Plateau-Generating Nerve Cells in Helix: Morphological and Electrophysiological Characteristics

We describe the anatomical and electrophysiological characteristics of a group of Helix nerve cells, styled P cells, that generate long-lasting depolarizations in response to repeated stimulations at low frequencies. Four neurones were identified in the perioesophageal ganglia of the snail Helix pomatia. Their structure was determined by intracellular injection of Lucifer Yellow, cobaltlysine or horseradish peroxidase. The soma was found to contain neurosecretory granules. These cells innervated the whole foot muscle and the mantle, but were not involved in muscle movement or locomotion. They may participate in mucus secretion. Upon depolarization they fired Ca2+-dependent spikes; at a critical firing rate (5–6 Hz), the spikes were converted into depolarized plateaus (+10 to +20 mV) lasting for several seconds. The plateau was Ca2+-dependent and persisted in Na+-free saline. It was sustained by a slowly inactivating Ca2+ current that produced a large intracellular Ca2+ accumulation (monitored with the Ca2+- sensitive dye Arsenazo III). The plateau was restricted to the soma and the proximal axon and may act as a driver potential inducing axon firing and prolonging the release of neurosecretory materials. Note: To whom reprint requests should be addressed.