What role do Ia afferents play in the stretch reflex?

• A. They synapse on the Alpha Motor Neuron of the Agonist and Send Collateral to an Inhibitory Interneuron
• B. They Directly Contract the Antagonist
• C. They Inhibit the Agonist Motor Neuron
• D. They Become Motor Neurons Themselves

Answer: (A)

A stretch reflex is driven by the spindle’s Ia sensory fibers, which carry the signal of muscle stretch into the spinal cord. They make a direct, monosynaptic connection with the alpha motor neuron that innervates the same (agonist) muscle, producing a quick contraction to resist the stretch. At the same time, they send a collateral to an inhibitory interneuron that dampens the motor neurons to the antagonist muscle, creating reciprocal inhibition so the opposite muscle relaxes as the stretched muscle shortens. This combination—directly exciting the agonist motor neuron and inhibiting the antagonist motor neurons—explains why Ia afferents both trigger the contraction of the stretched muscle and help suppress the opposing one. The other options don’t fit because Ia afferents don’t directly cause antagonist contraction, they don’t inhibit the agonist motor neuron, and they don’t become motor neurons themselves.