Lower Motor Apparatus Practice Test

When a muscle is stretched, the stretch reflex kicks in to resist that stretch by activating the stretched muscle. The first part of the reflex is the muscle spindle detecting the stretch and sending increased Ia afferent activity into the spinal cord. In the spinal cord, these Ia fibers form a direct, monosynaptic connection with the alpha motor neurons that innervate the same (agonist) muscle. This direct excitation causes the agonist to contract, which is the third step in the reflex sequence—the actual muscle activation that creates movement or resistance to the stretch.

Alongside this, the Ia input also helps shut down the opposite muscle by recruiting an inhibitory interneuron to suppress the antagonist, allowing smooth, coordinated action. The direct connection to the antagonist would not be correct for the primary step in this sequence; the agonist gets the direct monosynaptic excitation, while inhibition of the antagonist happens through an interneuron pathway.