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.
What kind of sensory receptor transduces mechanical energy into electrical energy?
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.
How does this differ from most other cells?
In most cells, the flow of potassium ions is outwards because the cell is higher in potassium than the surrounding medium. In the case of hair cells however, the endolymph surrounding the stereocilia has a higher level of potassium than the cell, and the flow is in the reverse direction.
In fact, when a hair bundle is displaced by a mechanical stimulus, its response depends on the direction and magnitude of the stimulus. In an unstimulated cell about 10 per cent of the ion channels are open. As a result, the cell's resting potential (about −50 mV) is determined, in part, by the inward flow of current. A positive stimulus that displaces the stereocilia towards the tall edge opens additional channels and the resultant influx of positive ions depolarises the cell by as much as tens of mV. A negative stimulus that displaces the stereocilia towards the short edge shuts the channels that are open at rest and hyperpolarises the cell (Figure 15).
This directional sensitivity of the cells, their arrangement on the organ of Corti and the hypothesised motion of the organ of Corti in response to a stimulus, means that an upward movement of the basilar membrane leads to depolarisation of the cells, whereas a downward deflection elicits hyperpolarisation.
The receptor potential of a hair cell is graded; as the stimulus amplitude increases, the receptor potential grows increasingly larger, up to a maximal point of saturation. The relationship between a bundle's deflection and the resulting electrical response is S-shaped (Figure 15d). This results in a high degree of sensitivity. A small displacement of only 100 nm (100 × 10−9 m) represents 90% of the response range of the hair cell (shaded part). Deflection of a hair cell by the width of a hydrogen atom is enough to make the cell respond.