What is the second step of the stretch reflex?

What is the second step of the stretch reflex?

• A. Monosynaptic Excitation of Alpha Motor Neuron (Monosynaptic Synapse)
• B. Activation of Agonist Muscle
• C. Inhibitory Interneuron Inhibits Antagonist
• D. Ia Afferent Sends Collateral to Inhibitory Interneuron

Answer: (A)

When the muscle is stretched, the primary sensory afferent (Ia) from the muscle spindle is activated. The second step in this rapid arc is the direct, monosynaptic excitation of the alpha motor neuron of the same muscle by that Ia afferent. This single synapse causes the stretched muscle to contract. The other elements, like collateral signals from the Ia fiber to an inhibitory interneuron that reduces activity in the opposing muscle, occur as part of reciprocal actions but are not the immediate second step. Activation of the agonist muscle happens as a result of that direct alpha motor neuron input.

When the muscle is stretched, the primary sensory afferent (Ia) from the muscle spindle is activated. The second step in this rapid arc is the direct, monosynaptic excitation of the alpha motor neuron of the same muscle by that Ia afferent. This single synapse causes the stretched muscle to contract. The other elements, like collateral signals from the Ia fiber to an inhibitory interneuron that reduces activity in the opposing muscle, occur as part of reciprocal actions but are not the immediate second step. Activation of the agonist muscle happens as a result of that direct alpha motor neuron input.