External and internal influences on muscle pathology.

Three cases of different types of neuromuscular diseases demonstrate different muscle responses to external stress or intrinsic muscle abnormalities. The first muscle biopsy shows stenosis of its vessels causing acute muscle ischemia, stress from an external vascular disease. The muscle response is similar to the cellular necrosis seen in primary muscle diseases (myopathies), but the histologic pattern is more focal than most myopathies. The second muscle biopsy demonstrates the effects of external motor nerve injury or disease causing groups of muscle fibers to atrophy. If a nerve reinnervates the muscle, it changes the fiber types in distinct patterns. The third muscle biopsy shows an intrinsic muscle abnormality causes chronic failure of the muscle fibers to thrive and repeated attempts by the fibers to regenerate, stimulating other tissue repair processes, like fibrosis, to change the muscle. Depending on the etiologic factor, muscle will respond to internal and external influences in different manners.