In the stretch reflex (myotatic reflex), Ia afferents from muscle spindles monosynaptically excite homonymous alpha motor neurons AND disynaptically inhibit antagonist motor neurons. The disynaptic inhibition is mediated by which interneuron?

• A Renshaw cell (recurrent inhibition)
• B Ib inhibitory interneuron (autogenic inhibition from Golgi tendon organ)
• C Commissural interneuron providing crossed inhibition
• D Ia inhibitory interneuron (disynaptic reciprocal inhibition) ✓

Explanation

The Ia inhibitory interneuron mediates reciprocal inhibition in the stretch reflex. When a Ia afferent fires (e.g., during patellar tap), it monosynaptically activates the quadriceps alpha motor neurons (agonist contraction) and simultaneously sends a collateral to the Ia inhibitory interneuron, which glycinergically inhibits hamstring (antagonist) alpha motor neurons. This prevents co-contraction of antagonists, producing coordinated joint movement. Renshaw cells mediate recurrent inhibition from motor neuron collaterals. Ib interneurons respond to Golgi tendon organ input (inhibiting agonists). This circuit is disrupted in spasticity and relevant to understanding the hyper-reflexia of upper motor neuron lesions.

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

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