What is the role of gamma motor neurons in NMES?

• A. Gamma motor neurons directly drive muscle contraction.
• B. NMES primarily activates alpha motor neurons to evoke muscle contraction; gamma motor neurons adjust spindle sensitivity but are not directly activated to drive contraction.
• C. Gamma neurons control autonomic responses.
• D. Gamma neurons are not involved in NMES at all.

Answer: (B)

NMES elicits muscle contraction primarily by depolarizing alpha motor neurons, which send signals to extrafusal muscle fibers to produce force. Gamma motor neurons, on the other hand, innervate the intrafusal fibers of the muscle spindle and adjust the spindle’s sensitivity to stretch. Their role is to modulate proprioceptive feedback and reflex gain, not to directly generate the contraction. Because NMES targets the pathways that produce force rather than the spindle-sensitivity loop, gamma motor neurons are not the direct drivers of NMES-induced contraction. They’re not autonomic controllers, and while they’re part of the muscle’s overall control system, the immediate contraction comes from activating alpha motor neurons.