Communication Topic 6: Photoreceptor Cells

1. CommunicationTopic 6: Photoreceptor Cells Biology in Focus, HSC Course Glenda Childrawi, Margaret Robson and Stephanie Hollis

2. DOT Point(s) • 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 • explain that colour blindness in humans results from the lack of one or more of the colour-sensitive pigments in the cones

3. Introduction 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. We are going to look in detail at this process. droualb.faculty.mjc.edu

4. Introduction We know the retina is the innermost coat of the eyeball and is very thin (1/10mm thick). It consists of several layers of nerve cells, one of which is the layer of visual receptors-the rods and cones. www.tumblr.com

5. Photoreceptors • Of all the nerve cells that make up the retina, only the rods and cones respond directly to light. This is the reason we call them photoreceptors. In humans, each retina contains approximately 125 million rods and 6 to 7 million cones. bocah.info

6. Position of Photoreceptors 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. www.clipart.dk.co.uk

7. Position of Photoreceptors 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. www.clipart.dk.co.uk

8. Light Images to Electrochemical Signals The rods and cones, when stimulated by light, perform 3 main functions: • They absorb light energy (this involves the visual pigment) • They convert light energy into electrochemical energy, generating a nerve impulse • They transmit this nerve impulse towards the bipolar cells of the retina www.dana.org

9. Position of Photoreceptors There are 5 main layers of nerve cells or neurones that are directly involved in the transmission of impulses in the retina: • The photoreceptor layer • Bipolar cell layer • Ganglion cells layer • Horizontal cells • Amacrine cells Handout Retina Cells Diagram www.clipart.dk.co.uk

10. Light Images to Electrochemical Signals 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. www.clipart.dk.co.uk

11. Light Images to Electrochemical Signals 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. scien.stanford.edu

12. Light Images to Electrochemical Signals Horizontal Cells These 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. www.austincc.edu

13. Light Images to Electrochemical Signals Amacrine Cells These cells occur at the junction between bipolar cells and ganglion cells. Amacrine cells are involved in processing or ‘summarising’ incoming visual information. www.webexhibits.org -

14. Interesting Information Studies of the eye show that several rods make synaptic contact with one bipolar neurone, which in turn connects a single cell body of an optic nerve fibre. This is termed retinal convergence and leads to a decrease in visual acuity in rods. Visual acuity is the ability to see a clear and precise image. universe-review.ca

15. Interesting Information Retinal convergence is, however, an advantage in giving the eye increased sensitivity to small amounts of light. If light of a low intensity hits a single rod cell it, as a single cell, may not receive enough energy to initiate a nerve impulse in a bipolar cell. However, if several rods each receives a small amount of light, they may have enough combined energy to initiate this response in the bipolar cell. www.timdashwood.com

16. Interesting Information Cones, which show little or no convergence, therefore have greater visual acuity, but are less sensitive to light. www.clipart.dk.co.uk

17. 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/or cones are stimulated • Contrast enhancement • Recognition of horizontal, vertical and diagonal lines • 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) www.clipart.dk.co.uk

18. Rods and Cones For this next section, draw a chart like the one below and fill in each section as we go through the next few slides. Handout Photoreceptor Diagrams

19. Rods and Cones Both rods and cones are elongated 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. The conducting part of the cell comprises a cell body containing a nucleus and an extension or process called the foot. This process conducts impulses to the next layer of neurones in the retina. clearsci.blogspot.com

20. Rods and Cones Structure 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 conical (cone-shaped). Most cones are broader than rods. askabiologist.asu.edu

21. Rods and Cones Pigment Rods and cones contain visual pigments, chemical substances that absorb light energy. These pigments, sometimes collectively termed visual purple, are stacked in layers of flattened membranes in the outer segment of each photoreceptor.

22. Rods and Cones Pigment Rhodopsin is the only pigment present in rods. Cones contain iodopsins. There are three different types of iodopsin, one found in each type of cone cell. Each type is sensitive to a different wavelength of light. eulep.pdn.cam.ac.uk

23. Rods and Cones Function The cone cells are therefore responsible for colour vision, while the rod cells can only see in black and white. 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. www.eyedesignbook.com

24. Distribution and Function Distribution 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. thebrain.mcgill.ca

25. Distribution and Function Rods are extremely sensitive to light, responding best to low light intensities. They can be stimulated (bleached) by very small quantities of light energy. The pigment can also be rapidly regenerated. The resulting sensitivity makes them able to operate well in semi-darkness. They are used for night vision and to detect light and shadow contrasts. damesek.blogspot.com

26. Distribution and Function Distribution Cones are distributed in groups throughout the retina, but there are fewer around the periphery of the retina, most being concentrated in the macula (yellow spot), an area of the retina that gives the central 10 degrees of vision. The fovea is a small pit in the middle of the macula and contains cones only. www.phys.ufl.edu

27. Distribution and Function The cones in the fovea are very densely packed and show no renal convergence. As a result they have a high degree of visual acuity, perceiving images central to the visual field clearly and precisely. www.positscience.com

28. Distribution and Function Cones are responsible for colour vision. Each cone contains one of three types of iodopsin pigment, and each type of iodopsin is sensitive to one of the primary colours of light. www.clipart.dk.co.uk

29. Distribution and Function The model of trichromatic colour vision proposes that all colours perceived by the eyes are a combination of the three basic colour, red, green and blue, to which the cones are sensitive. The brain interprets the combination of different types of cones that have been stimulated as the different colours we see. www.cgadvertising.com

30. Distribution and Function 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 high intensity light, giving them daytime vision. www.sciencedaily.com

31. Distribution and Function 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. www.wired.com

32. Colour Blindness • 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. www.clipart.dk.co.uk

33. Colour Blindness 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. www.heraldsun.com.au

34. Colour Blindness It is extremely rare that a person is monochromatic, unable to distinguish colours and seeing most things in shades of black, white and grey. anthro.palomar.edu

35. Colour Blindness Not all people who have defective genes for colour vision are colour blind. Some people 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 one on a colour vision plate like the one below.

36. Homework -Students to complete DOT Point