillary blood supply. Glycogen is distributed in small clusters of granules between myofibrils and among the thin filaments. With brief bursts of activity, it provides an important source of anaerobic energy that is not dependent on nutrient blood flow to the muscle fibre.

At the ends of the muscle fibre, where force is transmitted to adjacent connective tissue structures, the sarcolemma is folded into numerous finger-like projections that strengthen the junctional region by increasing the area of attachment. Tubular invaginations of the sarcolemma penetrate between the myofibrils in a transverse plane at the limit of each A-band (Fig. 23.10). The lumina of these transverse tubules (T-tubules) are thus in continuity with the extracellular space. T-tubules play an important role in excitation–contraction coupling.

Fig. 23.10 Three-dimensional reconstruction of a mammalian skeletal muscle fibre, showing in particular the organization of the transverse tubules (orange) and sarcoplasmic reticulum (buff). Mitochondria (blue) lie between the myofibrils and a muscle nucleus (green) at the periphery. Note that T-tubules are found at the level of the A–I junctions, where they form triads with the terminal cisternae of the sarcoplasmic reticulum.

(Artwork by Lesley Skeates.)

The sarcoplasmic reticulum is a specialized form of smooth endoplasmic reticulum. It consists of a plexus of anastomosing membrane cisternae that fill much of the space between myofibrils and expand into larger sacs, the junctional sarcoplasmic reticulum or terminal cisternae, where they come into close contact with T-tubules. At this point, they form part of a structure called a triad, consisting of a central T-tubule flanked on either side by two terminal cisternae, the latter filled with dense, granular material (see Fig. 23.6B). The membranes of the sarcoplasmic reticulum contain calcium–ATPase pumps that transport calcium ions into the terminal cisternae, where the ions are bound to calsequestrin, a protein with a high affinity for calcium, in dense storage granules. In this way, calcium can be accumulated and retained in the terminal cisternae at a much higher concentration than anywhere else in the sarcoplasm. Ca²⁺ release channels (made of ryanodine receptor molecules) are concentrated mainly in the terminal cisternae. They form half of the junctional ‘feet’ or ‘pillars’ that bridge the sarcoplasmic reticulum and T-tubules at the triads, forming a critical communication point between them. The other half of the junctional feet is the T-tubule receptor, which constitutes the voltage sensor.

Attachments

The forces developed by skeletal muscles are transferred to bones by connective tissue structures: tendons, aponeuroses and fasciae. The microstructure of tendons is considered here.

Tendons

Tendons take the forms of cords or straps that are round or oval in cross-section and consist of dense, regular connective tissue (Fig. 23.11). They contain fascicles of type I collagen that are oriented mainly parallel to the long axis but are, to some extent, interwoven. The fasciculi may be conspicuous enough to give tendons a longitudinally striated appearance to the unaided eye. Tendons generally have smooth surfaces, although large tendons may be ridged longitudinally by coarse fasciculi (e.g. the osseous aspect of the angulated tendon of obturator internus). Loose connective tissue between fascicles provides a pathway for small vessels and nerves and condenses on the surface as a sheath or epitendineum, which may contain elastic and irregularly arranged collagen fibres. The loose attachments between this sheath and the surrounding tissue present little resistance to movement of the tendon, but where greater freedom of movement is required, a tendon is separated from adjacent structures by a synovial sheath.

Fig. 23.11 Attachment of a tendon (orange) to skeletal muscle (pink). The regular dense connective tissue of the tendon consists of parallel bundles of type I collagen fibres that are oriented in the long axis of the tendon and the muscle to which it is attached. A few elongated fibroblast nuclei are visible.

(Photograph by Sarah-Jane Smith.)

Tendons are strongly attached to bones, both at the periosteum and through fasciculi (extrinsic collagen fibres), which continue deep into the bone cortex. Sections of fresh bone show that at sites of tendinous attachment there is often a smooth plate of white fibrocartilage, which may cushion and reinforce the attachment zone. Tendons are slightly elastic and may be stretched by up to 6% of their length without damage. Recovery of the elastic energy stored in tendons can make movement more economical. Although they resist extension, tendons are flexible. They can therefore be diverted around osseous surfaces or deflected under retinacula to redirect the angle of pull.

Because tendons are composed of collagen and their vascular supply is sparse, they appear white. However, the blood supply to tendons is not unimportant; small arterioles from adjacent muscle tissue pass longitudinally between the fascicles, branching and anastomosing freely, and are accompanied by venae commitantes and lymphatic vessels. This longitudinal plexus is augmented by small vessels from adjacent loose connective tissue or synovial sheaths. Vessels rarely pass between bone and tendon at osseous attachments, and the junctional surfaces are usually devoid of foramina. A notable exception is the calcaneal (Achilles) tendon, which receives a blood supply across the osseotendinous junction. During postnatal development, tendons enlarge by interstitial growth, particularly at myotendinous junctions, where there are high concentrations of fibroblasts. Growth decreases along the tendon from the muscle to the osseous attachments. The thickness finally attained by a tendon depends on the size and strength of the associated muscle, but it also appears to be influenced by other factors, such as the degree of pennation of the muscle. The metabolic rate of tendons is very low but increases during infection or injury. Repair involves the initial proliferation of fibroblasts, followed by interstitial deposition of new fibres.

The nerve supply to tendons is largely sensory, and there is no evidence of any capacity for vasomotor control. Specialized endings that are sensitive to force (Golgi tendon organs) are found near myotendinous junctions; their large myelinated afferent axons run centrally within branches of muscular nerves or in small rami of adjacent peripheral nerves.

Innervation

Every skeletal muscle is supplied by one or more nerves. In the limbs, face and neck there is usually a single nerve, although its axons may be derived from neurones in several spinal cord segments. Muscles such as those of the abdominal wall, which originate from several embryonic segments, are supplied by more than one nerve. In most cases, the nerve travels with the principal blood vessels within a neurovascular bundle, approaches the muscle near its least mobile attachment and enters the deep surface at a position that is more or less constant for each muscle.

Nerves supplying muscle are frequently referred to as ‘motor nerves,’ but they contain both motor and sensory components. The motor component is composed mainly of large, myelinated α-efferent axons that supply the muscle fibres, supplemented by small, thinly myelinated γ-efferents, or fusimotor fibres, that innervate the intrafusal muscle fibres of neuromuscular spindles and fine, non-myelinated autonomic efferents (C fibres) that innervate vascular smooth muscle. The sensory component consists of large, myelinated IA and smaller group II afferents from the neuromuscular spindles, large myelinated IB afferents from the Golgi tendon organs and fine myelinated and non-myelinated axons that convey pain and other sensations from free terminals in the connective tissue sheaths of the muscle.

Within muscles, nerves follow the connective tissue sheaths, coursing in the epimysial and perimysial septa before entering the fine endomysial tissue around the muscle fibres. The α motor axons branch repeatedly before they lose their myelinated sheaths and terminate near the middle of muscle fibres. These terminals tend to cluster in a narrow zone toward the centre of the muscle belly known as the motor point. Clinically, this is the place on the muscle where it is easiest to elicit a contraction with stimulating electrodes. Long muscles generally have two or more terminals, or end-plate bands, because many muscle fibres do not run the full length of the anatomical muscle.

A specialized synapse, the neuromuscular junction (Ch. 22), is formed where the terminal branch of an α motor axon contacts the muscle fibre. The axon terminal gives off several short, tortuous branches, each ending in an elliptical area, the motor end-plate. Within the underlying discoidal patch of sarcolemma—the sole plate or subneural apparatus—the sarcolemma is thrown into deep synaptic folds. This discrete type of neuromuscular junction is an example of an en plaque ending and is found on muscle fibres that are capable of propagating action potentials. A different type of ending is found on slow tonic muscle fibres that do not have this capability (e.g. the extrinsic ocular muscles), where these fibres form a minor component of the muscle. In this case, the propagation of excitation is taken over by the nerve terminals, which branch over an extended distance to form a number of small neuromuscular junctions (en grappe endings). Some muscle fibres of this type receive the terminal branches of more than one motor neurone. The terminals of the γ-efferents that innervate the intrafusal muscle fibres of the neuromuscular spindle also take a variety of forms.

The terminal branches of α motor axons are normally in a ‘one-to-one’ relationship with their muscle fibres: a muscle fibre receives only one branch, and any one branch innervates only one muscle fibre. When a motor neurone is excited, an action potential is propagated along the axon and all its branches to all the muscle fibres it supplies. The motor neurone and the muscle fibres it innervates can therefore be regarded as a functional unit, the motor unit, which accounts for the more or less simultaneous contraction of a number of fibres within the muscle.

The size of a motor unit varies considerably. In muscles employed for precision tasks (e.g. extraocular muscles, interossei and intrinsic laryngeal muscles), each motor neurone innervates only about 10 muscle fibres, whereas in a large limb muscle, a motor neurone may innervate several hundred muscle fibres. Within a muscle, the fibres belonging to one motor unit are distributed over a wide territory, without regard to fascicular boundaries, and they intermingle with the fibres of other motor units. The motor units become larger in cases of nerve damage, because denervated fibres induce collateral or terminal sprouting of the remaining axons. Each new branch can reinnervate a fibre, thus increasing the territory of its parent motor neurone.

Muscle Contraction: Basic Physiology

The arrival of an action potential at the motor end-plate causes acetylcholine (ACh) to be released from storage vesicles into the 30- to 50-nm synaptic cleft that separates the nerve ending from the sarcolemma. ACh is rapidly bound by receptor molecules located in the junctional folds, triggering an almost instantaneous increase in the permeability, and hence conductance, of the postsynaptic membrane. This generates a local depolarization (the end-plate potential), which initiates an action potential in the surrounding area of sarcolemma. The activity of the neurotransmitter is rapidly terminated by the enzyme acetylcholinesterase (AChE), which is bound to the basal lamina in the sarcolemmal junctional folds. The sarcolemma is an excitable membrane, and action potentials generated at the neuromuscular junction propagate rapidly over the entire surface of the muscle fibre.

The action potentials are conducted radially into the interior of the fibre via the T-tubules, which are extensions of the sarcolemma; this ensures that all parts of the muscle fibre are activated rapidly and almost synchronously. Excitation–contraction coupling is the process whereby an action potential triggers the release of calcium from the terminal cisternae of the sarcoplasmic reticulum into the cytosol. This activates a calcium-sensitive switch in the thin filaments and thus initiates contraction. At the end of excitation, the T-tubular membrane repolarizes, calcium release ceases, calcium ions are actively transported back to the calsequestrin stores by the calcium–ATPase pumps and the muscle relaxes.

Electron microscopy shows that the length of the thick and thin filaments does not change during muscle contraction. The sarcomere shortens by means of the thick and thin filaments sliding past each other, which draws the Z-discs toward the middle of each sarcomere (see Fig. 23.7). As the overlap increases, the I-bands and H zones narrow to extinction, while the width of the A-bands remains constant. Filament sliding depends on the making and breaking of bonds (cross-bridge cycling) between myosin head regions and actin filaments. Myosin heads ‘walk’ along actin filaments (sliding the filaments past each other) using a series of short power strokes, each resulting in a relative movement of 5 to 10 nm. Actin filament binding sites for myosin are revealed only in the presence of calcium, which is released into the sarcoplasm from the sarcoplasmic reticulum, with the consequent repositioning of the troponin–tropomyosin complex on actin (the calcium-sensitive switch). Both myosin head binding and release are energy dependent (adenosine triphosphate (ATP) binding is required for detachment of bound myosin heads as part of the normal cycle). In the absence of ATP (as occurs postmortem), the bound state is maintained and is responsible for the muscle stiffness known as rigor mortis.

The summation of myosin power strokes leads to an average sarcomere shortening of up to 1 µm. Because each muscle has thousands of sarcomeres in series along its length, the anatomical muscle shortens by a centimetre or more, depending on the muscle.

Slow Twitch versus Fast Twitch

The passage of a single action potential through a motor unit elicits a twitch contraction whereby peak force is reached within 25 to 100 msec, depending on the motor unit type involved. However, the motor neurone can deliver a second nervous impulse in less time than it takes for the muscle fibres to relax. When this happens, the muscle fibres contract again, building the tension to a higher level. Because of this mechanical summation, a sequence of impulses can evoke a larger force than a single impulse; within certain limits, the higher the impulse frequency, the more force is produced (‘rate recruitment’). The other strategy is to recruit more motor units. In practice, the two mechanisms appear to operate in parallel, but their relative importance may depend on the size or function of the muscle; in large muscles with many motor units, motor unit recruitment is probably the more important mechanism.

With the exception of rare tonic fibres, skeletal muscles are composed entirely of fibres of the twitch type. These fibres can all conduct action potentials, but they are not the same in other respects. Some fibres obtain their energy very efficiently by aerobic oxidation of substrates, particularly fats and fatty acids. They have large numbers of mitochondria and contain myoglobin, an oxygen-transport pigment related to haemoglobin. They are supported by a well-developed network of capillaries, which maintains a steady nutrient supply of oxygen and substrates. Such fibres are well suited to functions such as postural maintenance, in which moderate forces must be sustained for prolonged periods. At the other extreme are fibres that have few mitochondria, little myoglobin and a sparse capillary network. Their immediate energy requirements are met largely through anaerobic glycolysis, a route that provides prompt access to energy stores but is less sustainable than oxidative metabolism. Such fibres are capable of brief bursts of intense activity, but these must be separated by extended quiescent periods during which intracellular pH and phosphate concentrations, perturbed in fatigue, are restored to normal values, and glycogen and other reserves are replenished.

These types of fibres tend to be segregated into different muscles in some animals; thus, some muscles have a conspicuously red appearance, derived from the rich blood supply and high myoglobin content associated with a predominantly aerobic metabolism, whereas others have a much paler appearance, reflecting a more anaerobic character. These variations in colour led to the early classification of red and white muscles.

In humans, all muscles are, in fact, mixed, with fibres specialized for aerobic working conditions intermingled with fibres of a more anaerobic or intermediate metabolic character. The different types of fibres are not readily distinguished in routine histological preparations but are clear when specialized enzyme histochemical techniques are used. On the basis of metabolic differences, the individual fibres can be classified as predominantly oxidative, slow twitch (red) fibres or glycolytic, fast twitch (white) fibres. Muscles composed mainly of oxidative, slow twitch fibres thus correspond to the red muscles of classic descriptions. This classification has now been largely superseded by myosin-based typing and the presence of specific disease-related enzymes (see later).

Muscles that are predominantly oxidative in their metabolism contract and relax more slowly than muscles that rely on glycolytic metabolism. This difference in contractile speed is due in part to the activation mechanism (volume density of sarcotubular system and proteins of the calcium ‘switch’ mechanism) and in part to molecular differences between the myosin heavy chains of these types of muscle, which affect the ATPase activity of the myosin head; this in turn alters the kinetics of its interaction with actin and hence the rate of cross-bridge cycling. Differences between myosin isoforms may be detected histochemically; ATPase histochemistry continues to play a significant role in diagnostic typing (Table 23.2). Two main categories have been described: type I fibres, which are slow contracting, and type II fibres, which are fast contracting. Molecular analyses have revealed that type II fibres can be subdivided according to their content of myosin heavy-chain isoforms into types IIA, IIB and IIX (Schiaffino and Reggiani 1996). There is a correlation between categories and metabolism and therefore with fatigue resistance: type I fibres are generally oxidative (slow oxidative) and resistant to fatigue; type IIA are moderately oxidative, glycolytic (fast oxidative glycolytic) and fatigue resistant; and type IIB largely rely on glycolytic metabolism (fast glycolytic) and so are easily fatigued. Fibre type grouping (Fig. 23.12) occurs when there are repeated cycles of denervation and reinnervation and is seen in a variety of conditions, including disuse atrophy, ageing, demyelination neuropathies and some forms of muscular dystrophy.

Table 23-2 Physiological, structural and biochemical characteristics of the major histochemical fibre types

Fig. 23.12 ATPase stain of skeletal muscle demonstrating fibre type grouping. Muscle fibers are clustered into large groups by type: type I (dark) and type II (light).

(Courtesy of Mark Curtis, MD, Jefferson Medical College, Philadelphia, PA.)

CASE 3 Inclusion Body Myositis

A 62-year-old man has a 2-year history of progressive weakness of his hands and fingers, hindering his ability to perform some activities of daily living. In the last 8 months, he has experienced frequent falls due to leg weakness. Increased difficulty swallowing has led to a 10-pound weight loss.

On examination, there is decreased strength in the finger and wrist flexors, worse on the right, and bilateral quadriceps weakness. Neck flexors are weak. Creatine kinase is slightly elevated.

Discussion: Inclusion body myositis is the most common acquired inflammatory myopathy. The predominantly distal pattern of weakness helps distinguish this disease from other inflammatory myopathies that involve primarily proximal muscles. Muscle biopsy in such cases shows endomysial mononuclear cell inflammation; scattered muscle fibre degeneration and regeneration, with characteristic rimmed vacuoles (Fig. 23.13); clustering of small fibres; and both intranuclear and intracytoplasmic inclusions that stain for amyloid.

Fig. 23.13 Photomicrograph of rimmed vacuoles (arrow). Rimmed vacuoles are a typical finding in inclusion body myositis but may be seen in other pathological conditions, including polymyositis, congenital myopathies and some limb girdle muscular dystrophies.

(Courtesy of Mark Curtis, MD, Jefferson Medical College, Philadelphia, PA.)