Ionic permeabilities of an Aplysia giant neuron

In a giant neuron of Aplysia californica, permeabilities and conductances obtained by measuring net fluxes of Na+, K+ and Cl-minus with ion-specific microelectrodes were compared with those obtained by measuring transmembrane current and potential changes when the three ions were varied in the external solution. Net fluxes were measured with ion-specific microelectrodes, after blocking metabolic processes, thus allowing movement of ions down their electrochemical gradients. Premeabilities and conductances obtained from the "chemical" measurements (i.e., ion-specific electrodes) were generally comparable to the values obtained from "electrical" measurements (i.e., ion-specific electrodes) were generally comparable to the values obtained from "electrical" measurements. Where discrepancies occurred, they could be explained by showing that some of the assumptions necessary to use the "electrical" method were not quantitatively true in this system. The absolute magnitudes of the permeabilities are significantly less than those found in many axonal preparations. There is also a relatively high PNa/PK ratio. The selectivity of the membrane against ions such as Tris" and MeSO3-minus is not good, Tris+ being nearly as permeable as Na+ and MeSO3-minus about one-half as permeables as Cl-minus. These properties may be characteristic of somal membranes.