Functionning of a photoreceptor in diurnal vision. Phototransduction (transformation of luminous energy into nerve impulse) occurs in the external segment of photoreceptors (cones and rods). In the darkness (see. A flux of sodium ions (in violet), called dark current, thereby penetrates from the outside towards the inside of the photoreceptor. The membrane of the photoreceptor is then depolarized, creating an electrical signal, the nerve impulse, that will be transmitted to bipolar neurons, then to ganglion cells and eventually to the brain through the optic nerve. At the level of the external segment of a photoreceptor, the plasmic membrane forms folds (cone) or disks (rod). At the level of those folds photopigments are inserted (in red), that are composed of retinal (derivative of vitamine A, in pink) and opsin (rhodopsin in the rods, 3 photopsins in the cones). In absence of light, the retinal has a folded shape (cis-retinal), that enables it to embed into the opsin. In presence of light, the cis-retinal absorbs a photon. It then passes under a folded isomeric shape, the trans-retinal, and break away from the opsin. The release of trans-retinal is at the origin of a cascade of events leading to the degradation of cGMP linked to sodium channel, thereby at the closing of this channel and at the interruption of the Na+ ions flux. The membrane of the photoreceptor then hyperpolarize and the bipolar neurons are inhibited. See.
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