Calcium spikes in a leech nonspiking neuron.

The NS neurons are nonspiking cells, present as pairs in each midbody ganglion of the leech nervous system, which display a very extensive arborization. They were shown to regulate the coactivation of motoneurons. Here we have investigated the electrophysiological properties of these neurons under the hypothesis that transmission along the extensive neurites requires the aid of voltage-dependent conductances. The results indicate that NS neurons respond to electrical stimulation with a spike-like event, which was not an all-or-none but rather a graded phenomenon that depended on the intensity and duration of the electrical stimulus. The spike-like response was activated at a membrane potential of approximately -50 mV; its amplitude was a logarithmic function of the extracellular Ca2+ concentration and was unaffected by a broad range of changes in the extracellular Na+ concentration; intracellular application of tetraethylammonium (TEA) caused a large increase in its amplitude and duration. These data indicate that NS neurons bear voltage-dependent low-threshold Ca2+ and TEA-sensitive K+ conductances that could contribute to shaping synaptic signals, or transmission along the extensive neuritic tree.