The Auditory Pathway This graphic depicts the events in the stimulation of auditory receptors, from channeling sound waves into the external ear and onto the TM, to the transduction of those vibrations into local receptor potentials Sound waves enter the external auditory canal and strike the eardrum. The vibrations of the eardrum cause the ossicles to vibrate and the stapes pushes the membrane of the oval window in and out. The movement of the oval window sends fluid pressure waves into the perilymph of the scala vestibuli which then transmit them to the scala tympani and eventually to the round window (causing it to bulge outward into the middle ear). The pressure waves move into the endolymph of the cochlear duct and cause the basilar membrane to vibrate which moves the hair cells of the spiral organ against the tectorial membrane. This leads to bending of the stereocilia and ultimately to the generation of nerve impulses in first-order neurons in cochlear nerve fibers. Sound waves of various frequencies cause certain regions of the basilar membrane to vibrate more intensely than other regions. Each segment of the basilar membrane is “tuned” for a

Scala vestibuli Cochlear duct (contains endolymph) tympani Perilymph Basilar membrane Cochlea Sound waves Helicotrema Stapes vibrating in oval window Malleus Incus External auditory canal Tympanic Secondary tympanic membrane vibrating in round window Auditory tube Vestibular membrane Middle ear Tectorial membrane Spiral organ (organ of Corti) 1 2 3 4 5 6 7 Scala vestibuli Cochlear duct (contains endolymph) tympani Perilymph Basilar membrane Cochlea Sound waves Helicotrema Stapes vibrating in oval window Malleus Incus External auditory canal Tympanic Secondary tympanic membrane vibrating in round window Auditory tube Vestibular membrane Middle ear Tectorial membrane Spiral organ (organ of Corti) 1 2 3 4 5 6 7 8 Scala vestibuli Cochlear duct (contains endolymph) tympani Perilymph Basilar membrane Cochlea Sound waves Helicotrema Stapes vibrating in oval window Malleus Incus External auditory canal Tympanic Secondary tympanic membrane vibrating in round window Auditory tube Vestibular membrane Middle ear Tectorial membrane Spiral organ (organ of Corti) 1 2 3 4 5 6 7 8 9 Scala vestibuli Cochlear duct (contains endolymph) tympani Perilymph Basilar membrane Cochlea Sound waves Helicotrema Stapes vibrating in oval window Malleus Incus External auditory canal Tympanic Secondary tympanic membrane vibrating in round window Auditory tube Vestibular membrane Middle ear Tectorial membrane Spiral organ (organ of Corti) 1 2 3 4 5 6 Scala vestibuli Cochlear duct (contains endolymph) tympani Perilymph Basilar membrane Cochlea Sound waves Helicotrema Stapes vibrating in oval window Malleus Incus External auditory canal Tympanic Secondary tympanic membrane vibrating in round window Auditory tube Vestibular membrane Middle ear Tectorial membrane Spiral organ (organ of Corti) 1 2 3 4 5 Scala vestibuli Cochlear duct (contains endolymph) tympani Perilymph Basilar membrane Cochlea Sound waves Helicotrema Stapes vibrating in oval window Malleus Incus External auditory canal Tympanic Secondary tympanic membrane vibrating in round window Auditory tube Vestibular membrane Middle ear Tectorial membrane Spiral organ (organ of Corti) 1 2 Scala vestibuli Cochlear duct (contains endolymph) tympani Perilymph Basilar membrane Cochlea Sound waves Helicotrema Stapes vibrating in oval window Malleus Incus External auditory canal Tympanic Secondary tympanic membrane vibrating in round window Auditory tube Vestibular membrane Middle ear Tectorial membrane Spiral organ (organ of Corti) 1 2 3 Scala vestibuli Cochlear duct (contains endolymph) tympani Perilymph Basilar membrane Cochlea Sound waves Helicotrema Stapes vibrating in oval window Malleus Incus External auditory canal Tympanic Secondary tympanic membrane vibrating in round window Auditory tube Vestibular membrane Middle ear Tectorial membrane Spiral organ (organ of Corti) 1 2 3 4 Scala vestibuli Cochlear duct (contains endolymph) tympani Perilymph Basilar membrane Cochlea Sound waves Helicotrema Stapes vibrating in oval window Malleus Incus External auditory canal Tympanic Secondary tympanic membrane vibrating in round window Auditory tube Vestibular membrane Middle ear Tectorial membrane Spiral organ (organ of Corti) 1

The Auditory Pathway The cell bodies of the sensory neurons are located in the spiral ganglia. Nerve impulses pass along the axons of these neurons, which form the cochlear branch of the vestibulo- cochlear (VIII) nerve

The Auditory Pathway The nerve impulses follow CN VIII en route to the medulla, pons, midbrain, and thalamus, and finally to the primary auditory cortex in the temporal lobe. Slight differences in the timing of nerve impulses arriving from the two ears at the superior olivary nuclei in the pons allow us to locate the source of a sound

Equilibrium Equilibrium is another function of the inner ear - controlled by the vestibular apparatus (the saccule and utricle of the vestibule, and the 3 semicircular canals) Static equilibrium refers to a state of balance relative to the force of gravity Dynamic equilibrium involves the maintenance of balance during sudden movements

Static Equilibrium Static equilibrium is controlled by the sensory hairs within the macula of the utricle and saccule

The Inner Ear Movements of the hair cells in contact with the tectorial membrane transduce mechanical vibrations into electrical signals which generate nerve impulses along the cochlear branch of CN VIII

Static Equilibrium An otolithic membrane, studded with dense calcium carbonate crystals (otoliths), responds to gravity when head position is changed This movement opens transduction channels in the hair cells, producing local potentials which summate to form nerve AP