What are alpha motor neurons?

• A. Neurons which innervate skeletal muscles
• B. Neurons which innervate smooth muscles
• C. Neurons which sense muscle length
• D. Interneurons in the spinal cord

Answer: (A)

Alpha motor neurons are the somatic motor neurons that innervate skeletal muscles, specifically the extrafusal muscle fibers, and drive contraction by releasing acetylcholine at the neuromuscular junction. They reside in the ventral horn of the spinal cord (and in brainstem nuclei for cranial nerves) and are considered lower motor neurons because they directly activate muscle fibers to produce movement. This distinguishes them from neurons that target smooth muscle (autonomic motor neurons), from sensory neurons that detect muscle length (muscle spindle afferents), and from interneurons in the spinal cord that modulate circuits without directly causing muscle contraction. There are also gamma motor neurons that innervate intrafusal fibers within the muscle spindle, which work with alpha motor neurons to regulate muscle tone and reflex sensitivity.