Selective Recruitment of Interganglionic Interneurones During Different Motor Patterns in Pleurobranchaea

Abstract

ABSTRACT The buccal and anterior head structures of the mollusc, Pleurobranchaea, participate in several types of behaviour which are generated largely by rhythmic activity produced in the brain and buccal ganglia. This paper examines the neuronal mechanisms (1) by which motor output from these ganglia is coordinated, and, (2) which underlie the generation of different motor patterns. Preliminary experiments indicated that the neuronal circuitry which produces the rhythms is contained in each of the paired buccal ganglia. The timing characteristics of the rhythms arise in the buccal ganglia and the pattern and symmetry of motor output in the brain roots are determined by activity in buccal cerebral interneurones (BCI) within the buccal ganglia. Intracellular recordings showed that unique properties of individual cells within two pools of interganglionic interneurones, the BCI and paracerebral cells (PCC), determine in part the types of coordinated patterns that are produced. The BCI, which project to the brain, are a heterogeneous population of cells, based on : ( 1 ) the activity they evoke when they are depolarized ; and (2) their activity patterns during rhythms that are evoked by buccal nerve stimulation. Evidence is presented to show that BCI are selectively recruited to produce the distinguishing features of different motor patterns. Similarly, the PCC were found to be a diverse group of cells that exert both excitatory and inhibitory effects on the nervous system. Stimulation of excitatory PCC in isolated nervous systems elicited a variety of activity patterns. These data suggest that the PCC may selectively recruit the BCI to produce different motor patterns and that differences in patterns can be attributed, in part, to the unique properties of individual cells within each pool of interneurones.