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===Response to injury===
*Limited array of ways in which to respond to injury
**[[Muscles - degenerative#Degeneration|Degeneration]]
**[[Muscles - degenerative#Necrosis|Necrosis]]
**[[Muscles - normal#Regeneration|Regeneration]]
**[[Muscles - degenerative#Atrophy|Atrophy]]
**[[Muscles - hyperplastic and neoplastic#Hypertrophy|Hypertrophy]]
*Large number of factors indicing the changes above, e.g.:
**Trauma
**Toxins
**Infectious agents
**Nutritional deficiencies
**Ichaemia
**[[Muscles - developmental|Hereditary diseases]]
*Specific diagnosis is often not possible based on morphological or histological features alone
*Additional tests, clinical information and history are often required
===Regeneration===
[[Image:Muscle regeneration.jpg|right|thumb|100px|<small><center>Muscle regeneration (Image sourced from Bristol Biomed Image Archive with permission)</center></small>]]
*Skeletal muscle myofibres have substantial regenerative ability
*Success depends on:
**An intact '''sarcolemmal tube''' - to act as a support and guide
**Availability of '''satellite cells''' - to act as progenitor cells for new sarcoplasm production
**Macrophages to clear up cell debris
**If these conditions are not met (e.g. severe thermal damage) '''fibrosis''' will occur
*Stages:
#Nuclei in [[Muscles - degenerative#Necrosis|necrotic segement]] disappear, hyalinased sarcoplasm due to loss of normal myofibrillar structure, may separate from adjacent normal myofibrils and/or [[Muscles - degenerative#Calcification|mineralise]]
#Monocytes from capillaries -> macrophages in necrotic portion, satellite cells swell -> vesicular with prominent nucleoli -> mitosis (within 1-4 days after initial injury)
#Satellite cells move to centre
#Macrophages clear the sacrolemmal tube, plasmalemma disappears, shape maintained by basal lamina
#Satellite cells -> myoblasts (contain myosin) -> fuse forming myotubes with row of central nuclei; cytoplasmic processes fusing
#Growing and differentiating fibre, striations appear - formation of sarcomeres
#Nuclei move to peripheral position (2-3 weeks after initial injury)
*Regeneration by '''budding'''
**When conditions are not optimal, disrupted sacrolemma
**E.g. injection of irritating substance, trauma, [[Muscles - degenerative#Ischaemia|infarction]]
**Myoblasts proliferate -> sacrolamma bulges from cut part -> club-shaped with numerous central nuclei = muscle giant cells
*Monophasic lesions - all at same phase above
===Rigor Mortis===
*Muscles remain biochemically active after the death of an animal
*Following a period of relaxation, contraction and stiffening occurs
*Due to deficiency of ATP releasing myosin heads from their binding sites at end of power stroke
*Onset faster in ATP deprived animals (starvation, hunting, tetanus...)
*May be absent in cachetic animals
*Disappears due to autolysis or putrefaction
*See [[General Pathology - Post-Mortem Change#Rigor Mortis|general pathology]]
**Damage occured at one time, e.g. trauma or one toxin exposure
*Multiphasic lesions - different stages as described above
**Ongoing damage, e.g. vitamin E - selenium deficiency, continuous exposure to toxin