NBME Form 5 Practice Test

Lower motor neuron signs arise from degeneration of motor neurons that directly innervate muscle fibers. One classic manifestation is fasciculations—visible twitches caused by spontaneous discharges of denervated motor units. In ALS, you can see both UMN and LMN signs, but fasciculations specifically point to LMN involvement.

In contrast, signs like hyperreflexia, spasticity, and Babinski reflect upper motor neuron damage—loss of inhibitory control from the brain over the spinal cord—leading to increased reflexes, increased tone, and a Babinski response. So the presence of fasciculations best indicates lower motor neuron involvement.