Pattern generation in the buccal system of freely behaving Lymnaea stagnalis

Central pattern generators (CPGs) are neuronal circuits that drive active repeated movements such as walking or swimming. Although CPGs are, by definition, active in isolated central nervous systems, sensory input is thought play an important role in adjusting the output of the CPGs to meet specific behavioral requirements of intact animals. We investigated, in freely behaving snails (Lymnaea stagnalis), how the buccal CPG is used during two different behaviors, feeding and egg laying. Analysis of the relationship between unit activity recorded from buccal nerves and the movements of the buccal mass showed that electrical activity in laterobuccal/ventrobuccal (LB/VB) nerves was as predicted from in vitro data, but electrical activity in the posterior jugalis nerve was not. Autodensity and interval histograms showed that during feeding the CPG produces a much stronger rhythm than during egg laying. The phase relationship between electrical activity and buccal movement changed little between the two behaviors. Fitting the spike trains recorded during the two behaviors with a simple model revealed differences in the patterns of electrical activity produced by the buccal system during the two behaviors investigated. During egg laying the bursts contained less spikes, and the number of spikes per burst was significantly more variable than during feeding. The time between two bursts of in a spike train was longer during egg laying than during feeding. The data show what the qualitative and quantitative differences are between two motor patterns produced by the buccal system of freely behaving Lymnaea stagnalis.