Parts of the retina: layers and cells that compose it.

A description of the parts of the retina, at the microscopic and macroscopic level.

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

In this article we will answer questions related to the parts of the retina and how they functionThis article will answer questions related to the parts of the retina and its functioning, such as what type of cells compose it or which are the structures in charge of color processing.

What is the retina?

The retina is a complex sensory membrane located in 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 composed of a thin, transparent tissue made up of a collection 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 due to the 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 sight) and the center area for central vision (which is used to recognize faces or read).

All in all, it must be said that the retina is a fundamental structure of the human eye and our vision and eye health depend on it. and our eye health depends on it.

Parts of the retina

The parts of the retina and its anatomical composition can be described at two structural levels: the macroscopic level and the microscopic level.

Macroscopic structure

Several structures can be observed on the surface of the retina, as detailed below. 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 the axons of the retinal ganglion cells that form the optic nerve exit.. This area lacks sensitivity to light stimuli, hence it is also known as the "blind spot".

• You may be interested in "What are neuron axons?".

2. Macula

The ocular macula or macula lutea is the area responsible for central vision and the one that allows us to see with maximum acuity. that allows us to see with maximum visual acuity.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 detail vision and movement. Thanks to the macula we can distinguish faces, colors and all kinds of small objects.

3. Pfovea

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, as it is the receptor focus of the light rays that reach the retina, and only has cone photoreceptors, responsible for color perception.

4. Ora serrata

The ora serrata is the most anterior and peripheral part of the retina, where it comes into contact with the ciliary body, a structure responsible for the production of aqueous humor (a colorless liquid found in the anterior part of the eye) and for changing the shape of the crystalline lens to achieve accommodation or focusing. to achieve accommodation or correct ocular focusing..

Microscopic structure

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

1. Pigmented epithelium

It is the outermost layer of the retinaIt is composed of cubic cells that are not neurons and have melanin granules, a substance that gives them a characteristic pigmentation.

2. Photoreceptor cell layer

This layer is composed of the outermost segments of the cones (responsible for color differentiation or visual acuity) and rods (responsible for peripheral vision).

3. Outer limiting layer

It is composed of cell-cell junctions of the adherent zonule type (area surrounding the outer surface of the cell containing dense filamentous material) between photoreceptor cells and Müller cells (glial cells responsible for auxiliary functions).

4. Nuclear or outer granular layer

This layer is formed by the nuclei and bodies of the photoreceptor cells..

5. Outer plexiform layer

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

6. Granular or inner nuclear layer

It is formed by the nuclei of four types of cellsBipolar, horizontal, Müller cells and amacrine cells.

7. Internal 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..

9. Optic nerve fiber layer

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 separates the retina from the vitreous humor, a transparent, gelatinous fluid located between the retina and the crystalline lens.The vitreous humor, a clear, jelly-like fluid located between the retina and the lens that helps maintain the shape of the eyeball and assists in the clear reception of images.

Cell types: an inside look

In addition to having a layered structure, the retina is made up of three types of cells: pigment cells - responsible for the metabolism of photoreceptors, neurons and supporting 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 consist of two broad 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).

Rods are concentrated at the outer edges of the retina and are used for 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 combined provide information to all photoreceptor cells. Despite the number of cells with which they form synapses, these cells represent a relatively small population of retinal cells (less than 5% of the cells in the inner nuclear layer).

Still it is not known why there are two kinds of horizontal cells, but it is speculated that it may have to do with the identification of color differences in the red/green system.It is speculated that it may 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, 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 terminals of the bipolar cells and with the dendrites of the ganglion cells.

4. Bipolar cells

Bipolar cells connect photoreceptors with ganglion cells. Their function is to transmit signals from photoreceptors to ganglion cells, either directly or indirectly.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 photoreceptors or with cones (but not with both at the same time) and can also establish connections with horizontal cells.

5. Ganglion cells

Ganglion cells are the cells from which information from the retina originates. Their axons leave the eye, pass through the optic nerve and reach the brain to send the processed visual stimulus to the retina. to send the processed visual stimulus to the lateral geniculate nucleus (primary processing center of visual information).

When they reach this latter processing nucleus, they form synapses with neurons that project to the primary visual cortex, an area specialized in processing information from 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 captured by the retina are conducted through the optic nerve to the brain, where the information is processed and we actually "see" what is in front of our eyes.

When the optic nerves penetrate the skull, they intertwine to form the optic chiasm.. 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 optic ribbons until it reaches the geniculate nuclei, where the fibers are sorted in order to obtain the information needed for the visual field.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) passes through each cerebral hemisphere until it reaches the occipital lobe, the area at the back of the brain that is in charge of processing visual information.

The paradoxical thing about our brain is that it processes visual information in an inverted way; that is, images from the left side are "seen" in the right hemisphere and vice versa. Similarly, images seen on the upper side are processed in the lower hemisphere and vice versa. Mysteries of visual processing.

Bibliographical references:

• Richard S. Snell (2003). Clinical Neuroanatomy. Médica Panamericana.