What is the Retina?
When having a conversation with someone, for example, you are using the macula to look at the other person’s face and the peripheral retina when you notice someone else walking into the room. It follows that when any part of the retina is damaged or diseased, there is loss of vision in the area it serves; when people have macular problems, whatever they are looking at directly is unclear, missing, distorted or often a combination of these. Peripheral retinal problems may either not be noticed at all or affect the vision away from the centre.
The retina, which is actually part of the brain, is transparent and has several layers. The outermost layer consists of a sheet of up to 130 million photoreceptors (light-sensitive cells).
There are two basic types of photoreceptor: rods and cones. In simple terms, cones are concentrated in the macula, work best in bright light and are responsible for fine-detailed vision. Rods, conversely, are fairly evenly distributed throughout the retina (except the fovea, where they are absent), and are more sensitive to dim lights and adapt to the dark (dark adaptation) better and faster.
Photoreceptors contain pigments that absorb the light that enters the eye, resulting in chemical changes that then cause small electrical signals to be generated. These electrical signals pass through the other layers of the retina, which process the information, and into the optic nerve towards the occipital lobe of the brain. This is located inside the very back of the skull. The retina has its own blood supply that consists of a central retinal artery and its branches and the central retinal vein and its tributaries. These pass in and out of the eye, respectively, through the optic nerve. This blood supply only serves the inner two thirds or so of the retina, with the outer retina (most significantly the photoreceptors) gaining oxygen and nutrients from the choroid. This is a dense network of blood vessels just inside the sclera (tough white outer coat of the eye) and is separated from the retina by a single-layered sheet of cells called the retinal pigment epithelium (RPE).
The RPE is darkly pigmented and has several roles in acting as a ‘caretaker’ to the photoreceptors. One of the most important roles is in removing waste products from the photoreceptors, such as the parts of the photoreceptors that are shed throughout the day after absorbing light and generating the electrical signals going to the brain.Another function of the RPE is to absorb light once it has passed through the retina so that it doesn’t ‘bounce’ around inside the eye. This prevents glare and improves vision.
The inner limiting membrane (ILM) a sheet-like membrane normally covering the surface of the retina and is often peeled away during vitrectomy for macular disease. This is partly in order to allow the macula to return more easily towards its normal shape and partly to ensure that all of the vitreous is removed from the macula at the time of surgery.