Definition of Retina
One of the most complex and intricate structures of the eye is the retina. The tissue is so complex that it is categorized as neural tissue. In fact, the retina could be likened to one large neuron with a really long axon (tail).
This neuron is like the film in a camera. It takes an image, turns it into an electronic signal and sends it via the optic nerve to the occipital lobe of the brain.
The occipital lobe processes the signal into the image we see. Interestingly, images taken by the retina are actually upside down and flat. These images are turned around and given depth by the brain.
The Cells of the Retina
There are six different types of cells that make up the retina and help to transmit images to the brain. These cells are:
- Ganglion cells
- Amacrine cells
- Müller cells
- Bipolar cells
- Horizontal cells
- Photoreceptor cells
The ganglion cells are the only cells of the retina that are capable of firing an action potential, or signal, to the brain. Their axons make up the nerve fiber layer that becomes the optic nerve.
Amacrine and horizontal cells act as interneurons. Amacrine cells modulate the activity of the ganglion cells by exciting them. Horizontal cells are responsible for lateral inhibition. This means they turn off surrounding photoreceptor cells in order to block out excess visual information.
Müller cells are unique to the retina and act like the glial cells in the brain. They maintain the stability of retinal cells by cleaning up neurotransmitters and other cellular trash (phagocytosis), as well as regulating potassium levels, and maintaining the electrical insulation (myelin) of photoreceptor and ganglion cells.
Bipolar cells are the main link between the photoreceptor cells and the ganglion cells. The photoreceptors are what translate photons of light into signals sent to the brain to provide visual images. These cells essentially capture the images we see. The photoreceptor cells are divided into two types of cells:
- Rods - extremely sensitive to light requiring only one photon to activate, making rods the cells we use at night and in low light conditions. They are shaped like rods which maximizes the amount of photopigment (rhodopsin) they can hold.
- Cones - Shaped like little cones, they are not as sensitive to light as rods, requiring 100 photons of light to activate. However, they do provide fine detail and color vision, and recover more quickly after activation.
The cones are mainly condensed into a small area called the fovea located in the macula, which provides our central and most detailed vision. This detail is partially because there is a one to one ratio of cones to ganglion cells in the fovea. This essentially means that each cone has it's own axon in the nerve fiber layer that creates the optic nerve, making signal transmission extremely efficient. There are about 5 million cones in the human retina.
The rod cells are found in the periphery of the retina and are responsible for our peripheral and night vision. They are very sensitive to dim light, but the ratio of ganglion cells to rods is relatively high. There are typically about 15 to 30 rods to one ganglion cell. Because of this, the visual acuity provided by the rods is very poor. The human retina contains about 100 million rod cells.
It is interesting to note that rods and cones adapt differently to light. Both photoreceptors can become saturated with light, meaning they cease to function because the photopigment in the cell is used up. Once the photopigment is replenished the cells fire again.
For example, a deer in headlights. Rods are the only cells active when it is completely dark outside. But rods are easily saturated because they react very slowly to changing light conditions and are activated by just one photon of light. When a deer looks into the headlights of a car, their rods become completely saturated effectively blinding them, which is why they freeze. They can't see where to go.
Cones are difficult to saturate because they react very quickly to changes in lighting and are not as sensitive to light as rods. But they can be saturated. When you look into a bright light and then look away and see a purple, green or black circle in your central vision, you've saturated your cones. This also happens when looking at a certain color for long periods, because the photopsin stored in that color's photoreceptor cell is used up.
The Layers of the Retina
The retina has nine layers of highly specialized neuronal tissue. These layers are listed below in order of the back of the eye to the front of the eye, meaning the deepest layers of the retina are listed first.
- Retinal pigment epithelium – prevents the back scattering of light, also absorbs photoreceptor disk membrane and recycles the photopigments used by rods and cones.
- Photoreceptor Outer Segment – contains the light sensitive portions of the rods and cones that actually capture the image. Both rods and cones are associated with a protein called opsin. Each cone is associated with what is called a photopigment, or pigment of a certain color. There are three types of cones that are sensitive to different wavelengths of light: short, medium and long wavelengths.
- External Limiting Membrane – connects the photosensitive processes of the rods and cones to their cell bodies, Mϋller cells are also a part of this layer
- Outer Nuclear Layer – made up of the cell bodies of both the rods and the cones
- Outer plexiform layer – comprised of the axonal processes of the rods, cones and horizontal cells, as well as the dendrites of the bipolar cells. This layer begins the processing of the image.
- Inner Nuclear Layer – contains the cell bodies of the amacrine, horizontal and bipolar cells and continues processing the image
- Inner plexiform layer – contains the axons of the bipolar cells and the dendrites of the ganglion cells and finishes processing the image
- Ganglion cell layer – comprised of the ganglion cells that fire the signals to the brain, this layer sends the image through the nerve fiber layer
- Nerve fiber layer - essentially the axons, or tails, of the ganglion cells below it. It is quite literally the fibers that make up the entirety of the optic nerve. This layer sends the image through the optic nerve to the occipital lobe of the brain
Put simply, the retina generates and partially processes the visual image before it is sent to the occipital lobe of the brain.
As we said earlier, the macula of the retina is highly populated with cone cells. Rod cells are completely absent in this part of the retina.
The center of the macula is called the fovea and is even more saturated with cones than the macula itself. This area is also devoid of any blood vessels, making it extremely efficient in transmitting images. However, the fovea can also be easily damaged by disease leading to permanent central vision loss.
Diseases of the Retina
Macular degeneration is one such disease. It has been nicknamed the “faceless” disease because when we look at a person’s face, we are using the fovea of our macula. There are two types of macular degeneration: wet and dry.
As we get older, hyaline deposits, known as drusen, form under the retinal pigment epithelium (RPE) layer (the layer that focuses images) in what’s called Bruch’s membrane. Bruch’s membrane separates the choroid layer, the layer that provides the blood vessels for the retina, from the RPE layer.
In dry macular degeneration, there isn’t a lot of drusen. Instead, the theory is that the immune system mistakes retinal cells for foreign invaders and begins to attack them, destroying them and the vision they provide.
In wet macular degeneration, the drusen causes the RPE layer to become separated from the choroid, effectively disconnecting its blood supply. New blood vessels begin to grown in to compensate, but these blood vessels are usually quite weak and are prone to leakage. Any blood that leaks from these vessels causes damage to the neuronal cells of the retina, causing vision loss.
There are treatments available for this disease; however, they are no guarantee that vision will be saved, or restored.
The retina is an extremely complex part of the eye. Indeed, there have been numerous books written on the subject. This is just a brief overview of the retina and how it works, but even this overview gives you a greater appreciation for how delicate our eyes are.