Vision and signal transmission

When light enters the human eye it is refracted by the cornea, lens and both the aqueous and vitrious humor and is focused onto the eye's retina. This refraction is not entirely perfect however, and so the eye can use cillary muscles to contract the lense for further adjustments. When the image from reflected light is not focused properly, this can lead to either hyperopic or myopic conditions. These are what is more commonly known as far sightedness and near sightedness respecively. When an indavidual is far sighted the image is focused behind the eye. When they are near sighted the image is focused in front of the eye.

The retina of the eye is covered in photoreceptors. These come in two forms, rods and cones. Rods are very plentiful, outnumbering cones nearly 20:1. These are used mainly during situations of dim light. Cones will however detect color and are used in bright light. When light enters the photoreceptor cells a protein called rhodopsin is bleached, causing a conformational change. Rhodopsin will then activate a G-Protein called tansducin, and the signal continues to propogate until sodium channels in the photoreceptors are closed, which in turn halts the release of glutamate. Post synaptic biopolar cells will respond to this lack of glutamate by either turning on or off, depending on the type. These bipolar cells then signal (or don't) ganglion cells, which continues to propogate th single to the brain where it is processed.