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4.2 describe the differences in distribution, structure and function of the photoreceptor cells in the human eye
describe the differences in distribution, structure and function of the photoreceptor cells in the human eye
- The retina consists of a thin sheet of photoreceptor cells. These are light-sensitive cells which are activated by light energy to produce an impulse which travels along the neurons that link them to the brain.
- In the retina there are two types of photoreceptor cells: rods and cones. Both of these cells are modified neurones. They are not distributed around the retina uniformly.
- Rods are long rod-shaped cells, which are sensitive to low levels of light but are unable to discriminate between colours. The image formed by the brain using information form rod cells lacks detail. Rods are linked in groups to single neurones. Rods are found mainly around the periphery of the retina and there are none at the fovea. They are more suitable for night vision. When the pupil is dilated more rods will be exposed. Rods also detect movement very well.
- Cones are conical cells which contain a pigment which is only sensitive to high intensities of light but exist in three different forms so that these cells can distinguish between colours. They have extensive nerve connections with the brain and produce a more detailed image. The number of cones increases towards the centre of the back of the retina. At the centre of the retina is a small area, known as the fovea, which has densely packed cones only. The fovea corresponds to the region of maximum visual acuity.
- Cones are more suitable for day vision. In bright light, when the pupil is contracted, it will be mainly the cones that are activated. As cones require light of high intensity to stimulate them, it follows that we cannot see colours in poor light.
- Visual acuity is dependent on the number of cone cells per unit area. The more there are the greater the number of impulses which will pass to the brain and the more detailed the image.