The objective of the proposed research is to investigate what role changes in intracellular Ca2 ion play in determining the permeability of the soma membrane of neurons. Changes in intracellular Ca2 ion will be measured from changes in the absorbance of the Arsenazo III inside neuron soma in the abdominal ganglion of the gastropod mollusc Aplysia californica. Changes in the activity of K ion and Na ion ions will be measured with ion-selective electrodes. Mechanisms for the influx and efflux of Ca2 ion across the membrane will be investigated. The relationship between the Na-Ca exchange system and the Na pump will be determined. We will concentrate our efforts on the effects of changes in intracellular Ca2 ions on membrane K ion permeability during pacemaker activity, during post-tetanic hyperpolarizing responses, and in the presence of illumination. The long term goal of this research is to arrive at a more complete understanding of how the receptive membrane of the neuron's soma-dendrite complex integrates incoming information under normal and abnormal (disease-related) conditions.