Retinal is aldehyde of retmol, the retmol (which contains an alcoholic group, fig 7.14.1) being vit A Retinal is also known as retinene. In short reti

In rhodopsin, the retinal belongs to the cis variety. The particular type of opsin that occurs in the rhodopsin is sometimes called scotopsin. When light strikes the rods, the cis retinal is converted into trans retinal. This conversion, by some means, produces a local electrical change, i.e., causes development of a receptor potential (see sec XB2 chap 3, photoreceptor potential, for details) which marks the beginning of the nerve impulse. Subsequent fate of the retinal The trans form of the retinal can no longer remain in combination with the opsin, so separation of opsin and the trans form of the retinal occurs. Subsequently the trans form of retinal enters into the pigment layer of the retina (which lies immediately outside the layer of rods and cones) leaving the opsin within the rods. In the pigment layer, the trans form is converted into the cis form of retinal which then comes back to the rod and reunites with the opsin to form rhodopsin again. The whole process is called regeneration of the photopigment, rhodopsin The separation of opsin and trans retinal is called 'photo decomposition'. When photo decomposition occurs, the rhodopsin is said to be bleached. Cones Cones are necessary for (i) daylight or bright light (photopic) vision, (ii) color vision, and (iii) acuity of vision. In the macula, particularly in the fovea there are only cones (and no rods).