Neuronal Pathways and Synaptic Connexions in the Abdominal Cord of the Crayfish*

Abstract

ABSTRACT As material for the study of functional connexions within a central nervous system, invertebrates with ganglionated nerve cords appear to offer advantages absent elsewhere. In such animals it should be possible to lead off from the axons which transmit the information from one synaptic region to another at all levels in the connectives, provided there are no cells or synapses in these connectives. In the crayfish, it has been previously shown that such an analysis is indeed possible, at the level between the ‘brain ‘and the remainder of the ventral cord. It was found (Wiersma, Ripley & Christensen, 1955) that many interneurones respond to stimulation of several segments of the body, usually to stimulation of homologous areas of the individual segments. Three fundamentally different ways in which the inflow to different ganglia may become integrated in this way were suggested by Wiersma (1958), and are illustrated diagrammatically in Fig. 1. To gain further information about the occurrence of these different types, it was desirable to obtain recordings from interneurones at a lower level in the cord where the actual integration takes place. The isolated abdomen preparation proved highly suitable for this purpose. The number of fibres in each connective here is still smaller than that in each circumoesophageal commissure (about 1200 as against about 2000), and it was found possible to obtain from the cord preparations in which unit responses were clearly recognizable with not much more difficulty than from the commissure. Altogether it was possible to account for some 75 definite entities as against 100 in the commissure. In both places some of these are not single fibres but bundles of primary sensory fibres with similar properties. In the cord such bundles are more frequent than in the commissure. The nature and distribution of these entities will be described elsewhere; in the present paper their physiological properties will be discussed with special reference to the evidence they provide for the existence of these three possible types of integration. The results will show that the one in which a single interneurone collects impulses from different ganglia (scheme C of Fig. 1) is certainly realized in many of these interneurones.