The centrally determined sequence of impulses initiated from a ganglion of the calm Mya

Abstract

The origin of patterned sequences of motor impulses from the central nervous systems of lower animals is a problem which must be faced as we appreciate more and more that in a variety of different animals there can be found examples of co-ordinated sequences which are a product of the structure of the central ganglia, and not dependent on the form of peripheral excitation. In some cases such patterns are spontaneous, in other instances a background of afferent excitation is required to maintain them, and in yet other instances the pattern is initiated by afferent excitation which does not influence the pattern. Although many responses, in a variety of animals, are known to be peripherally controlled, yet the basic pattern of the response depends on central nervous factors, though this may be modified by peripheral factors. Moreover, a centrally determined mechanism must presumably be the basis of the first phase of any response, to avoid the uneconomical procedure of initially making a random preliminary movement and then correcting this proprioceptively. We may suppose, then, that a response can be initiated by a predetermined sequence of motor impulses, although in its continuation the response may also be modified to any extent by the afferent effects of its own earlier phases. Although of great importance, it is not the present concern that the predetermination of the sequence may be genetic or be derived from the animal's earlier experience. The present study is an exploration of the responses of one particular preparation which may prove of interest in this general problem. It is an advantage to use a primitive animal in which the normal number of efferent axons in one nerve is not too great for all to be identified separately and simultaneously and in which this total actuates a widespread and co-ordinated movement.