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BiologyMad.com. The Retina Contains photoreceptor cells (rods and cones) and associated interneurones and sensory neurones. BiologyMad.com.

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Presentation on theme: "BiologyMad.com. The Retina Contains photoreceptor cells (rods and cones) and associated interneurones and sensory neurones. BiologyMad.com."— Presentation transcript:

1 BiologyMad.com

2 The Retina

3 Contains photoreceptor cells (rods and cones) and associated interneurones and sensory neurones. BiologyMad.com

4 This is the process by which light initiates a nerve impulse. The structure of a rod cell: Visual Transduction BiologyMad.com Detection of light is carried out on the membrane disks These disks contain thousands of molecules of rhodopsin (photoreceptor molecule)

5 Rhodopsin consists of: –Opsin (membrane bound protein) –Retinal (covalently-bound prosthetic group) sensitive part Retinal is made from vitamin A Retinal is the light sensitive part - –exists in 2 forms: cis and trans forms Visual Transduction BiologyMad.com

6 In the dark retinal is in the cis form. When it absorbs a photon of light it quickly switches to the trans form. This changes the shape of the opsin protein – a process called bleaching Visual Transduction Rhodopsin with cis retinal Rhodopsin with trans retinal Light- fast (  s) BiologyMad.com

7 The reverse reaction (trans to cis) requires an enzyme reaction and is very slow (taking a few minutes) This process requires ATP, as rhodopsin has to be resynthesised Visual Transduction Rhodopsin with cis retinal Rhodopsin with trans retinal Light- fast (  s) Dark - slow (mins) BiologyMad.com

8 Bleaching of the rhodopsin in a rod cell  Alters the permeability of the membrane to Na+  nerve impulse  sensory neurone in the optic nerve  to the brain Visual Transduction BiologyMad.com

9 Rhodopsin controls sodium channels Rhodopsin with cis retinal opens sodium channels (absence of light) Rhodopsin with trans retinal closes sodium channels (light) Visual Transduction BiologyMad.com

10 In the Dark… In the dark the channel is open  Na+ flow in can cause rod cells to depolarise. –Therefore in total darkness, the membrane of a rod cell is polarised Therefore rod cells release neurotransmitter in the dark However the synapse with bipolar cells is an inhibitory synapse i.e. the neurotransmitter stops impulse BiologyMad.com

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12 In the Light… As cis retinal is converted to trans retinal, the Na+ channels begin to close  less neurotransmitter is produced. If the threshold is reached, the bipolar cell will be depolarised  forms an impulse which is then passed to the ganglion cells and then to the brain BiologyMad.com

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14 Rods and Cones RodsCones Outer segment is rod shapedOuter segment is cons shaped 10 9 cells per eye, distributed throughout the retina, so used for peripheral vision cells per eye, found mainly in the fovea, so can only detect images in centre of retina. Good sensitivityPoor sensitivity Only 1 type  monochromatic vision 3 types (R, G & B)  colour vision Many rods connected to one bipolar cell  poor acuity = poor resolution Each cone is connected to one bipolar cell  good acuity = good resolution BiologyMad.com

15 Colour Vision 3 different cone cells. Each have a different form of opsin (they have the same retinal) 3 forms of rhodopsin are sensitive to different parts of the spectrum –10% red cones –45% blue cones BiologyMad.com

16 Coloured light will stimulate these 3 cells differently - by comparing the nerve impulses from the 3 kinds of cones the brain can detect any colour –Red light  stimulates R cones –Yellow light  stimulates R and G cones equally –Cyan light  stimulates B and G cones equally –White light  stimulates all 3 cones equally Called the trichromatic theory of colour vision Colour Vision BiologyMad.com

17 When we look at something the image falls on the fovea and we see it in colour and sharp detail. Objects in the periphery of our field of view are not seen in colour, or detail. The fovea has high density of cones. Each cone has a synapse with one bipolar cell and one ganglion  each cone sends impulses to the brain about its own small area of the retina  high visual acuity Colour Vision

18 Refers to the ability of the eye to alter its focus so that clear images of both close and distant objects can be formed on the retina –The lens shape can be altered by suspensory ligaments and the ciliary muscles. This adjusts the focus Accommodation BiologyMad.com

19 Accommodation Distant objects: –Light rays are almost parallel so do not need much refraction to focus onto the retina. –The lens therefore needs to be thin and “weak” (i.e. have a long focal length). BiologyMad.com

20 Accommodation Close objects: –Light rays are likely to be diverging, so need more refraction to focus them onto the retina. –The lens therefore needs to be thick and “strong” (i.e. have a short focal length). BiologyMad.com

21 Regulates the amount of light entering the eye so that there is enough light to stimulate the cones, but not enough to damage them Composed of 2 sets of muscles: –Circular and radial  have opposite affects (antagonistic) The Iris BiologyMad.com

22 By contracting and relaxing these muscles the pupil can be constricted and dialeted The Iris BiologyMad.com

23 Is under control of the autonomic nervous system –Sympathetic Nerve  pupil dilation –Parasympathtic Nerve  pupil constriction –The drug atropine inhibits the parasympathetic nerve, causing pupil to dilate The Iris BiologyMad.com

24 The iris is a good example of a reflex arc: The Iris stimulus receptor coordinator effector response More Light Rods and Cones Brain Iris muscles Pupil constricts Sensory neurone Motor neurone BiologyMad.com


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