1. Communication Chapter 4:
Transformation in the retina: light signal to electrical impulse

2. The light signal reaching the retina is transformed into an electrical impulse
Once the eye has focused light on the retina, the light signal is transformed into an electrical impulse which is carried by the optic nerve to the brain, which interprets the signal as a ‘picture’.

3. Photoreceptor cells 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
The Retina
The retina is the innermost coat of the eyeball and is a very thin sheet of cells (1/10mm thick).
The retina consists of several layers of nerve cells, one of which is the layer of visual receptors - the rods and cones.

4. Photoreceptor cells 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
Of all the nerve cells, only the rods and cones respond directly to light, and for this reason are called photoreceptors.
In humans, each retina contains approximately million rods and 6-7 million cones.

5. Photoreceptor cells 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
Position of photoreceptors in the retina
The many layers of nerve cells in the retina are arranged ‘back to front’ compared with what one would expect. The rods and cones, which are sensitive to light, are the last layer of cells that the light reaches. Light coming into the eye passes through the entire retina before striking the rods and cones, which are closest to the choroid layer.
The photoreceptors generate impulses, which travel back along the various neuron layers of the retina to the optic nerve, which carries the signals to the brain.

6. Photoreceptor cells 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

7. Photoreceptor cells 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
There are 5 main layers of nerve cells or neurones that are directly involved in the transmission of impulses in the retina:
The photoreceptor cell layer
Bipolar cell layer
Ganglion cell layer
Horizontal cells
Amacrine cells

8. Photoreceptor cells 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
The Photoreceptor Cell Layer
The rods and cones, when stimulated by light, perform 3 main functions:
They absorb light energy (this involves the visual pigments).
They convert light energy into electrochemical energy, generating a nerve impulse.
They transmit this nerve impulse towards the bipolar cells.

9. Photoreceptor cells 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
Bipolar Cell Layer These sensory neurones receive electrochemical signals from the rods and cones and transmit the signal from these photoreceptors to the next layer of cells, the ganglion cells.

10. Photoreceptor cells 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
Ganglion Cell Layer The neurons in this layer receive electrochemical signals from the bipolar cells. The distal end of ganglion cells is extended into long processes that go on to form the fibres of the optic nerve. These neurons are responsible for carrying electrochemical signals from the retina to the brain.

11. Photoreceptor cells 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
Associated nerve cell layers
Horizontal Cells
Occur at the junction between photoreceptors and bipolar cells. They connect one group of rod and cone cells with another and then link them to bipolar cells.

12. Photoreceptor cells 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
Associated nerve cell layers
Amacrine Cells
Occur at the junction between bipolar cells and ganglion cells.
Horizontal and amacrine cells are involved in processing or ‘summarising’ incoming visual information.

13. Photoreceptor cells 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
Interesting information:
Visual acuity is the ability to see a clear and precise image.
Several rods make synaptic contact with one bipolar neurone. This is termed retinal convergence and leads to a decrease in visual acuity in rods, and increased sensitivity to small amounts of light.
Cones, which show little or no convergence have greater visual acuity, but are less sensitive to light.
How does retinal convergence increase sensitivity to light?
If several rods each receives a small amount of light, they may have enough combined energy to initiate a nerve impulse in the bipolar cell.

14. Photoreceptor cells 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
Interpretation of the Visual Signal
Although some information is processed in the retina, most of the interpretation of visual stimuli occurs in the brain, based on variables such as:
How strong the light is
How many rods and cones are stimulated
The combination of cones stimulated (leading to colour detection)
Differences in the image that falls on the retina of the left and right eye (depth perception)

15. Structure, distribution and function of rods and cones in the human eye Describe the differences in the structure, distribution and function of the photoreceptor cells in the human eye.
Both rods and cones are elongate cells that contain an outer segment (closer to the choroid layer of the eye) joined to an inner segment that leads to the conducting part of the cell.
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Conducting part of cell
Foot

16. Structure, distribution and function of rods and cones in the human eye Describe the differences in the structure, distribution and function of the photoreceptor cells in the human eye.
The conducting part of the cell comprises a cell body containing the nucleus and an extension or process called the foot. This process conducts impulses to the next layer of neurons in the retina.
Conducting part of cell
Foot

17. Structure, distribution and function of rods and cones in the human eye Describe the differences in the structure, distribution and function of the photoreceptor cells in the human eye.
Both rods and cones are named after the shape of their outer segments.
In rods, this segment is long and narrow.
Cones tend to have a shorter outer segment that is cone-shaped.

18. Structure, distribution and function of rods and cones in the human eye Describe the differences in the structure, distribution and function of the photoreceptor cells in the human eye.
Rods and cones contain visual pigments. These pigments (also called visual purple) are stacked in layers of flattened membranes in the outer segment.
The role of the visual pigments is to absorb light energy, which the rod or cone cell then converts into an electrochemical signal that the brain can interpret.

19. Structure, distribution and function of rods and cones in the human eye Describe the differences in the structure, distribution and function of the photoreceptor cells in the human eye.
Rhodopsin is the only pigment present in rods. Rod cells can only see in black and white.
Cones contain iodopsins. There are three types of iodopsins, one found in each type of cone cell. Each type is sensitive to a different wavelength of light (one of the primary colours of light – red, green, blue). Therefore, cone cells are responsible for colour vision.

20. Structure, distribution and function of rods and cones in the human eye Describe the differences in the structure, distribution and function of the photoreceptor cells in the human eye.
Distribution and Function
Rods are evenly distributed across most of the retina, but are absent from the fovea. As a result, rods are responsible for most peripheral vision, including the detection of movement.
The rods are not very tightly packed in the retina and many rods may connect with one bipolar neuron. This retinal convergence results in the rods having poorer visual acuity.

21. Structure, distribution and function of rods and cones in the human eye Describe the differences in the structure, distribution and function of the photoreceptor cells in the human eye.
Rods are extremely sensitive to light, and can be stimulated (bleached) by very small quantities of light energy. The pigment can also be rapidly regenerated.
They are used for night vision and to detect light and shadow contrasts.

22. Structure, distribution and function of rods and cones in the human eye Describe the differences in the structure, distribution and function of the photoreceptor cells in the human eye.
Cones are distributed in groups throughout the retina. Most cones are concentrated in the macula (yellow spot), an area that gives the central 10 degrees of vision, and fewer around the periphery of the retina.
The fovea is a small pit in the middle of the macula that contains densely packed cones which show no retinal convergence. As a result they have a high degree of visual acuity.

23. Structure, distribution and function of rods and cones in the human eye Describe the differences in the structure, distribution and function of the photoreceptor cells in the human eye.
The absence of blood vessels, neurone fibres and rod cells in the fovea lead to it being the area of most acute vision.
Because cones are less sensitive to light than rods are, they require larger quantities of light to stimulate (or bleach) them. As a result, cones function best in bright light, giving daytime vision. Cones take longer to regenerate once they have been bleached by light.
This bleaching effect is experienced when you are exposed to a ‘blinding’ flash of light. For example, when a bright flash of a light such as a camera flash goes off and we cannot seem to see anything for a brief period, usually only a fraction of a second.

24. Visual pigments identify that there are three types of cones, each containing a separate pigment sensitive to either blue, red or green light
All rods have only one type of pigment, rhodopsin. They are not sensitive to different colours. Rhodopsin is a broad spectrum pigment. Its peak sensitivity is in the 500nm wavelength region of the visible spectrum, but rods do not allow us to perceive any colour. Vision involving rhodopsin in rods allows us to see in shades of black, white and grey.
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25. Visual pigments outline the role of rhodopsins in rods
Cones contain one of three types of iodopsin pigments and are most sensitive to light in one of three wavelengths.

26. Visual pigments outline the role of rhodopsins in rods
These pigments result in cone cells being sensitive to:
Short wavelengths of blue light (peak 455nm)
Medium wavelengths of green light (peak 530nm)
Long wavelengths of red light (peak 625nm)

27. Visual pigments outline the role of rhodopsins in rods
Light of a particular wavelength may stimulate more than one cone which allows us to see a variety of colours. By comparing the rate at which various receptors respond, as well as the overlap in colours detected, the brain is able to interpret these signals as intermediate colours.

28. Biochemistry of vision – how the rods and cones work outline the role of rhodopsins in rods
The light-sensitive pigments rhodopsin and iodopsins are each made up of two parts:
A retinal (retinene) molecule derived from vitamin A
A protein called opsin
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If vitamin A is lacking, vision is affected and a condition known as ‘night blindness’ may result.

29. Biochemistry of vision – how the rods and cones work outline the role of rhodopsins in rods
It is the difference in the type of opsin molecules that determines whether the visual pigment is rhodopsin (rods) or iodopsins (cones):
Scotopsin = part of rhodopsin
Photopsin = part of iodopsins (three types – Opsin blue, Opsin green, Opsin red)

30. Biochemistry of vision – how the rods and cones work outline the role of rhodopsins in rods
The main function of the photochemical pigments rhodopsin is to absorb light. When light strikes rhodopsin, the light energy is absorbed and rhodopsin changes from its resting state to an excited state. This change is due to the activation of the retinal part of rhodopsin.

31. Biochemistry of vision – how the rods and cones work outline the role of rhodopsins in rods
This causes rhodopsin to split into its protein opsin part and a free retinal part. The split rhodopsin pigment is said to be ‘bleached’, but this change is temporary.

32. Biochemistry of vision – how the rods and cones work outline the role of rhodopsins in rods
This is the start of an electrical impulse that moves along the receptor to the brain by first triggering the release of a chemical substance known as a neurotransmitter.
The neurotransmitter then stimulates a bipolar cell, generating an impulse in this cell.
The bipolar cell transmits the electrochemical signal to the ganglion cells which in turn carry the signal to the brain.
NOTE: The signal is termed electrochemical because it involves both an electrical change in membrane and a chemical release of a neurotransmitter.

33. Biochemistry of vision – how the rods and cones work outline the role of rhodopsins in rods
Rhodopsin, which was temporarily bleached or broken down in the presence of light, is then regenerated so that is can be reused.
Retinal and opsin recombine with the help of enzymes. This allows a new image to be received. Vitamin A is essential for these steps to occur.
Night blindness is the difficulty for the eyes to adjust to dim light. Affected individuals are unable to distinguish images in low levels of illumination. People with night blindness have poor vision in the darkness, but see normally when adequate light is present.

34. Biochemistry of vision – how the rods and cones work process and analyse information from secondary sources to compare and describe the nature and functioning of photoreceptor cells in mammals, insects and in one other animal
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35. Biochemistry of vision – how the rods and 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
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36. Colour blindness in humans explain that colour blindness in humans results from the lack of one or more of the colour-sensitive pigments in the cones
Because cones detect colour, any defect or damage to the cones will affect the ability of the eye to perceive colour. Humans have three different forms of opsins present in cones – each colour is coded for by one gene.
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37. Colour blindness in humans explain that colour blindness in humans results from the lack of one or more of the colour-sensitive pigments in the cones
A mutation in a gene that codes for a cone pigment leads to the inability of this pigment to function correctly. As a result the person is unable to perceive colour in the normal trichromatic manner, and is said to be either colour blind or colour deficient.

38. Colour blindness in humans explain that colour blindness in humans results from the lack of one or more of the colour-sensitive pigments in the cones
In humans the genes that codes for red and green pigments are located on the sex chromosome (it is a sex-linked disorder). The gene for colour-blindness is recessive to the gene for normal vision.

39. Colour blindness in humans explain that colour blindness in humans results from the lack of one or more of the colour-sensitive pigments in the cones
A person who is termed ‘colour blind’ is not truly colour blind, but is usually able to see only two of the three primary colours of light. They lack one or more of the colour-sensitive pigments in the cones
Because of this, they perceive colours differently. Such individuals have dichromatic vision and interpret all colours based on combinations of the two primary colours that they are able to see.

40. Colour blindness in humans explain that colour blindness in humans results from the lack of one or more of the colour-sensitive pigments in the cones
People who are red-green colour blind find it difficult to distinguish between red and green objects placed adjacent to each other. They can often still distinguish red objects from green objects if there is a difference in the brightness of the objects.
It is extremely rare that a person is monochromatic, unable to distinguish colours and seeing most things in shades of black, white and grey.

41. Colour blindness in humans explain that colour blindness in humans results from the lack of one or more of the colour-sensitive pigments in the cones
Some people who have defective genes for colour vision may be colour deficient. A mutation in a gene for any one of the cone pigments may simply cause a change in the peak of spectral sensitivity of that cone. People who are colour deficient may not see the number on a colour vision plate like the one below.
Colour blindness and deficiency are detected by showing a person special pictures called Ishihara plates, made up of patterns of dots. Certain numbers are arranged in a particular pattern, often to form a number.