The action of light on the eye

THE optic nerve is unlike any other sensory nerve in that the receptor elements in the retina are not in imihediate connection with the fibres of the nerve but are linked to them through a chain of neurones and synapses. This arrangement can hardly fail to affect the nature of the optic nerve discharge and a knowledge of how it is affected, besides telling us something about the eye, might also give results of more general interest concerning the action of the synaptic junction. In previous papers(l) on the action currents in the eel's optic nerve we have called attention to two features of the nerve discharge which might depend on synaptic conduction, namely, the long and variable reaction time and its dependence on the area illuminated as well as on the intensity of the light. In discussing these points we suggested a temporal and spatial summation in the nervous paths as a possible explanation, but we could not exclude the alternative possibility that they were due to the preliminary changes caused by the light before any nervous excitation had taken place. Meanwhile a chance observation had given us evidence of synaptic activity of an entirely different kind and the investigation of this has now led to the conclusion that the reaction time is also dependent upon synaptic activity.