In the stretch reflex with reciprocal inhibition, which describes the second step?

• A. Ia Afferent Sends Collateral to Inhibitory Interneuron
• B. Alpha Motor Neuron Directly Activates Agonist
• C. Inhibitory Interneuron Inhibits Antagonist
• D. Golgi Tendon Activation Occurs

Answer: (A)

When a muscle is stretched, the muscle spindle detects the change and sends a signal via Ia afferents into the spinal cord. The immediate response is the monosynaptic activation of the stretched muscle’s alpha motor neurons, causing contraction. The second part of the reflex in reciprocal inhibition is that the Ia afferent also sends a collateral signal to an inhibitory interneuron, which then inhibits the motor neurons of the antagonist muscle. This inhibition of the antagonist allows the opposing muscle to relax, enabling the stretch reflex to proceed smoothly. The other options describe either the initial activation of the agonist or processes not involved in this particular reflex sequence.