All animals are sensitive to some extent to changes in their environment. Special parts of the body are responsive to some of these changes and feed information concerning them into the central nervous system. Information about the workings of the animal's own body, or about communication signals from other members of its species, may be acquired in a similar fashion. These specially sensitive structures are known as sense organs or sensory receptors. They are crucially important in the lives of animals.

Since there is a very great variety of different types of sense organ, the following pages must necessarily be no more than a selective introduction to their physiology. This chapter attempts to give a general outline of the properties of sensory receptors, the next four chapters survey some selected different types, with particular emphasis on their sensory cells and transduction processes. The books by Barlow & Mollon (1982), Dawson & Enoch (1984), Darian-Smith (1984b), Corey & Roper (1992) and Dusenbery (1992) are some of the many useful sources of further information.

The methods used for investigating receptors fall into two main categories. Firstly, there is the behavioural, or psychophysical, approach, where the receptor is investigated indirectly by observation of the response of the animal to a sensory stimulus. For example, suppose we are interested in the ability of an animal to discriminate colours. In the case of humans, the experiments are not too difficult, since we can ask the subject if two colours look different.