Mechanism of Sound Transduction
Opening and closure of transduction channel generate neural signals
Even though a sound of very high intensity can move the stereocilia by about 20 nm sideway, the smallest detectable sound might move the stereocilia merely 0.3 nm. This is an incredibly small distance considering the 500 nm diameter of a stereocilium, which is more than 1000 times greater than the distance moved.
What is the molecular mechanism employed by the hair cell to transduce such a small amount of sound energy?
It turns out that the response of hair cell depends on a unique type of cation channel called the TRPA1 channel, present on the tips of the stereocilia. TRPA1 channels belong to the transient captoror potential (TRP) family of ion channels. The bending of stereocilia is likely to cause these channels to open or close depending on the bending direction, thus inducing changes in the hair cell receptor potential. Each channel is connected to another channel on the adjacent cilium by an elastic filament, called a tip link. When stereocilia are bent in one direction, tip links connecting the channels at the tips are stretched, causing the linked channels to open. This allows inward K+ current which depolarizes the cell. Bending in the other direction relieves the tension, preventing K+ influx and thus leading to hyperpolarization.
In the case of depolarization, voltage-gated calcium channels are activated and allow calcium entry, which then evokes excitatory neurotransmitter release. Postsynaptic neurons (spiral ganglion fibers) are activated by the neurotransmitters and subsequently relay the impulse signal to the brain. In general, the action potential firing rate reflects the sound intensity, whereas the location of the inner hair cell which the signal comes from gives information about the sound pitch or frequency.
Figure 1: Bending of the stereocilia stretches tip links, causing the opening of TRPA1 channels to allow potassium ion entry. Consequently depolarization follows and activates voltage-gated calcium channels, allowing calcium ion entry which triggers neurotransmitter release and hence the activation of postsynaptic neuron.
Image courtesy of https://openlearn.open.ac.uk/mod/resource/view.php?id=263162 under a Creative Commons license.
The unusual potassium ion influx
It is worth to note that the depolarizing effect due the opening of K+ channel in the hair cell is unique because in most cases hyperpolarization happens instead. The reason is the unusually high K+ levels in endolymph, which results in a zero K+ equilibrium potential, compared to the typical equilibrium potential of -80 mV in other neurons. In addition, the 80 mV endocochlear potential creates a steep gradient that drives K+ movement into hair cells when K+ channels are open.