Extracellular fields within the cortex

A physically based theory of ionic currents associated with nervous activity is extended to provide a model of electrical activity in the cortex and its immediate neighbourhood. It is shown that the most significant variations of potential in the extracellular fluid are associated with fluctuations in density of calcium and potassium ions. Nervous activity may activate a calcium resonance at the external surface of a neural membrane, which in turn excites a potassium resonance. The resulting variations of electrical potential are sufficient to account for the event related potentials observed in the extracellular fluid. In addition, it is found that periodic variations of potential associated with the potassium resonance may be initiated by metabolic changes in ion concentrations and are in close correspondence with those identified experimentally with the alpha- and beta-rhythms. It is shown how to determine the effective conductance of the neural membrane from ionic theory, with a result comparable to that assumed by Hodgkin & Huxley, under the conditions of the voltage clamp.