STUDIES OF THE PUPILLARY LIGHT REFLEX PATHWAY OF RABBITS BY THE EVOKED POTENTIAL METHOD

Summary The present experiments aimed at examining the results obtainable both by the intersection of the posterior commissure and by the stimulation of the pretectal region, the midbrain and other parts with an intention of making the course of light reflex pathwxy more clear-cut and precise. In twenty adult rabbits the parts causing a pupilloconstriction by the electric stimulation were looked for, and the apperance of evoked pctential from these parts upon photic stimulus and that of responses from the parts not causing a pupillary symptom but responding to a photic stimulus were pursued by means of the cathode ray oscillograph. The results are summarized as follows:. 1. The parts which cause a pupillccon striction as the reaction to electric stimulation are divided into two groups, one accompanied by evoked potential. The parts at which an evoked potential is recorded can be thought to receive an impulse from retina and to form a part of the light reflex pathway. On the other hand, those at which no evoked potential appears are regarded as a pupillocon strictor pathway but as having nothing to do with the light reflex pathway, though there are found some parts with exceptional respons. 2. As the parts which cause the bilateral pupilloconstriction upon electric stimulaticn and do give rise to the evoked potential upon bilateral photic stimulation, are to be mentioned the midline part of the posterior commissure including its neighbouring parts, and the midline part on the base of menencephalic central gray matter. The parts which cause a pupilloconstriction of similar degree on either side give birth to an evoked potintial of nearly similar wave form to the bilateral photic stimulus. 3. The parts which cause the contralateral pupilloconstriction upon electric stimulation and which give rise to an obviously evoked potential upon photic stimulus given to the pupilloconstrictor side, are the pretectal region. especially Area pretfctalis propria, Nucl. lentiformis pars parvocel-lularis and Nucl. interstitials cormissurae posterioria. The nature of the evoked potentials reveals that these parts represent some sort of relay nuclei in the light reflex pathway. 4. The parts which cause the ipsilateral pupillcconstriction and at which an evoked potential appears, are in the main the descending peduncle of the posterior commissure and the pudloconstrictor nucleus. the evoked potential at the descending Feduncle of the posterior commisure shows a monophasic negative wave, while that at the pupilioconstrictor nucleus gives a triphaslc wave. A distinct difference is always Obse vable between them. 5. Even when evoked potentials upon photic stimulus can be recorded from some parts of the lateral geniculate body, the superior colliculus, the optic radiation and the mesencephalic reticular formation, there is missed any pupilloconstrictor response upon electric stimulation there. An evoked potential of the fibres reaching from the optic tract to the pretectal region, if any, must be overlappd and overturned by the above-mentioned potentials and, therefore, is indeterminable so far under this circumstance. From the above data, we believe. we could demonstrate the validity of conclusion obtained from intersection of the posterior commissure and from stimulation of the pretectal region and the midbrain in rabbits, that an afferent path of light reflex changes neuron in the pretectal region. makes a complete decusation at the posterior commissure, passes through the descending peduncle of the posterior commisure and ultimately reaches the pupilloconstrictor nucleus.