The Ia afferent fiber from the muscle spindle synapses on which type of neuron to produce the stretch reflex?

• A Gamma motor neurons exclusively, not alpha motor neurons
• B Alpha motor neurons of antagonist muscles to facilitate antagonist contraction
• C Alpha motor neurons of the same muscle (monosynaptic reflex) and inhibitory interneurons (Ia inhibitory interneurons) to antagonist muscles ✓
• D Renshaw cells only, producing autoinhibition of the same motor neuron pool

Explanation

The stretch reflex (myotatic reflex) is monosynaptic: Ia afferents from the nuclear bag and chain fibers of the muscle spindle project directly onto alpha motor neurons of the same (agonist) muscle, causing its contraction. Simultaneously, Ia afferents activate Ia inhibitory interneurons that synapse on alpha motor neurons of antagonist muscles, producing reciprocal inhibition. This is the basis for the knee jerk (L3-L4) and all deep tendon reflexes used clinically. Gamma motor neurons control spindle sensitivity but are not part of the monosynaptic arc.

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

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