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//SKELETAL MUSCLE

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. 5.36). 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 from unipennate (e.g. flexor pollicis longus) and bipennate (e.g. rectus femoris, dorsal interossei) to multipennate (e.g. deltoid). Fibres may pass obliquely between deep and superficial aponeuroses, in a type of ‘unipennate’ form (e.g. soleus), or muscle fibres may start from the walls of osteofascial compartments and converge obliquely on a central tendon in circumpennate fashion (e.g. tibialis anterior). Muscles may exhibit a spiral or twisted arrangement (e.g. sternocostal fibres of pectoralis major and latissimus dorsi, which undergo a 180° twist between their medial and lateral attachments), or may spiral around a bone (e.g. supinator, which winds obliquely around the proximal radial shaft), or may contain two or more planes of fibres arranged in differing 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 which correspond to contrasting, and in some cases independent, actions.

  

Fig. 5.36  Morphological ‘types’ of muscle based on their general form and fascicular architecture.

Muscle nomenclature

The names given to individual muscles are usually descriptive, based on their shape, size, number of heads or bellies, position, depth, attachments, or actions. The meanings of some of the terms used are given in Table 5.1.


Table 5.1   — Terms used in naming muscles.

Shape Number of heads or bellies Position

   Deltoid (triangular)
   Quadratus (square)
   Rhomboid (diamond-shaped)
   Teres (round)
   Gracilis (slender)
   Rectus (straight)
   Lumbrical (worm-like)

   Biceps (two heads)
   Triceps (three heads)
   Quadriceps (four heads)
   Digastric (two bellies)

   Anterior, posterior, medial, lateral, superior, inferior, supra-, infra-
   Interosseus (between bones)
   Dorsi (of the back)
   Abdominis (of the abdomen)
   Pectoralis (of the chest)
   Brachii (of the arm)
   Femoris (of the thigh)
   Oris (of the mouth)
   Oculi (of the eye)
Size Depth Action

   Major, minor, longus (long)
   Brevis (short)
   Latissimus (broadest)
   Longissimus (longest)

   Superficialis (superficial)
   Profundus (deep)
   Externus/externi (external)
   Internus/interni (internal)

   Extensor, flexor
   Abductor, adductor
   Levator, depressor
   Supinator, pronator
   Constrictor, dilator
  Attachment  
 

   Sternocleidomastoid (from sternum and clavicle to mastoid process)
   Coracobrachialis (from the coracoid process to the arm)
 

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

Microstructure of skeletal muscle

The cellular units of skeletal muscle are enormous multinucleate muscle fibres (Fig. 5.37, Fig. 5.38) which develop by fusion of individual myoblasts (see below). Individual muscle fibres are long, cylindrical structures that tend to be consistent in size within a given muscle, but in different muscles may range from 10 to 100 μm in diameter and from millimetres to many centimeters in length. The cytoplasm of each fibre, sarcoplasm, is surrounded by a plasma membrane that is often called the sarcolemma. The contractile machinery is concentrated into myofibrils, long narrow structures (1–2 μm in diameter) that extend the length of the fibre and form the bulk of the sarcoplasm. Numerous moderately euchromatic, oval nuclei usually occupy a thin transparent rim of sarcoplasm between the myofibrils and the sarcolemma, and are especially numerous in the region of the neuromuscular junction (see Fig. 3.37). A transverse section of a muscle fibre may only reveal one or two nuclei, but there may be several hundred along the length of an entire fibre. Myogenic satellite cells lie between the sarcolemma and the surrounding basal lamina (see below).

  

Fig. 5.37  Skeletal muscle fibres from human lateral rectus in longitudinal section, showing transverse striations representing the sarcomeric organization of actin and myosin filaments. The variation in fibre diameter is typical of extraocular muscles. Capillaries (C) and nerves (N) lie between the fibres, oriented mainly in parallel and so are also sectioned longitudinally. Toluidine blue stained resin section.
(Provided by courtesy of the Department of Optometry and Visual Science, City University, London.)

  

Fig. 5.38  Levels of organization within a skeletal muscle, from whole muscle to fasciculi, single fibres, myofibrils and myofilaments.

The myofibrils are too tightly packed to be visible by routine light microscopy (see below). Of greater significance are transverse striations, produced by the alignment across the fibre of repeating elements, the sarcomeres, within neighbouring myofibrils. These cross-striations are usually evident in conventionally stained histological sections, but may be demonstrated more effectively using special stains. They are even more striking under polarized light when they appear as a pattern of alternating dark and light bands. The darker, anisotropic or A-bands, are birefringent and rotate the plane of polarized light strongly. The lighter, isotropic or I-bands, rotate the plane of polarized light to a negligible degree. In transverse section, the profiles of the fibres are usually polygonal (Fig. 5.38; see Fig. 5.40). The sarcoplasm often has a stippled appearance, because the transversely sectioned myofibrils are resolved as dots. Their packing density varies. In some muscles, e.g. the extrinsic muscles of the larynx, the muscle fibres tend not to be tightly packed, whereas in others, e.g. the group of jaw closing muscles, the fibres are closely packed and have rounded profiles.

  

Fig. 5.40  The electron microscopic appearance of skeletal muscle in longitudinal section. A, Low-power view of parts of two adjacent muscle fibres, separated by endomysium (E) containing capillaries (C) and a peripherally-placed nucleus (N) in the fibre on the right. Mitochondria (arrows) are situated peripherally and between myofibrils (M). Myofibrils pack the cytoplasm, with their sarcomeres (contractile units) in register, as seen by the alignment of Z discs (dark transverse lines) across each muscle fibre. B, A sarcomere within a myofibril, and parts of two others. (A sarcomere is the distance between adjacent Z discs). Also seen are the A band, bisected by the M line, and I band, which here is almost obliterated in the contracted state (see Fig. 5.41). A triad is visible between myofibrils, comprised of a T-tubule (long arrow) and two terminal cisternae of sarcoplasmic reticulum (short arrows).
(Part A provided by courtesy of Professor Hans Hoppeler, Institute of Anatomy, University of Bern, Switzerland.)

In general, skeletal muscle fibres are large (there are a few exceptions, e.g. the intrinsic muscles of the larynx). This means that electron micrographs, unless of very low magnification, seldom show more than part of the interior of a fibre. Myofibrils, cylindrical structures about 1 μm diameter (Fig. 5.37), are the dominant ultrastructural feature of such micrographs. In longitudinal sections they appear as ribbons and are interrupted at regular intervals by thin, very densely stained transverse lines, which correspond to discsin the parent cylindrical structure. These are the Z-lines or, more properly, Z-discs (Zwischenscheiben = interval discs) that divide the myofibril into a linear series of repeating contractile units, sarcomeres, each of which is typically 2.2 μm long in resting muscle. At higher power, sarcomeres are seen to consist of two types of filament, thick and thin, organized into regular arrays (Fig. 5.38; Fig. 5.40). 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 electron density (as seen in the electron microscope) 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 (Fig. 5.40, Fig. 5.41). The central, paler region of the A-band, which is not penetrated by the thin filaments, is called the H-zone (Helle = light). At their centres, the thick filaments are linked together transversely by material that constitutes the M-line (Mittelscheibe = middle [of] disc), that is visible in most muscles. The I-band consists of the adjacent portions of two neighbouring sarcomeres in which the thin filaments are not overlapped by thick filaments. The thin filaments of adjacent sarcomeres are anchored in the Z-disc, which bisects the I-band. A third type of filament is composed of the elastic protein, titin.

  

Fig. 5.41  Sarcomeric structures. The drawings below the electron micrograph (of two myofibrils sectioned longitudinally and with their long axes, orientated transversely) indicate the corresponding arrangements of thick and thin filaments. Relaxed and contracted states are shown to illustrate the changes which occur during shortening. Insets at the top show the electron micrographic appearance of transverse sections through the myofibril 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 Professor Brenda Russell, Department of Physiology and Biophysics, University of Illinois at Chicago.)

The high degree of organization of the arrays of filaments is equally evident in electron micrographs of transverse sections (Fig. 5.41, Fig. 5.42). The thick myosin filaments form a hexagonal lattice. In the regions where they overlap 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. 5.42  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. The variation in the appearance of myofibrils in cross-section is explained in Fig. 5.41.
(Photograph by Professor Brenda Russell, Department of Physiology and Biophysics, University of Illinois at Chicago.)

The banded appearance of individual myofibrils is a function of the regular alternation of the thick and thin filament arrays. The size of myofibrils places them at the limit of resolution of light microscopy: cross-striations are only visible at that level because of the alignment in register of the bands in adjacent myofibrils across the width of the whole muscle fibre. In suitably stained relaxed material, the A-, I- and H-bands are quite distinct, whereas the Z-discs, which are such a prominent feature of electron micrographs, are thin and much less conspicuous in the light microscope, and M-lines cannot be resolved.

Muscle proteins

Myosin, the protein of the thick filament, constitutes 60% of the total myofibrillar protein and is the most abundant contractile protein. The thick filaments of skeletal and cardiac muscle are 1.5 μm long. Their composition from myosin heavy and light chain assemblies is described in Chapter 1. The other components of myosin, the regulatory proteins tropomyosin and troponin, play a major part in the control of contraction. Actin is the next most abundant contractile protein and constitutes 20% of the total myofibrillar protein. In its filamentous form, F-actin, it is the principal protein of the thin filaments. A number of congenital myopathies result from gene mutations in components of the thin filament assembly (reviewed in Clarkson et al 2004). The third type of long sarcomeric filament connects the thick filaments to the Z-disc, and is formed by the giant protein, titin, which has 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. They have a tethered portion in the A-band, where they are attached to thick filaments as far as the M-line, and an elastic portion in the I-band. The elastic properties of titin endow the relaxed muscle fibre with passive resistance to stretching and with elastic recoil.

A number of proteins which 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 which 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 lengths 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 and to the sarcolemma. 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, the dystroglycan/sarcoglycan complex, that spans the membrane and links specifically with merosin, the α2 laminin isoform of the muscle basal lamina. This stabilizes the muscle fibre and transmits forces generated internally on contraction to the extracellular matrix.

  

Fig. 5.39  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 Professor Stanley Salmons, from a specimen provided by courtesy of Tim Helliwell, Department of Pathology, University of Liverpool.)

Dystrophin is the product of the gene affected in Duchenne muscular dystrophy, a fatal disorder that develops when mutation of the gene leads to the absence of the protein. A milder form of the disease, Becker muscular dystrophy, is associated with a reduced size and/or abundance of dystrophin. Female carriers (heterozygous for the mutant gene) of Duchenne muscular dystrophy may also have mild symptoms of muscle weakness. At about 2500 kb, the gene is one of the largest yet discovered, which may account for the high mutation rate of Duchenne muscular dystrophy (approximately 35% of cases are new mutations). Other muscular dystrophies may involve deficiencies in proteins functionally associated with dystrophin, such as the dystroglycan/sarcoglycan complex or α2 laminin. The involvement in muscular dystrophy of defects in the dystrophin adhesion complex is reviewed in Batchelor & Winder (2006).

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 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. The number of mitochondria in an adult muscle fibre is not fixed, but 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 which have a high mitochondrial content and good capillary blood supply. Small clusters of glycogen granules are dispersed between myofibrils and among the thin filaments. In brief bursts of activity they provide an important source of anaerobic energy that is not dependent on blood flow to the muscle fibre.

Tubular invaginations of the sarcolemma penetrate between the myofibrils in a transverse plane at the limit of each A-band (Fig. 5.40). The lumina of these transverse (T-) tubules are thus in continuity with the extracellular space. 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.

The sarcoplasmic reticulum (SR) is a specialized form of smooth endoplasmic reticulum and forms a plexus of anastomosing membrane cisternae that fills much of the space between myofibrils (Fig. 5.43). The cisternae expand into larger sacs, junctional sarcoplasmic reticulum or terminal cisternae, where they come into close contact with T-tubules, forming structures called triads (Fig. 5.40; Fig. 5.43). The membranes of the SR 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 elsewhere in the sarcoplasm. Ca2+-release channels (ryanodine receptors) are concentrated mainly in the terminal cisternae and form one half of the junctional ‘feet’ or ‘pillars’ that bridge the SR and T-tubules at the triads. The other half of the junctional feet is the T-tubule receptor that constitutes the voltage sensor.

  

Fig. 5.43  Three-dimensional reconstruction of a mammalian skeletal muscle fibre, showing in particular the organization of the transverse tubules and sarcoplasmic reticulum. Mitochondria lie between the myofibrils. Note that transverse tubules are found at the level of the A/I junctions, where they form triads with the terminal cisternae of the sarcoplasmic reticulum.

Connective tissues of muscle

The endomysium is a delicate network of connective tissue that surrounds muscle fibres, and forms their immediate external environment. It is the site of metabolic exchange between muscle and blood, and contains capillaries and bundles of small nerve fibres. Ion fluxes associated with the electrical excitation of muscle fibres take place through its proteoglycan matrix. The endomysium is continuous with more substantial septa of connective tissue that constitute the perimysium. The latter ensheathes groups of muscle fibres to form parallel bundles or fasciculi, carries larger blood vessels and nerves and accommodates neuromuscular spindles. Perimysial septa are themselves the inward extensions of a collagenous sheath, the epimysium, which forms part of the fascia that invests whole muscle groups.

Epimysium consists mainly of type I collagen, perimysium contains type I and type III collagen, and endomysium contains collagen types III and IV. Collagen IV is associated particularly with the basal lamina that invests each muscle fibre.

The epimysial, perimysial and endomysial sheaths coalesce where the muscles connect to adjacent structures at tendons, aponeuroses, and fasciae (see below): this gives the attachments great strength, since the tensile forces are distributed in the form of shear stresses, which are more easily resisted. This principle is also seen at the ends of the muscle fibres, which divide into finger-like processes separated by insertions of tendinous collagen fibres. Although there are no desmosomal attachments at these myotendinous junctions, there are other specializations that assist in the transmission of force from the interior of the fibre to the extracellular matrix. Actin filaments from the adjacent sarcomeres, which would normally insert into a Z-disc at this point, instead penetrate a dense, subsarcolemmal filamentous matrix that provides attachment to the plasma membrane. This matrix is similar in character to the cytoplasmic face of an adherens junction. The structure as a whole is homologous to the intercalated discs of cardiac muscle. At the extracellular surface of the junctional sarcolemma, integrins provide contact with the basal lamina which in turn adheres closely to collagen and reticular fibres (type III collagen) of the adjacent tendon or other connective tissue structure.

Vascular supply and lymphatic drainage

In most muscles the major source artery enters on the deep surface, frequently in close association with the principal vein and nerve, which together form a neurovascular hilum. The vessels subsequently course and branch within the connective tissue framework of the muscle. The smaller arteries and arterioles ramify in the perimysial septa and give off capillaries which run in the endomysium: although the smaller vessels lie mainly parallel to the muscle fibres, they also branch and anastomose around the fibres, forming an elongated mesh.

Mathes & Nahai (1981) have classified the gross vascular anatomy of muscles into five types according to the number and relative dominance of vascular pedicles which enter the muscle (Fig. 5.44). This classification has important surgical relevance in determining which muscles will survive and therefore be useful for pedicled or free tissue transfer procedures using techniques of plastic and reconstructive surgery. Type I muscles possess a single vascular pedicle supplying the muscle belly, e.g. tensor faciae latae (supplied by the ascending branch of the lateral circumflex femoral artery) and gastrocnemius (supplied by the sural artery). Type II muscles are served by a single dominant vascular pedicle and several minor pedicles, and can be supported on a minor pedicle as well as the dominant pedicle, e.g. gracilis (supplied by the medial circumflex femoral artery in the dominant pedicle). Type III muscles are supplied by two separate dominant pedicles each from different source arteries, e.g. rectus abdominus (supplied by the superior and inferior epigastric arteries) and gluteus maximus (supplied by the superior and inferior gluteal arteries). Type IV muscles have multiple small pedicles which, in isolation, are not capable of supporting the whole muscle, e.g. sartorius and tibialis anterior: about 30% survive reduction onto a single vascular pedicle. Type V muscles have one dominant vascular pedicle and multiple secondary segmental pedicles, e.g. latissimus dorsi (supplied by the thoracodorsal artery as the primary pedicle, and thoracolumbar perforators from the lower six intercostal arteries and the lumbar arteries as the segmental supply), and pectoralis major (supplied by the pectoral branch of the thoracoacromial axis as the dominant pedicle, and anterior perforators from the internal thoracic vessels as the segmental supply).

  

Fig. 5.44  Classification of muscles according to their blood supply.
(By permission from Cormack GC, Lamberty BGH 1994 The Arterial Anatomy of Skin Flaps, 2nd edn. Edinburgh: Churchill Livingstone.)

In muscle cross-sections, the number of capillary profiles found adjacent to fibres usually varies from 0 to 3. Fibres that are involved in sustained activities, such as posture, are served by a denser capillary network than fibres that are recruited only infrequently. It is common for muscles to receive their arterial supply via more than one route. The accessory arteries penetrate the muscle at places other than the hilum, and ramify in the same way as the principal artery, forming vascular territories. The boundaries of adjacent territories are spanned by anastomotic vessels, sometimes at constant calibre, but more commonly through reduced-calibre arteries or arterioles which are referred to as ‘choke vessels’ (see Ch. 6). These arterial arcades link the territories into a continuous network.

Veins branch in a similar way, forming venous territories that correspond closely to the arterial territories. In the zones where the arterial territories are linked by choke vessels, the venous territories are linked by anastomosing veins, in this case without change of calibre. On either side of these venous bridges, the valves in the adjacent territories direct flow in opposite directions towards their respective pedicles, but the connecting veins themselves lack valves, and therefore permit flow in either direction.

Because of the potential for relative movement within muscle groups, vessels tend not to cross between muscles, but radiate to them from more stable sites or cross at points of fusion. Where a muscle underlies the skin, vessels bridge between the two. These may be primarily cutaneous vessels, i.e. they supply the skin directly, but contribute small branches to the muscle as they pass through it, or they may be the terminal branches of intramuscular vessels which leave the muscle to supplement the cutaneous blood supply. The latter are less frequent where the muscle is mobile under the deep fascia. The correspondence between the vascular territories in the skin and underlying tissues gave rise to the concept of angiosomes, whichare composite blocks of tissue supplied by named distributing arteries and drained by their companion veins (see Ch. 6).

The pressure exerted on valved intramuscular veins during muscular contraction functions as a ‘muscle pump’ that promotes venous return to the heart. In some cases this role appears to be amplified by veins which pass through the muscle after originating elsewhere in superficial or deep tissues (see Ch. 79). The extent to which the muscle capillary bed is perfused can be varied in accordance with functional demand. Arteriovenous anastomoses, through which blood can be returned directly to the venous system without traversing the capillaries, provide an alternative, regulated pathway.

The lymphatic drainage of muscles begins as lymphatic capillaries in epimysial and perimysial, but not endomysial, sheaths. These converge to form larger lymphatic vessels that accompany the veins and drain to the regional lymph nodes.

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 neurons located in several spinal cord segments and their associated ganglia. 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 to its least mobile attachment, and enters the deep surface at a position which 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 mainly composed of large, myelinated α-efferent axons, which supply the muscle fibres, supplemented by small, thinly myelinated γ-efferents, or fusimotor fibres, which innervate the intrafusal muscle fibres of neuromuscular spindles, and fine, non-myelinated autonomic efferents (C fibres), which 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 which convey pain and other sensations from free terminals in the connective tissue sheaths of the muscle.

Within muscles, nerves travel through the epimysial and perimysial septa before entering the fine endomysial tissue around the muscle fibres. α-Motor axons branch repeatedly before they lose their myelinated sheaths and terminate in a narrow zone towards the centre of the muscle belly known as the motor point. Clinically, this is the place on a muscle from which it is easiest to elicit a contraction with stimulating electrodes. Long muscles generally have two or more terminal, or end-plate bands, because many muscle fibres do not run the full length of an anatomical muscle. The terminal branch of an α-motor axon contacts a muscle fibre at a specialized synapse, the neuromuscular junction (see Fig. 3.37). It gives off several short, tortuous branches each ending in an elliptical area, the motor end plate. The underlying discoidal patch of sarcolemma, the sole plate or subneural apparatus, 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 which are capable of propagating action potentials. A different type of ending is found on slow tonic muscle fibres, which do not have this capability, e.g. in the extrinsic ocular muscles, where slow tonic fibres form a minor component of the anatomical 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 different 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 of its branches to all of the muscle fibres that it supplies. The motor neurone and the muscle fibres that it innervates can therefore be regarded as a functional unit, the motor unit: the arrangement 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 used for precision tasks, e.g. extraocular muscles, interossei and intrinsic laryngeal muscles, each motor neurone innervates perhaps 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 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 of a neuromuscular junction causes acetylcholine (ACh) to be released from storage vesicles into the highly infolded 30–50 nm synaptic cleft that separates the nerve ending from the sarcolemma (see Fig. 3.37). 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 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, extensions of the sarcolemma (see above), ensuring 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 (see below) and so 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 in the sarcoplasmic reticulum by the calcium–ATPase pumps, and the muscle relaxes.

The lengths of the thick and thin filaments do not change during muscle contraction. The sarcomere shortens by the sliding of thick and thin filaments past one another, which draws the Z-discs towards the middle of each sarcomere (Fig. 5.41). As the overlap increases, the I- and H-bands narrow to near 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’ or ‘row’ along actin filaments using a series of short power strokes, each resulting in a relative movement of 5–10 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, causing a repositioning of the troponin–tropomyosin complex on actin (the calcium-sensitive switch). Myosin head binding and release are both energy dependent (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: an anatomical muscle shortens by a centimetre or more, depending on the muscle, because each muscle has thousands of sarcomeres in series along its length. For further details of actin–myosin interactions in muscle contraction, see Alberts et al (2002) and Pollard & Earnshaw (2007) (see Bibliography of selected titles for publication details).

Slow twitch vs fast twitch

The passage of a single action potential through a motor unit elicits a twitch contraction where peak force is reached within 25–100 ms, 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 and, within certain limits, the higher the impulse frequency, the more force is produced (‘rate recruitment’). An alternative 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 and/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 differ in other respects. Some fibres obtain their energy very efficiently by aerobic oxidation of substrates, particularly of fats and fatty acids. They have large numbers of mitochondria; contain myoglobin, an oxygen-transport pigment related to haemoglobin; and are supported by a well-developed network of capillaries that maintains a steady nutrient supply of oxygen and substrates. Such fibres are well suited to functions such as postural maintenance, in which moderate forces need to be sustained for prolonged periods. At the other end of the spectrum, some fibres have few mitochondria, little myoglobin, and a sparse capillary network, and store energy as cytoplasmic glycogen granules. Their immediate energy requirements are met largely through anaerobic glycolysis, a route that provides prompt access to energy but that is less sustainable than oxidative metabolism. They are capable of brief bursts of intense activity that 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.

Different types of fibre tend to be segregated into different muscles in some animals: some muscles have a conspicuously red appearance, reflecting their 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 muscle into red and white types. This classification has now been largely superseded by myosin-based typing and the presence of specific disease-related enzymes.

In man, all muscles are mixed; fibres that are specialized for aerobic working conditions intermingle with fibres of a more anaerobic or intermediate metabolic character. The different types of fibre are not readily distinguished in routine histological preparations but are clear when specialized enzyme histochemical techniques are used. On the basis of metabolic differences, 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 correspond to the red muscles of classical descriptions. Muscles that are predominantly oxidative in their metabolism contract and relax more slowly than muscles relying 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. These differences affect the ATPase activity of the myosin head, which 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 5.2). Two main categories have been described: type I fibres, which are slow-contracting, and type II, which are fast-contracting. Molecular analyses have revealed that type II fibres may be further subdivided according to their content of myosin heavy-chain isoforms into types IIA, IIB and IIX (Schiaffino & Reggiani 1996). There is a correlation between categories and metabolism, and therefore with fatigue resistance, such that 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 IIB largely rely on glycolytic metabolism (fast glycolytic) and so are easily fatigued.


Table 5.2   — Physiological, structural and biochemical characteristics of the major histochemical fibre types.

Characteristic Fibre types
  Type I Type IIA   Type IIB
Phyiological        
Function Sustained forces, as in posture   Powerful, fast movements  
Motor neurone firing threshold Low Intermediate   High
Motor unit size Small Large   Large
Firing pattern Tonic, low-frequency   Phasic, high-frequency  
Maximum shortening velocity Slow Fast   Fast
Rate of relaxation Slow Fast   Fast
Resistance to fatigue Fatigue-resistant Fatigue-resistant   Fatigue-susceptible
Power output Low Intermediate   High
Structural        
Capillary density High     Low
Mitochondrial volume High Intermediate   Low
Z-band Broad Narrow   Narrow
T and SR systems Sparse     Extensive
Biochemical        
Myosin ATPase activity Low     High
Oxidative metabolism High Intermediate   Low
Anaerobic glycolysis Low Intermediate   High
Calcium transport ATPase Low     High

Fibre type transformation

The fibre type proportions in a named muscle may vary between individuals of different age or athletic ability. Fibre type grouping, where fibres with similar metabolic and contractile properties aggregate, increases after nerve damage and with age. It occurs as a result of reinnervation episodes, where denervated fibres are ‘taken over’ by a sprouting motor neurone and their type properties transformed under direction of the new motor neurone. If the nerves to fast white and slow red muscles are cut and cross-anastomosed in experimental animals, so that each muscle is reinnervated by the other’s nerve, the fast muscle becomes slower-contracting, and the slow muscle faster-contracting (Buller et al 1960). There is evidence that fibre type transformation may be a response to the patterns of impulse traffic in the nerves innervating the muscles. If fast muscles are stimulated continuously for several weeks at 10 Hz, a pattern similar to that normally experienced by slow muscles, they develop slow contractile characteristics and acquire a red appearance and a resistance to fatigue even greater than that of slow muscles.

The initial phase of slowing can be explained by less rapid cycling of calcium, the result of a reduction in the extent of the sarcoplasmic reticulum and changes in the amount and molecular type of proteins involved in calcium transport and binding. Chronic stimulation also triggers the synthesis of myosin heavy and light chain isoforms of the slow muscle type: the associated changes in cross-bridge kinetics result in a lower intrinsic speed of shortening. The muscle becomes more resistant to fatigue through changes in the metabolic pathways responsible for the generation of ATP and a reduced dependence on anaerobic glycolysis. There is a switch to oxidative pathways, particularly those involved in the breakdown of fat and fatty acids, and an associated increase in capillary density and in the fraction of the intracellular volume occupied by mitochondria. If stimulation is discontinued, the sequence of events is reversed and the muscle regains, over a period of weeks, all of its original characteristics. The reversibility of transformation is one of several lines of evidence that the changes take place within existing fibres, and not by a process of degeneration and regeneration.

Many of the changes in the protein profile of a muscle that are induced by stimulation are now known to be the result of transcriptional regulation. For example, analysis of the messenger RNA species encoding myosin heavy chain isoforms shows that expression of the fast myosin heavy chain mRNA is downregulated within a few days of the onset of chronic stimulation, while the slow myosin heavy chain mRNA is upregulated. Although myosin isoform expression is responsive to the increase in use induced by chronic stimulation, it tends to be stable under physiological conditions unless these involve a sustained departure from normal postural or locomotor behaviour.

Attachments of skeletal muscles

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

Tendons

Tendons (Fig. 5.45) take the form of cords or straps of round or oval cross-section, and consist of dense, regular connective tissue. They contain fascicles of type I collagen, orientated 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 conduit for small vessels and nerves, and condenses on the surface as a sheath or epitendineum, which may contain elastin and irregularly arranged collagen fibres. The loose attachments between this sheath and the surrounding tissue present little resistance to movements of the tendon, but in situations where greater freedom of movement is required, a tendon is separated from adjacent structures by a synovial sheath.

  

Fig. 5.45  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 which are orientated in the long axis of the tendon and the muscle to which it is attached. A few elongated fibroblast nuclei are visible in the tendon.

Tendons are strongly attached to bones, both at the periosteum and through fasciculi (extrinsic collagen fibres). Tendinous attachments (entheses or osteotendinous junctions) have been broadly categorized as either fibrocartilagenous or fibrous. In fibrocartilagenous entheses, four zones of tissue have been identified: pure dense fibrous connective tissue (continuous with and indistinguishable from the tendon), uncalcified fibrocartilage, calcified fibrocartilage and bone (continuous with and indistinguishable from the rest of the bone). There are no sharp boundaries between the zones, and the proportions of each component vary between entheses (Fig. 5.46A,B,D). At fibrous entheses, which are characteristic of the shafts of long bones, the tendon is attached to bone by dense fibrous connective tissue either directly or indirectly via the periosteum (Fig. 5.46C). It has been suggested that the greater area of the skeleton to which many fibrous entheses (e.g. pronator teres, deltoid) are attached compared with fibrocartilagenous entheses (e.g. rotator cuff tendons) is important in dissipating stress. (For a review of entheses and the concept of the ‘enthesis organ’, see Benjamin et al 2006.)

Tendons are slightly elastic and may be stretched by 6–10% of their length without damage. Recovery of the elastic ‘strain’ energy stored in tendons can make movement more economical. Although they resist extension, tendons are flexible and can therefore be diverted around osseous surfaces or deflected under retinacula to redirect the angle of pull. Since tendons are composed of collagen and their vascular supply is sparse, they appear white. However, their blood supply is not unimportant: small arterioles from adjacent muscle tissue pass longitudinally between the fascicles, branching and anastomosing freely, and accompanied by venae comitantes 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 tendon (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 appears to be influenced by additional 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 an initial proliferation of fibroblasts followed by interstitial deposition of new collagen fibres.

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

Form and function in skeletal muscles

Direction of action

Although muscles differ in their internal architecture, the resultant force is directed along the line of the tendon: any forces transverse to this direction must therefore be in balance (Fig. 5.36, Fig. 5.47). In strap-like muscles, the transverse component is negligible. In fusiform, bipennate and multipennate muscles, symmetry in the arrangement of the fibres produces a balanced opposition between transverse components, whereas in asymmetrical muscles, e.g. unipennate muscles, the fibres generate an unopposed lateral component of force which is balanced by intramuscular pressure.

  

Fig. 5.47  The ‘detorsion’ or untwisting which results from the contraction of a spirally arranged muscle.

Muscles that incorporate a twist in their geometry unwind it as they contract, so that they tend not only to approximate their attachments but also to bring them into the same plane. Muscles that spiral around a bone tend to reduce the spiral on contraction, imparting rotational force.

Force and range of contraction

The force developed by an active muscle is the summation of the tractive forces exerted by millions of cross-bridges as they work asynchronously in repeated cycles of attachment and detachment. This force depends on the amount of contractile machinery that is assembled in parallel, and therefore on the cross-sectional area of the muscle. The phrase ‘contractile machinery’ has been chosen deliberately here. Mechanically, it matters little that the myofilaments are assembled into myofibrils, the myofibrils into fibres, and the fibres into fascicles (see Fig. 5.38): the total area occupied by myofilamentous arrays determines the force. If the fibres are small, the force will be influenced only to the extent that more of the cross-sectional area will be occupied by non-contractile elements, such as endomysial connective tissue. If there are many small fascicles, the amount of perimysial connective tissue in the cross-section will increase.

The range of contraction generated by an active muscle depends on the relative motion that can take place between the overlapping arrays of thick and thin filaments in each sarcomere. In vertebrate muscle, the construction of the sarcomere sets a natural limit to the amount of shortening that can take place: the difference between the minimum overlap and the maximum overlap of the thick and thin filaments represents a shortening of about 30%. Since the sarcomeres are arranged in series, the muscle fibres shorten by the same percentage. The actual movement that takes place at the ends of the fibres will depend on the number of sarcomeres in series, i.e. it will be proportional to fibre length. By way of illustration, compare the behaviour of two muscles, fixed at one end, both having fibres parallel to the line of pull and the same cross-sectional area. If one muscle is twice as long as the other, then the force developed by each muscle will be the same, but the maximum movement produced at the free ends will be twice as much for the longer muscle. Muscles in which the fibres are predominantly parallel to the line of pull are often long and thin (strap-like): they develop rather low forces, but are capable of a large range of contraction. Where greater force is required the cross-sectional area must be increased, as occurs in a pennate construction (Fig. 5.48). Here, the fibres are set at an angle to the axis of the tendon (the angle of pennation). The range of contraction produced by such a muscle will be less than that of a strap-like muscle of the same mass, because the fibres are short and a smaller fraction of the shortening takes place in the direction of the tendon. The obliquely directed force can be resolved vectorially into two components, one acting along the axis of the tendon, and one at 90° to this. In symmetrical forms (Fig. 5.48), the transverse force is balanced by fibres on the opposite side of the tendon. The functionally significant component is the one acting along the axis of the tendon. As the lengths of the vectors show, less force is available in this direction than is developed by the fibres themselves. In practice, this loss is not very great: angles of pennation are usually less than 30°, and so the force in the direction of the tendon may be 90% or more of that in the fascicles (cos 30° = 0.87). Angulation of a set of fibres reduces both the force and range of contraction along the axis of the tendon. However, these negative consequences are outweighed by the design advantage conferred by pennation, i.e. the opportunity to extend the tendinous aponeurosis, and so increase the area available for the attachment of muscle fibres. A given mass of muscle can then be deployed as a large number of short fibres, increasing the total cross-sectional area, and hence the force, available. In a multipennate muscle, the effective cross-sectional area is larger still, and the fibres tend to be even shorter. The ‘gearing’ effect of pennation on a muscle therefore results from an internal exchange of fibre length for total fibre area: this allows much greater forces to be developed, but at the expense of a reduced range of contraction.

  

Fig. 5.48  Force vectors in an idealized pennate muscle. The increase in effective cross-sectional area made possible by this architecture outweighs the small reduction in the component of force acting in the direction of the tendon.

Although the terms power and strength are often used interchangeably with force, they are not synonymous. Power is the rate at which a muscle can perform external work and is equal to force × velocity. Since force depends on the total cross-sectional area of fibres, and velocity (the rate of shortening) depends on their length, power is related to the total mass of a muscle. Strength is usually measured on intact subjects in tasks which require the participation of several muscles, when it is as much an expression of the skillful activation and coordination of these muscles as it is a measure of the forces which they contribute individually. Thus it is possible for strength to increase without a concomitant increase in the true force-generating capacities of the muscles involved, especially during the early stages of training.

Muscles and movement

Historically, attempts were made to elucidate the actions of muscles by gross observation. The attachments were identified by dissection, and the probable action deduced from the line of pull. With the use of localized electrical stimulation it became possible to study systematically the actions of selected muscles in the living subject. This approach was pioneered above all by Duchenne de Boulogne in the mid 19th century. Such knowledge is necessarily incomplete: a study of isolated muscles, whether by dissection, postmortem or stimulation in vivo, cannot reveal the way in which those muscles behave in voluntary movements, in which several muscles may participate in a variety of synergistic and stabilizing roles. Duchenne appreciated this, and supplemented electrical stimulation with clinical observations on patients with partial paralysis to make more accurate deductions about the way in which muscles acted together in normal movement. Manual palpation can be used to detect contraction of muscles during the performance of a movement, but tends to be restricted to superficially placed muscles, with examination taking place under quasi-static conditions. Modern knowledge of muscle action has been acquired almost entirely by recording the electrical activity which accompanies mechanical contraction, a technique known as electromyography (EMG). This technique can be used to study voluntary activation of deep as well as superficial muscles, under static or dynamic conditions. Multiple channels of EMG can be used to examine coordination between the different muscles that participate in a movement. These data can be further supplemented by monitoring joint angle and ground reaction force, and by recording the movement on camera or with a three-dimensional motion analysis system.

Actions of muscles

Conventionally, the action of a muscle is defined as the movement that takes place when it contracts. However, this is an operational definition: equating ‘contraction’ with shortening, and ‘relaxation’ with lengthening is too simple in the context of whole muscles and real movements. Whether a muscle approximates its attachments on contraction depends on the degree to which it is activated, and the forces against which it has to act. The latter are generated by numerous factors: gravity and inertia, any external contact or impact, actively by opposing muscles, and passively by the elastic and viscous resistance of all the structures which undergo extension and deformation, some within the muscle itself, others in joints, inactive muscles and soft tissues. Depending on the conditions, an active muscle may therefore maintain its original length or shorten or lengthen, and during this time its tension may increase, decrease or not change. Movements that involve shortening of an active muscle are termed concentric, e.g. contraction of biceps/brachialis while raising a weight and flexing the elbow. Movements in which the active muscle undergoes lengthening are termed eccentric, e.g. in lowering the weight previously mentioned, biceps/brachialis ‘pays out’ length as the elbow extends. Eccentric contractions are associated with increased risk of muscle tears, especially in the hamstrings. Muscle contraction that does not involve change in muscle length is isometric.

Natural movements are accomplished by groups of muscles. Each muscle may be classified, according to its role in the movement, as a prime mover, antagonist, fixator or synergist. It is usually possible to identify one or more muscles which are consistently active in initiating and maintaining a movement: they are its prime movers. Muscles that wholly oppose the movement, or initiate and maintain the opposite movement, are antagonists, e.g. brachialis has the role of prime mover in elbow flexion, and triceps is the antagonist. To initiate a movement, a prime mover must overcome passive and active resistance and impart an angular acceleration to a limb segment until the required angular velocity is reached; it must then maintain a level of activity sufficient to complete the movement. Antagonists may be transiently active at the beginning of a movement, and thereafter they remain electrically quiescent until the deceleration phase, when units are activated to arrest motion. During the movement, the active prime movers are not completely unrestrained, and are balanced against the passive, inertial and gravitational forces mentioned above.

When prime movers and antagonists contract together they behave as fixators, stabilizing the corresponding joint by increased transarticular compression, and creating an immobile base on which other prime movers may act, e.g. flexors and extensors of the wrist co-contract to stabilize the wrist when an object is grasped tightly in the fingers. In some cases, sufficient joint stability can be afforded by gravity, acting either on its own, e.g. knee and hip joints when they are in or near the close-packed position in the erect posture, or in conjunction with a single prime mover, e.g. the shoulder joint when it is stabilized by supraspinatus with the arm pendent. In other cases, and whenever strong external forces are encountered, prime movers and antagonists contract together, holding the joint in any required position.

Acting across a uniaxial joint, a prime mover produces a simple movement. Acting at multiaxial joints, or across more than one joint, prime movers may produce more complex movements which contain elements that have to be eliminated by contraction of other muscles. The latter assist in accomplishing the movement, and are considered to be synergists, although they may act as fixators, or even as partial antagonists of the prime mover. For example, flexion of the fingers at the interphalangeal and metacarpophalangeal joints is brought about primarily by the long flexors, superficial and deep. However, these also cross intercarpal and radiocarpal joints, and if movement at these joints was unrestrained, finger flexion would be less efficient. Synergistic contraction of the carpal extensors eliminates this movement, and even produces some carpal extension, which increases the efficiency of the desired movement at the fingers.

In the context of different movements, a given muscle may act differently, as a prime mover, antagonist, fixator or synergist. Even the same movement may involve a muscle in different ways if it is assisted or opposed by gravity. For example, in thrusting out the hand, triceps is the prime mover responsible for extending the forearm at the elbow, and the flexor antagonists are largely inactive. However, when the hand lowers a heavy object, the extensor action of the triceps is replaced by gravity, and the movement is controlled by active lengthening, i.e. eccentric contraction, of the flexors. It is important to remember that all movements take place against the background of gravity, and its influence must not be overlooked.

Development of skeletal muscle

Most information about the early development of the skeletal musculature in man has been derived from other vertebrate species. However, where direct comparisons with the developing human embryo have been made, the patterns and mechanisms of muscle formation have been found to be the same.

A myogenic lineage, denoted by the expression of myogenic determination factors, can be demonstrated transiently in some cells shortly after their ingression through the primitive streak. Skeletal muscle found throughout the body is derived from this paraxial mesenchyme, which is formed from ingression at the streak and subsequently segmented into somites (see also the origin of extraocular muscles, Ch. 41).

Skeletal muscle originates from a pool of premyoblastic cells which arise in the dermatomyotome of the maturing somite and begin to differentiate into myoblasts at 4–5 weeks of gestation. By 6 weeks, cells have migrated from the dermatomyotomal compartment to form the myotome in the centre of the somite (see Fig. 44.3). These myotomal precursor cells are identified by the expression of myogenic determination factors; they will eventually differentiate within the somite to form the axial (or epaxial) musculature (erector spinae). A distinct cohort of precursor cells migrates away from the somite to invade the lateral regions of the embryo; there they form the muscles of the limbs (see Ch. 51), limb girdles and body wall (hypaxial musculature; see Fig. 44.3). Virtually all cells in the lateral half of the newly formed somite are destined to migrate in this way. Myogenic determination factors are not expressed in these cells until the muscle masses coalesce. The appearance of myotomal myoblasts, and the migration of myoblasts to the prospective limb region, occurs first in the occipital somites. Thereafter these processes follow the general craniocaudal progression of growth, differentiation and development of the embryo. The myoblastic cells from which the limb muscles develop do not arise in situ from local limb bud mesenchyme, as was once thought, but migrate from the ventrolateral border of those somites adjacent to the early limb buds.

Myogenic determination factors

The myogenic determination factors Myf-5, myogenin, MyoD and Myf-6 (herculin) are a family of nuclear phosphoproteins. They have in common a 70-amino-acid, basic helix-loop-helix (bHLH) domain that is essential for protein–protein interactions and DNA binding. Outside the bHLH domain there are sequence differences between the factors that probably confer some functional specificity. The myogenic bHLH factors play a crucial role in myogenesis. Forced expression of any of them diverts non-muscle cells to the myogenic lineage. They activate transcription of a wide variety of muscle-specific genes by binding directly to conserved DNA sequence motifs (–CANNTG–known as E-boxes) that occur in the regulatory regions (promoters and enhancers) of these genes. Their effect may be achieved cooperatively, and can be repressed, e.g. by some proto-oncogene products. Some of the bHLH proteins can activate their own expression. Accessory regulatory factors, whose expression is induced by the bHLH factors, provide an additional tier of control.

The myogenic factors do not all appear at the same stage of myogenesis (Buckingham et al 2003). In the somites, Myf-5 is expressed early, before myotome formation, and is followed by expression of myogenin. MyoD is expressed relatively late together with the contractile protein genes. Myf-6 is expressed transiently in the myotome and becomes the major transcript postnatally. Whether this specific timing is important for muscle development is not yet clear. The creation of mutant mice deficient in the bHLH proteins (gene ‘knock-out’) has shown that myogenin is crucial for the development of functional skeletal muscle, and that while neither Myf-5 nor MyoD is essential to myogenic differentiation on their own, lack of both results in a failure to form skeletal muscle. In the limb bud (see Ch. 51) the pattern of expression of the bHLH genes is generally later than in the somite: Myf-5 is expressed first but transiently, followed by myogenin and MyoD, and eventually Myf-6. These differences provide evidence at the molecular level for the existence of distinct muscle cell populations in the limb and somites. It may be that the myogenic cells that migrate to the limb differ at the outset from those that form the myotome, or their properties may diverge subsequently under the influence of local epigenetic factors.

Formation of muscle fibres

In both myotomes and limb buds, myogenesis proceeds in the following way. Myoblasts become spindle-shaped and begin to express muscle-specific proteins. The mononucleate myoblasts aggregate and fuse to form multinucleate cylindrical syncytia, or myotubes, in which the nuclei are aligned in a central chain (Fig. 5.49). These primary myotubes attach at each end to the tendons and developing skeleton. The initiation of fusion does not depend on the presence of nerve fibres, since these do not penetrate muscle primordia until after the formation of primary myotubes.

  

Fig. 5.49  Stages in formation of skeletal muscle. Mononucleate myoblasts fuse to form multinucleate primary myotubes, characterized initially by central nuclei. Subsequently, other myoblasts align along the primary myotubes and begin to fuse with one another, forming secondary myotubes. In large animals, such as man, further generations of new muscle fibres are similarly formed. As the contractile apparatus is assembled, the nuclei move to the periphery, cross-striations become visible and primitive features of the neuromuscular junction emerge. Later, small adult-type myoblasts – satellite cells – can be seen lying between the basal lamina and the sarcolemma of the mature muscle fibre. These too appear to be derived from cells that originated in the somites during early development.
(Redrawn from a figure provided by Terry Partridge, Department of Genetic Medicine, Children’s National Medical Center, Washington DC.)

Although synthesis of the contractile machinery is not dependent upon fusion of myoblasts, it proceeds much more rapidly after fusion. Sarcomere formation begins at the Z-disc, which binds actin filaments constituting the I-band to form I–Z–I complexes. The myosin filaments assemble on the I–Z–I complexes to form A-bands. Nebulin and titin are among the first myofibrillar proteins to be incorporated into the sarcomere, and may well determine the length and position of the contractile filaments. Desmin intermediate filaments connect the Z-discs to the sarcolemma at an early stage, and these connections are retained.

Myogenic cells continue to migrate and to divide, and during weeks 7–9 there is extensive de novo myotube formation. Myoblasts aggregate near the midpoint of the primary myotubes and fuse with each other to form secondary myotubes, a process that may be related to early neural contact. Several of these smaller diameter myotubes may be aligned in parallel with each of the primary myotubes. Each develops a separate basal lamina and makes independent contact with the tendon. Initially, the primary myotube provides a scaffold for the longitudinal growth of the secondary myotubes, but eventually they separate. At the time of their formation, the secondary myotubes express an ‘embryonic’ isoform of the myosin heavy chains, whereas the primary myotubes express a ‘slow’ muscle isoform apparently identical to that found in adult slow muscle fibres. In both primary and secondary myotubes, sarcomere assembly begins at the periphery of the myotube and progresses inwards towards its centre. Myofibrils are added constantly and lengthen by adding sarcomeres to their ends. T-tubules are formed and grow initially in a longitudinal direction: since they contain specific proteins not found in plasma membranes, they are probably assembled via a different pathway from that which supports the growth of the sarcolemma. The sarcoplasmic reticulum wraps around the myofibrils at the level of the I-bands.

By 9 weeks, the primordia of most muscle groups are well defined, contractile proteins have been synthesized and the primitive beginnings of neuromuscular junctions can be observed, confined initially to the primary myotubes. Although some secondary fibre formation can take place in the absence of a nerve, most is initiated at sites of innervation of the primary myotubes. The pioneering axons branch and establish contact with the secondary myotubes. By 10 weeks these nerve–muscle contacts have become functional neuromuscular junctions and the muscle fibres contract in response to impulse activity in the motor nerves. Under this new influence the secondary fibres express fetal (sometimes referred to as neonatal) isoforms of the myosin heavy chains. At this stage several crucial events take place which may be dependent on, or facilitated by, contractile activity. As the myofibrils encroach on the centre of the myotube, the nuclei move to the periphery, and the characteristic morphology of the adult skeletal muscle myofibre is established. The myofibrils become aligned laterally, and A- and I-bands in register across the myotube produce cross-striations that are visible at the light microscopic level. T-tubules change from a longitudinal to a transverse orientation and adopt their adult positions: they may be guided in this process by the sarcoplasmic reticulum, which is more strongly bound to the myofibrils.

The myotubes and myofibres are grouped into fascicles by growing connective tissue sheaths, and the fascicles are assembled to build up entire muscles. As development proceeds, the increase in intramuscular volume is accommodated by remodelling of the connective tissue matrix.

At 14–15 weeks, primary myotubes are still in the majority, but by 20 weeks the secondary myotubes predominate. During weeks 16–17, tertiary myotubes appear: they are small and adhere to the secondary myotubes, with which they share a basal lamina. They become independent by 18–23 weeks, their central nuclei move to the periphery, and they contribute a further generation of myofibres. The secondary and tertiary myofibres are always smaller and more numerous than the primary myofibres. In some large muscles, higher order generations of myotubes may be formed.

Late in fetal life, a final population of myoblasts appears which will become the satellite cells of adult muscle. These normally quiescent cells lie outside the sarcolemma beneath the basal lamina (Fig. 5.49, Fig. 5.50). M-cadherin, a cell adhesion protein of possible regulatory significance, occurs at the site of contact between a satellite cell and its muscle fibre. In a young individual, there is one satellite cell for every 5–10 muscle fibre nuclei. The latter are incapable of DNA synthesis and mitosis, and satellite cells are therefore important as the sole source of additional muscle fibre nuclei during postnatal growth of muscle (to maintain the ratio of cytoplasmic volume per nucleus as fibres increase in mass). After satellite cells divide, one of the daughter cells fuses with the growing fibre, the other remains as a satellite cell capable of further rounds of division. Similar events may take place to support exercise-induced hypertrophy of adult skeletal muscle. Satellite cells provide a reservoir of myoblasts capable of initiating regeneration of an adult muscle after damage. Other stem cell populations may also be induced to begin a myogenic differentiation pathway, e.g. bone marrow stem cells and processed lipoaspirate cells (Mizuno et al 2002).

  

Fig. 5.50  Electron micrograph of a satellite cell. Note the two plasma membranes that separate the cytoplasm of the satellite cell from that of the muscle fibre, and the basal lamina (arrows) of the transversely-sectioned muscle fibre, which continues over the satellite cell (see also Fig. 5.38). Compare this appearance with the normal muscle nucleus which is seen in the adjacent fibre (above).
(Photograph by Dr Michael Cullen, School of Neurosciences, University of Newcastle upon Tyne.)

The development of fibre types

Developing myotubes express an embryonic isoform of myosin which is subsequently replaced by fetal and adult myosin isoforms. The major isoform of sarcomeric actin in fetal skeletal muscle is cardiac α-actin; only later is this replaced by skeletal α-actin. The significance of these developmental sequences is not known.

The pattern of expression is fibre-specific as well as stage-specific. In primary myotubes, embryonic myosin is replaced by adult slow myosin from about 9 weeks onwards. In secondary and higher order myotubes the embryonic myosin isoform is superseded first by fetal and then by adult fast myosin, and a proportion go on to express adult slow myosin. Other fibre-specific, tissue-specific and species-specific patterns of myosin expression have been described in mammalian limb muscles and jaw muscles.

The origin of this diversity in the temporal patterns of expression of different fibres, even within the same muscle, is far from clear. It has been suggested that intrinsically different lineages of myoblast emerge at different stages of myogenesis or in response to different extracellular cues. If this is the case, their internal programmes may be retained or overridden when they fuse with other myoblasts or with fibres that have already formed. The fibres that emerge from this process go on to acquire a phenotype that will depend on the further influence of hormones and neural activity.

In man, unlike many smaller mammals, muscles are histologically mature at birth, but fibre type differentiation is far from complete. In postural muscles, the expression of type I myosin increases significantly over the first few years of life; during this period the fibre type proportions in other muscles become more divergent. The presence in adult muscles of a small proportion of fibres with an apparently transitional combination of protein isoforms reinforces the view that changes in fibre type continue to some extent in all muscles and throughout adult life. Fibre type transitions also occur in relation to damage or neuromuscular disease; under these conditions, the developmental sequence of myosins may be recapitulated in regenerating fibres.

Growth and regulation of fibre length

Muscle fibres grow in length by addition of sarcomeres to the ends of the myofibrils. It is important that the number of sarcomeres is regulated throughout life, so that the mean sarcomere length, and hence filament overlap, is optimized for maximum force. This is achieved by addition or removal of sarcomeres in response to any prolonged change of length. For example, if a limb is immobilized in a plaster cast, the fibres of muscles that have been fixed in a shortened position lose sarcomeres, while those that have been fixed in a lengthened position add sarcomeres; the reverse process occurs after the cast has been removed.

Satellite cells and muscle repair

Until the mid 20th century, the mechanisms responsible for the maintenance and repair of skeletal muscle were unclear. These issues were largely resolved by the almost simultaneous discovery that multinucleated muscle fibres were formed by the fusion of mononucleated precursors, myoblasts, and that a population of satellite cells, so-called because of their position on the edge of the fibre, existed between the basal lamina of the mature muscle fibre and its sarcolemma. Although satellite cells constitute 2–5% of the nuclei enclosed by the basal lamina, their role in repair and regeneration of muscle was not resolved until quite recently, because they could be identified only by their anatomical position and quiescent appearance. In contrast, active myoblasts at sites of muscle injury lose both of these features. In recent years, the discovery of a number of myogenic differentiation genes whose expression is retained in the quiescent satellite cell has elucidated their role.

Studies in mouse models, where genetic analysis is possible, have shown that the functional properties of postnatal satellite cells are dependent on the expression of the Pax7 gene, whereas the prenatal development of muscle is not similarly dependent. This implies that the satellite cells are not simply the relics of the prenatal myogenic population although they appear to be derived from the same embryonic source in the somites. Moreover, satellite cells are not a homogeneous population: no two differentiation markers concur completely. This is also the situation in human tissue (Fig. 5.51). It has yet to be determined whether this variation reflects a difference in position in the lineage, in functional status, or in the adjacent environment.

  

Fig. 5.51  Two adjacent sections, fluorescence-immunolabelled, of a regenerating muscle fibre in the trapezius muscle of a power-lifter. A, Anti-laminin antibody (red) shows basal lamina. Anti-CD56 (green) is a marker of myogenic cells and of newly-formed myotubes. B, Adjacent section, basal lamina (red) and myogenin-positive nuclei (green). Basal laminae outline the transversely-sectioned muscle fibres, including the original outline of the fibre being regenerated (centre field). Numerous small blood vessels also outlined by basement membranes, are present probably reflecting local inflammation. Within the CD56 +ve zone, several nuclei (one arrowed) are positive for myogenin, indicating their terminally-differentiated status. Numerous other nuclei (Hoechst dye, blue) within the basement membrane surrounding the area of regeneration probably include proliferating myogenic cells and inflammatory cells.
(Provided by courtesy of Ms Mona Lindström and Professor Lars-Eric Thornell, Department of Anatomy, Umea University, Sweden.)

The satellite cell been rigorously established in mice as being both necessary and sufficient for effective regeneration of damaged skeletal muscle. The cells proliferate to replace their resident region of muscle in 3–4 days and to replenish the quiescent satellite cell population (Collins & Partridge 2005). In man, there is histological evidence of the rapid accumulation of myoblasts, presumably derived from local satellite cells, at sites of muscle damage.

A point of wide pathological interest is the demonstration that the failing regenerative potency of satellite cells in aging muscle seems in large part to be attributable to age-related changes in the systemic environment rather than a decline in the intrinsic capabilities of the satellite cells themselves (Conboy et al 2005).

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