Calcium dynamics at a plastic synapse in Aplysia

Because certain primitive behavioral responses in the large sea snail Aplysia have recently been linked to neurophysiological events at a synaptic level, special interest attaches to the role played by calcium ions at such synapses. Using an extended version of the model applied earlier to trace the flow of energy and information through a ganglion of the medicinal leech (Triffet & Green, 1980), the authors investigate the electropotential effects of small transient localized changes in the calcium concentration near the inner membrane surface of a neuron in the resting state. When this state is well below the firing threshold, changes in Ca2+ concentration less than 10(-8)M are shown to result only in low-level harmonic background oscillations. When the potential of the neurons is closer to threshold, however, and/or the Ca2+ concentration is of the order of 10(-8)M, easily recognizable graded potentials appear, and these grow into firing peaks when the calcium concentration is increased still further. Though no attempt is made to deal with the amplification effects dependent on calcium-vesicle interactions and the related release of transmitter molecules, a unified mechanism for the underlying calcium ion dynamics is proposed. Graded potentials of increasing size are associated with a progressive localized thickening of the inner and outer Debye layers. Moreover, the transverse and longitudinal calcium currents set up in such regions prove adequate to account for both the depletion of Ca2+ ions necessary to achieve habituation, and the increase in their concentration required for sensitization.