The Retina

The retina is the light-sensitive layer lining the back of the eye. It contains two types of photoreceptor, rods and cones, together with neurones that begin to process the visual information.The retina has three main layers of cells:• the photoreceptors (rods and cones), at the back, next to the pigmented layer
• bipolar neurones, which receive signals from the photoreceptors across synapses
• ganglion cells, which receive signals from the bipolar neurones. Their axons form the optic nerveThe retina is inverted: light has to pass through the ganglion cells and bipolar neurones before it reaches the photoreceptors. Behind the photoreceptors, a layer of pigmented epithelium absorbs light that has passed through, so it does not reflect around the eye and blur the image.Because the ganglion cell axons run across the front of the retina, they must pass through it to leave the eye. At this point there is no room for photoreceptors, which forms the blind spot.Rods and cones are not spread evenly. The fovea contains only cones, packed very tightly, and the overlying neurones are pushed aside so light reaches the cones directly. Away from the fovea, rods become much more common and cones rarer. There are about 120 million rods but only about 6 million cones in each human retina.Other neurones in the retina link neighbouring photoreceptors and bipolar neurones, so the retina starts to process the image, for example by increasing contrast at edges, before impulses travel along the optic nerve to the brain.