What is a Retina?

The retina is a light tissue with a sensitive lining at the back of the eye. It covers about 65% of the inner surface. The retina consist of photosensitive cells known as rods and cones which transform the light energy into signals that are transferred to the brain through the optic nerve.

Gray's subject #225 1014
Artery central retinal artery
MeSH Retina
Dorlands/Elsevier Retina

About the Retina

At the center of the retina is a tiny dimple known as the fovea or fovea centralis. This is responsible to provide the sharpest vision as well as to perceive location of most colors. The retina is considered as the central nervous system as the optic nerve and the retina originate as outgrowths of the developing brain. It is also known to be the only part of the CNS which can be seen non-invasively.

The retina is a layered structure which is complex consists of several layers of neurons which are interconnected with synapses. The photoreceptor cells are the only neurons which are sensitive to direct light. They can be categorized into two types- the rods and cones.

The rods function well in dim light and are responsible for black and white vision. When it comes to color perception and daytime vision, the cones are responsible. The third type of photoreceptor is the photosensitive ganglion cell which helps in reflexive responses in bright daylight.

The rods and cones get neural signals and they therefore go through complex processing by the rest of the neurons of the retina. There are many features of visual perception which can be traced by the processing of light and retinal encoding.

In humans, the entire retina is 72% of a sphere and 22 mm in diameter. The retina in the human eye contains 7 million cones and between 75 to 150 million rods.

Vertebrate Retina

The vertebrate retina has ten main layers. They are:-

The Inner Limiting Membrane

Muller cell footplates

The Nerve Fiber Layer

It is important for the axons of the ganglion cell nuclei.

Ganglion Cell Layer

It is the layer that has the ganglion cells.

Inner Plexiform Layer

This part contains the synapse which lies between the bipolar cell axons and the dendrites of amacrine cells and ganglion.

Inner Nuclear Layer

This part contains the surrounding cell bodies and nuclei of bipolar cells. These are sensitive to touch and heat and pass signals to the spinal cord.

Outer Plexiform Layer

This is that part where the rods and the cones project out at the end of the rod.

Outer Nuclear Layer

Contains cell bodies and cones

External Limiting Membrane

It’s a fine layer that is a medium between the inner segment portions of the photoreceptors from the cell nucleus.

Photoreceptor Layer

Consists of rods and cones.

Retinal Pigment Epithelium

In animals like birds, the pectin has a complex shape that protrudes from the retina in the vitreous humour. It thus supplies nutrients and oxygen to the eye and also helps in vision. Even reptiles have similar and simpler structure which is known as papillary cone.