Synaptic inputs on a bimodal pacemaker neuron in Helix pomatia L.

1. Synaptic modification of the RPal neuron located in the right parietal ganglion of the snail Helix pomatia was investigated by applying single electric pulses to the right pallial, left pallial or anal nerves. 2) Stimulation caused short term changes in the bursting type activity of the neuron: polyphasic synaptic potential including both the depolarizing and hyperpolarizing phases was evoked. 3) The depolarizing phase, probably of polysynaptic origin, proved to be Na+- and Ca2+-dependent. The hyperpolarizing phase consisted of two components which could be differentiated on the basis of time course, dependence on membrane polarity and sensitivity to ionic environment. The fast component was Mg2+-sensitive while the slow one Ca2+- and Cl--sensitive. Both of these components seem to be connected to changes in K permeability of the membrane, however, in K channels responsible for the fast and slow components are probably not identical.