Muscles - Anatomy & Physiology

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Introduction

  • Skeletal muscle includes muscles of:
    • Posture
    • Movement
    • Respiration


  • Two basic types of skeletal myofibre:
    • Type I
      • Grossly red
      • High myoglobin level
      • Slow rate of contraction
      • High oxidative activity
      • Function - postural
    • Type II
      • Grossly white
      • Low myoglobin level
      • Fast rate of contraction
      • High glycolytic activity
      • Function - exercise


  • Each muscle is composed of multiple fascicles
    • Each fascicle is composed of multiple polygonal myofibres



Regeneration

Muscle regeneration (Image sourced from Bristol Biomed Image Archive with permission)
  • 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:
  1. Nuclei in necrotic segement disappear, hyalinased sarcoplasm due to loss of normal myofibrillar structure, may separate from adjacent normal myofibrils and/or mineralise
  2. Monocytes from capillaries -> macrophages in necrotic portion, satellite cells swell -> vesicular with prominent nucleoli -> mitosis (within 1-4 days after initial injury)
  3. Satellite cells move to centre
  4. Macrophages clear the sacrolemmal tube, plasmalemma disappears, shape maintained by basal lamina
  5. Satellite cells -> myoblasts (contain myosin) -> fuse forming myotubes with row of central nuclei; cytoplasmic processes fusing
  6. Growing and differentiating fibre, striations appear - formation of sarcomeres
  7. 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, infarction
    • Myoblasts proliferate -> sacrolamma bulges from cut part -> club-shaped with numerous central nuclei = muscle giant cells
  • Monophasic lesions - all at same phase above
    • 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


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


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