Introduction

Understanding the mechanisms which generate and control locomotory movements is fundamental to a complete knowledge of the neuronal control of behaviour. We can regard locomotion, such as jumping, walking or flying, as basic building blocks for much of an animal's behavioural repertoire; and we can pose three basic questions about the control of such movements. First, what mechanisms ensure that muscles contract in the appropriate sequence? In walking, for example, the basic pattern is repeated flexion and then extension of each leg, with flexion of the left leg coinciding with extension of the right. Second, how does a nervous system select, initiate and terminate a particular type of movement? For example, what initiates the pattern of walking; and how is walking rather than running or swimming selected? Third, how is the basic pattern for movement modulated appropriately? Stride pattern changes, for example, when a person walks up a flight of steps or turns a corner.

Experimental approaches to these questions have often involved work on invertebrates and lower vertebrates, animals in which the parts of the nervous system that generate programs for movement contain a limited number of neurons. This offers the opportunity to identify and characterise all the components involved in generating a particular movement. A specific question that has occupied many investigators is how to determine the source of rhythmical activity that underlies many regularly repeated movements, such as walking or flying.