Neuroendocrine cells of Aplysia brasiliana. II. Bag cell prepotentials and potentiation

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Abstract
After a period of repetitive firing the bag cells of A. brasiliana enter a resting state during which electrical stimuli to the ipsilateral pleurovisceral connective evoke prepotentials. Extracellular recording from the connectives indicated that actively-propagated bag cell spikes preceded both intrasomatic prepotentials and full soma action potentials. Prepotential amplitude increased when depolarizing current was introduced through an intrasomatic microelectrode. Hyperpolarizing current within a range of constant input resistance, decreased prepotential amplitude. The prepotentials are apparently blocked spikes and not chemical synaptic potentials. During potentiation of the prepotentials, the action potentials block closer to the bag cell somata. Components of the extracellular bag cell action potentials develop first at distal sites and then in proximal neurites. Direct stimulation of the neurites can induce prepotential potentiation which does not require temporal stsummation of charge on membrane capacitance. The bag cells occasionally show plasticity analogous to post-tetanic potentiation. Repeated stimulus trains eventually cause fatigue. Repetitive activation in other neural systems with regions of probabilistic conduction may cause spikes to propagate progressively through the network and thereby enhance output.