How the Eye Responds to Light

An image on the retina stimulates photoreceptors, cells which are specialised to detect light. This is a chemical event: pigments in the photoreceptors are broken down by light and this results in a generator potential. When threshold is reached, action potentials are created and nerve impulses begin to travel towards the brain.

The two photoreceptors in the human eye are the rods and cones:

Rods

Rods contain a reddish-purple compound called rhodopsin, or visual purple. This contains opsin, a protein and retinene, a light absorbing compound which is derived from vitamin A. When retinene is exposed to light, it changes shape (it is actually converted into an isomer) and this causes the rhodopsin to break down. The free opsin acts as an enzyme and, through a series of reactions, activates a neurotransmitter called cyclic GMP.

Cyclic GMP closes membrane pores in the rod cell and the negative charge inside the cell increases. This causes the membrane of the cell to hyperpolarise (become more negative) and sets up signals in nearby nerve cells. Nerve impulses then pass the to brain for decoding. In the absence of further light stimulation the original retinene molecule reforms and this then recombines with opsin to form rhodopsin.

Rhodopsin is very sensitive to light and so rods are mainly used in dim light. In strong light the rhodopsin is broken down quicker than it can be reformed but in dim light, production is able to keep pace with the slower rate of breakdown.

Cones

Cone cells contain the photosensitive pigment iodopsin, a photosensitive pigment is made up of a photopsin, a different protein to that found in rods, and retinene. The events that occur in cone cells stimulated by light are basically the same as those that occur in rod cells. There are, however, three different types of cone. Each one contains a slightly different pigment and responds to a different wavelength of light. One responds to red light, one to green light and one to blue light, so allowing us to see in colour.

The colour that we 'see', or more accurately perceive, depends on which cones and stimulated. When all the cones are fully stimulated we see 'white'. When very few are stimulated we see black. Stimulation of separate types produces red, green or blue and all colours in between are produced by combinations of different levels of stimulation of the three types together.

Related Eye Links:

Eye Structure;
Colourblindness Test;
Snellen chart.

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