SPREAD OF STIMULATING CURRENT IN THE CORTICAL GREY-MATTER OF RAT VISUAL-CORTEX STUDIED ON A NEW IN-VITRO SLICE PREPARATION

Extracellular electrical stimulation of the cortical grey matter is ve ry often used in electrophysiological studies, but the parameters of t he 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 r elied on a protocol making use of several of the means known to limit the effects of ischaemia: Halothane anaesthesia was used during the su rgery and intracardiac perfusion was employed to reduce the brain temp erature, to increase the intracerebral concentration of glucose and ma gnesium and to decrease that of calcium. The spread of stimulating cur rent has been determined from strength-distance relationships establis hed for the activation of axons. The strength-distance curves could be fitted by a quadratic relationship, indicating that the threshold cur rent for the activation of an axon increases as the square of the dist ance separating it from the tip of the stimulating electrode. The slop e of the regression line between threshold intensity and squared dista nce (k coefficient) is highly variable from one axon to another (range 2100-27 500 mu A/mm(2), median 8850 mu A/mm(2)). Part of this variabi lity is related to differences in conduction velocity. The theoretical number of axonal branches and axon initial segments activated by a gi ven current intensity has been extrapolated from these experimental re sults.