INTRODUCTORY LECTURE ON THE PHYSIOLOGY OF VISION

1. INTRODUCTORY LECTURE ON THE PHYSIOLOGY OF VISION S. I. OGUNGBEMI DEPARTMENT OF PHYSIOLOGY UNIVERSITY OF LAGOS

2. SPECIAL SENSES • Special senses are: • Vision • Audition • Olfaction • Gustation and • Special proprioception (Vestibular apparatus) • They serve as tools for learning and formation of memory

3. CHAPTERS OF THE LECTURE • Functional Anatomy of the Eye • Optics and Optical Defects • The Visual Pathway • Photoreceptor Mechanism of Retina

4. Visual System • Eye • Accessory structures • Eyebrows, eyelids, eyelashes, tear glands • Protect eyes from sunlight and damaging particles • Optic nerve (II) • Tracts • Pathways • Eyes respond to light and initiate afferent action potentials

5. Accessory Structures of Eye • Eyebrows • Prevent running perspiration into eyes • Shade • Eyelids or palpebrae • Consist of 5 tissue layers • Protect and lubricate • Conjunctiva • Covers inner eyelid and anterior part of eye • Lacrimal apparatus • Extrinsic eye muscles

6. Lacrimal Apparatus • Lacrimal apparatus • Lacrimal Gland: Produces tears to moisten, lubricate, wash • Lacrimal Canaliculi • Collects excess tears • Punctum • Lacrimal Sac • Nasolacrimal duct • Opens into nasal cavity

7. Extrinsic Eye Muscles

8. Anatomy of the Eye • Three coats or tunics • Fibrous: Consists of sclera and cornea • Vascular: Consists of choroid, ciliary body, iris • Nervous: Consists of retina

9. Fibrous tunic: Outer Sclera: White outer layer, maintains shape, protects internal structures, provides muscle attachment point, continuous with cornea Cornea: Avascular, transparent, allows light to enter eye and bends and refracts light Vascular tunic: Middle Iris: Controls light entering pupil; smooth muscle Ciliary muscles: Control lens shape; smooth muscle Retina: Inner Contains neurons sensitive to light Macula lutea or fovea centralis: Area of greatest visual acuity Optic disc: Blind spot Compartments Anterior: Aqueous humor Posterior: Vitreous humor Lens Held by suspensory ligaments attached to ciliary muscles Transparent, biconvex Anatomy of the Eye

10. Horizontal section of the right eye. AP, anterior pole; PP, posterior pole; VA, visual axis

11. Functional Anatomy of the Eye • The eye has 3 concentric coats or layers for the specialisedsense of vision. • The sclera– the outer protective layer of fibrous coat which is transparent anteriorly as the corneai.e. ⅙of sclera • The choroid– Middle melanin-pigmented layer which contains the blood vessels which nourish the eye. The specialised anterior portions are ciliary body and iris • The retina– the innermost layer which contains the photoreceptor cells i.e. the rods and cones.

12. The Sclera • Sclera is the tough posterior outer coat. • It is composed of tightly bound elastic and collagen fibres. • It provides adequate protection for internal contents and components of the eye. • It withstands high intraocular pressure of about 20 mmHg. • The high intraocular pressure keeps structures involved in vision in proper shape and position.

13. The Cornea • The cornea is more convex than sclera. • It is the anterior surface of the eyeball. • It is made of collagen fibrils. • It is covered anteriorly by stratified epithelium. • It is continuous with conjunctiva covering the exposed sclera. • It transmits and focuses incident light to the retina.

14. Its epithelium utilises lacrymal secretion to keep cornea in hydrated state. • If the cornea is not hydrated, it dries up and looses its transparency → xerophthalmia. • It has its free nerve endings from trigeminal nerve. • It is avascular – i.e. it has no blood vessel in itself. • Being most, it receives oxygen from its own metabolism directly from atmosphere.

15. The Choroid, Ciliary Body and Iris. • Choroid comprises an outer pigmented and inner vascular layer. • Blood vessels of the choroid nourish the inner layers of retina by simple diffusion. • Melanocytes are abundant in the pigmented layer. • Black melanin pigments serve to absorb light rays thereby preventing their reflection back to the retina, which would blur the optical image.

16. Ciliary bodyarises from the anterior end of the choroid coat. • The ciliary apparatus consists of the ciliary muscles, the iris and the suspensory ligament which suspends the lens. • Ciliary muscle consists of an outer ciliary muscle and inner ciliary processes. • It is made up of radial (dilator) and circular (constrictor) muscles. • Motor supply to the ciliary muscle is parasympathetic with cell bodies of the preganglionic neurons in the Edinger-Westphal nucleus of oculomotor nerve CN III

17. Postganglionic neurons are in the ciliary ganglion. • Contraction of ciliary muscle makes the lens more convex. • The iris- thin pigmented contractile diaphragm arising from the ciliary body. • The iris varies the aperture (or pupil) of the lens in response to contraction of the ciliary muscles. • The pupil is the visible coloured aperture of the eye. • The pupil which varies the amount of light entering the eyes. • It contains radial and circular multiunit smooth muscles which control the size of the pupil.

18. Parasympathetic stimulation causes contraction of the ciliary circular muscles (sphincter pupillae in the iris) and constriction of the pupil. • Sympathetic stimulation causes contraction of radial muscles (dilator pupillae in the iris) and dilation of the pupil. • Sympathetic fibres relay via the superior cervical ganglion. • The Aqueous Humour • It is a clear liquid located between cornea and lens • It is continuously formed by active transport and diffusion by the processes of the ciliary body.

19. It nourishes the lens and cornea and also buffers acids produced by the anaerobic glycolysis taking place in the lens. • Its composition is similar to CSF or plasma without proteins. • It is reabsorbed through the Canal of Schlemn into the intrascleral veins at The junction of the iris with the cornea. • Blockage of canal increases aqueous humour volume and intraocular pressure (which is normally 15 – 20 mmHg), leading to glaucoma.

20. Glaucoma can damage the retina and optic nerve and blindness may result. • The Lens • It is a biconvex, transparent and elastic solid disc. • It is held in position by suspensory ligament between the iris (in front) and the vitreous humor (behind). • The suspensory ligament attaches it to the posterior surfaces of the ciliary processes. • Reduction of tension in the ligament by contraction of ciliary muscles increases the refractive power of the lens.

21. With advancing age, the lens becomes less elastic resulting in presbyopia. • The function of lens is to provide a fine adjustment to the focus. • No blood vessels in the lens. • The central artery which supplied it before birth atrophies and remains as an end-artery supplying only the retina. • The Retina • The retina is inverted.

22. It consists of an outer pigmented layer, that separate it from the choroid, and an inner nervous layer towards the vitreous humour. • The Pigmented layer of the retina and choroid are single sheets of melanin-containing epithelial cells (melanocytes). • The functions of the pigmented layer of the retina are: • Absorption of light to prevent reflection blurred image in the eye. • Phagocytosis of degenerating membrane discs and shelves from rods and cones. • Storage of large quantities of vitamin A which is required for synthesis of visual pigment.

23. Retinal detachment :detachment of pigmented layer from the nervous layer causes blindness. • Treatment → laser surgical attachment of the 2 layers. • The Nervous Layer • It comprises rods (for poor light vision) and cones (bright light vision) - outermost, light sensitive and abuts on the pigmented epithelium. • The rods and cones are the visual receptors. • Bipolar cells – middle layer • Ganglion cells – innermost layer

24. Schematic diagram of a rod and a cone

25. In each retina, there are about: • 100 million rods • 7 million cones and • 1 million ganglion cells. • There are also 2 groups of interneurones • The horizontal cells which interconnect adjacent rods and cones. • The amacrine cells which interconnect the ganglion cells.

26. Interneurones have only dendrites but no axons • The links with other neurones are both presynaptic and postsynaptic. • Only the ganglion cells have axons. • Incident light must therefore pass through layers of cells, axons, and blood vessels before reaching the rods and cones (photoreceptors). • Fovea Centralis • It is located in the center of the macula lutea (yellow spot) as a small depression in the eye visual axis. • Fovea centralis is point of highest visual acuity in daylight.

27. Neural components of the extrafoveal portion of the retina. C, cone; R, rod; MB, RB, and FB, midget, rod, and flat bipolar cells; DG and MG, diffuse and midget ganglion cells; H, horizontal cells; A, amacrine cells

28. Nerve fibers and blood vessels which pass to and from the optic disc do not pass over the macula lutea but around it. • Its photoreceptors are cones only. • The Optic Disk (Blind Spot) • Ganglion cells converge to form the optic nerve • Optic nerve leaves the eye at the optic disk. • It is situated about 3 mm to the nasal (medial) side of the fovea centralis.

29. It is white in colour because axons are myelinated. • No photoreceptors at optic disk hence blind spot. • The Vitreous Humor • It is a transparent embryonic tissue of gelatinous consistency. • It fills the posterior chamber between the lens and retina.

30. Rod and cone density along the horizontal meridian through the human retina. A plot of the relative acuity of vision in the various parts of the light-adapted eye would parallel the cone density curve; a similar plot of relative acuity of the dark-adapted eye would parallel the rod density curve.

31. Retina seen through the ophthalmoscope in a normal human. The diagram on the left identifies the landmarks in the photograph on the right.

32. Functions of the Complete Eye • Visible light: Portion of electromagnetic spectrum detected by human eye • Refraction: Bending of light • Divergence: Light striking a concave surface • Convergence: Light striking a convex surface • Focal point: Point where light rays converge and cross

33. OPTICS AND OPTICAL DEFECTS • Image Formation on the Retina • Refractory surfaces of the eye from the front are: • Anterior and posterior surfaces of the cornea • Aqueous humour • Anterior surface of the lens • Posterior surface of the lens • Vitreous humour • When all these refractive surfaces are resolved to a single plane, it forms the principal plane. • It lies about 1.5 mm behind the cornea just in front of the lens. • The diopteric power of the eye is about 60 D.

34. Near point of the eye is the minimum distance (from the eye) from which an object can be properly focused on the retina. • It is about 10 cm in children, 25 cm in adult, and it increases with age. • The recession of the near point with age (beyond 40 cm due to increase plasticity of the lens) is called presbyopia. • The eye focuses a distant object on the retina. • The image that is formed on the retina is inverted, but is interpreted into the upright position by the brain. • The other defects due to refractive errors of the eye are myopia, hypermetropia and astigmatism

35. One is legally blind when his visual acuity falls below 20/200 feet. • In myopia (i.e. short-sightedness), the image is formed in front of the retina due to long eye balls or increased curvature of the cornea. • This defect is corrected by a concave lens which diverges the in-coming light-rays and so allows the lens to focus the image on the retina. • In hypermetropia (i.e. long-sightedness), the image is formed behind the retina due to short eyeballs or reduced curvature of the cornea.

36. The correction is by convex lens which converges the incident rays and allows the lens of the eye to focus the image on the retina. • Astigmatism is due to irregular curvature of the cornea so that the image is refracted to different foci, causing blurring. • It is corrected by a cylindrical lens. It may be associated with myopia. • Presbyopia (i.e. far-sightedness) is due to old age and has been described earlier. It is due to recession of the near point (i.e.) moving away from the eyes. • Visual acuity is tested by Snellen’s Charts.

38. Common defects of the optical system of the eye. In hyperopia, the eyeball is too short and light rays come to a focus behind the retina. A biconvex lens corrects this by adding to the refractive power of the lens of the eye. In myopia, the eyeball is too long and light rays focus in front of the retina. Placing a biconcave lens in front of the eye causes the light rays to diverge slightly before striking the eye, so that they are brought to a focus on the retina.

41. Other Visual Tests • Visual Acuity • In visual acuity, distance vision is tested using Snellen’s chart. • Snellen’s chart is a board on which rows of letters are printed with the larger letters at the top. • The figure that mark each of the rows indicates the distance at which the thickness of the line of the individual letter subtends an angle of 1’’ and the height of the letters 5’’ at the eye. • At that distance, the normal eye conveniently distinguish the letters of that particular row.

42. The test is read at a distance of 6 m at which the normal eye will read the row of the letters marked with figure 6. • The card is placed in a good white light. • One eye is tested at a time while the other eye is kept covered with an opaque disc in a spectacle frame. • Visual acuity is expressed by a fraction of which numerator represents the test distance in meters i.e. the reciprocal of the visual angle in minute. • The denominator the smallest row of letters which can be read at the distance.

43. Thus normal vision is ⁶⁄₆ viz. 6 is the smallest row of letters that can be read at 6 meters. at which distance. • If the subject has hypermetropia, he will be able to read the same line with and without a convex (or positive) lens. • If the subject has myopia, a negative (or concave) lens will improve his acuity. • Inversion of the Retinal Image • Prick a hole in a piece of black paper, hold it in the left hand about 3 inches from one while the other eye is closed. • Look at the sky through the hole.

44. Hold a pin in your right hand so that its head is close to the eye between the paper and the eye. • It appears to be upside down. • The light coming through the hole in the paper casts a direct shadow of the pin’s head on the retina and this shadow is the same way up as the pin itself. • Near Point • Hold an open book in front of the bare eye and bring it nearer just before the point it can no longer be seen clearly. • Measure the distance to the eye.

45. Focusing and Accommodation Process • Hold a pencil between one eye and the corner of the room and keep the other eye closed. • Attempt to focus both the corner of the room and pencil at the same time. • We can only focus on one thing at a time. • If an open book is placed about 2 feet from the eyes and the viewer sees it through a screen or fine net held at 6 inches from the eyes. • He can see either the mesh of the screen or net or the letters in the book with clarity, but not at the same time.

46. When the mesh is in sharp focus, the book is blurred but when the letters are in sharp focus, the mesh is blurred. • Accommodation enables man to shift his gaze from near to far objects with ease and greater speed than the finest camera of a most skilled photographer. • Retina Vessels • Look at the sky through a pin hole in a piece of black paper held close to the eye. • Close the other eye. • Move the paper up and down and side to side.

47. The shadow of retina vessels will be seen • The group of blood vessels seen depends on the direction of the movement. • Blind Spot • Make 2 black circles of ⅛ in diameter and 4 in apart. • Hold up the paper in front of the right eye at arm’s length. • Close the left eye and fix the right eye on the left arm mark and bring the paper slowly to the face. • The right hand mark will disappear (i.e. when focused on blind spot) and then reappear as it is brought nearer (i.e. when focused on the fovea centralis).

48. Scheiner’s Experiment • Make 2 pin holes in a card of about 2 mm apart (i.e. less than pupil diameter). • Place the card close to one eye while the other is closed and look through the hole at a distant object, say, across the window. • Hold a pencil with its point about 10 ins in front of the eye such that it comes into the field of vision. • While looking at the distant object, the pencil point appears double. • Carefully, slide along an opaque card to cover only one of the pin holes and then focus on the pencil point.

49. The window cross-piece will then appear double. • Again slide in a card to obscure the same pin hole as before and notice that the double images disappear. • After-Images: after-images are of same shape and size with original stimulus • Look intently at a bright circle of white light for 20-30 s in the dark room. • Turn off the light and look fixedly at a black surface where an after-image of the original stimulus will be seen. • This is positive after-image owing to the lack of second stimulus.

50. Repeat the stimulation and quickly transfer the gaze to the centre of white area larger than the original source of light. • Owing to the second stimulus (the white area), the after-image will be negative and appear as a dark area on the white ground. • Put a piece of red glass in the illuminated box and transfer the gaze to a white area and the after-image will still be negative and tinged with complementary colour of the first stimulus. • The negative after-image of white is black; of red is green; of blue is yellow, and the longer and stronger the primary stimulus, the stronger will be the after-image.