What is the first step of the stretch reflex?

• A. Inhibition of alpha motor neurons
• B. Rapid stretch of muscle spindle (1a afferent)
• C. Activation of gamma motor neurons
• D. Slowing of muscle contraction

Answer: (B)

The initial trigger of the stretch reflex is the rapid stretch of the muscle spindle. When the muscle is quickly stretched, the muscle spindle (a proprioceptive receptor inside the muscle) is deformed, activating its Type Ia afferent fibers. These Ia signals quickly enter the spinal cord and directly excite the alpha motor neurons that innervate the same muscle, causing a rapid contraction to resist the stretch. This reflex arc may also involve interneurons that inhibit the antagonist muscle to enhance the response. The gamma motor system, which adjusts spindle sensitivity, helps keep the spindle responsive during movement, but it isn’t the first step in the reflex.