The retina is a layer at the back of the eye that detects light. It consists of different types of specialised cells to help generate a visual image (what we see). One of these cells is called photoreceptors. They detect light coming into the eye, converting it into electrical signals which is then transmitted to the brain through a series of other specialised cells in the retina and the optic nerve.
Cells of the retina
The retina is composed of different types of cells, each having their own role in generating vision.
The photoreceptors are the main cells that help us generate vision. They are differentiated into two types, rods and cones.
Rods are the most abundant photoreceptor cell in the retina, responsible for vision in dim light and peripheral vision. Cone cells are responsible for our central vision, along with helping us to see colour and objects in detail under bright light. The number of cone cells in human is less compared to rods and they are mainly situated in the fovea.
Most inherited retinal conditions are due to progressive degeneration of the photoreceptors as a result of mutations in any one of more than 300 genes, which eventually lead to sight loss.
Retinal pigment epithelium (RPE)
Below the photoreceptors, there is a specialised thin layer of cells called the retinal pigment epithelium (RPE). These cells play a supportive role to the photoreceptors by providing nutrients and removing toxic waste material generated by them.
The bipolar, horizontal, amacrine and ganglion cells help transmit the electrical signals converted by the photoreceptors. The Mueller cells also play a supportive role by maintaining the structural and functional stability of all the other retinal cells.
Macula and fovea
The macula is the central part of the retina where incoming light is mainly focused on. It is responsible for our central vision. Our ability to thread a needle, distinguish facial characteristics and read books relies on the proper function of photoreceptors in the macula. As part of the retina, it also has specialised light detection cells called photoreceptors.
The fovea is a pit at the centre of the macula which only contains cone cells. Their density is the highest here compared to the rest of the retina and thus, our vision is the sharpest when light is focused here. The fovea is best visualised with a special scan called the optical coherence tomography (OCT) scan.