The Retina

Layers of photoreceptors with light sensitive pigments.

What are photoreceptors?

Photoreceptor cells are special types of neurons which are sensitive to light.


In a human eye there are two types of photoreceptors:


  • rods - which contain the light sensitive pigment rhodopsin, and
  • cones - which contain the light sensitive pigment photopsin.

What is their purpose?

These photo-sensitive pigments are responsible for changing light signals into electrochemical signals which are interpreted by the brain as a certain image.

How do they do this?

The process is relatively complicated but the basic steps are:

  1. As rods and cones are not in the first layer of the retina, the light or collection of light known as photons pass through the layer of the ganglion cells, then the bipolar cells and then finally reach the rods and cones.
  2. In the rods the rhodopsin absorbs the light causing the retinal part of the molecule to change shape.
  3. This change in the rhodopsin molecule causes a change in the opsin molecule which then activates a relay molecule known as transducin.
  4. This results in less inhibitory neurotransmitters being released between the synapses between the rods and bipolar cells, and the bipolar cells and ganglion cells, (it enables messages to pass through).
  5. The electrochemical signal is passed through the rods to the bipolar cells, then from the bipolar cells to the ganglion cells, then from the ganglion cells to the optic nerve which delivers the electrochemical signal to the brain for interpretation.

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Distribution, Structure and Function

Cones

Structure

  • Elongated cells with a synaptic terminal and a shorter outer terminal containing fewer discs than rods. Similar to the basic structure of a nerve cell.
  • Contains photopsin pigments


Distribution

  • 6 Million cones in the human eye.
  • They are largely concentrated on the fovea where most of the day light is focused.


Function

  • Provide detailed (colour) images.
  • Not sensitive to light - (sacrifices sensitivity for colour detail).
  • Images not sharp in low light conditions as photons may not cause an electrochemical impulse to the brain.

Rods

Structure

  • Elongated cells with a synaptic terminal and an outer terminal containing discs. Similar to the basic structure of a nerve cell.
  • Contains Rhodopsin pigment.


Distribution

  • 125 Million rods in the human eye.
  • They are evenly distributed around the surface of the whole retina.


Function

  • Rods are very sensitive to light.
  • Responsible for "night vision" seeing black and white.


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What is the role of Rhodopsin?

  • Rhodopsin is a light absorbing pigment found in the rods of the human eye.
  • Composed of a derivative of vitamin A called retinal bonded to a protein called opsin.
  • Retinal part of the rhodopsin molecule is responsible for the initial absorption of light.
  • Different opsin molecules affect the light – absorbing ability of the retinal.
  • Rhodopsin is sensitive to lower wavelengths in the visible spectrum, (blue – green).
  • Rhodopsin is more active during low light or darkness.

Colour in cones

There are three types of cones found in the human eye which are all sensitive to different wavelengths of the visible spectrum. This is due to the type of opsin molecule which best absorbs that certain colour. The opsin molecule and its absorbing wavelength are as follows:

  • Blue region, maximum of 420 nanometres.
  • Green region, maximum of 530 nanometres.
  • Red region, maximum of 560 nanometres.

These cones are responsible for all the colours of the spectrum which we see.

Photoreceptors in Mammals, Insects and Planarian Worm

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What is the purpose of Colour vision in animals?

The animal kingdom heavily relies on vision as a means of communication. In particular is the use of colour vision. Each organism varies in its capabilities of seeing certain objects. In order to have colour vision an organism must have cones, a photoreceptor found in the retina of the eye. Bees are an example of an organism which uses colour vision as a means of communication. Bees can see the ultraviolet end of the spectrum which enables the bees to see certain patterns on flowers. This results in the bee distinguishing between which plants to access its food from.


Many members of the five classes of animals; amphibians, fish, reptiles, birds and mammals can see using colour vision. This adaptation enables the organism to communicate within its environment, aid in reproduction as well as reproductive behaviour and eat certain foods which they are attracted to due to the colour of the food. For example many primates can detect the ripeness of the food they eat based on the colour of the food.


Nocturnal animals do not rely on colour vision. Nocturnal animals are largely active during the night and hence have a larger number of rods in their eye structure. This enables the nocturnal organism to see sharper images during the night time.


Birds use colour vision as a means of communicating with other birds and detecting food. Birds are strongly attracted to the colour red. To enable birds to see the red colour more readily they have a higher concentration of red pigment photoreceptors in their eye. The result of this is that birds can see red more readily then any other colour resulting in finding food at a much more efficient rate.


Ultimately colour vision is a useful tool for all organisms in communicating within their own environment.

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