Introduction

Human eyes are a great gift of nature through which we can perceive different colours and objects present around us. This sense of vision makes our surroundings so beautiful. Eyes help us enjoy the splendid landscapes of nature.

Our eyes consist of eyeballs fitted in the bony socket called the orbit. The eyeball is made up of three layers; the fibrous layer, vascular layer, and the retina.

The retina is the innermost layer of the eyeball responsible for detecting light and signalling the visual cortex.  It is composed of photoreceptors that detect light photons and generate nerve impulses, carried by the neurons to the visual cortex.

In this article, we will study the anatomical structure, development, physiology as well as clinical conditions associated with this inner layer of the eyeball i.e. the human retina.

Structure of Retina

The structure of the retina can be studied at two-level; gross anatomical structure and histological structure.

Gross Anatomy

As mentioned earlier, the retina is the innermost layer of the eyeball. Grossly, it can be divided into two parts; optic part and non-visual part.

The optic retina is sensitive to light and consists of two layers; a neural layer and a pigmented layer.  The neural layer detects light while the pigmented layer serves to decrease the scattering of light within the eyeball. The optic retina occupies the posterior two-thirds of the eyeball and terminates at the posterior border of the ciliary body.

The non-visual part occupies the anterior one-thirds. It is an extension of the pigmented layer of the retina over the ciliary body and the posterior surface of the iris.

Fundus and Optic Disc

The internal part of the optic retina where the light is focused is called the fundus of the eyeball. The fundus has a small circular area through which the blood vessels and nerve fibres enter the eyeball. This area is called the optic disc or blind spot as it contains no sensory cells.

Macula

The macula is a yellow spot present just lateral to the optic disk. It appears yellow when observed with red light. It is an oval area having only cones and is specialized for visual acuity i.e. clearness of vision.  There is a small depression at the centre of the macula called fovea centralis. It is regarded as the point of most acute vision.

Histological Structure

From the histological point of view, the human retina is divided into ten distinct layers. Each of these layers has a role in the process of vision. A brief detail of all these layers is as follows.

1.     Pigmented Layer

The pigmented layer of the human retina is made up of cuboidal or low columnar epithelial cells with basal nuclei. It is the only non-neural layer and surrounds the remaining neural layers of the retina. It is present on the most inner side i.e. light hits this layer after passing through the other nine layers of the retina.

The cells of the pigmented layer have numerous vacuoles and smooth endoplasmic reticulum to store vitamin A that helps in the regeneration of the photoreceptor pigment. These cells have small apical projections surrounding the photoreceptor cells present in the next layer.

The main function of the pigmented layer is to prevent the scattering of light by absorbing it after it has passed through the neural layers of the retina. The tight junctions among the cells in this layer form a blood-retina barrier, protecting the photoreceptors from the high-flow blood in the underlying vascular choroid.

They are also responsible for the regeneration of photopigments, phagocytosis of any foreign particles as well as the removal of free radicals.

2.     Rode and Cone Layer

This layer is present just deep to the pigmented cell layer of the retina. The outer segments of the rods and cones, the photoreceptor cells, are present in this layer. The light rays are detected by photoreceptors in this layer of the retina.

3.      Outer Limiting Layer

It is a faint but well-defined layer between the outer nuclear layer and the rod and cone layer. This layer consists of junctional complexes that hold together the photoreceptor cells and the processes of Muller cells, the glial cells present in the retina.

4.     Outer Nuclear Layer

It is the first of the neuronal layer containing the cell bodies. It is present just deep to the outer limiting layer. It contains the cell bodies of the photoreceptor cells i.e. rods and cons. The structure of these cells will be discussed under the subsequent heading.

5.      Outer Plexiform Layer

It is a wide layer containing nerve fibres. This plexiform layer consists of the axons of the photoreceptor cells (rods and cones) and the dendrites of the bipolar neurons present in the inner nuclear layer. The fibres of photoreceptors and bipolar neurons make synapses in this layer of the retina.

6.     Inner Nuclear Layer

This layer contains the cell bodies of association neurons like bipolar neurons, amacrine cells, and horizontal cells. These cells make neuronal connections with the cells present in the other layers of the retina, integrating and processing the signals from rods and cones.

7.     Inner Plexiform Layer

 This layer contains the fibres arising from the neurons present in the inner nuclear layer and the ganglionic layer. The synapse between the ganglionic cells and the bipolar cells are formed in this fibrous layer.

8.     Ganglionic layer

This layer contains the cell bodies of ganglionic cells, the neurons responsible for carrying the nerve impulse from the retina to the brain. This layer has a maximum thickness at the optic disc where no photoreceptors are present. The thickness of the ganglionic layer goes on decreasing towards the periphery. The ganglionic cells have very large axons that form the next layer, the nerve fibre layer.

9.     Nerve Fiber Layer

This layer consists of the axons of the ganglionic cells. These axons converge on the optic disk from all over the retina to form the optic nerve. The optic nerve leaves the eyeball at the optic disk.

10.Inner Limiting Layer

This layer bounds all the layers of the retina on the inner side. It is the first layer where the light strikes. It consists of the processes of the Muller cells and collagenous fibres that cover the membrane of the vitreous body present in the eyeball. This layer is in direct contact with the vitreous body.

Cellular Structures

Having understood the histological layout of the human retina, it is important to study the structure of some important cells found in the retinal layers. The structure of important cells is discussed below.

Rods

These cells are extremely sensitive to light. These cells are divided into two segments; an outer segment and an inner segment.

The outer segment consists of flattened membranous discs that are stacked like coins, surrounded by a plasma membrane. These discs contain a visual pigment called rhodopsin. It detects the light stimulus and initiates the action potential in the rods. These are called rods because of the rod-shaped appearance of the outer segment.

The inner segment consists of the nucleus, mitochondria, ribosomes, etc. This segment is present in the outer nuclear layer of the retina. It is the biosynthetic segment of the cell.

A small constriction is present between the inner and the outer segment called the connecting stalk.

The inner segment of rods gives rise to an axon that communicates with the dendrites of the bipolar cells in the outer plexiform layer of the retina. 

These cells are responsible for black and white vision.

Cones

These cells are responsible for colour vision. There are three similar types of cones for detecting the three primary colours. They have a structure similar to that of rods having an inner segment, an outer segment, and a connecting stalk.

The outer segment of cones has a cone-like appearance. It consists of stacked flattened membranes that are filled with a visual pigment called iodopsin. There are three distinct types of iodopsins present in the three types of cones.

The inner segment contains the synthetic organelles and nucleus.

An axon arising from the inner segment enters the outer plexiform layer to make synapses with the bipolar neurons.

Muller Cells

These are the glial cells present most abundantly in the retina. These cells have cell bodies located in the inner nuclear layer while the processes of these cells span the entire retina.

These cells provide metabolic support to the cells in the retina. They maintain the extracellular environment of the cells, remove the debris, maintain ionic concentrations, and store glycogen.

Bipolar Cells

These cells have an axon and a dendrite arising from the cell body located in the centre. The cell bodies of such cells are also present in the inner nuclear layer. One cellular process extends in the inner plexiform layer while the other extends into the outer plexiform layer.

These cells are responsible for integrating the nerve signals. They also communicate the nerve impulses from the photoreceptors to the ganglionic cells.

Horizontal Cells

These cells have multiple processes that communicate with the photoreceptors as well as the bipolar cells. They are responsible for the lateral inhibition of the nerve impulses arising in the retina.

Development of Retina

The retina develops from the walls of the optic cup, an outgrowth of the forebrain. The walls of the optic cup divide into two layers; an outer layer and an inner layer.

The outer layer of the optic cup differentiates into the pigment layer of the retina. The inner layer is thick and gives rise to all the neural layers.

During the early embryonic periods, the two layers are separated by a space called the intraretinal space. This space is derived from the cavity of the optic cup. This space soon disappears resulting in the fusion of the two layers. However, the fusion of not firm at this stage.

The inner layer of the optic cup receives signals from the lens and proliferates, giving rise to the neuroepithelium of the retina. This neuroepithelium later gives rise to all the cells present in the neural retina.

The optic cup is connected to the forebrain via an optic stalk. The cavity of the optic stalk is obliterated by the growth of the axons from the glial cells. these axons then collectively form the optic nerve.

Physiology

In this section, we will discuss the physiological process of vision. It involves the detection of light by photoreceptors, generation of the action potential, and signalling to the visual cortex.

Detection of Light

This is the first step in the vision process. Light is detected by the visual pigments present in the outer segments of rods and cones.

Rhodopsin is the visual pigment present in rods. It is composed of a protein called scotopsin and a pigment called retinal. The 11-cis-retinal isomer is present in rhodopsin.

When a photon of light falls on rhodopsin, it activates an electron in the molecule. This photoactivated electron causes rapid decomposition of rhodopsin to scotopsin and all-trans-retinal.

The rhodopsin is regenerated from its components in the absence of light.

A similar process occurs in cones when they are exposed to the light of a certain wavelength specified by the type of iodopsin present in them.

Generation of Action Potential

The action potential is generated by photoreceptors by hyperpolarization, instead of depolarization in the rest of the neurons.

The inner segment of rods continuously pumps the sodium ions outwards while the sodium ions are pumped into it. Potassium ions continue to leave the photoreceptor cells via non-gated potassium channels present in the inner segment.

The outer segment of photoreceptors has cGMP gated sodium channels that allow the sodium ions to diffuse into the cell in the dark. It is because the cGMP levels are high in the cell in the absence of light.

When a photon of light falls on the outer segment, it activates the rhodopsin, generating retinal. This retinal activates transducing, a G-protein present in the outer segment of photoreceptors. It, in turn, activates the cGMP phosphodiesterase enzyme that cleaves the cGMP to 5’-GMP. The cGMP levels begin to fall in the cell, the gated sodium channels in the outer segment close, and a negative potential begins to develop in the cells.

The cells become hyperpolarized and an action potential is generated that is carried by the neuronal networks of the retina to the glial cells.

Visual Pathway

The nerve signals arising in the retina are carried to the visual cortex along the visual pathways. The components included in the visual pathway are as follows.

• Nerve fibers from the nasal and the temporal halves of retina travel in the optic nerve to reach the optic chiasma.
• At optic chiasma, the fibers coming from the nasal of the retina cross each other i.e. fibers from the nasal half of left retina cross and join the right optic path and vice versa.
• The nerve fibers from the nasal half of the opposite side and the temporal half of the same side join together to form the optic tract.
• Optic tracts on both sides terminate at the lateral geniculate body, a part of the thalamus.
• The lateral geniculate body gives rise to optic radiations on each side of the brain.
• Optic radiations finally terminate at the visual cortex in each cerebral hemisphere.

Once the nerve signals reach the visual cortex, they are integrated and processed, and the final image of the object is perceived by the brain.

Blood Supply of Retina

Retina has a dual blood supply.

• The neural layers are supplied by the central retinal artery. It enters the retina through the optic disc and divides into multiple branches. The central retinal artery is a blind blood channel that does not anastomose with other blood vessels.
• The non-neural layer i.e. the pigmented retina is the outermost layer adherent to the choroid, the vascular layer of the eyeball having abundant blood vessels. The blood supply of this part of the retina is derived from the choroid via diffusion. The outer segments of rods and cones in the rod and cone layer are also dependent on blood diffusion from the choroid blood vessels.

Clinical Conditions

The human retina is very essential for vision as it converts the light signals into nerve signals. Any injury or defect in this layer of the eyeball impairs the vision of a person. Some important clinical conditions associated with the human retina are as follows.

Retinal Detachment

The major clinical condition associated with the human retina is retinal detachment. It is the condition in which the neural retina detaches from the pigmented retina, the non-neural layer.

Retinal detachment may occur due to an injury to the eyeball that causes the collection of fluid or blood between the neural retina and the pigmented retina. Sometimes, it is due to the contraction of collagenous fibres present in the vitreous body that pull the neural retina towards the centre of the eyeball.

Retinal detachment is a medical emergency and the detached layers must be replaced surgically in their normal position before degeneration. These layers can resist degeneration for days due to independent blood supply. However, if the surgery is not performed soon, the retina will degenerate and will not function even after surgical repair.

Night Blindness

It is a condition in which the person fails to have adequate vision at night. Night blindness is seen in patients with vitamin A deficiency. Vitamin A is required to regenerate rhodopsin after it has been cleaved by light. In vitamin A-deficient people, enough rhodopsin is not present to detect the low light at night.

Night blindness is treatable with vitamin A therapy. It can be reversed by giving intravenous vitamin A injection within one hour of its development. The person should remain on a vitamin A-rich diet for months to make it available to the eyes if the IV injection is not given.

Color Blindness

It is another commonly seen clinical condition associated with the human retina. Humans have three types of cones that detect three primary colours i.e red, green, and blue. The absence of any one type of cone results in colour blindness.

Red-green colour blindness is more common. In this condition, one group of the colour receptive cones is missing. Such a person is unable to differentiate some colours from others. In most cases, the person is unable to distinguish red from green and thus called red-green colour blindness.

Blue weakness is a rare condition in which the blue cones are missing. The person is unable to perceive the blue colour.

Both these conditions are hereditary.

Summary

The human retina is the innermost layer of the eyeball that detects light and converts light signals into an action potential.

Anatomically, it is divided into two parts; the optic retina and the non-visual retina. The optic part occupies the posterior two-thirds of the eyeball while the non-visual part occupies the anterior one-third, beneath the ciliary body and the iris.

In histological studies, the optic retina is seen to be made of ten distinct layers.

• The pigmented layer is the outermost layer present just deep to the choroid. It is the non-neural layer.
• The other nine layers are the neural layers having different types of neurons, glial cells, and neural networks.
• Rods and cones are the photoreceptor cells in the retina.
• Bipolar cells, amacrine cells, and horizontal cells are responsible for integrating the nerve impulses and transferring them to the ganglionic cells.
• The axons of ganglionic cells carry the nerve impulses to the visual cortex.

The human retina is developed from the two layers of the optic cup, a projection of the developing forebrain.

The vison process has three steps;

• Detection of light as it falls on the outer segments of photoreceptors and breakdown the visual pigments
• Generation of action potential via hyperpolarization caused by decreased cGMP in the cells due to the breakdown product of the visual pigments
• Carrying of the action potential by visual pathways to the visual cortex

The blood supply of the retina comes from two sources; the non-neural layer is supplied by the blood diffusing from the choroid while the neural layers are supplied by the central retinal artery.

Retinal detachment is a medical emergency associated with the injury to the retina. Night blindness and colour blindness are two other important clinical conditions.

Read more about Temporal and Spatial Summation

Frequently Asked Questions

What is the retina in the human eye?

The retina is the innermost layer of the human eye that detects lights and signals the brain about visual stimuli. It consists of photoreceptors that can detect light as well as colour. 

How does the human retina work?

The human retina is a photosensitive layer that consists of millions of photoreceptors. The photoreceptors are of two types; rods for black and white vision and cons for colour vision. These photoreceptors detect light and can convert it into electrical signals or nerve impulses that travel along the optic nerve to the visual cortex present in the brain. 

Can you see without a retina?

Humans need the retina to detect light or see any object because the photoreceptors in the retina are responsible for vision. In the absence of the retina, the eye is wasted as the person cannot perceive light or see anything.

What is the blind spot of the retina?

It is the part of the retina where the optic nerve enters or leaves it. This part of the retina does not have any photoreceptors. Therefore, any light falling at this spot cannot be detected. This is why it is known as the blind spot.

References

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3. Kolb, Helga. “Simple Anatomy of the Retina”. Webvision. PMID 21413391. Retrieved 1 January 2018.
4. Kolb, Helga. “Photoreceptors”. Webvision. Retrieved 11 January 2018.
5. Bringmann A, Syrbe S, Görner K, Kacza J, Francke M, Wiedemann P, Reichenbach A (2018). “The primate fovea: Structure, function and development”. Prog Retin Eye Res. 66: 49–84. doi:10.1016/j.preteyeres.2018.03.006. PMID 29609042.