Physiology of Vision and vision pathway.pptx

This describes the pathway of vision and physiology of vision which is a sense in the body

1. Physiology of Vision Dr. Senthamizselvan. R

2. Sensory Organs Vision, hearing and equilibrium, smell and taste comprises the special sensations. These special senses are detected by special sensory organs starting from 1.Eye 2.Ear 3.Nose 4.Tongue

3. Vision Physiological anatomy of the Eye The Eye consists of: • The orbit • The globe or eyeball • Eyelids • Extra-ocular muscles

4. Orbit Orbit: Bony Cavity present in the skull which contains the Eyeball and its appendages( Muscles, Nerves, Etc..,) Main function is to protect the Eye from mechanical trauma.

5. Eyelids Formed by a thin fold of skin, to cover the eye Key function of the Eye lid is to keep the cornea moist, Blinking of the eyelids spreads the tears over the cornea and keeps it moistened always, Prevents drying of cornea and conjunctiva during sleep.

6. Extraocular muscles Produce movements of the eyeball to fix vision on an object.

7. Eyeball The eyeball has three layers, Outer layer:- Sclera and cornea Middle vascular layer :- Choroid Inner neural layer:- Retina

8. Sclera and Cornea Sclera forms the outer layer of the eyeball. Posterior 5/6 th of the sclera is transopaque, anterior 1/6 th is transparent and forms the cornea The cornea is transparent and allows light to enter the eyeball. Cornea causes refraction of the light from distant objects .

9. Choroid Choroid constitutes the middle pigmented vascular layer, Supplies nutrients to the all layers of the eyeball, except cornea(Avascular), Regulates retinal temperature, Choroid pigments prevent reflection of light within the eye , Iris and ciliary body are part of the choroid.

10. Iris and Pupil Pigmented part of the eyeball and iris controls the amount of light enters the retina by contraction and relaxation of iris muscles, and thereby regulating the aperture(Pupil) diameter.

11. Lens Cornea refracts the light that enters the eyeball, Lens further focus the light from the cornea on to the retina, Lens is held in place by the suspensory ligaments that arise from the ciliary body. Curvature of the lens can be increased and decreased, with the help of the ciliary muscles, The ability of the lens to change its power enables a person to focus on objects at different distances.

12. Retina The innermost layer of the eyeball which contains the receptors(Rods and Cones) for visual processing,

13. Compartments of the Eyeball Lens and the zonules divide the eyeball in anterior and posterior compartment, The Anterior compartment contains the aqueous humour The posterior compartment contains the vitreous humour These maintain the shape of the eyeball.

14. Optic Nerve Optic nerve carries the visual information generated by the retina to the brain. The optic nerve leaves the retina, at the level of Optic disc, The optic disc is called as Blind spot, because it is devoid of rods and cones.

15. Refraction of Light by the Eye Light entering the eye is refracted by the cornea, aqueous humour, lens and vitreous humour, Most of the refraction occurs at the anterior surface of the cornea and the remaining in the lens.

16. Inverted image on the Retina The image formed on the retina after refraction will be inverted

17. Accommodation Accommodation is the process by which the diverging rays from a near object is focused onto the retina The steps involved in the accommodation are Convergence of the eyeballs , Constriction of the pupils, The contraction of the ciliary muscles releases the suspensory ligaments, releasing the tension exerted on the lens matter and allows the lens to assume a more spherical shape with greater refractive power, The anterior curvature of the lens increases, and the refractive power of the lens also increases, as a result image is focused on to the retina.

18. Accommodation

19. Presbyopia Age related disorder, Loss of accommodation in old age, because of decrease in elasticity of the lens, Fatiguability of ciliary muscles, Patient have trouble in near work and reading.

20. Refractive Errors of the Eye Refractive Errors of the Eyeball occurs due to a discrepancy in the size of the globe and the total refractive power of the eye at rest. Refractive power is the degree to which a lens, or the optical system converges or diverges light( also known as Optical power). The unit of refractive power is Diopter. Most common, 1. Myopia( Near sightedness) 2. Hypermetropia( Long sightedness) 3. Astigmatism 4. Presbyopia(Age related)

21. Myopia( Near sightedness) Refractive power at rest is more than normal or the eyeball is bigger than normal. Hence images of distant objects are focused in front of retina at rest. This causes blurred image on retina. There is difficulty in seeing distant objects clearly. Near objects may be seen clearly.

22. Hypermetropia( Long sightedness) Refractive power of eye at rest is less than normal or the eyeball is small than normal. Hence at rest, the eye brings parallel rays of light to a focus behind the retina, so image on the retina is blurred. The patient will not have difficulty in seeing distant objects. But there will be strain on doing close work as the available accommodation will be used for focusing the distant object.

23. Astigmatism Different meridians (lines at different angles ) of cornea have different curvatures. Corneal plane with steeper curve has greater refractive power; flatter curve – has lesser refractive power. Thus, a point of light cannot be brought to a point focus on the retina. In some meridians, the light will be focused behind the retina and in some of the meridians, in front of the retina and so on. Hence the image formed will be blurred.

24. Correction of Refractive errors Spectacle correction: Concave lens in myopia (to diverge the light rays), Convex lens in hypermetropia (to converge the light rays), Cylindrical lens in astigmatism (to correct the refractory error in the different meridian), Bifocal lens for presbyopia (convex lens for reading). Contact lens: Ideal correction, especially for Astigmatism and high Myopia Surgery

25. Correction of Refractive errors

26. Correction of Refractive errors

27. Retina Retina has 10 layers and contains the photoreceptors for visual processing – rods and cones

28. Optic disc The Optic disc is an area in the retina where there are no rods and cones (blind spot). The nerve fibers converge here to form the optic nerve. The retinal blood vessels enter the globe through optic disc.

29. Macula lutea and fovea Macula lutea is situated near the center of the retina in human eye, It is responsible for the visual acuity ( sharp central vision), It has a central depression called fovea centralis , where the concentration of cones is very high and visual acuity also high.

30. Macula lutea and fovea . Images of objects of focus fall on the macula and it is the region of highest visual resolution, because it has high concentration of cones,

31. Rods and cones Rods contain the visual pigment rhodopsin and are involved in vision in dim light. When light falls on rods, the rhodopsin gets broken down and bleached, generating neural signals in the retina.

32. Night Blindness(Nyctalopia) Night blindness is the inability of a person to see properly during night and in dim light. Rhodopsin contains retinal, a vitamin A derivative. Hence night blindness occurs in Vitamin A deficiency.

33. Colour vision Cones are responsible for the colour vision There are 3 primary cone pigments , blue, green, red . Each pigment maximally absorbs blue light, green light and red light, respectively.

34. Trichromatic theory There are several theories of colour vision, the most accepted one is trichromatic theory, According to the Trichromatic theory, the perception of any colour is produced by the relative ratios of absorption of light by the 3 primary cone pigments. The appropriate combination of stimulation of the 3 cone pigments (colours) can produce the sensation of any colour. Normal people are thus said to be trichromats.

35. Colour blindness Colour vision is tested with the Ishihara’s chart. Colour blindness is due to a genetic defect in the cone pigments. The affected person will be unable to differentiate different colors.

36. Types of Colour blindness

37. Thank you