Cones and Rods

Cones

Contain a photochemical pigment called iodopsin which perceives light of high intensity.

In the presence of light, iodopsin breaks down to iodine and opsin.

This breakdown brings about depolarization of the cell membrane of the cones.

Thus, a nerve impulse is generated and transmitted along the neurons to the optic nerve, which carries the impulses to the brain for interpretation.

Greater concentration of cones occurs in the fovea centralis or yellow spot.

Each cone has its own bipolar neurone which in turn links it with an optic nerve fibre, i.e. cones lacks retinal convergence.

This property of the cones enables them to have high visual acuity.

Visual acuity is the ability of the eye to distinguish objects clearly.

There are three types of cone cells which enable higher animals to differentiate the different colours they perceive.

These are the blue, green and red cones.

Rods

Contain a photochemical pigment called rhodopsin which perceives light of low intensity, but is not sensitive to colour.

Have retinal convergence, therefore they have low visual acuity, hence cannot distinguish fine detail.

Rhodopsin degenerates to opsin and retinine (a derivative of vitamin A) to bring about depolarization of the cell membrane and then triggers off an impulse.

The amount of rhodopsin in the rods is increased in the dark, raising the sensitivity of the rods to dim light.

Resynthesis of rhodopsin occurs slowly in the dark for continued photochemical reaction in the rods.

Rods are in greater concentration round the periphery of the retina and are absent in the fovea centralis.

Their greater number in the retina tends to confirm their special value in dim light.

In retina of nocturnal animals, there are more rods than cones. This is because the rods are sensitive to light of low intensity.

The combination of rods and cones enables the eye to be efficient under all light conditions.

In the retina there is an area called blind spot where the optic nerve enters the eyeball.

Blind spot has neither rods nor cones, so images from objects falling on the blind spot cannot be perceived.

Differences between Cones and Rods