Describe the Initiation of Visual Impulse
When light falls on the retina it is absorbed by the photosensitive pigments of rods (rhodopsin) and cones (photopsin). It initiates photochemical changes such as rhodopsin bleaching and regeneration that stimulate electrical changes. Rhodopsin consists of a protein (opsin) and a carotenoid-11-cis retinal. Light that is absorbed by rhodopsin converts 11-cis retinal into trans retinal. This process is an isomerisation that occurs through the formation of many chain reactions and the reduction of c-GMP. The reduction of c-GMP is responsible for closure of sodium channel, leading to hyperpolarisation and electric response in bipolar cells and other neural cells. Thus producing electric response. Then all trans retinal reisomerise into 11-cis retinal and then combine with opsin to form rhodopsin. A similar process occurs for photopsin.