Light that strikes the retina trigger nerve impulses which travel through the optic nerve to the occipital lobes of the brain. Embryologically, the retina and the optic nerve originate as outgrowths of the developing brain, so the retina is considered part of the central nervous system.
The retina is made up of several layers of interconnected neurons. The only neurons that are sensitive to light are called photoreceptor cells which consists of two types; the rods and cones. Rods function in dim light, while cones are stimulated by the stronger light of daytime. There is also a third type of photoreceptor, the photosensitive ganglion cell.
Nervous signals from the rods and cones are processed by other neurons of the retina. The output takes the form of action potentials in retinal ganglion cells whose axons form the optic nerve.