Rhodopsin or Visual Purple is a puplish red photosensitive pigment
present in the outer segment of the rods (120 million rods). It is made up of protein portion, an opsin (scotopsin) combined with an aldehyde of vitamin
A called Retinene 1, which is referred to as a retino. On exposure to light,
rhodopsin is bleached, ie., broken down to retinene and opsin, but is
resynthesised in the dark. Some of the retinene recombines with scotopsin to form rhodopsin while some are reduced to vitamin A. The rods are extremely sensitive to light and are responsible for vision in dim light. This is called SCOTOPIC VISION.
Cones also contain visual pigments made up of retinene, combined
with a protein opsin (photopsin). Three pigments each responding to
different wavelength are found in man. There are three primary colors namely red, green and blue. Color vision is a function of bright light vision and cones are responsible for color perceptions. In bright light maximum perception of colors is at the fovea region of the retina where rods are absent and only cones are present. In dim light when the rods in the extra foveal retina function, colors are not perceived and the various colors appear as shades of grey. Cones function in bright light and the system has more acuity and can perceive colors (PHOTOPIC VISION).
Photochemical basis of retinal function is a basis of conversion of
light energy into nerve impulses. It is this process that excites the nerve fibres and sets up nerve impulses.
The impulses generated in the receptor neurons in response to generator potentials in the cones are interpreted by the brain as the appropriate intermediate colour. The interpretation or perception of colour pictures seen by our eyes is a complex function of the brain. It is located in the occipital lobe of the cerebral cortex.


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