Final answer:
Hair cells with hair-like stereocilia are essential for vertebrate hearing and equilibrium. The stereocilia bend in response to movements of the basilar membrane and trigger nerve impulses when ion channels in the hair cell membrane open. This depolarization enables hearing and equilibrium.
Step-by-step explanation:
Vertebrate hearing and equilibrium require the activity of hair cells, which are named for the hair-like stereocilia extending from the cell's apical surfaces. The stereocilia are an array of microvilli-like structures arranged tallest to shortest. Protein fibers tether adjacent hairs together within each array, such that the array will bend in response to movements of the basilar membrane. When the stereocilia bend, tension in the protein tethers opens ion channels in the hair cell membrane, triggering nerve impulses that travel down the afferent nerve fibers attached to the hair cells. This process depolarizes the hair cell membrane, enabling hearing and equilibrium.