Through the retina of our eyes, this fragile membrane sensitive to light, we are able to perceive images that we will always remember.
This article will answer questions related to parts of the retina and how they work, For example, what type of cells makes it up or what structures are responsible for processing color.
What is the retina?
The retina is a complex sensory membrane located on the posterior surface of the innermost layer of the eyeball. This area of the eye is responsible for receiving images from the outside to transform them into nerve signals that will be transmitted to the brain through the optic nerve.
Almost all parts of the retina are made up of thin, transparent tissue made up of a collection of nerve fibers and photoreceptor cells, which are specialized cells responsible for converting light into signals sent to the brain.
The retina is usually reddish or orange because there are a large number of blood vessels located just behind it. The periphery or the outer part of the retina is responsible for peripheral vision (which covers up to almost 180 ° with the eye) and the central area of central vision (which helps us recognize the faces of people or to read).
However, it must be said that the retina is a fundamental structure of the human eye and our vision depends on it and our eye health.
Parts of the retina
The parts of the retina and their anatomical composition can be described from two structural levels: the macroscopic level and the microscopic level.
On the surface of the retina, several structures can be observed detailed below:
1. Papilla or optic disc
The papilla or optic disc is a circular area located in the central area of the retina. From this structure originate the axons of the retinal ganglion cells which form the optic nerve.. This area has no sensitivity to light stimuli, which is why it is also known as the “blind spot”.
The ocular macula or oriole macula is the area responsible for central vision and that this allows us to see with maximum visual acuity, Being the area of the retina with the highest density of photoreceptor cells.
Located in the center of the retina, it is responsible for detailed vision and movement. Thanks to the macula, we can distinguish faces, colors and all kinds of small objects.
The fovea is a shallow crack located in the center of the ocular macula. This structure is responsible for most of the total visual acuity, being the focal point of reception of light rays reaching the retina, and has only conical photoreceptors, responsible for color perception.
4. Now tense
Ora serrata is the most anterior and peripheral part of the retina, where it comes in contact with the ciliary body, a structure responsible for the production of aqueous humor (a colorless fluid found in the anterior part of the retina) . and the change in lens shape to get the correct eye accommodation or focus.
If we go to a microscopic level, we can see how different parts of the retina are grouped together in layers. We can differentiate up to 10 parallel layers, which are as follows (from the most superficial to the least):
1. Pigmented epithelium
It is the outermost layer of the retinaIt is made up of cubic cells that are not neurons and enjoy granules of melanin, a substance that gives them a characteristic pigmentation.
2. Coverage of photoreceptor cells
This layer is made up of the outermost segments of the cones (responsible for color differentiation or visual acuity) and rods (responsible for peripheral vision).
3. External limitation cover
It is made up of junctions between cells of the adherent zonule type (area surrounding the outer surface of the cell and containing dense filamentous material) between photoreceptor cells and Müller cells (glial cells of cells responsible for auxiliary functions).
4. Nuclear or granular outer layer
This layer is formed by the nuclei and bodies of photoreceptor cells.
5. Outer plexiform layer
In this layer, the synapse between photoreceptor cells and bipolar cells is made.
6. Grainily cover the inner core
It is formed by the nuclei of four types of cells: Bipolar, horizontal, Müller and amacrine cells.
7. Internal plexiform layer
It is the region of synaptic connection between bipolar, amacrine and ganglion cells. This layer is formed by a dense fabric of fibrils arranged in a network.
8. Layer of ganglion cells
This layer is formed by the nuclei of the ganglion cells. Located on the inner surface of the retina, receive information from photoreceptors via bipolar, horizontal and amacrine intermediate neurons.
9. Layer of optical nerve fibers
In this layer of the retina, we can find axons of ganglion cells which are the ones that form the optic nerve itself.
10. Internal limitation plug
This last layer is the one that separates the retina from the vitreous humorA clear, gelatinous liquid located between the retina and the lens that helps maintain the shape of the eyeball and helps make image reception clear.
Cell types: an inside look
In addition to having a layered structure, the retina is made up of three types of cells: pigmented cells – responsible for photoreceptor metabolism -, neurons and support cells – such as astrocytes and Müller cells, the function is to support other nerve cells.
The five main types of retinal neurons are described in more detail below:
1. Photoreceptor cells
They consist of two major classes of cells: cones and rods. Cones are more concentrated in the center of the retina and are the only type of photoreceptor cell found in the center of the retina (the fovea). They are responsible for color vision (also called photopic vision).
The rods are concentrated on the outer edges of the retina and are used in peripheral vision. These photoreceptors are more sensitive to light than cones and are responsible for almost all night vision (also called scotopic vision).
2. Horizontal cells
There appear to be two types of horizontal cells, each with a different shape, which when combined provide information to all photoreceptor cells. Despite the number of cells with which they form synapses, this type of cell represents a relatively small population of retinal cells (less than 5% of the cells in the inner nuclear layer).
again the reason why there are two classes of horizontal cells is not knownBut it is speculated that it might have to do with identifying color differences in the red / green system.
3. Amacrine cells
Amacrine cells allow ganglion cells to send temporally correlated signals to the brain; that is, the information transmitted by the same amacrine cell to two different ganglion cells would cause these ganglion cells to send signals at the same time.
These cells generate synaptic connections with axonal terminations of bipolar cells and with dendrites of ganglion cells.
4. Bipolar cells
Bipolar cells connect to photoreceptors with ganglion cells. Its function is to transmit signals from photoreceptors to ganglion cells, Either directly or indirectly.
This type of cell has a central cell body from which two different groups of neurites (axons and dendrites) extend. They can connect with rod or cone photoreceptors (but not both at the same time) and can also define connections with horizontal cells.
5. Nodal cells
ganglion cells are the cells from which information comes from the retina. Their axons leave the eye, pass through the optic nerve and reach the brain to send the previously processed visual stimulus to the lateral geniculate nucleus (primary visual information processing center).
When they reach this final processing nucleus, they form synapses with neurons projecting into the primary visual cortex, an area specializing in processing information about static and moving objects, as well as pattern recognition, and the “ visual stimulus is finally interpreted.
From eye to brain: how visual information travels
Light stimuli picked up by the retina are conducted through the optic nerve to the brain, where the information is processed and we actually “see” what we have in front of our eyes.
When the optic nerves enter the skull, they cross to form the optic chiasmus. This structure exchanges part of the fibers of each nerve to the opposite side, so that those that carry the vision of the right half and the left half of our visual field are grouped separately.
The perceived information continues through the optical ribbons until it reaches the geniculate nuclei, Where fibers are classified so that every point in the optical field is recorded more accurately. From the geniculate nuclei originate a bundle of nerve fibers (optical radiation) that cross each cerebral hemisphere to reach the occipital lobe, the posterior area of the brain which is responsible for processing visual information.
The paradox of our brain is that it processes visual information in reverse; that is, images on the left side are “seen” in the right hemisphere and vice versa. Likewise, the images seen at the top are processed at the bottom of the hemispheres and vice versa. Mysteries of visual processing.
- Richard S. Snell (2003). Clinical neuroanatomy. Pan American Medical.