A full reaction of degeneration indicates which of the following conditions?

• A. Lower motor neuron lesion
• B. Upper motor neuron lesion
• C. Recent CVA
• D. Long-standing peripheral nerve injury

Answer: (A)

A full reaction of degeneration reflects complete denervation of a motor unit. When a lower motor neuron or peripheral nerve is damaged and no adequate reinnervation occurs, all the muscle fibers within that motor unit undergo degeneration, producing a full reaction. In contrast, upper motor neuron lesions do not denervate the muscle—the nerve supply to the muscle remains intact—so you don’t see a full reaction. A recent cerebrovascular accident is an acute upper motor neuron issue, so the same applies. In long-standing peripheral nerve injury, nearby healthy axons often sprout to reinnervate fibers, leading to partial rather than complete degeneration.