In reptiles such as turtles, the anatomy of the colchlea is somewhat different, and the basilar membrane plays a minor part in frequency discrimination. The whole burden of selectivity falls on the auditory hair cells, and it appears that each one of these is individually tuned to respond to a certain frequency of vibration. Evidence of such tuning has come from recording of the electrical behaviour of auditory hair cells in the ear of the turtle. An intracellular electrode can be used both to monitor the potential inside a hair cell and to inject current into it. When a small steady current is abruptly switched on or abruptly switched off, the membrane potential is set oscillating (or “ringing”) like a gong that has been banged (figure 18-49). The cell behaves, in other words, as a damped electrical resonator. The caracteristics frequency of sound to wich it is most receptive. The mecanism of the oscillation is unknown, though it is possible in principle for certain combinations of voltage-gated ion channels to give rise to such behaviour.

The precise nature of the coupling between the electrical response and the mechanical deformation in the cochlea remains a mystery and a source of astonishment. This transducing system enables the mammalian ear to operate over a range of sound intensities spanning more than seven orders of magnitude and allows us to hear sounds so faint that they make the basilar membrane vibrate with an amplitude of no more than a fraction of the diameter of a hydrogen atom.

Rod Cells Can Detect a Single Photon³⁵

The transducers by wich we perceive light are also phenomenally sensitive, and their functioning is much better understood. As descibed in Chapter 16, the photoreceptor in the vertebrate eye are of two classes : the cone celle, which serve for color vision and require fairly bright ligth ; and the rod cells, which provide for monochromatic vision in dim light.? A rod cell can produce a measurable electrical signal in response to a single photon - a human being can perceive five photons as a flash of light. Rods and cones appear to operate on similar principles, but rods have been more intensively studied.

B.Alberts, D.Bray, J.Lewis, M.Raff

MOLECULAR BIOLOGY OF THE CELL

Médecine-Sciences, Falmmarion

Paris. 1986. p.1063