FUNCTION OF RETINA
Light perception is the main function of the retina. The light sensitive cells are the rods and cones. Light rays cause chemical changes in photosensitive pigments in these cells and they emit nerve impulses which pass to the occipital lobes of cerebrum via the optic nerves.
The rods are more sensitive than the cones. They are stimulated even in low intensity or dim light, e.g. by the dim light in the interior of a darkened room (scotopic vision).
The cones are only sensitive to bright light; therefore, colour can only be seen in daylight. The different wavelengths of light stimulate photosensitive pigments in the cones, resulting in the perception of different colours. In bright lighting, the light rays are focused on the macula lutea.
The rods are more numerous towards the periphery of the retina. Rhodopsin (Visual purple) is a photosensitive pigment present only in the rods. It is bleached by bright light and when this occurs the rods cannot be stimulated. Rhodopsin is quickly reconstituted when an adequate supply of
vitamin A is available. When the individual moves from an area of bright light to one of dim light, there is a time period when it is difficult to see.
The rate at which dark adaptation takes place is dependent upon the rate of reconstitution of rhodopsin. In dim evening light, different colours cannot be distinguished because the light intensity is insufficient to stimulate colour sensitive pigments in cones.