Ia afferent fibers from muscle spindles make monosynaptic excitatory connections with alpha motor neurons of the same muscle. Simultaneously, they inhibit antagonist muscles through interneurons. What type of interneuron mediates this reciprocal inhibition?

• A Ia inhibitory interneurons (Renshaw-independent) that hyperpolarize antagonist alpha motor neurons ✓
• B Renshaw cells that receive recurrent collaterals from alpha motor neurons and inhibit antagonist motor neurons
• C Golgi tendon organ (Ib) interneurons that mediate autogenic inhibition of the antagonist
• D Gamma motor neurons that reduce spindle sensitivity of the antagonist muscle

Explanation

Ia afferents from muscle spindles project monosynaptically to homonymous alpha motor neurons (stretch reflex arc). Collaterals of the same Ia afferents also synapse on Ia inhibitory interneurons, which in turn project to antagonist motor neurons and inhibit them. This reciprocal inhibition (Sherrington's law of reciprocal innervation) ensures that when an agonist contracts reflexively, the antagonist relaxes. Renshaw cells mediate recurrent inhibition of the agonist's own motor neurons via recurrent alpha motor neuron collaterals — a different circuit.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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