Renshaw cells in the spinal cord provide recurrent collateral inhibition to α-motor neurons. What neurotransmitter do Renshaw cells use, and which receptor mediates their activation by motor neuron collaterals?

• A Renshaw cells release GABA to inhibit α-motor neurons; they are activated by muscarinic receptors
• B Renshaw cells release dopamine to modulate α-motor neurons; they are activated by NMDA receptors
• C Renshaw cells release serotonin to inhibit α-motor neurons; they receive input from descending corticospinal fibers
• D Renshaw cells release glycine to inhibit α-motor neurons; they are activated by nicotinic acetylcholine receptors on motor neuron recurrent collaterals ✓

Explanation

Renshaw cells are small interneurons located in the ventral horn that receive excitatory input from recurrent collaterals of α-motor neurons via nicotinic acetylcholine receptors (the same motor neuron that drives muscle also feeds back to Renshaw cells). Renshaw cells in turn release glycine (not GABA) to inhibit the originating α-motor neurons, creating a negative feedback loop that limits excessive firing and stabilizes motor output. Strychnine poisoning (glycine antagonist) abolishes this inhibition, causing tetanic muscle spasms. This circuit is distinct from Ia reciprocal inhibition.

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

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