Further Links Contact Us Search Help

Syllabus | Exams | Exam techniques | Resources | Teachers | Feedback

Biology

Home > Biology > Options > Communication > Communication: 4. The retina

9.5 Option – Communication: 4. The retina

Syllabus reference (October 2002 version)
4.The light signal reaching the retina is transformed into an electrical impulse	Students learn to: 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   describe the differences in distribution, structure and function of the photoreceptor cells in the human eye   outline the role of rhodopsins in rods   identify that there are three types of cones, each containing a separate pigment sensitive to either blue, red or green light   explain that colour blindness in humans results from the lack of one or more of the colour sensitive pigments in the cones	Students: 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   process and analyse information from secondary sources to describe and analyse the use of colour for communication in animals and relate this to the occurrence of colour vision in animals

Extract from Biology Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.
[Edit 17 Aug 04]

Background: A nerve impulse is an electrochemical disturbance which spreads along the neurone from one end to the other. This impulse is accompanied by minute changes in the electrical conductivity of the neurone.

Nerve impulses generated by the eye are carried by connector neurones to the nerve fibres that make up the optic nerve. The impulses are taken to a part of the brain called the thalamus and from there passed to the visual areas in the cerebral cortex. When we look at an object, the brain receives impulses from the eye and these are interpreted.

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

• 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.

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

• Try to gather information from a range of resources, including popular scientific journals, digital technologies like CD-ROMs and the Internet.

• When processing and analysing information you need to be aware that the concentration and nature of photoreceptor cells varies amongst species. Not all species posses colour vision, and many less complex animals possess simple light receptors. Look for generalisations such as all visual systems are based on the same photochemical, rhodopsin.

• A table like the one below is an effective tool to assist you to gather information. Well-designed tables assist you to identify useful information. Some suggested answers are given as a model.

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.

process and analyse information from secondary sources to describe and analyse the use of colour for communication in animals and relate this to the occurrence of colour vision in animals

• Try to gather information from a range of resources, including popular scientific journals, digital technologies like CD-ROMs and the Internet.

• Research strategies such as camouflage, mimicry, sexual dimorphism and reproductive behaviour. Types of animals which use these strategies include amphibians, reptiles, mammals and birds.

• Focus on identifying at least two examples of the use of colour for communication, describe how it is used and analyse how the strategy benefits the organism.

outline the role of rhodopsins in rods

• 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.

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

• 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.

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

• 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.

Colour blindness Howard Hughes Medical Institute, Chevy Chase, Maryland, USA.