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The Eye and the Nervous System

The Eye and the Nervous System. BiologyMad.com. The Retina. BiologyMad.com. The Retina. Contains photoreceptor cells (rods and cones) and associated interneurones and sensory neurones. BiologyMad.com. Visual Transduction. This is the process by which light initiates a nerve impulse.

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The Eye and the Nervous System

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  1. The Eye and the Nervous System BiologyMad.com

  2. The Retina

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

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

  5. BiologyMad.com Visual Transduction • 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

  6. BiologyMad.com Light- fast (ms) Rhodopsin with cis retinal Rhodopsin with trans retinal Visual Transduction • 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

  7. BiologyMad.com Light- fast (ms) Rhodopsin with cis retinal Rhodopsin with trans retinal Dark - slow (mins) Visual Transduction • 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

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

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

  10. BiologyMad.com 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

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

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  14. BiologyMad.com Rods and Cones

  15. BiologyMad.com 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 • 45% blue cones

  16. BiologyMad.com Colour Vision • 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

  17. Colour Vision • 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

  18. BiologyMad.com Accommodation • 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

  19. BiologyMad.com 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).

  20. BiologyMad.com 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).

  21. BiologyMad.com The Iris • 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)

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

  23. BiologyMad.com The Iris • 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

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

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