Muscle Atrophy

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Atrophic muscle fibres (Image sourced from Bristol Biomed Image Archive with permission)
  • Decreased myofibre or whole muscle diameter
  • Myofibrils removed by disintegration -> sacrolemma too large -> forms folds
  • Caused by:
    • Disuse atrophy (e.g. fracture, failure to use limb, recumbency)
      • Slower than denervation atrophy
      • Reversible unless too prolonger or severe to cause loss of myofibres
    • Pressure atrophy
      • Any prolonged pressure on muscles resulting in muscle atrophy
        • Abscesses, neoplasms, parasitic cysts
    • Denervation atrophy
      • Any interference or damage to its nerve supply results in muscle atrophy
        • Can be rapid - over 50% of muscle mass may be lost in a few weeks e.g. roarer horses with laryngeal hemiplegia
      • May be reversible if innervation re-established
      • Histologically:
        • Fibres become rounded in cross section unless compressed by normal fibres
        • Increased concentration of nuclei as they take much longer to disintegrate
        • Fibrous stroma of epimysium and endomysium condenses -> more prominent
        • End result in muscle consisting of almost only fibrous tissue
      • Sometimes replaced by fat tissue -> increased size of muscle = pseudohypertrophy
      • Muscle may have a mixture of atrophied and hypertrophied (due to increased work load) fibres if some motor units are not damaged
    • Nutritional atrophy for nutrients during:
      • Malnutrition, cachexia, senility
      • Gradual onset except for some febrile diseases causing cachexia
      • Postural muscles are not affected, sometimes even hypertrophy
      • Histologically:
        • Some nuclei disappear as myofibre volume is decreased
      • Grossly:
        • Smaller, darker, thinner muscles
      • Senile atrophy
        • Similar to nutritional atrophy.
        • Lipofuscin pigmentation is common
          • Grossly:
            • Yellow-brown / dark brown colour (esp in diaphragm)