Interactions of pattern-generating interneurons controlling feeding in Lymnaea stagnalis.

Intracellular recordings were made from rhythm-generating interneurons in the Lymnaea feeding system. The feeding pattern is a three-phase rhythm of interneuronal activity (N1, N2, N3) corresponding to protraction, rasp, and swallow. We describe the firing pattern and anatomy of the premotor interneurons, each of which fires a predominant burst in only one phase of the feeding rhythm. The rhythm can be driven by steady depolarization of N1 cells. The phase of the rhythm is reset by brief stimulation of N2 or N3 interneurons. N1 neurons excite the N2 interneurons, and these in turn inhibit the N1 cells. This recurrent inhibitory pathway can account for the switch from the N1 phase of the feeding cycles to the N2 phase. The endogenous properties of the N2 interneurons are apparently responsible for the termination of N2 bursts. N3 interneurons display postinhibitory rebound (PIR), and this probably contributes to their burst after the end of the N2 inhibitory input. N2 and N3 interneurons inhibit the N1 cells. When the N3 burst dies away, activity in N1 cells resumes under the stimulus of depolarizing current. Interactions between interneurons are mainly by discrete, monophasic postsynaptic potentials, that follow 1:1. They have relatively short latency (2-12 ms) and duration (up to 100 ms). The synaptic connections between the three types of premotor interneurons are sufficient to account for the sequence of activity seen during feeding.