What are gamma motoneurons?

• A. The sensory neurons that detect stretch
• B. The interneurons in spinal cord interneural loops
• C. The motor neurons innervating muscle spindles
• D. The autonomic neurons controlling blood flow

Answer: (C)

Gamma motoneurons are the motor neurons that innervate the intrafusal fibers inside a muscle spindle, adjusting its sensitivity to stretch. While alpha motor neurons drive the main muscle fibers (extrafusal fibers) to contract, gamma motor neurons tighten the spindle by activating the polar regions of the intrafusal fibers. This keeps the sensory endings (Ia and II afferents) responsive to length changes even as the muscle shortens or lengthens, so reflexes and motor control remain accurate. There are static and dynamic gamma neurons, contributing to sensing overall muscle length and the rate of stretch. This coordination with alpha motor neurons—often described as the gamma loop—ensures continuous, appropriate sensory feedback during movement. The other options describe the sensory detectors of stretch, spinal interneurons, or autonomic control, not gamma motor neurons.