Which statement correctly contrasts UMN and LMN lesions according to the material?

• A. UMN lesions cause paresis or weakness; LMN lesions cause no motor innervation
• B. UMN lesions cause paralysis; LMN lesions cause sensory loss
• C. UMN lesions cause spasticity; LMN lesions cause tremor
• D. UMN lesions cause numbness; LMN lesions cause reflex changes

Answer: (A)

When the issue is where the motor signal is coming from, the pattern of weakness changes. An upper motor neuron (UMN) lesion disrupts the commands sent from the brain to the spinal cord but leaves the final motor pathway to muscles intact. That mainly shows up as paresis, a partial weakness, because the muscles can still be activated but not with normal control. Over time, you also see signs like spasticity and hyperreflexia due to loss of inhibitory input from higher centers. A lower motor neuron (LMN) lesion, by contrast, damages the neurons or nerves that actually innervate the muscle, so the muscle loses its motor innervation. That leads to weakness or paralysis of the affected muscles, often with rapid muscle atrophy and fasciculations, and reflexes are reduced or absent. So the statement contrasts UMN weakness with LMN loss of motor innervation, which cleanly captures the fundamental difference in how motor output is disrupted in each type of lesion.