1. Eye

2. Structure The most important structures to learn the function of are… Retina Lens –Ciliary Muscles Iris

4. The Iris and Pupil The iris controls the amount of light entering the eye and able to reach the retina High light intensities are able to damage the retina (rods). The iris consists of two types of muscle, Circular (parasympathetic) and radial (sympathetic). Circular muscles contracted, Radial Relaxed Circular muscles relaxed, radial contracted

5. Forming an image Most refraction happens at cornea Lens makes fine adjustments. This is called accommodation Visual association area perceives image right way up

6. Accommodation Far object Parallel rays Little refraction needed Ciliary muscles relax More tension in suspensory ligaments Lens pulled thinner Near object Diverging rays Much refraction needed Ciliary muscles contract Tension lost in suspensory ligaments Lens more spherical (natural shape)

7. Eye Problems: myopia (near/short sight) Parallel rays refracted too much Image focussed in front of retina Treated with diverging (concave) lens Increases focal length

8. Hypermetropia (far/long sight) Diverging rays refracted too little Image focussed behind retina Treated with converging (convex) lens Decreases focal length

9. Retina 3 layers –Ganglion cells (sensory neurone) –Bipolar neurones –Rods & Cones Fovea has more cones Periphery has more rods Optic nerve head (blind spot) has neither

10. Rods & Cones Contain light sensitive chemical (pigment) Convert light into electrical impulse

11. Rods and cones Like neurones but… Membrane is depolarized at rest Gated Na + channels are open (Inhibitory) neurotransmitter is constantly released Keeps bipolar neurone hyperpolarized

12. Converting light into electrical impulse Light changes 11-cis-retinal into all trans-retinal Rhodopsin → opsin + all trans-retinal Opsin closes gated Na + channels Membrane is hyperpolarized Inhibitory neurotransmitter release is reduced Bipolar neurone is depolarized If depolarization reaches threshold an action potential is generated in the bipolar neurone

13. Adaptation Dark adapted –Large amounts of rhodopsin, so… –low levels of light can break some down but.., –High levels of light break down large amounts of rhodopsin (bleaching) –Bright light hurts Light adapted –Low levels of rhodopsin –Bright light is able to break down some but… –Dim light does not break down enough to generate an action potential In the dark human eyes become “dark adapted” in the light they become “light adapted”

14. Summary of differences RodsCones Pigment is rhodopsin3 types of iodopsin (pigment B, G, R) Sensitive to dim lightOnly sensitive to bright light Poor visual acuityHigh visual acuity No colour visionColour vision Share connections to bipolar neurone Single connection to bipolar neurone Spread around periphery of retina Mostly in fovea

15. Retin Ə l convergence Cones have individual connection to bipolar neurone Several rods share a bipolar neurone

16. Sensitivity Rhodopsin is more easily broken down than iodopsin Retinal convergence –In dim light only a small amount of pigment may be broken down in an individual rod or cone –This may not be enough alone to trigger an action potential in a bipolar neurone –However, the reduction in neurotransmitter release from several rods at once may be enough to trigger and AP in the bipolar neurone

17. Visual Acuity When an impulse reaches the visual sensory area from a ganglion cell it can either have come from one cone or several rods. The brain cannot tell which The area of retina represented by several rods is larger The image from rods is less resolved, more blurry (pixelated)

18. The Trichromatic theory of colour vision 3 types of cone 3 types of iodopsin –Pigments B, G, R –Sensitive to different wavelengths of light Different wavelengths break down different proportions of pigment Brain perceives colour as relative stimulation from each type of cone E.g. a mixture of pigment G and R breakdown looks yellow

19. Parallel processing 3 pathways along optic nerve –Colour –Shape –Movement Impulse sent to visual sensory area Association area processes inputs from sensory area in light of other inputs from memory and other senses

20. Ageing Cataracts –Lens becomes cloudy/opaque –Lens can be surgically removed or replaced by artificial lens –Corrective lenses are needed to allow focussing on near/far objects Macular degeneration –Loss of rods and cones –Dry Cells die Untreatable –Wet Blood vessels grow through retina and burst Treatable