Communication: The Retina

Question 1

describe the differences in distribution, structure and function of the photoreceptor cells in the human eye

Answer 1

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.

Question 2

outline the role of rhodopsins in rods

Answer 2

Rhodopsins are light-sensitive pigments, which consist of two molecules bonded together, opsin and retinal. When light enters a rod cell, it splits rhodopsin molecules into its two components. This reaction results in an impulse in the neurone attached to the rod or cone. The two products slowly recombine, ready to be split again by more light. This is known as the visual cycle.
Rods contain rhodopsin that is sensitive to blue–green light. The figure below shows that this molecule has maximum absorbency at a wavelength of 498 nm, which corresponds to the light of a blue-green colour.

Question 3

identify that there are three types of cones, each containing a separate pigment sensitive to either blue, red or green light

Answer 3

The cones contain three different photopigments. The trichromatic theory of colour vision suggests that each is sensitive to a different range of wavelengths, corresponding to the three primary colours red, blue and green. The sensitivity of these photopigments is broad enough to allow them to cover the full spectrum of visible light. Each pigment is thought to be located in different cones, and different colours are perceived in the brain from the sensory input from combinations of the three cone types. Thus the brain builds up a colour picture according to the number of impulses received from the three types of cones.

Question 4

explain that colour blindness in humans results from the lack of one or more of the colour sensitive pigments in the cones

Answer 4

There are three colour sensitive types of cones in humans. Colour blindness in humans occurs because one or more of the three types of photopigments in cones is either absent or does not function properly. Complete inability to distinguish colours is rare. The most common form of colour blindness is the failure to discriminate between brown, red and green.

Question 5

process and analyse information from secondary sources to compare and describe the nature of photoreceptor cells in mammals, insects and in simple light receptors in one other animal

Answer 5

Type of animal 
Example
Distribution of photoreceptors	Ability to perceive colour
Mammal	
humans

located in the eye, different types of receptors; rods and cones; three types of cones are responsible for colour vision and are located more towards the centre of the retina; rods are located more towards the periphery of the eye and are best for movement detection

three colour vision system

dog

located in the eye; two types of cones

red-green colour blind

Invertebrate
bee

compound eyes, hundreds off optical units called ommatidia.

three colour vision but can see ultra-violet light and not red light

flatworm

located in cup eyes

directional information only; no colour

Question 6

Identify photoreceptor cells as those containing light sensitive pigments and explain that these cells convert light images into electrochemical signals that the brain can interpret

Answer 6

Photoreceptor cells contain light sensitive pigments. These cells convert light into electrochemical signals that the brain can interpret. An electrochemical signal consists of a wave of sodium and potassium ions which move across the cell membrane of the neurone.