A hallmark of lower motor neuron involvement is which combination of findings?

• A. Hyperreflexia and spasticity
• B. Hypotonia, hyporeflexia, and muscle atrophy
• C. Tremor at rest
• D. Hyperactive babinski

Answer: (B)

Loss of input from motor neurons to the muscle produces a flaccid, weak muscle with reduced tone and reflexes, and often muscle wasting. This combination—low muscle tone (hypotonia), diminished reflexes (hyporeflexia), and muscle atrophy—is characteristic of lower motor neuron involvement because the final common pathway from the spinal cord or brainstem to the muscle is damaged or interrupted.

In contrast, signs of upper motor neuron involvement, such as increased tone with spasticity, hyperreflexia, and a positive Babinski sign, arise when the brain or its descending pathways fail to regulate spinal reflexes. Tremor at rest points to basal ganglia dysfunction, not LMN disease. A hyperactive Babinski is also an upper motor neuron sign.