Of all the sensory receptors in the body, 70% are in the eye. The photoreceptors sense and encode the patterns made by light in our surroundings. The massive optic tracts that carry the coded information from the eyes to the brain contain over a million nerve fibres. Our brain invests the signals from our eyes with meaning, fashioning images of the world around us.
This illustration shows the first step of the visual process. Photons of light focus on the retina causing isomerisation at a double bond between two carbon atoms in rhodopsin. Image taken from www.biovisuals.com/rhodopsin.html without permission.
Once light has been focused on the retina the photoreceptors come in to play. Photoreception is the process by which the eye detects light energy. To approach this subject several areas need to be discussed
The functional anatomy of photoreceptor cells.
The location and packaging of the visual pigments that absorb the light stimulus.
The chemistry of the visual pigments and their response to light.
Activation of the photoreceptors and their response to varying levels of light intensity.