What are the two main types of motor responses generated by NMES?

• A. A direct motor response through peripheral pathways (motor volley) and a reflex motor response involving spinal cord reflexes (sensory volley)
• B. A cortical motor response only
• C. A spinal tremor response
• D. A peripheral reflex only

Answer: (A)

The main idea is that NMES can generate two kinds of muscle contractions by different pathways. One is a direct motor response: the electrical pulse directly depolarizes motor neurons in the peripheral nerves, causing a straightforward, immediate contraction of the target muscle. This is the motor volley, a fast, direct activation at the stimulation site.

The other is a reflex motor response: the stimulus also activates sensory afferents that enter the spinal cord and trigger reflex circuits, producing a contraction mediated by spinal reflexes. This is the sensory volley, and it tends to have a longer latency than the direct response because it involves synaptic processing in the spinal cord.

Cortical motor activation isn’t the primary route here, since NMES primarily engages peripheral nerves and spinal circuits rather than generating a significant cortical motor command through the stimulation. A spinal tremor isn’t the typical mechanism produced by NMES, and a peripheral reflex only would miss the direct activation component. In practice, NMES can produce both direct and reflex contractions, depending on how and where the nerves are stimulated.