What is the result of the monosynaptic excitation step in the stretch reflex?

• A. Activation of the Alpha Motor Neuron in the Agonist
• B. Inhibition of the Antagonist
• C. Activation of the Golgi Tendon Organ
• D. Activation of the Interneurons to the Antagonist

Answer: (A)

The main idea is that the stretch reflex uses a direct, monosynaptic path from the muscle spindle to the muscle’s own motor neuron. When a muscle is stretched, the spindle’s Ia sensory fibers enter the spinal cord and connect directly onto the alpha motor neuron that innervates that same muscle (the agonist). This direct excitation causes the alpha motor neuron to fire and the muscle to contract, opposing the stretch. That rapid, monosynaptic connection is what makes the agonist activate immediately.

The other options refer to different parts of reflex circuitry. Inhibition of the antagonist occurs via a separate, indirect (disynaptic) pathway through Ia inhibitory interneurons, not the direct monosynaptic step. The Golgi tendon organ and activation of interneurons to the antagonist relate to other protective or reciprocal mechanisms, not the immediate monosynaptic excitation of the agonist.