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 place 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 initial electrical signal is then relayed onto the functional cells of the retina. The bipolar cells are the first associated neurons, responding to the light stimulus by either hyperpolarization (OFF), or depolarization (ON). The second associated neurons are the ganglion cells where 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 transmission, horizontal cells delimit excitatory (center) and inhibitory (surround) zones at the origin of the receptive field. In scotopic vision, however, i) there is only a single class of bipolar cells, that depolarize in response to light, and ii) the bipolar-ganglion cell connection is not direct. Here, the AII amacrine cells are responsible for the inhibition of the OFF ganglion cells directly connected to them or for the excitation of the ON ganglion cells via ON bipolar cells of the cone circuit. Finally, in mesopic vision, the sensory message originates in rods, and is subsequently relayed by the cone circuit via gap junctions between photoreceptors.