The stretch reflex arc (knee jerk) bypasses interneurons. Which unique anatomical arrangement within the ventral horn explains why the agonist contracts while the antagonist simultaneously relaxes?

• A Ia afferents monosynaptically inhibit both agonist and antagonist motor neurons with a net effect of agonist contraction
• B Ib afferents from Golgi tendon organs provide the monosynaptic excitation of agonist motor neurons
• C Ia afferents from the agonist spindle monosynaptically excite agonist α-motor neurons AND synapse on inhibitory Ia interneurons that inhibit antagonist α-motor neurons (reciprocal inhibition) ✓
• D γ-motor neurons directly activate agonist α-motor neurons via gap junctions in the ventral horn

Explanation

Ia afferents from muscle spindles (annulospiral endings) project monosynaptically to homonymous (agonist) α-motor neurons, causing contraction. Collateral branches of the same Ia afferents synapse on inhibitory interneurons (Ia inhibitory interneurons / Renshaw-independent inhibitory interneurons) that suppress antagonist α-motor neurons. This disynaptic reciprocal inhibition ensures coordinated movement without co-contraction. The Golgi tendon organ Ib afferents, by contrast, provide autogenic inhibition of the agonist via disynaptic pathways (Ib interneurons) and facilitate antagonists.

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

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