Science, Maths & Technology

### Become an OU student

Hearing

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

# 3.5.1 Hair cells transform mechanical energy into neural signals

The tectorial membrane runs parallel to the basilar membrane, so when the basilar membrane vibrates up and down in response to motion at the stapes, so does the tectorial membrane. However, as shown in Figure 14, the displacement of the membranes causes them to pivot about different hinging points and this creates a shearing force between the hair cell stereocilia embedded in the tectorial membrane and the hair cells themselves which rest on the basilar membrane. Shearing is a particular form of bending in which, in this case, the top moves more than the bottom. It is this shearing force that transduces mechanical energy into electrical energy which is transmitted to the auditory nerve fibres.

## Activity

What kind of sensory receptor transduces mechanical energy into electrical energy?

Mechanoreceptors.

Figure 14 Schematic diagrams of shearing forces created between the hair cells and the tectorial membrane as a result of basilar membrane displacement. (a) Shearing force that results from displacement of the basilar membrane towards the scala vestibuli when the basilar membrane is driven upwards. (b) Relationship between hair cells and tectorial membrane with no stimulation. (c) Shearing forces in the direction opposite to that shown in (a) after displacement in the opposite direction

In order for the hair cell to transduce stereocilia shearing (mechanical) forces into an electrical (neural) response, the permeability of the hair cell membrane must change. This happens when the shearing motion, which is a mechanical stimulus, opens ion channels in the cell's plasma membrane and the current flowing through these channels alters the cell's membrane potential (this is the electrical response). So, in response to a mechanical stimulus, there is an influx of ions into the cell which disturbs the resting potential of the cell membrane, driving the membrane potential to a new level called the receptor potential. The channels are relatively non-selective about which ions they allow to pass through them. However, you should recall from Section 3.2 and from The mechanics of hearing by Jonathan Ashmore, that potassium is very plentiful in the endolymph. The stereocilia of the hair cells are bathed in endolymph whereas the basal region of the cell is bathed in perilymph (which is relatively low in potassium). So once the channels are opened, potassium ions flow into the hair cell.

## Activity

How does this differ from most other cells?