THE SHORT LATENCY DISCHARGE OF HIGH LIGHT-THRESHOLD IN THE RABBIT OPTIC NERVE

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  • 米村 大蔵
    Department of Ophthalmology, The School of Medicine, Kanazawa University
  • 土田 豊
    Department of Ophthalmology, The School of Medicine, Kanazawa University

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With single FLashes from a xenon discharge tube the electroretinogram (ERG) and light-induced optic nerve responses were recorded in the albino rabbit.The experiments were carried out in a dark room.<BR>1. The early receptor potential (ERP) in the rabbit consisted of a small positive potential followed by a large negative swing.The second phase of the ERP was succeeded by the a-wave with a latency which is as short as about 1.6msec.<BR>2. The conventional “compound spike potential of the optic nerve”, when induced with flash light, contains 1) the late-on-wave which is present only on low illumination and 2) the early-on-wave on which rhythmic wavelets are superimposed.The threshold of the wavelets appeared to approximate that of the oscillatory potential in the ERG.With increase of stimulus intensity the latency of the early-on-wave tended to decrease, finally approaching the absolute minimum latency (about 11msec).<BR>3. When the stimulus intensity was raised up to an level sufficient for producing an ERP of considerable size, a rapid potential with a latency of about 4.5-5.0msec, at a tentative estimate, was found to emerge preceding the earlyon- wave in the optic nerve potential.The new potential which may be called provisionally the short latency optic nerve potential (SLONP) was consistently recordable in the central and/or ventral area of the optic nerve, but scarcely detectable in the dorsal area.The amplitude of the SLONP was linearly related to the log of stimulus intensity in the dynamic range where the slope of amplitude vs log intensity is steepest.In this range the amplitude of the negative phase of the ERP was found to be proportional to stimulus intensity.<BR>4. The SLONP, along with the subsequent optic nerve potentials, were abolished with both nerve blockaders (lidocaine and tetrodotoxin) in low concentrations on their application to the retina.During this time the ERG components including the ERP were little affected.<BR>5. The foregoing results, coupled with other data inclusive of the findings obtained by the microelectrode experiments described in this study, indicate that the SLONP is the population response due to a short latency optic nerve discharge arising from particular optic nerve fibers, most of which leave the retina to pass through the contralateral optic nerve tract towards the brain.<BR>6. Differences between the SLONP and the early-on-wave in latency, lightthreshold, density function, vulnerability to anoxia, all suggest that the two potentials are distinct components.<BR>7. It was shown that there is some correlation between the amplitude of ERP and that of the SLONP.The functional significance of the SLONP was discussed.<BR>8. The comparative studies of the SLONP and the b-wave, which were obtained with regard to density functions and other aspects, may indicate that the two events behave independently.<BR>9. Picrotoxin in low concentration, when applied topically to the retina from the vitreous side, could give rise to enhancing effects on optic nerve potentials. The possibility was discussed that, in the retina, there might be a presynaptic inhibitory mechanism which can be depressed by picrotoxin.

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