The vestibular hair cell bundles consist of which structures?

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

Answer: (C)

Hair cell bundles in the vestibular system are made up of many stereocilia arranged in a staircase pattern, plus a single kinocilium on the apical surface. Stereocilia are actin-based projections, while the kinocilium is a true cilium that helps define the bundle’s polarity. When the bundle is deflected toward the kinocilium, tension on the tip links between stereocilia opens mechanosensitive channels, leading to depolarization and neurotransmitter release; deflection away reduces tension and causes hyperpolarization. This combination of stereocilia and kinocilium is what enables the vestibular hair cells to detect direction and strength of motion. Therefore, the correct description is that the vestibular hair cell bundle consists of stereocilia and a kinocilium.