Introduction

Most animals are active organisms and need up-to-date information about their environment if they are to behave appropriately. Much information is potentially available in the many forms of energy and chemicals that impinge on the surface of the organism and act as stimuli. An animal must be able to detect the various forms of energy and to sort them all out, a job that is carried out by its sense organs, which act as instruments monitoring stimuli coming in from the environment. Sense organs are thus an animal's mechanism for gathering up-to-date information, and as such it is hard to exaggerate their importance in behaviour.

Clearly, a monitoring instrument is useful only if it measures one particular form of stimulus; having an instrument that responded indiscriminately to all forms of stimuli would be nearly as uninformative as having no monitoring facility at all. Hence, one of the most fundamental properties of sense organs is selectivity. Each sense organ contains specific receptor cells that are tuned to be sensitive to one particular stimulus. In many sense organs, the receptor cells are also nerve cells, having axons that convey their information to the central nervous system, and receptor cells of this kind are, therefore, often called sensory neurons. In other sense organs, such as the eyes of vertebrates and insects, the receptor cells do not have long axons themselves but instead make synaptic contact with separate nerve cells that send trains of spikes to the central nervous system.