Vibration of the basilar membrane is converted into a mechanical push on the hair cell bundles in the organ of Corti. This causes the stereocilia to shear against the tectorial membrane, bending them. That bending opens mechanically gated ion channels in the hair cells, allowing ions from the surrounding endolymph to enter and depolarize the hair cell. The depolarization opens voltage-gated calcium channels, triggering release of neurotransmitter onto the afferent spiral ganglion (cochlear) neurons. The activated neurons then fire action potentials that travel along the cochlear nerve to the brain. Hair cells themselves don’t fire typical action potentials; their receptor potentials drive neurotransmitter release to drive the auditory nerve. This mechanism underlies how frequency information is mapped along the basilar membrane (tonotopy). The other options don’t fit: cortex is reached after several relays rather than directly from the basilar membrane; the semicircular canals respond to head movement, not basilar membrane vibration; hair cells are essential for transduction, and the cochlear nerve action potentials come from neurotransmitter-driven firing of spiral ganglion neurons, not directly from hair cells.