Describe the mechanism by which hair cell stereocilia transduce mechanical motion into neural signals in the vestibular system.

• A. Deflection away from kinocilium opens channels and decreases firing; toward kinocilium increases firing.
• B. Motion directly depolarizes the nerve without hair cell involvement.
• C. Deflection toward the kinocilium opens mechanotransduction channels, increasing neurotransmitter release and afferent firing; opposite deflection reduces firing.
• D. Deflection toward the kinocilium inhibits neurotransmitter release; away from kinocilium excites firing.

Answer: (C)

The key idea is how bending the hair-cell bundle translates into a change in transmitter release onto the vestibular nerve. Each hair cell has a group of stereocilia with a single kinocilium. When the bundle is deflected toward the kinocilium, tension on the tip links between stereocilia increases, opening mechanotransduction channels at the tips. These channels let positively charged ions enter from the surrounding endolymph (which is rich in potassium), depolarizing the hair cell. That depolarization opens voltage-gated calcium channels, triggering the release of neurotransmitter onto the afferent fiber and increasing its firing rate. If the bundle is deflected away from the kinocilium, the tip-link tension decreases, the channels close, the cell hyperpolarizes, transmitter release falls, and the afferent firing rate decreases. This directional response allows the vestibular system to encode head motion and orientation.