The action potentials leave the eye via the optic nerve to the visual cortex of the brain where they are interpreted into patterns and images . Each retina has around 6 million cones and 120 million rods. The rods are more sensitive than the cones and are stimulated by low intensity or dim light. They enable us to see at night but only in black and white. The rods also allow us to discriminate between different shades of dark and light and allow us to see shapes and movement.
Cones provide colour vision and high visual acuity in bright light.
It takes more light to stimulate cones, they therefore do not function in night vision. The conversion of light into action potentials occurs in the outer segment of the photoreceptor, this is where photopigments are present. Photopigments are coloured proteins that undergo structural changes upon light absorption, initiating events that lead to the production of action potentials. All photopigments contain two parts, a derivative of vitamin A called retinal which is the light absorbing portion and a glycoprotein called opsin which is bound to retinal. In the human retina there are four different opsins that differ in their structure between photoreceptors.