A study of soma isopotentiality in Aplysia neurons

Soma isopotentiality in small, approximately spherical nerve cells during an action potential results in the total transmembrane current being proportional to the first time derivative of the membrane potential. The analytical results of earlier workers are used to obtain quantitative estimates of the deviations in soma isopotentiality to be expected when cells R 2 and R 15 in the abdominal ganglion of Aplysia californica are subjected to subthreshold stimulation. The deviations in isopotentiality during an action potential are determined experimentally in a small number of cells by noting the instantaneous potential difference between two widely spaced microelectrodes inserted in the cell soma. These analytical and experimental investigations showed that the soma of the endogenously active cell R 15 could be considered isopotential both for subthreshold stimulation as well as during an action potential. On the other hand, a clear deviation from isopotentiality was observed in the silent cell R 2 where activity was elicited by antidromic stimulation of the axon. Accordingly for the spontaneous action potentials observed in cell R 15 , a phase–plane diagram transforms into a current–voltage plot and is discussed from this point of view. The lack of soma isopotentiality for antidromic stimulation is presumably due to differing contributions of the triggering axon spike at the two separate microelectrode locations.