Parts Of The Retina: Layers And Cells That Make It Up
Through the retina of our eyes, that fragile membrane sensitive to light, we are able to perceive images that we will always remember.
This article will answer questions related to the parts of the retina and how they function such as what type of cells it is made up of or what structures are responsible for processing color.
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 nervous signals that will be transmitted to the brain through the optic nerve.
Almost all parts of the retina are made up of a thin, transparent tissue made up of a set of nerve fibers and photoreceptor cells, which are specialized cells responsible for converting light into signals that are sent to the brain.
The retina usually appears reddish or orange because there are a large number of blood vessels located just behind it. The periphery or outer part of the retina is responsible for peripheral vision (which allows us to see up to almost 180º with our eyesight) and the area in the center of central vision (which helps us recognize people’s faces or 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 its anatomical composition can be described from two structural levels: the macroscopic level and the microscopic level.
macroscopic structure
Various structures can be seen on the surface of the retina detailed below:
1. Papilla or optic disc
The optic disc or optic disc is a circular area located in the central area of the retina. From this structure emerge the axons of the retinal ganglion cells that form the optic nerve This area lacks sensitivity to light stimuli, which is why it is also known as the “blind spot.”
2. Spot
The ocular macula or macula lutea is the area responsible for central vision and the one that 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 vision in detail and movement. Thanks to the macula we can distinguish faces, colors and all kinds of small objects.
3. Fovea
The fovea is a shallow indentation located in the center of the eye’s macula This structure is responsible for most of the total visual acuity, as it is the receiving focus of the light rays that reach the retina, and it only has cone photoreceptors, responsible for the perception of colors.
4. Ora serrata
The ora serrata is the most anterior and peripheral part of the retina, in which it comes into contact with the ciliary body, a structure responsible for the production of aqueous humor (a colorless liquid found in the front part of the eye) and of the change in the shape of the lens to achieve correct eye accommodation or focus
microscopic structure
If we get down to a microscopic level, we can see how various parts of the retina are grouped into layers. We can differentiate up to 10 parallel layers, which are the following (from most superficial to least):
1. Pigmented epithelium
It is the outermost layer of the retina is made up of cubic cells that are not neurons and have melanin granules, a substance that gives them characteristic pigmentation.
2. Photoreceptor cell layer
This layer is made up of the outermost segments of the cones (responsible for color differentiation or visual acuity) and the rods (responsible for peripheral vision).
3. External limiting layer
It is composed of junctions between cells of the zonula adherens type (area that surrounds the external surface of the cell and contains dense filamentous material) between photoreceptor cells and Müller cells (glial cells responsible for auxiliary functions).
4. Nuclear or external granular layer
This layer is formed by the nuclei and bodies of photoreceptor cells
5. Outer plexiform layer
In this layer the synapse is made between the photoreceptor cells and the bipolar cells.
6. Granular or inner nuclear layer
It is made up of the nuclei of four types of cells: bipolar, horizontal, Müller and amacrine cells.
7. Inner plexiform layer
This is the region of synaptic connection between bipolar, amacrine and ganglion cells. This layer is formed by a dense tissue of fibrils arranged in a network.
8. Ganglion cell layer
This layer is formed by the nuclei of the ganglion cells. Located on the inner surface of the retina, receive information from photoreceptors through intermediate bipolar, horizontal and amacrine neurons
In this layer of the retina we can find axons of ganglion cells that form the optic nerve itself.
10. Internal limiting layer
This last layer is the one that separates the retina and the vitreous humor a transparent, gelatinous liquid located between the retina and the lens that helps maintain the shape of the eyeball and helps to receive clear images.
Types of cells: an inside look
In addition to having a layered structure, the retina is made up of three types of cells: pigmented cells – responsible for the metabolism of photoreceptors -, neurons and support cells – such as astrocytes and Müller cells, whose function is support other nerve cells.
The five main types of retinal neurons are described in more detail below:
1. Photoreceptor cells
They are made up of two broad types of cells: cones and rods Cones are most 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 the cones and are responsible for almost all night vision (also called scotopic vision).
2. Horizontal cells
It seems that there are two types of horizontal cells, each with a different shape, which together offer information to all the photoreceptor cells. Despite the number of cells with which they form synapses, these types of cells represent a relatively small population of retinal cells (less than 5% of the cells in the inner nuclear layer).
Still The reason why there are two types of horizontal cells is not known but it is speculated that it could have to do with the identification of 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 those ganglion cells to send signals at the same time.
These cells generate synaptic connections with the axon endings of bipolar cells and with the dendrites of ganglion cells.
Bipolar cells connect photoreceptors to 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 they can also establish connections with horizontal cells.
5. Ganglion cells
Ganglion cells are the cells from which the information coming from the retina originates. Its axons leave the eye, pass through the optic nerve and reach the brain to send the visual stimulus already processed to the lateral geniculate nucleus (primary processing center for visual information).
When they reach this last processing nucleus, they form synapses with neurons that project to the primary visual cortex, an area specialized in processing information about static and moving objects, as well as in pattern recognition, and the visual stimulus is finally interpreted.
From the eye to the brain: how visual information travels
The light stimuli that the retina captures are conducted through the optic nerve to the brain, where the information is processed and we really “see” what we have in front of our eyes.
When the optic nerves penetrate the skull, They intersect to form the optic chiasm This structure exchanges part of the fibers of each nerve towards 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 optic tracts until it reaches the geniculate nuclei, where the fibers are classified so that each point of the optical field is recorded with greater precision. From the geniculate nuclei comes a bundle of nerve fibers (optic radiation) that cross each cerebral hemisphere until it reaches the occipital lobe, the posterior area of the brain that is responsible for processing visual information.
The paradoxical thing about our brain is that it processes visual information in an inverted way; that is, images on the left side are “seen” in the right hemisphere and vice versa. Similarly, images seen at the top are processed at the bottom of the hemispheres and vice versa. Mysteries of visual processing.
