Which of the following best describes the typical signs of upper motor neuron lesions compared with lower motor neuron lesions?

• A. UMN: spasticity, hyperreflexia, Babinski sign; LMN: flaccid weakness, atrophy, hyporeflexia
• B. UMN: atrophy and fasciculations; LMN: spasticity and hyperreflexia
• C. UMN: hyporeflexia and weakness; LMN: hyperreflexia and spasticity
• D. UMN: fasciculations and fibrillations; LMN: Babinski sign and rigidity

Answer: (A)

When upper motor neuron systems are damaged, the brain loses its ability to modulate reflexes, so muscles become tense and reflexes run high. You see spasticity, hyperreflexia, and a Babinski sign because the corticospinal tract can no longer exert its inhibitory influence on spinal reflexes. Lower motor neuron lesions affect the final common pathway to the muscle, leading to weakness with decreased tone (flaccidity), muscle wasting (atrophy), and reduced reflexes (hyporeflexia). The combination described—spasticity, hyperreflexia, Babinski—fits the upper motor neuron pattern, while weakness with flaccidity, muscle atrophy, and hyporeflexia fits the lower motor neuron pattern. The other options mix signs that belong to the opposite type (for example, fasciculations are LMN signs, while Babinski is an UMN sign, and spasticity is UMN rather than LMN).