The Eye and the Nervous System

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