3.5.3 Mechano-electrical transduction is rapid
Many other sensory receptors, such as photoreceptors and olfactory neurons, employ second messengers in the transduction process. This is not true for hair cells. The rapidity with which they respond makes this impossible. In order to deal with the frequencies of biologically relevant stimuli, transduction must be rapid. The highest frequency humans can hear is about 20 000 Hz. This in effect means that hair cells must be able to turn current on and off 20 000 times per second (200 000 times per second for a bat). Also, localisation of sound sources (Section 12) requires that animals are able to resolve very small time differences, in the order of 10 μs.