 Dear students, In this topic we shall discuss the mechanism of sound transduction by cochlear hair cells. The cochlea receives sound vibrations at the oval window through steps. This causes the displacement of basilar membrane. As a result, tactorial membrane slides across the tips of hair cells in the cochlear duct. When basilar membrane moves, the tactorial membrane also moves, but the tactorial membrane moves over the tips of hair cells. This movement exerts a shearing force at the tips of stereocilia. As a result, the tactorial membrane moves over the tips of hair cells. This is the shearing force which puts the tactorial membrane and as a result, the stereocilia bends laterally. Now, the lateral bending of the stereocilia causes the transduction event to start and happen. Mechanical deflection of stereocilia results in the conformational changes and opening of transduction ion channels in the tips of the cilia. Mechanical stimuli opens ion channels. A deflection of only 0.1 nanometer. This is the threshold for cochlear hair cells. When ion channels open, potassium ions enter the cell from the endolium present in the cochlear duct. This inward potassium current depolarizes that is excites the hair cells and produces the hair cell receptor potential. The hair cell excitation also results in the opening of voltage-gated calcium channels. As a result, calcium ions also influx and this influx causes the transmitter release from the basal end. Usually, the transmitter is the glutamate and this is released from the basal end on to the auditory nerve endings which are attacked with the hair cells. These auditory nerves then take the electrical signal along the cochlear nerve and then onward brain. When the potassium ions enter through the transduction ion channels, that also results in the opening of two types of channels at the basal end of the hair cell. The voltage-gated calcium channels open at the basal end and the potassium channels which are located on the basal end also open. As a result, from the basal end, the outflux of potassium ions and calcium ions starts. As a result, the cell becomes repolarized. Dear students, you may have noted that the depolarization of the hair cells that have occurred here is because of the potassium ions. That is a very unusual phenomenon because normally the ions involved in the depolarization are sodium ions. This unusual behavior of the cochlear hair cells is a very unusual adaptation. It is because the cochlear hair cells are located inside the cochlear duct. Now, we had seen in the structure that both sides of the cochlear duct have perilymph filled cavities. The basal and apical surfaces of the hair cells are exposed to different extracellular ionic concentrations because of this condition in which the hair cells are present in the endolymph and perilymph on both sides. The apical end is present in the cochlear duct and the cochlear duct is in the endolymph. The endolymph is potassium rich and sodium poor. The start of the depolarization from the endolymph side has a lot of potassium in it while it does not have so much sodium. The basal end of the hair cell is exposed to perilymph in the tympanic duct. This perilymph is just like other extracellular fluids which are potassium poor and sodium rich. This difference in perilymph endolymph composition results in a potential which is called endocochlear potential. This potential is 80 millivolt more positive in cochlear duct which has endolymph as compared to the perilymph in the tympanic duct. If we see that the resting membrane potential of the hair cell is about minus 60 millivolt, then the inside of the hair cell is about 45 millivolt more negative compared to the perilymph in the tympanic duct and the basal end of the hair cell. In comparison, it is about 140 millivolt more negative as compared to the endolymph which has stereociliary endolymph. This large electrical gradient across the stereocilia is the reason that drives potassium ions through the open transduction channels into the apical portion of hair cells. Although the resting membrane potential of the cell is due to the potassium ion difference, it has a lot of potassium ions but this potassium ion is still less compared to the endolymph. As a result, potassium ions still move into the cell and cause depolarization of the cell.