Introduction

There are very few instances in which complete neuronal pathways can be traced from the level of sense organs all the way to that of motor neurons. Notable exceptions are some startle behaviours like those described in Chapter 3, in which the size of the giant neurons involved makes experimental study relatively easy and in which the links between sensory processing and motor control are short. However, most of an animal's behavioural repertoire is not performed with the same urgency as escape movements. Much sensory analysis, particularly in visual and auditory pathways, involves several different stages, distributed over different regions of a brain. How are different sensory messages identified, and how are appropriate motor programs selected? The type of problem can be illustrated with a simple example. If a fly lands on your cheek, or if the skin of your knee itches, you can move your hand without thinking to those locations to remove the source of annoyance. This might seem like a trivial example of behaviour, but the neuronal mechanisms that allow us to perform such an act are far from being understood. It is relatively straightforward to map the locations of sensory receptors in the skin in an orderly manner within the brain, which generates a somatotopic map. However, it is not simple to generate the correct commands that will generate the correct balance of activation in different muscles of a jointed limb so that its end arrives at a specific location on the body surface.