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 appropriate environment for the normal functioning of the ganglion cells.
Bipolar cells are the main link between the photoreceptor cells and the ganglion cells. The photoreceptors are what deliver photons of light 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 (used for night vision)
- Cones (used for color vision and allow us to see fine details)
The cones are mainly condensed into a small area called the fovea located in the macula, which provides our central and most detailed vision. These cells work in the daytime because they need a lot of light to be activated.
These cells are actually shaped like little cones, which gives them a much faster reaction time than the rod cells. This translates into much better visual acuity. 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 their reaction time is much slower than that of the cones. Because of this the visual acuity provided by the rods is very poor. The human retina contains about 100 million rod cells.
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 – acts like a filter to block out any light that is not captured by the photoreceptor cells, in essence the retinal pigment epithelium sharpens the image, it is also where phagocytosis (removal of cellular trash) takes place
- 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: red, blue and green.
- 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 to the optic nerve
- 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 to the occipital lobe of the brain
The retina essentially 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.
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.
Each cone is associated with its own photopigment in one of three colors: red, green or blue. This provides us with our color vision. In those who are color blind, one or more of these photopigments is missing, which pigment is missing determines the type of colorblindness the person has. There is no treatment for color blindness.
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.