Control of feeding movements in the freshwater snail Planorbis corneus. III. Organization of the feeding rhythm generator.

(1) Neurons of different groups (for group classification, see Arshavsky et al. 1988a) have been polarized through an intracellular recording microelectrode in Planorbis corneus buccal ganglia during feeding rhythm generation. Group 1 neurons, active in the quiescence (Q) and in the protractor (P) phases of the cycle, and also group 2 and 4 neurons, active in the retractor (R) phase, have proved to be "influential", i.e., altering the rhythm generator operation. (2) Injection of a depolarizing current into group 1 neurons caused an increase of the rate of depolarization that neurons of this group exhibit in the Q- and P-phases of the feeding cycle. As a result, Q-phase shortened, the P-phase became longer, and the feeding rhythm accelerated. Opposite effects occurred when a hyperpolarizing current was injected into group 1 neurons. In some of the experiments, the hyperpolarization of group 1 neurons resulted in cessation of both their activity and the activity of all other protractor neurons. As a result, the P-phase of the cycle disappeared, i.e., the rhythm generator transited from A mode of operation to B mode. (3) With hyperpolarization of individual group 2 or 4 neurons, excitation of the R-phase neurons was delayed and the feeding rhythm phase shifted. This delay was accompanied by the enhanced activity of protractor neurons. (4) A generator model is considered in which two groups (1 and 2) of endogeneously active neurons are coordinated by the excitatory effect of group 1 on group 2 and the inhibitory action of group 2 on group 1. (5) Evidence is given that the different modes of rhythm generator operation (A, B and C, see Arshavsky et al. 1988a) are determined by different tonic inflow to group 1 neurons.