In the semicircular canals, the cupula deflects in response to angular acceleration. Deflection of hair cells in the horizontal (lateral) canal towards the kinocilium causes:

• A Depolarisation of the hair cell, increased neurotransmitter release, and increased firing rate in the vestibular nerve (excitatory response) ✓
• B Hyperpolarisation of the hair cell via K+ channel closure, decreasing vestibular nerve firing
• C Depolarisation via Cl- channel opening as the stereocilia bend toward the kinocilium
• D Release of histamine from type II hair cells causing nausea and reflex eye movements

Explanation

Hair cell mechanotransduction in the semicircular canals works through tip-link gating: deflection of stereocilia toward the tall kinocilium increases tension on tip links, opens mechanosensitive cation channels (predominantly K+, since endolymph is rich in K+ with a high endocochlear/endolabyrinthine potential), causing K+ influx and membrane depolarisation. Depolarisation opens basolateral voltage-gated Ca2+ channels, triggering glutamate vesicle release and increased action potential firing in the inferior/superior vestibular nerve. Deflection away from the kinocilium closes tip-link channels, causes hyperpolarisation, and decreases vestibular nerve firing. The ampullofugal (utriculofugal) flow in the horizontal canal (head rotation toward the ipsilateral side) deflects the cupula and stereocilia toward the kinocilium on the same side. Option B describes movement away from the kinocilium. Option C is incorrect (K+, not Cl-). Option D is incorrect.

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

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