Muscle Atrophy

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 (e.g. fracture, failure to use limb, recumbency)
    • Slower than denervation atrophy
    • Reversible unless too prolonger or severe to cause loss of myofibres
  • Denervation
    • 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
  • Metabolisation of muscle protein 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