Extracellular electrical stimulation of the cortical grey matter is very often used in electrophysiological studies, but the parameters of the stimulation itself have received only little attention. This study addresses the issue of the spread of stimulating current in rat visual areas 17 and 18a maintained in vitro. The preparation of the slices relied on a protocol making use of several of the means known to limit the effects of ischaemia: Halothane anaesthesia was used during the surgery and intracardiac perfusion was employed to reduce the brain temperature, to increase the intracerebral concentration of glucose and magnesium and to decrease that of calcium. The spread of stimulating current has been determined from strength-distance relationships established for the activation of axons. The strength-distance curves could be fitted by a quadratic relationship, indicating that the threshold current for the activation of an axon increases as the square of the distance separating it from the tip of the stimulating electrode. The slope of the regression line between threshold intensity and squared distance (k coefficient) is highly variable from one axon to another (range 2100-27 500 microA/mm2, median 8850 microA/mm2). Part of this variability is related to differences in conduction velocity. The theoretical number of axonal branches and axon initial segments activated by a given current intensity has been extrapolated from these experimental results.
Action Potentials Animals