Photoreceptors and Visual Pathways

The retina contains two major types of photoreceptors— rods and cones.

Their inner segment, which contains most cell organelles, is situated in the neural layer.

The outer segment is embedded in the pigmented layer and contains discs that hold two main types of photopigments—retinal —a Vitamin A derivative, and opsin —a glycoprotein.

All rods contain one form of opsin and retinal, providing noncolor vision. Since rods are highly sensitive to light, they aid in dim light vision.

Depending on the opsin type, the cones are of three types—red, green and blue. These cells provide color vision by selectively activating the different photopigments.

The light signal detected by photoreceptors is converted to an electrical signal by bipolar cells and passed on to the ganglion cells.

These cells form the optic nerve that carries the signal through optic chiasma to the primary visual cortex in the brain for visual perception.

Depletion of photopigments or their irregular synthesis leads to nyctalopia, commonly known as night blindness; while the congenital absence of cone pigments causes multiple types of color blindness.