Which sign is typical of a lower motor neuron lesion?

• A. Weakness
• B. Marked atrophy
• C. Fasciculations
• D. Reduced tone

Answer: (C)

Fasciculations reflect spontaneous firing of motor units after the nerve supplying a muscle is damaged. When the alpha motor neuron is lost or injured, the muscle fibers it used to control become denervated and electrically unstable, leading to small, visible twitches of the muscle. This spontaneous activity is a hallmark of lower motor neuron involvement.

Weakness, reduced reflexes, and decreased tone can also occur with lower motor neuron lesions, but they’re less specific and can appear in other conditions as well. The presence of ongoing fasciculations helps distinguish a lower motor neuron problem from upper motor neuron issues, which typically produce spasticity and hyperreflexia rather than these spontaneous twitches.