Surface versus Intracellular Polarization of Cortical Cells

Abstract

ALTHOUGH it is almost 100 yr since the original report of Fritsch and Hitzig¹, many of the effects of direct current stimulation (polarization) of the surface of motor cortex on cellular activity remain unclear. Electrophysiological research has shown that surface anodal polarization increases while surface cathodal polarization decreases the rate of discharge of cortical cells^2–5. These results have been interpreted in terms of a model developed to explain the effects of direct current applied extracellularly on isolated nerve terminals^6,7. In its simplest form, increases and decreases in discharge rate have been related to polarization induced displacements in membrane potential of cells subjected to a longitudinal potential gradient^8–12. Presumably some fraction of diffuse extracellular applied current enters areas of superficial membrane close to the focal polarizing electrode. The chief effect of this current, however, is exerted as it leaves from distant parts of the excitable membrane. Accordingly, in the cortex, a surface anode provides inward current as it enters superficial dendritic regions of vertically oriented cells which subtend several cortical layers. At the same time the anode generates an outward depolarizing current as it passes out through deep-lying areas of excitable membrane in the soma or axon–hillock region of the cell. The sequence of events is presumably reversed when a surface cathode is used.