The type II hair cells in the vestibular system differ from type I hair cells in their afferent innervation pattern. Type II hair cells have bouton (en passant) endings while type I are surrounded by calyx endings. The efferent innervation by olivovestibular fibers has what net effect on vestibular type I hair cell transmission?

• A Efferent fibers directly hyperpolarize type I hair cells via GABA-A receptors, reducing spontaneous firing during active movement
• B Efferent fibers release dopamine onto the calyx terminal to increase sensitivity during head rotation
• C Efferent fibers from the cerebellum directly stimulate type I hair cells via glutamate to match expected head movement with actual rotation
• D Efferent fibers terminate predominantly on the calyx ending (not the hair cell itself) and release ACh acting on α9/α10-nAChRs on the calyx, hyperpolarizing it and reducing afferent gain — a form of efferent-mediated gain control ✓

Explanation

The vestibular efferent system (olivovestibular bundle) releases ACh onto both type I and type II hair cells, but uniquely also onto the calyx ending surrounding type I hair cells. ACh acts on α9/α10 nicotinic acetylcholine receptors (nAChRs) on the calyx membrane; these have Ca²⁺ permeability and can activate KCa channels, hyperpolarizing the calyx and reducing the gain (sensitivity) of the afferent. This efferent modulation is thought to prevent overload of vestibular afferents during active self-generated head movements (efference copy principle), allowing the system to better detect unexpected external perturbations.

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

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