When light enters the eye, it first passes through the cornea, then the aqueous humor, lens and vitreous humor. Ultimately it reaches the retina, which is the light-sensing structure of the eye. The retina contains two types of cells, called rods and cones. Rods handle vision in low light, and cones handle color vision and detail. When light contacts these two types of cells, a series of complex chemical reactions occurs. The chemical that is formed (activated rhodopsin) creates electrical impulses in the optic nerve. Generally, the outer segment of rods are long and thin, whereas the outer segment of cones are more, well, cone shaped. Below is an example of a rod and a cone:
The outer segment of a rod or a cone contains the photosensitive chemicals. In rods, this chemical is called rhodopsin; in cones, these chemicals are called color pigments. The retina contains 100 million rods and 7 million cones. The retina is lined with black pigment called melanin -- just as the inside of a camera is black -- to lessen the amount of reflection. The retina has a central area, called the macula, that contains a high concentration of only cones. This area is responsible for sharp, detailed vision.
When light enters the eye, it comes in contact with the photosensitive chemical rhodopsin (also called visual purple). Rhodopsin is a mixture of a protein called scotopsin and 11-cis-retinal -- the latter is derived from vitamin A (which is why a lack of vitamin A causes vision problems). Rhodopsin decomposes when it is exposed to light because light causes a physical change in the 11-cis-retinal portion of the rhodopsin, changing it to all-trans retinal. This first reaction takes only a few trillionths of a second. The 11-cis-retinal is an angulated molecule, while all-trans retinal is a straight molecule. This makes the chemical unstable. Rhodopsin breaks down into several intermediate compounds, but eventually (in less than a second) forms metarhodopsin II (activated rhodopsin). This chemical causes electrical impulses that are transmitted to the brain and interpreted as light. Here is a diagram of the chemical reaction we just discussed:
Activated rhodopsin causes electrical impulses in the following way:
- The cell membrane (outer layer) of a rod cell has an electric charge. When light activates rhodopsin, it causes a reduction in cyclic GMP, which causes this electric charge to increase. This produces an electric current along the cell. When more light is detected, more rhodopsin is activated and more electric current is produced.
- This electric impulse eventually reaches a ganglion cell, and then the optic nerve.
- The nerves reach the optic chasm, where the nerve fibers from the inside half of each retina cross to the other side of the brain, but the nerve fibers from the outside half of the retina stay on the same side of the brain.
- These fibers eventually reach the back of the brain (occipital lobe). This is where vision is interpreted and is called the primary visual cortex. Some of the visual fibers go to other parts of the brain to help to control eye movements, response of the pupils and iris, and behavior.
Eventually, rhodopsin needs to be re-formed so that the process can recur. The all-trans retinal is converted to 11-cis-retinal, which then recombines with scotopsin to form rhodopsin to begin the process again when exposed to light.