Axonal Localization of An Excitatory Post-Synaptic Potential in A Molluscan Neurone

Abstract

Stimulation of the lateral nerve gives rise to an excitatory post-synaptic potential (EPSP) in the Anisodoris G cell. The amplitude of this EPSP is either unchanged or slightly reduced by moderate hyperpolarization of the soma membrane, whereas stronger hyperpolarization produces an increase. Two factors contribute to this paradoxical behaviour. First, the conductance of the G cell membrane increases when it is hyperpolarized. Second, the apparent changes in cell conductance associated with the EPSP are small relative to the input conductance of the G cell. There are two major regions of the G cell where presumed synaptic contacts can be demonstrated anatomically. The first is on branches of the stem process of the soma in the gastro-oesophageal ganglion neuropile, and the other is on the main trunk of the axon and on axonal branches in the gastro-oesophageal nerve. Presynaptic fibres from the lateral nerve are present in the latter region. No synaptic contacts are found on the soma surface. A comparison of the effects of axonal and somatic polarization on the behaviour of the lateral nerve EPSP indicates that the EPSP occurs on the axon or on axonal branches.