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 work , such as what type of cells compose it or what are the structures in charge of processing color.

What is the retina?

The retina is a complex sensory membrane located on the back 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 formed by a set of nerve fibers and photoreceptor cells, which are specialized cells in charge of converting light into signals that are sent to the brain.

The retina is usually reddish or orange in color because there are so many 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 180 degrees) and the central vision area (which helps us to recognize people’s faces or read).

However, it can be said that the retina is a fundamental structure of the human eye and on it depends our vision 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, as detailed below:

1. Papilla or optical disk

The papilla or optical disk is a circular area located in the central part of the retina. From this structure come the axons of the ganglion cells of the retina that form the optic nerve . This area lacks sensitivity to light stimuli, hence it is also known as the “blind spot”.

2. Stain

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 greatest density of photoreceptor cells.

Located in the center of the retina, it takes care of detailed vision and movement. Thanks to the macula we can distinguish faces, colors and all kinds of small objects.

3. Fovea

The phovea 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 it only possesses conical photoreceptors, responsible for the perception of colours.

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 anterior part of the eye) and the change in the shape of the lens to achieve the correct ocular accommodation or focus .

Microscopic structure

If we go into 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 more superficial to less):

1. Pigmented epithelium

It is the outermost layer of the retina , composed of cubic cells that are not neurons and have granules of melanin, 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 the rods (responsible for peripheral vision).

3. External boundary layer

It consists of adhesion zone cells (zone surrounding the outer surface of the cell and containing dense filamentous material) between the photoreceptor cells and the 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. External plexiform layer

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

6. Granular or nuclear inner layer

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

7. Internal plexiform layer

This is the synaptic connection region between bipolar, amacrine and ganglion cells. This layer is made up of a dense tissue 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, they receive information from photoreceptors through bipolar, horizontal intermediate neurons and amacrines .

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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 boundary layer

This last layer is the one that separates the retina from the vitreous humour , a transparent, gelatinous liquid located between the retina and the crystalline lens that helps to maintain the shape of the eyeball and helps to ensure that images are received 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 . The cones are more concentrated in the centre of the retina and are the only type of photoreceptor cell found in the centre of the retina (the pitting). They are responsible for color vision (also called photopic vision).

The 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 scopic vision).

2. Horizontal cells

It appears that there are two types of horizontal cells, each with a different shape, which combined offer information to all photoreceptor cells. Despite the number of cells with which they form synapses, these types of cells represent a relatively small population of cells in the retina (less than 5% of the cells in the inner nuclear layer).

It is not yet known why there are two classes of horizontal cells , but it is speculated that it may have to do with the identification of colour differences in the red/green system.

3. Amacrine cells

Amacrine cells allow ganglion cells to send temporarily 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 to ganglion cells. Their function is to transmit signals from the photoreceptors to the 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 photoreceptors rods or cones (but not both at the same time) and can also establish connections with horizontal cells.

5. Ganglion 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 already processed visual stimulus to the lateral geniculated nucleus (primary processing center of visual information).

When they reach the latter processing core, they form synapses with neurons that project to the primary visual cortex, an area specialized in information processing of 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 actually “see” what we have 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 fibres of each nerve towards the opposite side, so that those that carry the vision of the right and left half of our visual field are grouped separately.

The perceived information continues through the optical ribbons until reaching the genulated nuclei , where the fibres are classified so that each point of the optical field is recorded with greater precision.
From the genuated nuclei, a bundle of nerve fibres (optical radiation) passes through each brain hemisphere to the occipital lobe, the area at the back of the brain responsible for processing visual information.

The paradox of our brain is that it processes visual information in an inverted way; that is, the images on the left side are “seen” in the right hemisphere and vice versa. Similarly, images seen on the upper side are processed in the lower hemispheres and vice versa. Mysteries of visual processing.

Bibliographic references:

  • Richard S. Snell (2003). Clinical neuroanatomy. Pan-American Medicine.