Which of the following is directly involved in sound transduction?

Shearing of the hair cell stereocilia against the tectorial membrane results in depolarization, and the release of neurotransmitter onto afferent endings of the auditory cranial nerve leads to initiation of an action potential. Sound waves are directed toward the tympanic membrane by the pinna and the external auditory canal of the external ear. The vibration of the tympanic membrane is transmitted to the oval window by way of the ossicles of the middle ear. Induction of waves in the perilymph results in the movement of the basilar and vestibular membranes toward the scala tympani and causes the round window to bulge outward. The movement of the hair cells is facilitated because the tectorial membrane is rigid and the pillar cells form a pivot. The stabilization of the pressure between the middle ear and the nasopharynx is not directly related to the mechanism of sound transmission.