Coupled neuronal oscillators in the snail Lymnaea stagnalis: endogenous cellular properties and network interactions.

The role of endogenous cellular properties and network interactions due to electrotonic coupling were investigated in two bilateral populations of 2-7 peripheral neurones ('Peripheral Bursters') in the snail Lymnaea stagnalis. 1. These cells are endogenously capable of bursting. Their burst frequency does not depend on the level of steady membrane polarization. Short hyperpolarizing current pulses injected during the bursting cycle induce phase advance and no phase delay in subsequent cycles, the phase advance being a function of the phase of stimulus application. Phase response and inter-burst interval curves have been constructed for short hyperpolarizing current pulses. Their shape depends on the intensity and sign of tonically injected current. This property of Peripheral Bursters is one reason for the independence of period duration from membrane polarization. 2. Coordination of burst activity of Peripheral Bursters has been studied as a function of coupling strength: whereas homolateral cells are relatively strongly electrotonically coupled and always burst synchronously, thus forming a single oscillatory unit, heterolateral cells are only weakly electrotonically coupled. Accordingly, they show more variable, though coordinated, patterns of activity. Most commonly, periods with 1:1 entrainment alternate with periods with 1:2 entrainment. The duration and frequency of occurrence of either mode of entrainment are highly variable in different preparations.