The effects of polarizing currents on cell potentials and their significance in the interpretation of central nervous system activity

Abstract

Since no potential could be recorded from the medium surrounding a cell which was uniformly and synchronously active over its whole surface, potentials recorded across cell layers represent the difference in activity at apical and basal regions. Since potentials led from simple cell layers are effectively monophasic, the conduction time along the soma is a minor factor in determining the form of the record. Diphasic records are inferred to be summations of cell discharges some of which, represented predominantly in one phase, have a difference of potential of one polarity, others represented chiefly in the other phase having a potential difference of opposite polarity. If this is valid, potential differences of opposite polarity must be exhibited by cortical pyramids with a uniform anatomical orientation. A difference in the distribution or intensity of response of apical and basal dendrites is suggested as a possible factor. Polarization from an external source across a simple cell layer may either increase the amplitude of the monophasic difference of potential led from across it, or decrease and finally reverse its sign; presumably by altering the relative amplitudes of electrical activity at the two poles of the cells, but without blocking conduction from pre- to post-synaptic axons. The interpretation of this fact involves that polarizing currents act on cells essentially as they do on peripheral nerve axons. The change in polarity of record is in a direction opposite to the sign of the polarizing current. Polarization of optic tract axons may result in prolonged after-effects, negative under anodal polarization and positive under cathodal. This is interpreted as representing a repolarization, after discharge during the spike of the externally applied component of membrane charge, along with its resting metabolically induced component. This effect also occurs at axon end-arborizations, and changes in synaptic conduction may be assigned to such effects of sufficient intensity as well as to polarization of post-synaptic dendrites in the same region. Under polarizing currents the changes in amplitude and reversals of sign of the cortical alpha rhythm, of evoked potentials from shocks to the optic nerve including the evoked alpha waves, evoked potentials from the superior colliculus, strychnine spikes, and paroxysmal spikes of cortex set up by repetitive stimulation or otherwise all may be accounted for by the interpretation offered for the effects of polarization at the geniculate.