Skeletal Muscle Introduction PDF

CH APTER 15 Skeletal Musclea Beth A. Valentine Key Readings Index Structure, 992 Function, 993 Dysfunction/Responses to Injury, 997 Portals of Entry/Pathways of Spread, 1006 Defense Mechanisms/Barrier Systems, 1007 Diseases Affecting Multiple Species of Domestic Animals, 1007 Diseases of Horses, 101...

CH APTER 15 Skeletal Musclea Beth A. Valentine Key Readings Index Structure, 992 Function, 993 Dysfunction/Responses to Injury, 997 Portals of Entry/Pathways of Spread, 1006 Defense Mechanisms/Barrier Systems, 1007 Diseases Affecting Multiple Species of Domestic Animals, 1007 Diseases of Horses, 1016 Diseases of Ruminants (Cattle, Sheep, and Goats), 1024 Diseases of Cattle, 1024 Structure Normal Skeletal Muscle Understanding the normal structure and function of muscle, including gross, histologic, biochemical, physiologic, electrophysiologic, and ultrastructural features, is critical to understanding muscle disease. Structure of Myofibers Structural and physiologic features of skeletal muscle determine much of its response to injury. Although muscle cells are frequently called muscle fibers or myofibers, they are in fact multinucleated cells of considerable length, which in some animals may approach 1 m. Myonuclei are located peripherally in the cylindrical myofiber (Fig. 15.1) and direct the physiologic processes of the cellular constituents in their area through a process known as nuclear domains. This anatomic arrangement allows segments of the cell to react independently of other portions of the cell. Myonuclei are considered terminally differentiated, with little or no capacity for mitosis and thus for regeneration. Associated with myofibers are the satellite cells, also known as resting myoblasts (E-Fig. 15.1). These cells are distributed along the length of the myofiber, between the plasma membrane (sarcolemma) and the basal lamina. Satellite cells in skeletal muscle are very different from cells of the same name found within the peripheral nervous system. Muscle satellite cells are fully capable of dividing, fusing, and reforming mature myofibers. Thus, under favorable conditions, muscle cells (myofibers) are able to fully restore themselves after damage. Recent studies have found that pluripotent cells derived from bone marrow can also contribute to skeletal muscle repair, albeit only to a very small degree. Each myofiber is surrounded by a basal lamina and outside of this by the endomysium, a thin layer of connective tissue containing capillaries. Myofibers are organized into fascicles surrounded by the perimysium, a slightly more robust layer of connective tissue (EFig. 15.2). Entire muscles are encased in the epimysium, a protective fascia that merges with the muscle tendon. This connective tissue aFor a glossary of abbreviations and terms used in this chapter, see E-Glossary 15.1. 992 Diseases of Sheep and Goats, 1027 Diseases of Pigs, 1028 Diseases of Dogs, 1029 Diseases of Cats, 1035 framework is not inert but in fact forms an integral part of the contractile function of muscle by storing and relaying force generated by myofiber contraction. Ultrastructural examination reveals that skeletal muscle is a highly and rigidly organized tissue, with what are perhaps the most highly structured cells in the body. Each myofiber is composed of many closely packed myofibrils containing actin and myosin filaments. The striations visible with light microscopy (Fig. 15.2) represent the sarcomeric arrangement of muscle cells, in which actin and myosin filaments attached to transverse Z bands form the framework, and other organelles and intracytoplasmic materials are interspersed within this framework (Fig. 15.3). The endoplasmic reticulum of myofibers is called the sarcoplasmic reticulum and is modified to contain terminal cisternae that sequester the calcium ions necessary to initiate actin and myosin interaction and thus contraction. Sarcolemmal invaginations that traverse the cell, the T (for transverse) tubules, allow rapid dispersion of a sarcolemmal action potential to all portions of the myofiber. The terminal cisternae of two adjacent sarcomeres and the T tubule form what is called the triad (Fig. 15.3, A). Neuromuscular junctions can only be visualized using electron microscopy or other specialized procedures (Fig. 15.4). Neuromuscular junctions occur only in specific zones within the muscle, usually forming an irregular circumferential “band” midway between myofiber origin and insertion. Types of Myofibers Mammalian muscles are composed of muscle fibers of different contractile properties. A common classification of these fibers is based on three major physiologic features: (1) rates of contraction (fast or slow), (2) rates of fatigue (fast or slow), and (3) types of metabolism (oxidative, glycolytic, or mixed). These physiologic differences form the basis of histochemical methods that demonstrate fiber types. There are several fiber-type classifications. Classification of fibers into type 1, type 2A, and type 2B (Table 15.1) has proved to have practical application in muscle pathology. It is the classification used in this text. Type 1 fibers are rich in mitochondria, rely heavily on oxidative metabolism, and are slow contracting and slow fatiguing. Type 2 fibers have fewer mitochondria and are glycolytic, fast contracting, and more easily fatigable. In most species, type 2 fibers can be subdivided into type 2A and type 2B. Type 2B fibers are the fastcontracting, fast-fatiguing, glycolytic fibers that depend on glycogen CHAPTER 15 Skeletal Muscle 992.e1 E-Glossary 15.1 Glossary of Abbreviations and Terms Acetylcholine receptors Postsynaptic receptors within the sarcolemma at the neuromuscular junctions. Binding of acetylcholine released from terminal axons causes sodium influx to generate a muscle action potential. ALT Alanine aminotransferase ANA Antinuclear antibody AST Aspartate aminotransferase ATP Adenosine triphosphate ATPase Adenosine triphosphatase; an enzyme that catalyzes the hydrolysis of ATP Basal lamina A layer of extracellular matrix encircling myofibers and peripheral nerve fibers Cachexia Generalized muscle atrophy resulting from disease or malnutrition Chronic myopathic A variety of changes occurring in prolonged change myopathic or neuropathic conditions, including cytoarchitectural changes, myofiber diameter changes, and infiltration by fat or connective tissue CK Creatine kinase Compartment Ischemic necrosis of muscle following syndrome swelling in a nonexpandable compartment Congenital myopathy Muscle disease present at birth Degenerative myopathy Muscle disease characterized by muscle necrosis Denervating disease Disease caused by motor neuron death or peripheral nerve axonal degeneration Denervation atrophy Muscle atrophy caused by motor neuron death or peripheral nerve axonal degeneration Disuse atrophy Muscle atrophy caused by lack of muscular activity DNA Deoxyribonucleic acid Electromyography An electrodiagnostic method to evaluate (EMG) skeletal muscle and peripheral structure and function Endocrine myopathy Muscle disease, typically atrophy of type 2 fibers, caused by hypothyroidism or hypercortisolism Enzyme histochemistry A panel of reactions for microscopic evaluation of skeletal muscle Exertional Severe sudden skeletal muscle necrosis rhabdomyolysis caused by overexertion Fiber A myofiber; a skeletal muscle cell Fiber type Physiologic characteristics of skeletal myofibers, ranging from oxidative and slow twitch (type 1) to glycolytic and fast twitch (type 2). Fiber type is primarily determined by the activity of the innervating motor neuron. Fiber-type grouping Alteration of the normal mosaic pattern of intermingled type 1 and type 2 fibers, resulting in groups of a single fiber type. Fiber-type grouping indicates prior denervation and reinnervation by a different type of neuron. FIV Feline immunodeficiency virus GBE Glycogen branching enzyme GYS1 Glycogen synthase 1 H&E stain Hematoxylin and eosin stain HIV Human immunodeficiency virus HYPP Hyperkalemic periodic paralysis IgA Immunoglobulin A IFG-1 Insulin-like growth factor 1 IL-1 Interleukin-1 IL-6 Interleukin-6 Inflammatory Muscle disease characterized by myopathy inflammation, caused by infection or (myositis) immune-mediated disease Ischemic myopathy Muscle necrosis resulting from inadequate circulation LD50 LDH Lipomatosis Masticatory myositis Metabolic myopathy MH Monophasic muscle necrosis Motor neuronopathy Motor unit Muscular dystrophy Myoblast Myodegeneration Myofiber Myoglobinuria Myopathy Myostatin Myotonia Myotube Na+-K+ ATPase NADH Neuromuscular junction PAS Peripheral neuropathy Polymyositis Polyphasic myofiber necrosis Pseudohypertrophy PTAH Rhabdomyolysis RNA Sarcolemma Sarcolemmal tube Sarcopenia Lethal dose for 50% of animals exposed Lactic dehydrogenase Adipose tissue infiltrating skeletal muscle; steatosis An immune-mediated inflammatory myopathy confined to masticatory muscles in dogs Muscle disease caused by defects in muscle energy metabolism—for example, defective glycolysis, glycogenolysis, or mitochondrial enzymes Malignant hyperthermia Muscle necrosis caused by a single nonpersistent injury Disease of motor neurons All muscle fibers innervated by a single neuron An inherited, progressive myopathy characterized by ongoing myofiber necrosis and regeneration A mitotically active skeletal muscle precursor, derived from skeletal muscle satellite cells, capable of migration and fusion to form myotube during embryologic development and muscle regeneration Muscle fiber necrosis A muscle fiber; a muscle cell Passage of urine containing large amounts of myoglobin causing urine to be red. This occurs when there is severe sudden injury to a large amount of skeletal muscle. Disease of skeletal muscle A protein that limits the number of myofibers formed in embryologic development and also limits the diameter of mature myofibers Prolonged contraction following stimulation; most often caused by ion channel dysfunction An immature myofiber, present during embryologic development and during muscle regeneration A sodium potassium exchanger on the muscle sarcolemma Nicotinamide adenine dinucleotide, reduced form A synaptic zone of the muscle membrane containing acetylcholine receptors Periodic acid–Schiff Disease of peripheral nerves An immune-mediated inflammatory myopathy affecting multiple muscle groups Muscle necrosis caused by repeated or continuous injury Muscle that is grossly enlarged by infiltrating fibrosis and/or fat Phosphotungstic acid hematoxylin Necrosis (lysis) of skeletal muscle; most often used for sudden, severe muscle necrosis such as that caused by overexertion Ribonucleic acid The skeletal muscle cell plasma membrane The basal lamina remaining following skeletal muscle cell necrosis that guides the regenerating myofiber Age-related muscle atrophy and weakness Continued 992.e2 SECTION II Pathology of Organ Systems E-Glossary 15.1 Glossary of Abbreviations and Terms—cont’d Sarcoplasmic reticulum Satellite cell SDH Segmental necrosis Steatosis The skeletal muscle endoplasmic reticulum, modified to store calcium A resting myoblast, located between the sarcolemma and the basal lamina. Satellite cells undergo mitosis, migration, fusion, and eventual maturation resulting in skeletal muscle regeneration. Succinate dehydrogenase Necrosis involving segments of the myofiber but not the entire cell A condition in which myofibers are replaced by mature adipocytes E-Figure 15.1 Skeletal Muscle Myofibers, Transverse Section. Note the satellite cell (resting myoblast) located between the sarcolemma (arrow) and the basal lamina (arrowhead). Transmission electron microscopy (TEM). ­Uranyl acetate and lead citrate stain. (Courtesy Dr. B.A. Valentine, College of Veterinary Medicine, Oregon State ­University.) T tubule TNF-α Type 1 fiber Type 2 fiber Transverse tubule Tumor necrosis factor-α Oxidative, slow-twitch, fatigue-resistant myofibers. A high percentage of type 1 fibers are found in postural muscles. Glycolytic, fast-twitch, fatigue-sensitive myofibers. Locomotory muscles contain many type 2 fibers. Type 2 fibers can be further divided into subgroups; for example, type 2A are mixed oxidative glycolytic, and type 2B are purely glycolytic. E-Figure 15.2 Skeletal Muscle, Transverse Section, Normal Mammalian Muscle. Each myofiber is surrounded by an endomysium of fine collagenous connective tissue. Myofibers are organized into fascicles, which are surrounded by a slightly thicker perimysium. Frozen section, reticulin stain. (Courtesy Dr. B.A. Valentine, College of Veterinary Medicine, Oregon State University.) CHAPTER 15 Skeletal Muscle 993 slow, prolonged activity, such as those that maintain posture. Type 1 predominant postural muscles are most often located deep in the limb. Within the same muscle, the percentage of type 1 fibers often increases in the deeper portions. Muscles that contract quickly and for short periods of time, such as those designed for sprinting, contain more type 2B fibers. Only rarely are muscles composed of only one fiber type (e.g., the ovine vastus intermedius is type 1). Athletic training causes some type 2B fibers to be converted to 2A. There are also variations within breeds and differences in the same muscle in different species. For example, the dog has no type 2B purely glycolytic fibers; all canine fibers have strong oxidative capacity (see Fig. 15.6, B). Innervation and Motor Units Figure 15.1 Skeletal Muscle, Isolated Intact Myofiber. Note the multiple peripherally located nuclei (arrows). Phase contrast microscopy. (Courtesy Dr. B.A. Valentine, College of Veterinary Medicine, Oregon State University.) The axons of the peripheral nerve trunks contain terminal branches that innervate multiple myofibers. The terminal branches form synapses with the myofibers at the neuromuscular junction. The myofibers innervated by a single axon form a motor unit, all fibers of which will contract simultaneously after stimulation. Different muscles have different-sized motor units that relate to their function. For example, extraocular muscle function does not call for forceful contraction but, rather, for many fine movements to smoothly move the globe. Therefore, these muscles have very small motor units, with only a small number of myofibers (1 to 4) innervated by each axon. In contrast, the quadriceps muscle is not designed for fine movement but instead is designed for generation of force; therefore, motor units are quite large, with many myofibers (100 to 150 or more) innervated by a single axon. Function Figure 15.2 Skeletal Muscle, Longitudinal Section, Normal Mammalian Muscle, Cytoarchitectural Characteristics. Note the peripherally located myofiber nuclei and cross-striations on the muscle fibers. The cross-striations correspond to the A bands (dark lines) and I bands (light lines) in the transmission electron micrograph of Fig. 15.3, B. Myofibers are surrounded by an extensive capillary network (arrow). Hematoxylin and eosin (H&E) stain. (Courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee.) for their energy supply. Type 2A fibers are mixed oxidative-glycolytic and therefore, although fast contracting, are also slow fatiguing. Thus, type 2A fibers are “intermediate” in the concentration of mitochondria, fat, and glycogen between type 1 and type 2B. Most muscles contain both type 1 and type 2 fibers, and these can be demonstrated by the myosin adenosine triphosphatase (ATPase) reaction (Fig. 15.5, A). Notice that the different fiber types are normally intermingled, forming what is called a mosaic pattern of fiber types. In most mature muscles, the staining pattern of the ATPase reaction reverses when sections are preincubated in an acid rather than an alkaline solution. There are examples of both patterns in the illustrations in this section. Acid preincubation can also be used to distinguish type 2A and type 2B fibers (Fig. 15.5, B). Regenerating fibers, classified as type 2C fibers, stain darkly in both acid and alkaline preparations, which is a distinguishing feature. In most species, oxidative enzyme reactions to demonstrate mitochondria also demonstrate fiber types to some degree (Fig. 15.6, A). Fiber typing can also be done with immunohistochemical procedures to identify specific myosin isoforms. The percentage of each fiber type varies from muscle to muscle (Fig. 15.7). Type 1 fibers (slow contracting, slow fatiguing, and oxidative) are plentiful in those muscles in which the main function is Skeletal muscle has many functions in the body. Some obvious and major functions are maintaining posture and enabling movement, including locomotion. The rhythmic contraction of the respiratory muscles (the intercostal muscles and the diaphragm) is essential for life. In addition, muscles play a major role in whole-body homeostasis and are involved in glucose metabolism and maintenance of body temperature. On a purely esthetic level, muscle contributes to pleasing body contours. The function of skeletal muscle is intimately related to the function of the peripheral nervous system. The physiologic attributes of a muscle fiber—its rate of contraction and type of metabolism (oxidative, anaerobic, or mixed)—are determined not by the muscle cell itself but by the motor neuron responsible for its innervation (Fig. 15.8). This fact is significant in evaluating histologic changes in muscle fibers. It is possible to divide changes in muscle fibers into two major classes: neuropathic and myopathic. Neuropathic changes are those that are determined by the effect or the absence of the nerve supply (e.g., atrophy after denervation). The term myopathy should be reserved for those muscle diseases in which the primary change takes place in the muscle cell, not in the interstitial tissue and not secondary to effects from the nerve supply. The term neuromuscular disease encompasses disorders involving lower motor neurons, peripheral nerves, neuromuscular junctions, and muscles. Metabolism and Ionic Homeostasis Myofibers require a great deal of energy in the form of adenosine triphosphate (ATP) to generate force and movement. Type 1 oxidative and type 2A oxidative-glycolytic fibers use aerobic metabolism of glucose, stored in the muscle as glycogen, and fat. Type 2B glycolytic fibers rely primarily on anaerobic metabolism of glycogen for energy. Inherent or acquired metabolic defects that reduce skeletal muscle energy production can result in severe muscle dysfunction. A commonly encountered postmortem change, rigor 994 SECTION II Pathology of Organ Systems A band Z disk I band Myofibrils Sarcotubules Terminal cisternae Mitochondrion Sarcoplasmic reticulum Transverse tubule Mt A band I band H zone I band G Z disk M line Z disk M line Sarcomere Z disk Actin-tropomyosin Cytoskeletal proteins Overlap A Thin filament lattice Thick filament lattice Myosin Titin Z disk B A band Z line I band Center of sarcomere Myofilaments in cross-section Figure 15.3 Myofiber Structure. A, Schematic diagram of skeletal muscle illustrating the sarcomeric arrangement of myofilaments that form myofibrils, cytoskeletal proteins, and interspersed organelles. B, Skeletal muscle, longitudinal section, mammalian skeletal muscle. Sarcomeres are defined by Z lines, thick myosin filaments form A bands, and thin actin filaments form I bands. Bisecting the A bands are dense M lines with adjacent clear H zones. Elongate mitochondria (Mt) and granular glycogen (G) are interspersed between the myofibrils. Transmission electron microscopy (TEM). Uranyl acetate and lead citrate stain. (B courtesy Dr. B.A. Valentine, College of Veterinary Medicine, Oregon State University.) CHAPTER 15 Skeletal Muscle 995 A A B Figure 15.5 Muscle Fiber Typing, Myofibrillar Adenosine Triphosphatase (ATPase) Reaction, Normal Skeletal Muscle, Transverse Section. A, Dog. Type 1 (light) and type 2 (dark) fibers are arranged in a mosaic pattern. Frozen section, ATPase pH 10.0. B, Horse. Acid preincubation allows differentiation of three fiber types: type 1 (dark), type 2A (light), and type 2B (intermediate = gray). Frozen section, ATPase 4.35. (Courtesy Dr. B.A. Valentine, College of Veterinary Medicine, Oregon State University.) B Figure 15.4 Neuromuscular Junctions. A, An intramuscular nerve (arrowhead) has given off axons, which terminate on a myofiber at a neuromuscular junction (arrow). Teased preparation, silver impregnation method. B, Neuromuscular junctions, transverse section through the center region of normal mammalian muscle. The neuromuscular junctions (red-brown stain) form a cluster. Nonspecific esterase stain, frozen section. (A courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee. B courtesy Dr. B.A. Valentine, College of Veterinary Medicine, Oregon State University.) Table 15.1 Skeletal Muscle Fiber Types Fiber Type Physiologic Characteristics Morphologic Characteristics 1 Slow twitch, oxidative, fatigue resistant, “red muscle,” aerobic Fast twitch, oxidative and glycolytic, fatigue resistant Fast twitch, fatigue sensitive, glycolytic, “white muscle,” anaerobic High mitochondrial content, high fat content, low glycogen content Intermediate mitochondria, fat, and glycogen content Low mitochondrial and fat content, high glycogen content 2A 2B mortis, illustrates the importance of ATP generation within skeletal muscle. The muscle contractile apparatus is still active immediately after death. ATP is necessary for the release of actin from myosin, the interaction that results in the sliding of myofilaments and contraction of muscle. After death, the absence of adequate ATP production causes the muscle fibers to undergo sustained contraction, which is known as rigor mortis. Rigor mortis eventually disappears because of muscle structural breakdown caused by autolysis or putrefaction (bacterial decomposition). The period of time for onset and release of rigor mortis varies, depending on physiologic (glycogen stores at the time of death) and environmental factors such as the environmental temperature (see Chapter 1, Mechanisms and Morphology of Cellular Injury, Adaptation, and Death). Skeletal muscle is also excitable tissue, similar to that of the nervous system. Maintenance of proper ionic gradients across the sarcolemma is essential for initiation of the action potential. Internal ionic gradients, especially of calcium ions, are critical for initiation and termination of contraction. Alterations of ionic fluxes across the sarcolemma, or within the sarcoplasmic reticulum, can have a serious negative impact on myofiber function. Examination of Muscle: Clinical, Gross, and Microscopic The decision to closely examine muscle, either by a biopsy or at necropsy, relies on recognition of indicators of neuromuscular dysfunction. A summary of clinical signs of muscle disease is provided in Box 15.1. Clinical Findings Information on the clinical signs of muscular disease is available at www.expertconsult.com. Clinicopathologic Findings Information on this topic is available at www.expertconsult.com. CHAPTER 15 Skeletal Muscle Clinical signs of muscular disease are variable (see Box 15.1). The most common manifestations are alteration in muscle size, muscle weakness, and abnormal gait. Depending on the nature of the disorder, clinical signs can be localized, multifocal, or generalized. Alteration in muscle size is readily detected with careful physical examination. Unilateral atrophy is best appreciated by comparing muscles on both sides of the body. In cases of generalized atrophy, it is important to bear in mind the normal muscling of the breed. For example, the muscling of dairy cattle is less prominent than that of beef cattle, and mild generalized muscle atrophy in a draft horse breed is more difficult to detect than in a light horse breed. Weakness can be obvious, as in an animal that is unable to rise or prefers to remain recumbent, or can be manifested primarily as exercise intolerance. Special attention should be paid to gait analysis. The gait of an animal with generalized weakness caused by muscle or peripheral nerve dysfunction will have a short stride and often be stiff, and all four legs are often positioned well under the body for support while standing. The abnormal gait of an animal with neuromuscular disease must be distinguished from a similar gait that can occur because of musculoskeletal disease (which is a misnomer because these disorders affect bone and joint, not muscle). Muscle or peripheral nerve dysfunction in the horse, with this species’ unique biomechanics of the pelvic limb, can result in mechanical lameness that can be mistaken for neurologic disease. Odd equine hind limb gaits designated with such terms as shivers, stringhalt, and fibrotic myopathy are caused by muscle or peripheral nerve disorders. A fibrotic myopathy-like condition also occurs less commonly in the dog and can involve the forelimb. Severe denervating or progressive myopathic conditions that begin in utero or at an early age can cause joint contractures and limb deviation (see Fig. 15.43). Animals with myotonia often exhibit a stiff gait and develop episodic muscle spasms that can lead to collapse. Percussion of muscle groups can cause a persistent muscle contraction known as dimpling. In dogs, horses, and ruminants, the esophagus contains a large percentage of skeletal muscle. In dogs and camelids, myopathic, neuropathic, and neuromuscular junction disorders can involve these muscles, causing esophageal dysfunction and megaesophagus. Denervation can also contribute to esophageal dysfunction in cattle with vagal indigestion. As far as can be determined by clinical evaluation and extrapolation from similar conditions in other species, most neuromuscular disorders in animals are not associated with pain. Muscle cramps, caused by either primary myopathy or partial denervation, and muscle swelling are exceptions to this rule. 995.e1 If the plasma membrane of the myofiber is damaged or a segment of the myofiber becomes necrotic, some of the contents of the muscle cell will “leak out” and be taken up into the blood. The concentrations of some of these components in serum are used as an index of the extent of myofiber damage. The most commonly used is creatine kinase (CK). Aspartate aminotransferase (AST) and lactic dehydrogenase (LDH) are also released but are not specific indicators of muscle damage because they are also present in other tissues. Because CK has a low renal threshold, it is quickly excreted in the urine. The half-life of circulating CK varies somewhat between species but is generally approximately 6 to 12 hours. The half-life of AST and LDH in the serum is much longer, and serum AST and LDH concentrations remain elevated for several days after muscle injury. Serum concentration of alanine aminotransferase (ALT) will also increase in all species from severe muscle cell necrosis. Other serum indicators of skeletal muscle injury include carbonic anhydrase III and fatty acid binding protein, but these latter proteins are not part of a routine serum chemistry panel. It has been speculated that the sarcolemma can become “leaky,” leading to release of CK and other enzymes, without the affected segment becoming overtly necrotic. This possibility is very difficult to prove or disprove. Although the laboratory testing for CK and AST is relatively standardized, laboratory normal ranges may vary considerably within and among laboratories. Determining the normal range of blood values for animals is a difficult task. Normal serum CK concentration in animals is generally less than 500 U/L. Normal serum concentrations of AST, ALT, and LDH vary greatly between species. Tests included in chemistry panels also vary in different laboratories. Some laboratories do not include CK in small animal chemistry panels, which can result in a misdiagnosis of hepatic disease in a dog or cat with a persistent increase in serum AST and ALT concentrations because of degenerative muscle disease. For the purposes of discussion in this chapter, a mild increase in CK or AST is considered to be up to 2 to 3 times normal, a moderate increase is 4 to 10 times normal, and a severe increase is 10 times normal or more. It should be emphasized that myofibers can be dysfunctional without undergoing necrosis. Myopathic and neuropathic conditions that cause atrophy, weakness, spasm, stiffness, or myotonia rarely result in significant increase in serum muscle enzyme concentrations. At this time, there is no biochemical parameter that will assess muscle fiber function; only morphologic or structural muscle fiber integrity can be assessed. 996 SECTION II Pathology of Organ Systems Upper humerus Mid-femur Cranial C SCR B BB TLA Lateral Medial RF VL VI TM TA SP DP PE AL BF TLO SM LD ST TF A Mid-radius and ulna Figure 15.6 Mitochondria, NADH Reaction (Blue Stained), Skeletal Myocytes, Normal Skeletal Muscle, Transverse Section. A, Horse. Type 1 fibers contain the most mitochondria, type 2B the least, and mitochondrial content of type 2A fibers is intermediate between type 1 and type 2B. Frozen section, nicotinamide adenine dinucleotide dehydrogenase (NADH) reaction. B, Dog. All fiber types have a similar mitochondrial content; therefore, this reaction cannot be used to identify different types of myofibers in canine muscle. Frozen section, NADH reaction. (Courtesy Dr. B.A. Valentine, College of Veterinary Medicine, Oregon State University.) Electromyography Information on this topic is available at www.expertconsult.com. Methods of Gross and Microscopic Examination of Muscle A variety of examination techniques are often necessary to best appreciate changes occurring in muscle. Gross Examination of Muscle Gross examination includes evaluation of changes in size (atrophied, hypertrophied, or normal), color, and texture. The gross pathologic appearance of skeletal muscle can be deceiving. What appear to be mild changes in muscle on gross examination often can be severe on microscopic examination, and what appear to be severe changes on gross examination can turn out to be artifact. Subjective evaluation of size can be highly unreliable unless control muscles (e.g., from normal animals or from the opposite sides) are available for weighing and measuring. Color changes are common. The intensity of the red color of muscle varies, depending on the type of muscle, the age and species of animal, and the extent of blood perfusion. Pale muscle can indicate necrosis (Fig. 15.9, A and B; see Figs. 15.25; 15.33, A; 15.35, A; and 15.39) or denervation (Fig. 15.9, C; see Fig. 15.36) but is also G Mid-tibia ER % Type I CD PT CT LDE 0-15 PL P 16-30 FR L UL DDF 31-45 DDF DDF SDF 46-60 BF 61-75 GL GM FU SF 76-90 SDF 91-100 Figure 15.7 Percentage of Type 1 and Type 2 Myofibers in Limb Muscles in the Dog. There is a wide variation from muscle to muscle. Deeply located muscles have the most type 1 myofibers, indicative of their function in maintaining posture. (Redrawn from Armstrong RB, Sauber CW, Seeherman HJ, Taylor CR: Distribution of fiber types in locomotory muscles of dogs. Am J Anat 163:87-98, 1987.) LAL B VM Caudal PQ Neuromuscular junctions Motor neurons Spinal cord Muscle fibers Motor unit 1 Motor unit 2 Axon Figure 15.8 Motor Units of a Muscle. Motor neuron cell bodies within the ventral horn of the spinal cord give rise to axons that often travel long distances (meters) and eventually branch to innervate multiple skeletal muscle fibers at neuromuscular junctions. Box 15.1  Clinical Signs of Muscle Disease Muscle atrophy Muscle hypertrophy Muscle swelling Weakness Muscle spasm Abnormal gait Esophageal dysfunction (dogs, cats, camelids) common in young animals and anemic animals. Pale streaking of muscle most often reflects myofiber necrosis and mineralization (see Fig. 15.9, A and B) or infiltration by collagen or fat (see Fig. 15.9, C and D), and it is one of the more reliable indicators of gross pathologic changes. Muscle parasites can be grossly visible as discrete, round to oval, pale and slightly firm zones (see Figs. 15.40 and 15.41, A). Dark red mottling of skeletal muscle can indicate congestion, hemorrhage, hemorrhagic necrosis (see Figs. 15.31, A, and Figs. 15.31, A, and 15.37), inflammation, or myoglobin staining after massive CHAPTER 15 Skeletal Muscle Electromyography (EMG) can be a valuable tool when evaluating patients with suspected neuromuscular disease. Concentric needle EMG studies search for abnormal spontaneous activity generated by myofibers. In contrast to other electrodiagnostic studies, a flat line generated by a noncontracting muscle indicates a healthy muscle. Abnormal spontaneous activity includes wave forms designated as positive sharp waves, fibrillations, and myotonic bursts. These abnormal spontaneous electrical events are associated with characteristic sounds emitted by the EMG machine. Abnormal spontaneous activity, typically dense and sustained fibrillations and sharp waves, is generated in denervated muscle because of alteration in sodium channel activity in the membrane of denervated fibers. Spontaneous activity in degenerative myopathies, usually scattered fibrillations, positive sharp waves, and myotonic bursts, is likely related to 996.e1 ionic disturbances associated with fiber degeneration and regeneration; functional denervation after segmental necrosis of the segment containing the neuromuscular junction is also possible. Myotonic conditions result in notably abnormal ionic fluxes leading to waxing and waning of spontaneous potentials with a characteristic “dive bomber” sound. Severe denervating and degenerative disorders and canine cushingoid myotonia can be accompanied by myotonic bursts that start and stop abruptly, characteristic of pseudomyotonia. Nerve conduction velocity studies evaluate the integrity and function of the peripheral nervous system. Primary demyelinating disorders result in severe reduction of nerve conduction velocity, but axons are intact, and muscles are still technically innervated; therefore, spontaneous activity does not occur. Repetitive nerve stimulation tests the function of the neuromuscular junctions. CHAPTER 15 Skeletal Muscle A B C D 997 Figure 15.9 Pathologic Changes Resulting in Pale Skeletal Muscle. A, Pale streaks, necrosis and mineralization, degenerative myopathy, canine X-linked muscular dystrophy, diaphragm (left side), dog. B, Localized pallor, necrosis, injection site of an irritant substance, semitendinosus muscle, cow. The irritant was injected just under the perimysium and caused necrosis and disruption of the myofibers. Some irritant seeped down between the fascicles to cause necrosis, but the fascicles of myofibers are still in place. C, Overall pale muscle with pale streaks from collagen and fat infiltration, denervation atrophy, equine motor neuron disease, horse. Equine motor neuron disease muscle (right) compared with normal muscle (left). D, Enlargement and pallor, steatosis, longissimus muscles, neonatal calf. The majority of the muscles have been replaced by fat. (A courtesy Dr. B.A. Valentine, College of Veterinary Medicine, Oregon State University. B and D courtesy Dr. M.D. McGavin, College of Veterinary Medicine, University of Tennessee. C courtesy Dr. A. de Lahunta, College of Veterinary Medicine, Cornell University.) muscle damage (see Fig. 15.35, A) or can simply reflect vascular stasis (hypostatic congestion) after death. Hemorrhagic streaks within the diaphragm often accompany death caused by acute exsanguination. A green discoloration can indicate either eosinophilic inflammation (Fig. 15.10) or severe putrefaction. Lipofuscin accumulation in old animals, especially cattle, can cause a tan-brown discoloration of muscle. Black discoloration of the fascia occurs in calves with melanosis as an incidental finding and in older gray horses with metastasis of dermal melanoma to muscle fascia. Evaluation of texture is also important. Severely thickened and often calcified fascia occurs in cats with fibrodysplasia ossificans progressiva. Fat infiltration or necrosis can result in abnormally soft muscle. Decreased or increased muscle tone can be caused by denervation. Decreased tone can also occur as a result of a lack of muscle conditioning or postmortem autolysis. Careful microscopic examination of multiple muscles is often required to detect lesions. In cases of suspected neuromuscular disease, multiple muscle samples should include active muscle (tongue, diaphragm, intercostal muscles, and masticatory muscles), proximal muscle (lateral triceps, biceps femoris, semimembranosus, semitendinosus, and gluteal), and distal muscle (extensor carpi radialis and cranial tibial). For purposes of a biopsy, certain muscles (e.g., lateral triceps, biceps femoris, cranial tibial, semimembranosus, and semitendinosus) are easier to sample because of their parallel myofiber orientation. The ideal samples will also vary, depending on the suspected disorder, such as a type 1 predominant postural muscle for diagnosis of equine motor neuron disease, a type 2 predominant locomotory muscle for diagnosis of equine polysaccharide storage myopathy and temporal or masseter muscle for diagnosis of masticatory myositis in dogs and masseter myopathy in horses. Short fibers, such as those in the intercostal muscle, are preferred for physiologic studies in which intact muscle fibers are necessary and for studies of neuromuscular junction zones. Sampling of Muscle for Examination Information on this topic is available at www.expertconsult.com. Microscopic Examination Information on this topic is available at www.expertconsult.com. Enzyme Histochemistry and Immunohistochemistry Information on this topic is available at www.expertconsult.com. Electron Microscopy Information on this topic is available at www.expertconsult.com. Other Methods of Evaluation Information on this topic is available at www.expertconsult.com. Dysfunction/Responses to Injury It is often said that the range of response of muscle to injury is limited, consisting primarily of necrosis and regeneration. Actually, muscle is a remarkably adaptive tissue, with a wide range of response