During a stretch reflex, muscle spindle Ia afferents synapse monosynaptically on homonymous motor neurons and simultaneously activate Ia inhibitory interneurons that inhibit antagonist motor neurons. This simultaneous facilitation of agonist and inhibition of antagonist is termed reciprocal inhibition. Which is the inhibitory neurotransmitter released by the Ia inhibitory interneuron onto the antagonist motor neuron?

• A GABA — all spinal cord interneurons use GABAergic inhibition via GABAA receptors
• B Dopamine — spinal dopaminergic interneurons modulate the stretch reflex arc
• C Glycine — Ia inhibitory interneurons are glycinergic; the inhibitory postsynaptic potential they generate is mediated by glycine receptor (GlyR)-associated Cl⁻ channels that hyperpolarise the antagonist motor neuron ✓
• D Nitric oxide — NO diffuses from Ia afferents to directly hyperpolarise antagonist motor neurons

Explanation

Ia inhibitory interneurons in the spinal cord ventral horn use glycine as their inhibitory neurotransmitter. Glycine activates strychnine-sensitive GlyR chloride channels on antagonist alpha motor neurons, hyperpolarising them and preventing their firing during agonist contraction. This is the mechanism of reciprocal inhibition (Sherrington's law of reciprocal innervation). Strychnine poisoning blocks GlyR, abolishing this inhibition and causing uncontrolled simultaneous contraction of agonists and antagonists — tetanic convulsions. GABA (both GABAA and GABAB) is used by other inhibitory spinal interneurons (e.g., Renshaw cells use glycine and GABA), but the specific Ia inhibitory neurotransmitter is glycine.

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

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