The Musculature of the Rat

Robert Lewis Maynard , Noel Downes , in Anatomy and Histology of the Laboratory Rat in Toxicology and Biomedical Research, 2019

Structure of a Whole Skeletal Muscle

A single muscle is wrapped in an epimysium, a connective tissue sheath containing collagen fibres, the usual cells of loose connective tissue including fat cells, capillaries and nerve fibres. Septae of connective tissue radiate from the epimysium into the muscle dividing it into bundles of muscle fibres known as the fascicles. Each fascicle is wrapped in connective tissue: the perimysium. The process is repeated within the fascicle, with each muscle fibre surrounded by an endomysium. Blood vessels and nerve fibres enter the muscle via this connective tissue network. The epimysium blends with the connective tissue of tendon if the muscle is inserted onto bone via tendon, but if inserted directly onto bone then the epimysium blends with the periosteum of the bone (Fig. 6.4).

Figure 6.4. Microscopy of muscle. Although the bands are just about visible in the H and E section, electron microscopy is required for proper visualisation.

The sarcomere structure is common to skeletal and cardiac muscle. The band structure of the sarcomere is just discernible at high power in H and E of skeletal muscle, but can be clearly seen in the electron micrograph of cardiac muscle on the left.

Skeletal muscle is one of the more 'difficult' tissues for the light microscopist. A frame for holding the muscle at its normal length to avoid contraction when placed in fixative, rapid but thorough embedding in wax (vacuum embedding is recommended) and thin sections are needed if the subcellular structure of the cells is to be made out clearly. Muscle stains well with H&E, and the connective tissue stains (e.g. Masson's trichrome) show the connective tissue clearly, but iron haematoxylin stains (Heidenhain's or Weigert's) are best for examination of the cross banding of the cells, the nuclei and mitochondria. Really good sections allow the deeply staining A bands (include the full length of the thick filaments and part of the length of the thin filaments) and the paler staining I bands (containing only thin filaments) to be identified. In very good sections, the Z lines at the midpoints of the I bands can be clearly seen as thin dark lines. Transverse sections allow the fascicles to be identified and the nuclei to be recognised under the cell membrane of the fibres.

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Neurology

Georgina Barone , in The Cat, 2012

Fibrodysplasia Ossificans Progressiva

Fibrodysplasia ossificans progressiva is a disorder affecting the epimysium, tendons, and fascia with marked proliferation of fibrovascular connective tissue and associated chondroid and osseous metaplasia. 143 Clinical signs are characterized by progressive stiffness in the gait, enlargement of the proximal limb musculature, pruritus, and joint pain and are typically seen in young to middle-aged cats of both sexes. Radiographically, multiple mineralized densities (Figure 27-32) can be seen. Muscle biopsy shows collagen proliferation, focal areas of lymphocytic infiltration, and areas of cartilage and ectopic bone formation within the muscle tissue, with the pathologic abnormalities appearing to have originated from the fascial connective tissue. The clinical course progresses rapidly, and there is no known treatment. Prognosis is grave.

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Sonography of muscle injury

Justin C. Lee , Jeremiah C. Healy , in Clinical Ultrasound (Third Edition), 2011

Normal sonographic muscle appearance

During ultrasound scanning in the transverse plane, the epimysium is identified as an echogenic envelope surrounding the muscle belly, whilst the perimysium is seen as dot echoes, or short lines scattered throughout the hypoechoic background, representing the bulk of the muscle fibres ( Fig. 60.2). The intermuscular septa and aponeuroses are brightly echogenic linear structures in all imaging planes. Similarly, intramuscular extensions of tendons are identified as thick, fibrillar, echogenic structures. In the longitudinal plane, the perimysium is seen as oblique, parallel echogenic striae against a hypoechoic background of muscle fibres (Fig. 60.2). During contraction muscle alters shape and becomes hypoechoic with increased angulation of the echogenic septa in relation to the central tendon.

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Muscular ultrasound – introduction

Ian Beggs , in Clinical Ultrasound (Third Edition), 2011

Muscles

Skeletal muscle is composed of elongated muscle fibres that are covered by thin connective tissue, the endomysium. Muscle fibres are grouped together in muscle bundles or fascicles and are surrounded by perimysium, or fibro-adipose septa, which contains vessels and nerves. A thicker connective tissue sheath, the epimysium, surrounds the whole muscle. Muscles lie deep to the deep fascia.

The arrangement of muscle bundles in an individual muscle depends on muscle function and shape. A parallel arrangement is seen in strap muscles such as sartorius and also in rectus abdominis which is divided into segments by transverse tendinous intersections that are echogenic. In fusiform muscles the bundles are almost parallel in mid-substance then converge as they run to the tendon. In pennate muscles the fascicles run oblique to the line of traction. This increases the insertional area of the fascicles and the force of contraction of the muscle. Unipennate muscles (e.g. flexor pollicis longus) have a peripheral aponeurosis or tendon; bipennate muscles (e.g. rectus femoris) have a central tendon; and multipennate muscles (e.g. deltoid) have more than one tendon in the muscle substance. Muscles may have more than one muscle belly (e.g. biceps femoris) or have a spiral course (e.g. pectoralis major). 4

Normal muscle bundles are hypoechoic but become hyperechoic with fatty infiltration.

Fibro-adipose septa, intramuscular tendons and aponeuroses and epimysium are hyperechoic. Fibro-adipose septa appear as linear, almost parallel bands of increased echogenicity on longitudinal scans (Fig. 51.4) and as small echogenic 'dots' on transverse scans (Fig. 51.5). Echogenicity is lost if the ultrasound beam is not perpendicular to the fibro-adipose septa and the muscle then appears artefactually hypoechoic. Intramuscular tendons and aponeuroses are well defined and echogenic. They are better assessed on transverse scans.

Muscles

•

Muscle bundles are hypoechoic but become echogenic with fatty infiltration.

•

Fibro-adipose septa, tendons and aponeuroses are echogenic and are anisotropic.

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CONVENTIONAL WORKHORSE FLAPS

Jacques Baudet , ... Philippe Caix , in Flaps and Reconstructive Surgery, 2009

Flap modifications/flap handling

Expansion of the width of the flap

The width can be increased by longitudinal parallel incisions along the superficial aponeurosis of the muscle.

Extension of the length of the flap

The length can be increased by multiple transverse incisions through the epimysium of the muscle. Also, the deep surface of the muscle can be scored to increase the effective length of the muscle. This is performed very carefully as too much dissection will compromise the blood supply to the flap.

Free soleus muscle flap

Usually when a free soleus flap is harvested, it is part of the fibula flap based on the peroneal vessels. Several modifications of this flap can be performed, which include the soleus muscle with or without a skin island or other tissue in the region. We have no experience with free transfer of the fibula and soleus without harvesting the peroneal vessels. When soleus muscle is harvested as a free flap, it is usually performed as part of the fibula flap harvest where a cuff of soleus is included. The soleus muscle can be harvested as a free flap by including the posterior tibial vessels as the donor vessels of the flap; however, this is rarely indicated.

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Systems Toxicologic Pathology

Brian R. Berridge , ... Eugene Herman , in Haschek and Rousseaux's Handbook of Toxicologic Pathology (Third Edition), 2013

10 Structure and Function

10.1 Gross and Microscopic Anatomy

Muscles are surrounded and subdivided by connective tissue sheaths. The epimysium envelops entire muscles. Groups of muscle fibers are arranged in fasciculi that are separated by the perimysium, which is contiguous with the epimysium. The endomysium is a delicate network of connective tissue fibers, blood vessels, lymphatic vessels, and nerves that surrounds individual muscle fibers.

10.2 Cellular Content of Skeletal Muscle: Biology and Clinical Relevance

Skeletal muscle arises from mesodermal somites in the embryo during the first trimester of gestation. The somites give rise to myotomes, sites where embryonic muscle cells or myoblasts aggregate, that roughly correspond to the segments of the vertebral column; each somite receives a spinal nerve. Skeletal muscles of the adult will often contain muscle fibers of several myotomes following migration and fusion of embryonic myoblasts, and thus will also receive nerve supply from several myotomes. Myoblasts, the primitive mononuclear precursor cells of skeletal muscle fibers, elongate and fuse with each other to form myotubes. These cells rapidly form the early cytoplasmic components of mature muscle cells by production of thin (actin) and thick (myosin) myofilaments and Z-band material that aggregate into sarcomeres; the sarcotubular membrane systems are also formed at this stage. Myotubes subsequently fuse with each other and sarcomerogenesis continues as nuclei migrate to the subsarcolemmal positions. Finally, innervation occurs and fibers become organized for contractile function. Further growth of these fibers in width and length occurs during fetal life. Increased numbers of muscle fibers are produced by waves of growth in late gestation that are presumed to be the result of proliferation and activation of satellite cells which recapitulate the events of muscle fiber development and maturation as just described.

Skeletal Muscle Fibers
Microscopic Appearance

Muscle fibers or cells generally extend from tendon to tendon in a muscle and do not branch or form syncytia. In cross-sections, fibers have a polygonal or multifaceted shape in muscle of adults. Many factors, such as species, breed, age, weight, sex, plane of nutrition, position and function of the muscle, and exercise, influence the diameter of muscle fibers. Measurements of fibers in individual muscles will show variability in fiber size that will be reflected as a bell-shaped curve on a histogram. Differences in fiber size in various species are not directly related to body weight (pig   >   horse, cow, rabbit   >   sheep). Fiber size is greater in males than females, and tends to increase with age to maturity.

The cellular features of skeletal muscle fibers are best appreciated in longitudinal sections. The fibers are bounded by the plasma membrane or sarcolemma, which is covered by an external lamina (stained by periodic acid-Schiff reaction). The thin, elongated nuclei are generally positioned beneath the sarcolemma in a spiral pattern spaced 10–50   μm apart. At myotendinous junctions, muscle fibers have numerous centrally located nuclei. Nuclei of satellite cells are positioned between the sarcolemma and the external lamina. Fibers contain hundreds of longitudinally aligned myofibrils composed of repeating sarcomeres. The characteristic transverse striation of skeletal muscle fibers results from parallel alignment of the bands in adjacent myofibrils. The largest bands, termed according to their appearance in polarized light, are A bands (anisotropic or birefringent, appear bright) and I bands (iostropic, appear dark). The I bands, composed of thin myofilaments, are bisected by Z lines (disks, bands) that form the end of each sarcomere; the A bands, composed of thick filaments, are bisected by the less-birefringent H bands. The banding pattern, named for the appearance in polarized light, is reversed when studied by light microscopy with phase contrast optics, light microscopy with conventional optics on sections stained with the usual cationic dyes, or transmission electron microscopy.

Application of histochemical stains such as ATPase or NADH-TR to frozen sections of skeletal muscle will allow demonstration of various fiber-type populations that cannot be distinguished in paraffin-embedded sections stained with the usual stains such as hematoxylin and eosin; however, fiber types are recognized by ultrastructural study. The histochemical uniqueness of these fiber types correlates with differences in their physiologic features, such as contraction speed and fatigability; their biochemical and metabolic activities; their gross color; and their structure as revealed ultrastructurally. Table 46.5 summarizes these features.

TABLE 46.5. Characteristics of Major Mammalian Skeletal Muscle Fiber Types

Type I Type IIA Type IIB
Morphologic characteristics:
Natural color Dark Dark Pale
Glycogen content Low High High
Myoglobin content High High Low
Lipid globules Numerous Numerous Few
Mitochrondrial content High Intermediate Low
Physiological features:
Twitch speed Slow Fast Fast
Fatiguability Resistant Resistant Susceptible

Table reproduced from Fundamentals of Toxicologic Pathology, 2nd Ed. W. M. Haschek, C. G. Rousseaux and M. A. Wallig, eds. (2010) Academic Press, Table 12.9, p. 366, with permission

Most muscles will have a mixture of all fiber types to produce the so-called checkerboard pattern of differential histochemical staining. Fibers innervated by the same nerves will have the same fiber type, and reinnervated fibers may show reversal of fiber types. Some muscles will have a preponderance of one fiber type – for example, the soleus (red muscle high in type I fibers and capable of sustained action or weight bearing) and the gastrocnemius (white muscle high in type II fibers and capable of sudden action and purposeful motion). The proportions of the various fiber types may vary with species, breed, age, and exercise, and in certain muscular diseases.

Ultrastructural Appearance

The fiber surface is covered by the plasma membrane (sarcolemma) and external lamina. The elongated subsarcolemmal nuclei are surrounded by accumulation of mitochondria, lipid droplets, glycogen granules, elements of sarcoplasmic reticulum, and the Golgi apparatus. The fiber contains abundant contractile material that is organized as many myofibrils of 0.5–1.0   μm in diameter. Myofibrils are composed of repeating units, termed sarcomeres, of 2–3   μm in length. Sarcomeres end at dense Z lines that contain αα-actinin, actin, and tropomyosin. Thin 60-Å diameter myofilaments containing actin, troponin, and tropomyosin extend on both sides of the Z line to form I bands. The middle half of the sarcomere contains thick (160-Å diameter) myofilaments that are composed of myosin and interdigitate with the adjacent thin filaments. The center of the sarcomere, with only thick filaments, is the H band and is bisected by the relatively dense M line. Fiber contraction results in shortening of sarcomeres due to sliding of thick and thin filaments over each other to produce narrowed I and H bands. Cross-sections of myofibrils show variable appearance depending on the location in the sarcomere, but the edges of the A band will have thick filaments surrounded by a hexagonal array of thin filaments. The sarcoplasm surrounding myofibrils contains elements of the transverse (T) tubular system and sarcoplasmic reticulum (SR), mitochondria, lipid droplets, glycogen granules, and cytosol. The T tubules are invaginations of the sarcolemma that are often seen at the edge of the I band with two adjacent elements of SR to form a "triad." The T-tubular system functions as a channel to allow rapid spread of an electrical impulse from the motor end plate to the rest of the fiber to elicit release of calcium stored in the SR, with subsequent binding to regulatory proteins and interaction of actin and myosin to initiate contraction.

Satellite Cells

Satellite cells are thin cells with a nucleus and a scant amount of sarcoplasm interposed between the sarcolemma of muscle fibers and the external lamina. They are abundant in newborn animals; in muscle of mature animals, 3–5% of nuclei in muscle fibers belong to satellite cells. The cells play an important role in normal development of fibers and in regeneration of damaged fibers by serving as stem cells that can be activated to undergo mitosis in adult life and subsequently differentiate to myoblasts, myotubes, and, eventually, mature myofibers.

Motor End Plates

Motor end plates (neuromuscular junctions), generally recognized only by use of special techniques such as metallic impregnation, intravital dyes, histochemical procedures, or electron microscopy, represent a complex and intimate attachment site of the motor nerve fiber on the surface of the skeletal muscle fiber. The end of the nerve fiber is unmyelinated and branches into axon terminals that invaginate into a thickened zone of subsarcolemmal sarcoplasm with numerous nuclei as synaptic clefts. The axon terminal has abundant synaptic vesicles containing the neurotransmitter acetylcholine.

Muscle Spindles

Muscle spindles are fusiform structures 0.5–3.0   mm in length found longitudinally oriented at the edge of muscle fasciculi. The spindle has a thick fibrous capsule and contains multiple small variably sized intrafusal muscle fibers, nerve fibers, specialized nerve endings, and blood vessels. Muscle spindles have sensory function, and serve to maintain muscle tone by responding to stretch.

Connective Tissue

The interstitial connective tissue of muscle is subdivided into the epimysium (surrounds the entire muscle), perimysium (surrounds large angular fascicles divided into primary fascicles of 10–100 fibers), and endomysium (surrounds individual muscle fibers). The endomysium contains capillaries, nerve fibers, fibroblasts, and collagen fibrils. Larger amounts of collagen fibrils and large blood vessels and nerves are in the perimysium.

10.3 Physiology and Functional Considerations

The unique structural differentiation of skeletal muscle fibers is closely integrated with their highly developed specialized contractile function for locomotion and maintenance of posture by conversion of chemical energy into mechanical energy. Further specialization in form and function is provided by the differentiation of myofibers into various fiber types, each of which is specifically suited for certain physiologic applications.

The functional unit of the neuromuscular system is the motor unit, consisting of (1) nerve cell bodies in the ventral horns or brainstem, (2) axons of these neurons that course to the muscles and terminate as a motor end plate, and (3) the group of specific histochemical-type muscle fibers that are innervated by the neuron. The number of muscle fibers supplied by a neuron of a motor unit may vary widely depending on the degree of refined movement needed by the muscle (e.g., 10 fibers per neuron in extrinsic eye muscles to 2000 fibers per neuron in large limb muscles).

Contraction of muscle is the result of sarcomere shortening with interdigitation of thin and thick myofilaments. According to the sliding filament hypothesis of contraction, the force of contraction is generated by the movement of cross-bridges that project from myosin molecules along actin molecules. The chemical energy for contraction is supplied by high-energy phosphate compounds that are largely generated in type I fibers by mitochondrial oxidative phosphorylation via the electron transport system following the oxidation of fatty acids and glucose via the Krebs cycle, and in type II fibers by sarcoplasmic anaerobic glycolysis and glycogenolysis. Thus, the metabolic differences of the various fiber types are associated with differences in their functional features, such as speed of contraction and resistance to fatigue.

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Muscle

Philip Robinson , Richard J. Wakefield , in Essential Applications of Musculoskeletal Ultrasound in Rheumatology, 2010

Muscle Hernia

A muscle hernia is a protrusion of muscular tissue through a defect in the containing epimysium (i.e., fascia). 66 This commonly occurs in the anterior and lateral muscle groups of the lower leg (especially the tibialis anterior) but is also recognized in the rectus femoris and the hamstrings (Fig. 12-17). 23 It is thought that the fascia overlying tibialis anterior has an area of potential weakness due to penetrating branches of the peroneal nerve and associated vasculature. 42

There may be a history of previous trauma or surgery, but this is unusual. The hernia usually manifests as a mass that may appear only after exercise or on standing. 23 The hernia may be painful on exertion, but the main problem frequently is cosmetic, and it must be remembered that surgical treatment is not without complications. 67 The clinical differential diagnosis includes an incompetent perforating vein.

Ultrasound can accurately identify the thick echogenic muscle fascia, and any defect is seen as a hypoechoic gap (see Fig. 12-17). 23 Dynamic maneuvers can be performed to reproduce the muscle hernia if it is reduced. In an acute herniation, the muscle may appear hyperechoic due to compression of the fascial planes within it. However, if chronic, it may appear hypoechoic due to some degree of edema or necrosis. 23 Because of its small size and variable presentation on dynamic maneuvers, MRI can be relatively ineffective in demonstrating these lesions. 9,23

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Striated Muscle Dynamics

S.K. Gollapudi , ... M. Chandra , in Reference Module in Biomedical Sciences, 2014

Fast Skeletal Muscle: Structure and Organization

Skeletal muscles contain three distinct layers of connective tissue (Figure 1):

Figure 1. Structural organization of skeletal muscles.

 Illustrated pictures are reproduced from Martini, F., 2006. Fundamentals of Anatomy and Physiology, seventh ed. Pearson Education Inc., San Francisco, CA, p. 285, with permission.
1.

The epimysium is the dense connective tissue that surrounds the entire muscle tissue. The epimysium usually contains many bundles (fascicles) of muscle fibers.

2.

The perimysium is the connective tissue that surrounds each bundle of muscle fibers.

3.

The endomysium is the connective tissue that covers each single muscle fiber or myofiber or muscle cell.

Connective tissue links individual skeletal muscle cells in a bundle, which aids in transmitting force to the tendons. Another important function of various layers of connective tissue is that they provide a pathway for blood vessels, lymphatics, and nerve fibers. We will now review the molecular structure of myofibers that provides the basis for skeletal muscle function.

Myofibers

A myofiber is ∼100   μm in diameter and spans the whole muscle length (ML). The cell membrane of the myofiber is called the sarcolemma, which surrounds the sarcoplasm (Figure 2). The sarcolemma invaginates at regular intervals, forming tubular structure called the transverse tubule (T-tubule). On either side of the T-tubule, saclike structures called the terminal cisternae are present. Terminal cisternae are basically the enlarged portions of the sarcoplasmic reticulum (SR), where the intercellular Ca2+ is stored. Interposed between the SR and the T-tubule are dihydropyridine receptors and ryanodine receptors, which, upon activation, allow the release of Ca2+ from the SR into the sarcoplasm. T-tubules surround cylindrical structures called myofibrils, which contain many contractile filaments. Myofibrils are ∼1–2   μm in diameter and extend along the whole ML.

Figure 2. Structural organization of a skeletal muscle fiber.

 Illustrated pictures are reproduced from Martini, F., 2006. Fundamentals of Anatomy and Physiology, seventh ed. Pearson Education Inc., San Francisco, CA, p. 286, with permission.

Myofibrils

These are contractile structures within each muscle fiber. A single myofibril is made up of many short, repeating structural contractile units known as sarcomeres (Figure 3). Sarcomeres are arranged end to end. When viewed under the microscope, sarcomeres reveal two distinct bands: the dark band known as the A-band (anisotropic) and the light band known as the I-band (isotropic). Alternating A- and I-bands in the sarcomere lead to the striation pattern observed under the light microscope. The A-band is the region where the thick and thin filaments overlap, whereas the I-band contains only a portion of thin filaments. Thick filaments originate from the M-line and are connected to the Z-line by a giant muscle protein called titin (see Figure 3). The region flanked by two adjacent Z-lines makes up the sarcomere, which has the ability to produce force and contract. The contractile ability of the sarcomere results from interactions between thick and thin filaments located within the sarcomere.

Figure 3. Structure of the sarcomere. (a) A schematic representation. (b) An electron micrograph.

 Illustrated pictures are reproduced from Martini, F., 2006. Fundamentals of Anatomy and Physiology, seventh ed. Pearson Education Inc., San Francisco, CA, p. 288, with permission.

Thick Filaments

The main component of the thick filament is myosin-II (two myosin heads); each myosin-II consists of two identical myosin heavy chains (MHC). Each MHC consists of a globular head region connected to a long coiled-coil tail region via an elastic hinge region (Figure 4). Coiled-coil tail regions of myosin-II polymerize to form the thick filament, while the globular head regions protrude from the thick filament at regular intervals. These globular myosin heads can interact with actin to form crossbridges during muscle activation. Myosin heads are oriented toward the Z-line, while the coiled-coil tails are directed toward the M-line. An important feature of myosin polymerization is that myosin-II molecules are oriented in opposite directions in each half of the sarcomere. This orientation results in polarized thick filaments that help in directing force toward the center of the sarcomere. Each thick filament is made up of ∼300 myosin-II molecules. Each globular head of myosin contains an ATPase site and an actin-binding site. The energy released from the hydrolysis of adenosine triphosphate (ATP) and the structural rotation of myosin heads, with respect to the thin filament, produce force and shortening.

Figure 4. Structural organization of thick and thin filament proteins.

 Illustrated pictures are reproduced from Martini, F., 2006. Fundamentals of Anatomy and Physiology, seventh ed. Pearson Education Inc., San Francisco, CA, p. 291, with permission.

Thin Filaments

The thin filament contains several important contractile regulatory proteins (Figure 4). The main component is the actin filament, which is formed from the polymerization of globular actin molecules. Each globular actin monomer contains a binding site for the globular myosin head. Along the entire length of the thin filament, actin monomers spiral around a structural protein called nebulin. An important feature of the actin filaments is that they have polarity; that is, all actin monomers orient toward the M-line. This polarization of the actin filament, together with that of the thick filaments, plays an important role in directing contractile force toward the center of the sarcomere. Present along the entire length of the actin filament is another filamentous structure formed by the head-to-tail polymerization of coiled-coil dimers of tropomyosin (Tm) (Bailey, 1946). Each Tm dimer is helically arranged on the actin filament, spanning seven actin monomers. Located close to this head-to-tail overlap region of Tm is the troponin (Tn) complex (Ebashi and Kodama, 1965), which is made up of three individual protein subunits: TnC, TnI, and TnT (Greaser and Gergely, 1973; Hartshorne and Mueller, 1968). TnC is the Ca2+-binding subunit that serves as the trigger for muscle contraction, TnI serves as the inhibitor of actin–myosin interactions, and TnT is the Tm-binding subunit. In the absence of Ca2+, Tn holds Tm in a configuration that physically blocks the myosin-binding site on actin. Thus, the thin filament is said to be in the blocked state in the absence of Ca2+. When Ca2+ binds to TnC, a cascade of allosteric changes takes place within the Tn–Tm complex, which results in the movement of Tm on the actin filament, thereby exposing the myosin-binding sites on actin. This Ca2+-induced movement of Tm causes the thin filament to transition from the blocked- to the closed-state (McKillop and Geeves, 1993); because such a transition is mediated by Ca2+-mediated changes in the thin filament, it is known as the Ca2+-mediated activation of thin filaments. Now the myosin heads can interact strongly with actin and undergo conformational change to produce force. Thus, the Tn–Tm complex acts as an on/off switch to regulate actin–myosin interactions and hence is called the regulatory unit (RU).

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Skeletal Muscle1

Beth A. Valentine , in Pathologic Basis of Veterinary Disease (Sixth Edition), 2017

Defense Mechanisms/Barrier Systems

Defense mechanisms and barrier systems are summarized in Box 15-7 . The thick encircling fascia (epimysium) of many muscles provides some protection from penetrating injuries and from extension of adjacent infection. This fascia can, however, also contribute to injury under circumstances that lead to increased intramuscular pressure causing hypoxia (compartment syndrome). Tissue macrophages are not typically found in normal muscle but are recruited rapidly from circulating monocytes in the vasculature. Macrophages can cross even an intact basal lamina and effectively clear debris from damaged portions of myofibers, allowing for rapid restoration of the myocyte through satellite cell activation. Neutrophils and other inflammatory cells are also recruited from the bloodstream in response to injury or infection. The extensive vascular network of muscle includes extensive collateral circulatory pathways that render muscle relatively resistant to ischemic damage caused by thrombosis or thromboembolism. Despite the high vascular density of muscle, metastasis of neoplasms to muscle is quite rare. There is evidence that the capillary endothelium of skeletal muscle is inherently resistant to neoplastic cell adhesion and invasion.

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Overtraining Syndrome and the Use of Muscle in Exercise

Yun-tao Ma , in Acupuncture for Sports and Trauma Rehabilitation, 2011

SKELETAL MUSCLE

Skeletal muscle is surrounded by a layer of connective tissue, which is called the fascia, or epimysium, of the muscle; it consists mainly of collagen fibers. The fascia is built up in the same way as the outer layer of a joint capsule. It provides a surface against which the surrounding muscles can glide, and it gives muscles their form. Thus a muscle and its fascia are anatomically and physiologically bound together. When a muscle is fatigued, inflamed, or injured, the muscle is shortened and resists any stretching, and the same happens to the fascia. In fact, damage to the fascia creates additional problems. An inflamed fascia may adhere to other fascia, which makes muscle movement difficult or impossible. Scar tissue forms, and the lack of mobility can become permanent. This is one of the major sources of chronic soft tissue dysfunction and pain.

A muscle is further made up of small cell bundles, the fasciculi. Each fasciculus is surrounded by a thin layer of connective tissue, the perimysium. In the perimysium—which is made up of both collagenous and elastic fibers—the nerve and blood vessels branch off before finally reaching the actual muscle fibers. Each fasciculus consists of a number of muscle fibers, or muscle cells. Each muscle fiber is surrounded by a very thin layer of connective tissue, which is called endomysium (Fig. 5-1).

The structure and function of muscle fibers is described thoroughly in textbooks on physiology. The rest of this section is a brief review.

A muscle fiber is composed of small structures called muscle fibrils or myofibrils. The fibrils lie in parallel and give the muscle cell a striated appearance. Fibrils are made up of smaller regularly aligned components called myofilaments, which are chains of protein molecules. The striated appearance is attributable to the presence of two types of myofilament: actin and myosin. When the muscle contracts, the actin filaments move longitudinally between the myosin filaments. As a consequence, the myofibrils shorten and thicken.

The connective tissue surrounding the muscle, the epimysium, extends and is continuous with the muscle's tendon. The muscles of the body have very different shapes (Fig. 5-2). When a muscle contracts, it produces a force, F, that affects the origin and insertion of the muscle equally but in opposite directions. A muscle and its fascia become shortened when a muscle is fatigued, inflamed, or injured, which may create a static force on tissues of both origin and insertion. If the shortened muscle is forced to stretch, the muscle creates warning pain and conveys the stretching stress to the tendons of both origin and insertion. The consequence is tendinitis, which is a symptom of tendons, muscles, and related soft tissues, including nerves, blood vessels, and fascia. This condition can become more serious in athletes when medication is used to block or suppress the warning pain signals.

The structure of the muscle exactly serves its function. For example, strap-shaped muscle is found in places where it is necessary to execute large ranges of movement quickly. Pinnate-shaped muscle can be found where movements over a small range but of great strength are required. To assess the effect of a muscle, the clinician must also know where it is attached in relation to the joint. The alignment of force of a muscle is dependent on its physiologic cross-section. The ability of a muscle to create a force to do work depends on two factors: its physiologic cross-section and its position in relation to the joint. This knowledge is very important in the treatment of muscular symptoms related to movement.

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