Response of muscle to injury

What is the delicate supporting tissue between each muscle fiber? Fine connective tissue stroma.

endomysium

What is the loose collagenous tissue around each fascicle? Organizes myofibers into fascicles.

perimysium

What is the dense collagenous tissue around the entire muscle?

epimysium

What is the term for muscle fibers group into bundles?

fasciculi

What is the smallest functional unit of striated muscle?

sarcomere

What causes the repeated pattern of cross striations on each myofibril?

highly ordered arrangement of contractile proteins within each myofibril

What is muscle fiber? (myofiber)

multi-nucleated cells form a nuclear domain; terminally differentiated with little or no capacity for mitosis

What are satellite cells? (resting myoblasts)

between plasma membrane (sarcolemma) and basal lamina; fully capable of dividing, fusing, and reforming mature myocyte when basal lamina is intact

**What two things are needed for muscle regeneration?

presence of an intact basal lamina and availability of viable satellite cells

What does a longitudinal section of muscle histology look like?

elongated, unbranched cylindrical cells with flattened nuclei just beneath the sarcolemma (plasma membrane)

What does a transverse section of muscle histology look like?

shows muscle fibers as polyhedral cells with flattening of adjacent cells

What is a type 1 fiber?

slow twitch, oxidative, fatigue resistant, aerobic, high mitochondrial and fat content, and low glycogen content

What is a type 2b fiber?

fast twitch, glycolytic, fatigue sensitive, anaerobic, low mitochondrial and fat content, and high glycogen content

What is a type 2a fiber?

intermediate; fast twitch, fatigue resistant, oxidative and glycolytic, with intermediate mitochondria, fat, and glycogen content

What describes the skeletal muscle blood supply?

large blood vessels enter epimysium and divide throughout muscle in perimysium - fine branches arise so that each fiber has a surrounding capillary network

What is muscle contraction controlled by?

motor unit and includes the motor neuron consisting of cell body and axon in a peripheral nerve; the neuromuscular junction; and myofibers innervated by motoneuron

Any one motor nerve supplies fiber of ____ type(s).

one

What are the responses of muscle injury?

degeneration, necrosis, regeneration, atrophy, hypertrophy

What is degeneration?

muscle fiber swells and looses striations, may see vacuoles - if not reverses progresses to necrosis

What is necrosis?

fiber fragments and becomes very eosinophilic invaded by macrophages (lyse and phagocytose necrotic debris and form clear space in sarcolemmal tube)

Because of their multinucleate nature, what do myofibers usually undergo?

segmental necrosis, with involvement of only one or several continuous segments within the cell

What are nuclear domains?

segments of the cell react independently of other portions of the cell

How long does regeneration take in healthy animals?

greater or equal to 3 weeks; believed to recapitulate the embryologic development of skeletal muscle

What are the steps of muscle fiber regeneration?

segmental coagulation necrosis is followed by fragmentation of necrotic segment after becoming floccular (hyalinized). Satellite cells enlarge and move towards center along with invading macrophages. Satellite cells differentiate into myoblasts and form a myotube to produce sarcoplasm. The regenerating myofiber has a reduction in myofiber diameter with central rowing nuclei until it continues to grow and differentiate increases in diameter. It takes several days to weeks for nuclei to move to periphery.

What maintains the integrity of the myofiber during regeneration?

sarcolemmal tube formed by basal lamina and endomysium

What is successful regeneration?

satellite cells move into necrotic tissue and become myoblasts - form a tube (myotube) that produces sarcoplasm, rows of central nuclei appear, sarcomeres formed (striations)

What is ineffectual regeneration?

if muscle insult damages basal lamina of myofiber, regeneration is by budding; myoblasts proliferate and extend to end of ruptured tube, sarcoplasm bulges from cut end, formation of multinucleated muscle giant cells; large gap between ruptured myotubes heals by fibrosis

What are examples of possible successful regeneration?

metabolic, nutritional, toxic myopathies; myofiber degeneration with basal lamina preserved and sufficient satellite cells

What are examples of possible ineffectual regeneration?

infarction, trauma, intense inflammation, heat, infiltrative neoplasm; destroyed basal lamina and necrosis of satellite cells causes muscle giant cells and healing by fibrosis (impairs muscle function)

What are the 5 categories of degenerative myopathies?

ischemic, nutritional, toxic, exertional, traumatic

What is monophasic?

a single insult

What is polyphasic?

ongoing degenerative process

What is focal monophasic?

single traumatic incident like IM injection

What is multifocal monophasic?

exertional myopathy, toxin (one occasion)

What is multifocal polyphasic?

muscular dystrophy, selenium deficiency myopathy

Nutritional, toxic, and exertional damages are known to repair well. What damages repair poorly? Especially if greater than 6 hours?

ischemic damages

What are the main causes of muscle ischemia?

occlusion of a major blood vessel, external pressure on a muscle, swelling of a muscle in a non-expandable compartment, vasculitis/vasculopathy

What is ischemia myopathy due to occlusion of major blood vessel?

vascular occlusive syndromes such as thromboembolism of terminal aorta or iliac arteries in cat with cardiomyopathy

What is ischemia myopathy due to external pressure on a muscle?

downer cow syndrome, tight-fitting plaster casts and bandages, prolonged anesthesia (post anesthetic myopathy in horses)

What is downer cow syndrome?

external compression of relaxed muscles can cause ischemia necrosis in following circumstances

What is ischemia myopathy due to swelling of a muscle in a non-expandable compartment?

compartment syndrome; muscles surrounded by tight fascia or by phone are vulnerable to ischemia when they contract vigorously (increased intramusclar pressure is created if muscle cannot expand → collapse of veins and arteries → ischemia)

Muscle in full contraction may increase up to ___% in volume

20%

What are examples of compartment syndrome?

vigorous wing flapping causes infarction of supracoracoid muscles in boiler chickens and turkeys, anterior tibial syndrome

What is ischemia myopathy due to vasculalitis?

multifocal muscle infarction in sheep with virulent bluetongue obivirus infection or in equine purpura hemorrhagica with strep equi subsp equi Ag/Ab complex

What is nutritional myopathy?

white muscle disease caused by deficiency of vitamin E and/or selenium due to free radicals damaging cell membranes and mitochondria

What type of fibers are primarily affected in nutritional myopathy?

type 1 oxidative fibers; pale, flabby, and wet skeletal muscle

What are the common associated toxins in toxic myopathies?

gossypol (cotton seeds), Cassia spp (coffee senna), monensin (ionophore antimicrobials)-horses most sensitive, iron

What is exertional myopathy?

myofiber damage occuring due to exercise stress as the primary cause results in coagulative necrosis and accumulation of interstitial fluids (edema)

What are examples of exertional myopathies?

equine exertional rhabdomyolysis, capture myopathy; can progress to secondary renal failure is severe

Why are traumatic lesions known to repair by fibrosis with reduced function in muscle?

often associated with extensive disruption of basal laminae; hit by car, traumatic diaphragmatic hernia

What is muscle atrophy?

a reduction in the volume of the muscle as a whole or in the diameter of the myofiber

What is physiologic muscle atrophy?

type II fibers affected because of disuse, cachexia, and aging

What is muscle atrophy secondary to endocrine disease?

type II fibers affected because of hyperthyroidism or hypercortisolism

What is denervation atrophy?

type I and II fibers affected; maintenance of normal myofibers diameter is dependent of trophic factors generated by an intact associated nerve

What is the histological pattern of denervation atrophy?

a mixture of very small angular and very large fibers; nerve damage

What can follow denervation?

renervation?

What is the hallmark of denervation followed by reinnervation?

return to normal myofiber diameter (fiber type grouping)

What is fiber type grouping?

since the motor neuron determines the myofiber type, the newly innervated myofiber takes on the fiber determined by the innervating neuron

What are examples of denervation muscle atrophy?

polyneuritis equi, equine protozoal myeloencephalitis, laryngeal hemiplegia, sweeney

What is muscle hypertrophy?

increased myofiber diameter caused by increased workload,

What is work hypertrophy?

due to increase in normal physiologic work

What is compensatory hypertrophy?

increased workload on individual fibers caused by loss of myofibers in an adjacent muscle due to physical injury or partial denervation atrophy, myopathy, etc