Final answer:
In the stretch reflex arc, reciprocal inhibition causes the antagonist muscle to relax while the agonist muscle contracts, with inhibitory synapses occurring in the grey matter of the spinal cord.
Step-by-step explanation:
In a stretch reflex arc, when the agonist muscle contracts, reciprocal inhibition occurs in its antagonist muscle. This means that as a muscle, like the biceps brachii, contracts, its antagonist muscle, such as the triceps brachii, relaxes to allow for a smooth movement. This coordination is managed by the nervous system, where synapses, whether excitatory or inhibitory, occur in the grey matter of the spinal cord. Specifically, the dorsal horn of the spinal cord contains interneurons that facilitate inhibitory synapses, leading to the relaxation of the antagonist muscles during a reflex.