Central Pattern Generation: A Concept Under Scrutiny

Summary

What are the mechanisms that generate the motor patterns for rhythmic movements in animals? Until recently it has widely been accepted that the relative timing of motor activity is determined by central neuronal circuits termed central pattern generators (CPGs). The validity of this view is examined for four motor systems in which there is a high degree of sensory regulation: the walking systems of the cat, cockroach and stick insect, and the flight system of the locust. In none of these systems is the evidence sufficient to conclude that a CPG is primarily responsible for establishing the timing of motor activity. In the stick insect there is no evidence for the existence of a CPG for walking at all, while in the cockroach the deafferented motor pattern differs from the normal walking pattern to such an extent that phasic afferent input must play an important part in establishing the timing of motor activity. Reflex pathways have been identified which appear to function to produce some aspects of the normal motor pattern for walking in insects. No firm conclusion can be reached regarding the role of central pattern generators in establishing the timing of motor activity in the walking system of the cat because the motor patterns in completely deafferented preparations have not been analysed in sufficient detail to allow an adequate comparison with the intact motor pattern. Finally, in the flight system of the locust it has recently been found that one entire phase of the normal flight cycle depends on phasic afferent input from wing receptors, and that sensory feedback is involved in the generation of rhythmicity. It is now clear that CPGs as defined in deafferented preparations do not form the basis for establishing the timing of activity in all rhythmic motor systems. Thus central pattern generation cannot be regarded as universal functional principle for rhythmic motor systems.