Which neurons innervate extrafusal fibers?

• A. Alpha Motor Neurons
• B. Gamma Motor Neurons
• C. Interneurons
• D. Sensory Neurons

Answer: (A)

Alpha motor neurons are the neurons that innervate extrafusal muscle fibers. These are the main contractile fibers of a skeletal muscle, and when an alpha motor neuron fires, its axon reaches the neuromuscular junction on an extrafusal fiber and releases acetylcholine to trigger contraction. Gamma motor neurons, by contrast, innervate intrafusal fibers inside the muscle spool (the spindle) to adjust its sensitivity to stretch, not to cause contraction of the extrafusal fibers. Interneurons modulate motor output within the spinal cord but do not directly innervate extrafusal fibers, and sensory neurons provide afferent feedback from muscle spindles and Golgi tendon organs rather than causing contraction. So the neuron that directly drives extrafusal fiber contraction is the alpha motor neuron.