The vestibular hair cells are equipped with which structures?

• A. Stereocilia and kinocilia
• B. Stereocilia only
• C. Kinocilia only
• D. Cilia and flagella

Answer: (C)

Vestibular hair cells detect movement with a hair bundle made of multiple stereocilia arranged in a graduated staircase, plus a single kinocilium on the tallest edge. The stereocilia are actin-based projections, while the kinocilium is a true cilium. When the head moves, the bundle deflects toward the kinocilium, which pulls on tip links between adjacent stereocilia and opens mechanosensitive channels. The resulting influx of ions depolarizes the cell and increases neurotransmitter release to the afferent nerve, signaling motion. Deflection away from the kinocilium reduces channel opening and hyperpolarizes the cell. This combined presence of stereocilia and a kinocilium—not just kinocilia, nor cilia/flagella—gives vestibular hair cells their directional sensitivity.