How do stereocilia in hair cells respond to bending?

• A. They always close ion channels
• B. They are insensitive to bending
• C. They open ion channels when bent toward the shortest member
• D. They open ion channels when bent toward the tallest member and close when bent toward the shortest member

Answer: (D)

Mechanical gating of ion channels in hair cells is driven by tension in tip links that connect adjacent stereocilia. When the hair bundle is deflected toward the tallest stereocilium, those tip links tighten, pulling open the ion channels at the tips of the shorter stereocilia. That influx of ions (primarily K+ from the endolymph) depolarizes the cell and increases neurotransmitter release to the nerve. If the bundle is deflected toward the shortest stereocilium, the tip links slacken, the channels close, and the cell hyperpolarizes with reduced transmitter release. This directional opening and closing of channels in response to bending toward the tallest versus shortest members is what converts mechanical motion into an electrical signal.