Final answer:
The extensions on hair cells in the spiral organ are called stereocilia, which bend in response to sound vibrations and transduce these into electrical signals.
Step-by-step explanation:
The hair cells in the spiral organ have extensions called stereocilia, which respond to mechanical stimuli. These stereocilia are arrays of apical membrane extensions that emanate from the hair cell's surface. They are crucial for the process of hearing, transducing mechanical movements into electrical signals. When the basilar membrane moves due to sound vibrations, it causes the tectorial membrane to slide across the stereocilia, bending them. This bending causes protein fibers, which tether the stereocilia together, to open ion channels as they bend towards the tallest member of the array and close them when they bend towards the shortest member. Such activity depolarizes the hair cell membrane, initiating nerve impulses that are transmitted via the afferent nerve fibers to the cochlear nerve.