Parts of the retina: layers and cells that make it up

Through the retina of our eyes, that fragile light-sensitive membrane, we are capable of perceiving images that we will always remember.

This article will answer questions related to the parts of the retina and how they work, such as what type of cells make it up or what are the structures in charge of processing color.

• Related article: "The 11 parts of the eye and their functions"

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 a thin, transparent tissue made up of a bundle 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 in color because there are a large number of blood vessels located just behind it. The periphery or outer part of the retina is in charge of peripheral vision (which allows us to cover up to almost 180º with the view) and the zone of the center of the central vision (the one that helps us to recognize faces of people or read).

Nevertheless, it can 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.

macroscopic structure

Various structures can be observed on the surface of the retina. detailed below:

1. optic disc or papilla

The papilla or optic disc is a circular area located in the central area of ​​the retina. From this structure exit 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”.

• You may be interested in: "What are the axons of neurons?"

2. Taint

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, as it is 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 macula of the eye. This structure is responsible for most of the total visual acuity, being the receiving focus of the rays of light that reach the retina, and 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 for changing the shape of the crystalline to achieve the correct ocular accommodation or focus.

microscopic structure

If we go down to a microscopic level, we can see how various parts of the retina are grouped together in layers. We can differentiate up to 10 parallel layers, which are the following (from the most superficial to the least):

1. pigmented epithelium

It is the outermost layer of the retina, is made up of cuboidal cells that are not neurons and have melanin granules, a substance that gives them a 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. outer limiting layer

It is made up of junctions between cells of the adherent zonule type (an area that surrounds the external surface of the cell and contains dense filamentous material) between photoreceptor cells and Müller cells (glial cells in charge of photoreceptor functions). auxiliaries).

4. outer nuclear or granular layer

This layer is made up of the nuclei and bodies of photoreceptor cells.

5. outer plexiform layer

In this layer the synapse between the photoreceptor cells and the bipolar cells takes place.

6. Inner granular or 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 made up of the ganglion cell nuclei. Located on the inner surface of the retina receive information from photoreceptors via intermediate bipolar, horizontal, and amacrine neurons.

9. optic nerve fiber layer

In this layer of the retina we can find axons of ganglion cells that are the ones that form the optic nerve itself.

• You may be interested in: "Optic nerve: parts, course and related diseases"

10. internal limiting layer

This last layer is what separates the retina and the vitreous humor., a transparent and gelatinous liquid located between the retina and the crystalline lens that helps maintain the shape of the eyeball and helps to receive images clearly.

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 the metabolism of photoreceptors-, neurons and support cells -such as astrocytes and Müller cells, whose function is to 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 classes 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).

Rods are concentrated at the outer edges of the retina and are used for peripheral vision. These photoreceptors are more sensitive to light than the cones and are responsible for almost all of 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 combined provide information to all photoreceptor cells. Despite the number of cells with which they form synapses, these types of cells represent a population relatively small number of cells in the retina (less than 5% of cells in the nuclear layer internal).

Still the reason why there are two classes 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 enable ganglion cells to send temporally correlated signals to the brain; that is, 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 axonal endings of bipolar cells and with the dendrites of ganglion cells.

4. bipolar cells

Bipolar cells connect 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 establish connections with horizontal cells.

5. ganglion cells

Ganglion cells are the cells from where the information coming from the retina starts. Its axons leave the eye, pass through the optic nerve and reach the brain. to send the already processed visual stimulus to the lateral geniculate nucleus (primary processing center for visual information).

When they reach this latter processing nucleus, they form synapses with neurons that project to the primary visual cortex, a specialized area in the brain. information processing of static and moving objects, as well as in pattern recognition, and visual stimulation 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 enter the skull, intersect to form the optic chiasm. This structure exchanges part of the fibers of each nerve to the opposite side, so that they are they group separately those that carry the vision of the right half and the left half of our field visual.

Perceived information continues through the optic tracts to reach the geniculate nuclei., where the fibers are classified so that each point of the optical field is registered with greater precision. From the geniculate nuclei a bundle of nerve fibers (optic radiations) exits and crosses each hemisphere. brain until reaching the occipital lobe, the posterior area of ​​the brain that is responsible for processing information visual.

The paradoxical thing about our brain is that it processes visual information in an inverted way; that is, the images of the left side are "seen" in the right hemisphere and vice versa. In the same way, the images that are seen in the upper part are processed in the lower part of the hemispheres and vice versa. Visual processing mysteries.

Bibliographic references:

• Richard S. Snell (2003). clinical neuroanatomy. Pan American Medical.