3.6 Synaptic transmission from hair cells
In addition to being sensory receptors, hair cells are also presynaptic terminals. The membrane at the base of each hair cell contains several presynaptic active zones, where chemical neurotransmitter is released. When the hair cells are depolarised, chemical transmitter is released from the hair cells to the cells of the auditory nerve fibres. Excited by this chemical transmitter, the afferent nerve fibres contacting the hair cells fire a pattern of action potentials that encode features of the stimulus. We will return to how this information is encoded in Section 6. As in other synapses, the depolarisation that leads to transmitter release acts through an intermediary, namely calcium ions. Depolarisation opens channels at the base of the hair cell (voltage-gated calcium channels), which allow calcium ions to enter from the surrounding perilymph resulting in the release of transmitter (Figure 17). Calcium also has another function: it opens potassium channels, called calcium-activated potassium channels, which allow potassium ions to leave the cells because the perilymph on the other side is low in potassium. The potassium ions leaving the hair cell via the calcium-activated channels results in repolarisation of the cell. The identity of the neurotransmitter is controversial. Glutamate appears to be the transmitter in some cases but there is also evidence for another, as yet unidentified, substance.