TRANSDUCTION OF THE LIGHT MESSAGE - FROM THE PHOTON TO THE OPTIC-NERVE

Retinal transduction consists of the conversion of a physical stimulus , light, into an electrophysiological signal. This conversion takes pl ace in several stages. First of all, at the photoreceptor level, via a sequence of molecular activations and deactivations, the detection of light results in an hyperpolarization of the cell membrane. This init ial electrical signal is then relayed onto the functional cells of the retina. The bipolar cells are the first associated neurons, respondin g to the light stimulus by either hyperpolarization (OFF), or depolari zation (ON). The second associated neurons are the ganglion cells wher e the ON-OFF duality also operates and whose fibers make up the optic nerve. In coloured photopic vision, the photoreceptor - bipolar cell - ganglion cell circuit is direct. For the cone-bipolar cell transmissi on, horizontal cells delimit excitatory (center) and inhibitory (surro und) zones at the origin of the receptive field. In scotopic vision, h owever, i) there is only a single class of bipolar cells, that depolar ize in response to light, and ii) the bipolar-ganglion cell connection is not direct. Here, the All amacrine cells are responsible for the i nhibition of the OFF ganglion cells directly connected to them or for the excitation of the ON ganglion cells via ON bipolar cells of the co ne circuit. Finally, in mesopic vision, the sensory message originates in rods, and is subsequently relayed by the cone circuit via gap junc tions between photoreceptors.