Positive Potentials Recorded from the Superior Colliculus. ∗

Abstract

A needle electrode thrust into an active nerve pathway may so kill nerve fibers in its vicinity that one obtains an effective dead-end lead, with a positive potential. When 2 electrodes are effectively on opposite sides of a region of synapses, the endings of fibers at synapses act similarly. In the superior colliculus of the cat one regularly finds that a probe electrode thrust downward encounters a shallow negative post-synaptic potential, following stimuli to the optic nerve, then a deeper positive potential, usually of considerably higher voltage. Records obtained as the electrode is withdrawn may show, on the other hand, a reversal of the initial upper negativity, giving a positive spike at any level. The potentials encountered in the colliculus may be explained by conventional physiological considerations without the inference of a relative positivity at an active region of a tissue.

From the central region of synapses with optic tract fibers, in the colliculus, pathways radiate out in several directions. In the extreme case, of fibers conducting in all directions from a center, one can consider as an element the cone whose base is a small surface area, subtending a solid angle at the center. Since in the symmetrical case activity in elements adjacent to this will prevent current from flowing across the boundary of such a cone, it may be considered as if insulated in air, and for a small surface area, this element approaches a linearly arranged nerve fiber bundle. The critical feature of the solid figure to be retained is that the shunting material between the fibers will increase, and the resistance decrease, as the square of the distance from the center.

The potential distribution along the surface of such an element will be triphasic, and the positive phase toward the center will be of higher amplitude, because less shunted, than the peripheral positivity. From electrodes placed at the center and at the periphery, therefore, an overall central positivity will be recorded, the more so as the peripheral electrode will usually be at some distance from the ends of active fibers.

The negative spike recorded peripherally will be increasingly shunted, and therefore decrease in recorded amplitude, as the electrode is moved away from the central synaptic region, and the transition from negative to positive phases should be correspondingly abrupt at the central ends of the conducting elements.