The inner layer

Retina

The retina is the inner layer of tissue which extends across the back portion of the eye. It is this part of the eye which is light sensitive and acts to convert information from light waves to nerves impulses, which can then be sent to the brain. The retina is a complex structure which contains many cells types arranged in a layered system.

The cells which have the ability to detect light are called the photoreceptors and in the human eye there are two types. Both contain an outer, inner and synaptic region. However, there are slight differences between the two:

Rods

• The rods are slightly narrower than the cones and have their pigment contained in many stacked disks in the outer segment. This allows a high area for pigment expression and so increases their ability of light absorption.

• Rod cells only have one type of pigment, they therefore provide very little contribution to colour vision. However, they are a hundred times more sensitive to light compared with cones. They possess the characteristic of being able to respond to just one photon of light. This makes rods ideal for providing vision informational at night (scotopic vision).     

Rod cell diagram courtesy of Wikimedia Commons. This image is in the public domain thus free of any copyright restrictions.

Cones

• The cone cells structure varies slightly from the rod cells as they are shorter in length and have a tapered shape. This means they are wider at one end which enables them to filter incoming light. 

• They are similar to rods in that they also have membranous discs containing pigments. However, in cones these are still attached to their cell membrane whereas in rods, they are dissociated from the membrane and exist seperately.

• In comparison to rods, cone cells work best during times of bright light (photopic vision). Although they require a higher threshold for activation, they provide us with both colour vision and finer detail in our images.

• The reason as to how we can percieve colour is due to the existence of three types of cones cells. The first, L, responds to long wavelengths of light which generally peak in the yellow region. The second responds to medium length waves of light which are green in nature and are so called M. The third type is called S and these respond to short wavelengths which are typically in the violet region. All of these contain different photopsins (analogues of rhodopsin) which allows them to respond to different variations of colour, creating trichromatic vision.                                        

Cone cell diagram courtesy of Wikimedia Commons. This image is in the public domain thus free of any copyright restrictions. (Adapted).

The retina includes both neurons that respond to light and intricate neural circuits that perform the initial stages of image processing. The optic nerve is the final pathway of the eye, which then sends this information to the brain for more processing and visual perception.

Most would suppose that the photoreceptors (cells which respond to light) would be situated at the inner surface of the retina. Here they would be in contact with the vitreous humour and consequently, the first of the cells to receive light. However, they are instead located on its outer surface, meaning light has to pass through the entire layer of cells before it can activate the pigment molecules. This arrangement occurs so that the photoreceptors are in contact with the choroid layer, which supplies them with nutrients and the crucial molecule retinal. 

                       Retina image courtesy of Webvision. All rights reserved. 

 Fovea

The fovea is the part of the eye located in the centre of the macula region of the retina which allows for maximum acuity of vision. It is believed that around 50% of the nerve fibers in the optic nerve carry information from this region.

The human fovea has a diameter of about 1.0 mm and has a high concentration of cone photoreceptors. In the central 0.2 mm of the fovea, denoted the foveola, there is only cone cells present. There are rod cells on the edges of the fovea; however, most are located in the periphery.