tip links

Meaning: Tip links are tiny protein filaments that join the tip of each stereocilium, the hair-like projection on a hair cell, to the side of its taller neighbor. Sound makes the basilar membrane move, which bends the stereocilia. Bending toward the tallest stereocilia stretches the tip links and pulls open mechanically gated ion channels. K⁺ enters from the K⁺-rich endolymph and the cell depolarizes. That opens voltage-gated Ca²⁺ channels, so the cell releases more neurotransmitter onto the auditory nerve. Bending the other way slackens the links, closes channels, and hyperpolarizes the cell.

On the MCAT: Expect the hearing pathway: tympanic membrane, ossicles, oval window, cochlear fluid, basilar membrane, hair cells. Questions ask you to predict direction: bending toward the tall side means depolarization and more firing; bending the opposite way means less. Vestibular hair cells use the same mechanism for balance.

Common confusion: Tip links join stereocilia within one hair cell; they are not synapses or links between cells. The channels open by mechanical force, not by a ligand or by voltage; the voltage-gated step comes later. Hair cells release neurotransmitter, and the auditory nerve neurons fire the action potentials.