Membrane Characteristics of Bursting Pacemaker Neurones in Aplysia

The group of endogenously active molluscan neurones, classified as bursting or oscillatory pacemakers, regularly fire bursts of three to fifteen impulses separated by periods of silence associated with membrane hyperpolarizations (Fig. 1a). The abdominal ganglion of the marine mollusc Aplysia californica contains six bursting pacemakers, L2-L6 (L-group) and R15¹; these cells maintain their membrane potential oscillations in the absence of synaptic input^2,3 and spike generation⁴. They also have K⁺-dependent inhibitory postsynaptic potentials (IPSPs) which last for some seconds after single or multiple discharges in the presynaptic neurones^5–8. In the L-group these IPSPs are mixed with short-lasting Cl^?-dependent IPSPs. Although there is a different transmitter responsible for the generation of these IPSPs in R15 as opposed to those in the L-group, somewhat similar difficulties were encountered in attempts to determine the mechanism(s) underlying their generation, activation of an electrogenic Na⁺ pump having been initially implicated in both cases^5,8. Just why their duration is prolonged remains uncertain, but the voltage clamp studies of Wachtel and Wilson⁹ on the L-group have again implicated a regenerative current source.