An upper motor neuron lesion typically leads to which cluster of muscle signs?

• A. Hypotonia and hyporeflexia
• B. Hypertonicity, hyperreflexia, and spasticity
• C. Muscle atrophy
• D. Fasciculations

Answer: (B)

An upper motor neuron lesion disrupts the brain’s ability to inhibit spinal reflex circuits. Without that inhibition, motor neurons become more excitable, leading to increased muscle tone (hypertonicity) and exaggerated reflex responses (hyperreflexia). The combination of these changes, along with a velocity-dependent increase in resistance to passive movement (spasticity), is the typical cluster seen with UMN injury. In contrast, signs like muscle atrophy and fasciculations are more characteristic of lower motor neuron damage, and hypotonia with hyporeflexia can occur with LMN injury or acute spinal shock. So the described signs—increased tone, brisk reflexes, and spasticity—best reflect an upper motor neuron lesion.