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Why is muscle atrophy from a nerve injury usually more severe than muscle atrophy from a cast?

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atrophy One example of atrophy is the progressive loss of bone that occurs in osteoporosis (normal bone shown on left; osteoporotic bone shown on right).

While both nerve injury and immobilization (such as using a cast) result in muscular atrophy, the severity differs due to the underlying mechanism of the tissue's inactivity.1

When a limb is placed in a cast, the muscle undergoes disuse atrophy because it is not being actively engaged, though the nerve supply remains intact. In contrast, a nerve injury involves the loss of the motor nerve supply to the muscle. When the nerve cells in the spinal cord that normally activate the muscle are destroyed, the muscle becomes limp and paralyzed.1

This loss of neural input leads to extreme inactivity, causing the muscle fibers to shrink rapidly. Over several months, this process progresses to the fragmentation and disappearance of muscle fibers, which are then replaced by fat cells and connective tissue. Because nerve injury removes the essential signals required for muscle maintenance and function, the resulting atrophy is generally more severe than that caused by simple immobilization.1