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

2. Special proprioception (Vestibular apparatus)
SPECIAL SENSES
Special senses are:
Vision
Audition
Olfaction
Gustation and
Special proprioception (Vestibular apparatus)
They serve as tools for learning and formation of memory
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3. CHAPTERS OF THE LECTURE Functional Anatomy of the Eye
Optics and Optical Defects
The Visual Pathway
Photoreceptor Mechanism of Retina
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4. Eyebrows, eyelids, eyelashes, tear glands
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. Anatomy of the Eye 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
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

10. Horizontal section of the right eye
Horizontal section of the right eye. AP, anterior pole; PP, posterior pole; VA, visual axis
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11. Functional Anatomy of the Eye
The eye has 3 concentric coats or layers for the specialised sense of vision.
The sclera – the outer protective layer of fibrous coat which is transparent anteriorly as the cornea i.e. ⅙ of 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.
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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.
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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.
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14. It has its free nerve endings from trigeminal nerve.
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.
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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.
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16. Ciliary body arises 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
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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.
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18. Sympathetic fibres relay via the superior cervical ganglion.
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.
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19. Its composition is similar to CSF or plasma without proteins.
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.
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20. It is a biconvex, transparent and elastic solid disc.
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.
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21. The function of lens is to provide a fine adjustment to the focus.
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.
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22. The functions of the pigmented layer of the retina are:
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.
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23. Treatment → laser surgical attachment of the 2 layers.
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
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24. Schematic diagram of a rod and a cone
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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.
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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.
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27. Neural components of the extrafoveal portion of the retina
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
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28. Its photoreceptors are cones only. The Optic Disk (Blind Spot)
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.
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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.
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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.
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31. Retina seen through the ophthalmoscope in a normal human
Retina seen through the ophthalmoscope in a normal human. The diagram on the left identifies the landmarks in the photograph on the right.
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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.
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34. The eye focuses a distant object on the retina.
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
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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.
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36. Visual acuity is tested by Snellen’s Charts.
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.
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38. Common defects of the optical system of the eye
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.
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41. In visual acuity, distance vision is tested using Snellen’s chart.
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.
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42. The card is placed in a good white light.
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.
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43. Inversion of the Retinal Image
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.
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44. It appears to be upside down.
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.
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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.
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46. Move the paper up and down and side to side.
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.
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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).
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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.
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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 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.
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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.
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51. Thread a needle, note how you do it and how long it takes.
Streoscope Vision
Thread a needle, note how you do it and how long it takes.
Close one eye and repeat this experiment without your hand touching each other along visual axis and at right angle to the visual axis.
It is easier done by the 2 eyes.
Field of Vision
The subject stands facing the examiner with his back to the light at 2 ft.
Each eye must be examined separately.
The examiner closes his eye opposite the closed eye of the subject.
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52. Thus, the examiner’s field is compared with the that of the subject.
The subject open eye should look fix on the examiner’s open eye while the examiner holds his hand midway between himself and the subject.
The examiner then moves his outstretched forefinger from the periphery towards the centre of the visual field.
The subject is then asked to say when he sees the movement of the finger as the same moment as the examiner, provided they both have normal visual field.
The movements of the hand are repeated in all the different meridian of the field.
Thus, the examiner’s field is compared with the that of the subject.
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53. The perimeter is a more accurate tool for measuring visual field.
The examiner constantly observe the subject’s eye in case of any wandering from the point of fixation, which should be corrected immediately.
The perimeter is a more accurate tool for measuring visual field.
Chromatic Aberration
Look at a bright circle of light through a piece of cobalt glass, which transmit only red and blue rays.
A halo of 1 colour will be seen round the light because, the eye cannot focus at once long and short wavelengths.
The halo will be elliptical in shape if the eye has astigmatism.
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54. Cover 1 eye and look at the astigmatic fan with the other eye.
Astigmatism
Cover 1 eye and look at the astigmatic fan with the other eye.
The set of radiating lines on the card is astigmatic fan.
All line should appear black, but if some lines appear grey or blurred, astigmatism is present.
It is easier to say which lines are better seen than which lines appeared blurred.
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55. Simultaneous and Successive Contrasts
Lay a piece of grey paper on a large piece of blue paper and cover both with tissue paper.
The grey will be tinged with yellow other colour background could be used – simultaneous contrast.
Look fixedly at the centre of a small red square on a grey background; then look quickly at a piece of grey paper.
The after image of the red square will be blue-green – successive contrast.
Other colours can also be used.
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56. Lateral half of the visual field is called the temporal field
The Visual Pathways
Visual Field is the view seen by one eye without the movement of the head.
Lateral half of the visual field is called the temporal field
Medial half of the visual field is called the nasal field.
The retinal fibres also are similarly divided into nasal and temporal fibres.
Light from the temporal half of the visual field falls on the nasal half of the retina.
Light from the nasal half of the visual field falls on the temporal half of the retina.
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57. Light rays from upper right quadrant of the visual field fall on the lower left quadrant of the retina.
Light from the center of the visual field falls on the macula lutea (fovea centralis).
In this way, there is a total inversion of the image formed at the retina.
Field of Vision
Place field of vision disk it against your forehead and stare intently, straight ahead, at the target on the disk.
Have your partner move the mobile target, first to one side, then to the other while you continue to stare straight ahead.
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58. Functional Structure of the Visual Pathways
Record the exact angle at which the mobile target completely disappears from your field of vision on the Data/Analysis Sheet.
Functional Structure of the Visual Pathways
Fibers from the nasal half of each retina decussate at the optic chiasma and run in the contralateral optic tract.
Fibers from the temporal half of each retina (rays from the nasal field) run in the ipsilateral optic tract.
Fibers of each optic tract synapse in the lateral geniculate body of the thalamus.
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59. Geniculocalcarine fibers pass by way of optic radiations (geniculocalcarine tract) to the primary visual cortex calcarine sulcus occipital lobe (Broadman’s area 17, also known as V1).
Ganglion cell projections from the right hemiretina of each eye to the right lateral geniculate body and from this nucleus to the right primary visual cortex. Note the six layers of the geniculate. P ganglion cells project to layers 3–6, and M ganglion cells project to layers 1 and 2. The ipsilateral (I) and contralateral (C) eyes project to alternate layers. Not shown are the interlaminar area cells, which project via a separate component of the P pathway to blobs in the visual cortex.
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60. Lesions of the Optic Pathway
Optic nerve lesion results in total blindness in the affected eye (monocular blindness).
Optic tract lesion interrupts crossed nasal fibres from the opposite eye and temporal fibres from the same side.
The result is loss of visual field in the temporal side of the opposite eye (nasal fibres) and nasal field of the same side (temporal fibres) resulting in Homonymous hemianopia.
If the right or left optic tract is damaged, it results in left or right homonymous hemianopia respectively.
Homonymous hemianopia refers to the side of the body viz. left- or right-sided visual sides of the body.
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61. Optic chiasma lesion interrupts fibres from nasal part of both retinae and hence both temporal fields resulting in bitemporal hemianopia.
Bitemporal hemianopia in optic chiasma lesions is an example of heteronymous hemianopia since both temporal fields are affected i.e. the left temporal field is left of the body and the right temporal field is the right side of the body i.e. left and right sided or heteronymous.
Optic radiation – results in homonymous hemianopia but the macula fibres are not affected. The macular fibres run separately from the peripheral visual fields and they also have large number of fibres and so are not easily affected by occipital lesions.
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62. Visual pathways. Transection of the pathways at the locations indicated by the letters causes the visual field defects shown in the diagrams on the right (see text). Occipital lesions may spare the fibers from the macula (as in D) because of the separation in the brain of these fibers from the others subserving vision .
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63. Visual Pathways

64. Eye Disorders Myopia: Nearsightedness Retinal detachment
Focal point too near lens, image focused in front of retina
Hyperopia: Farsightedness
Image focused behind retina
Presbyopia
Degeneration of accommodation, corrected by reading glasses
Astigmatism: Cornea or lens not uniformly curved
Strabismus: Lack of parallelism of light paths through eyes
Retinal detachment
Can result in complete blindness
Glaucoma
Increased intraocular pressure by aqueous humor buildup
Cataract
Clouding of lens
Macular degeneration
Common in older people, loss in acute vision
Diabetes
Dysfunction of peripheral circulation

65. Connections of the Optic Tract or Visual fibres
As above to the lateral geniculate body and then to the calcarine/visual cortex for vision.
Pass from the optic chiasma to the suprachiasmatic nucleus of the hypothalamus for control of circadian rhythm.
From the lateral geniculate nucleus to the pretectal nucleus of the midbrain which then relays through the occulomotor nerve and Edinger-Westphal (pre-tectal) nucleus to mediate the pupillary and consensual light reflexes.
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66. From the lateral geniculate nucleus to the superior colliculus which then relays to the visual cortex and medial longitudinal bundle to cause eye movements synchronous with that of the body during postural adjustments.
Into the ventral lateral geniculate nucleus of the thalamus, to help control some of the body’s behavioral functions.
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67. Responses in the Visual Pathways
2 types of ganglion cells are found in the retina
Large ganglion cells (magno or M cells) are concerned with movement and sterropsis
Small ganglion cells (parvo or P cells) are concerned with color, texture and shape.
Axons of ganglion cells (optic nerve) project a detailed spatial representation of retina on the lateral geniculate body.
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68. Each geniculate body or nucleus contains 6 well defined layers
Layers 1 and 2 have large cells and are called magnocellular
Layers 3-6 have small cells and are called parvocellular
On each side, layers 1, 4 & 6 receive input from the contralateral eye
Layers 2, 3 & 5 receive input from the ipsilateral eye
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69. Major inputs occur from the visual cortex and other brain regions.
In each layer there is a precise point–to–point representation of the retina.
Only 10-20% of inputs to the lateral geniculate nucleus comes from the retina.
Major inputs occur from the visual cortex and other brain regions.
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70. Magno-cellular laminae (layers 1 & 2)
Ganglion cells P M
Magno-cellular laminae (layers 1 & 2)
Parvo-cellular laminae (layers 3-6)
Inter-laminar region
Via dendrites
Lateral geniculate
Visual Cortex
Superficial layer 4C
Blobs
Deep layer 4C
Color, texture, shape, fine detail
Function
Movement, depth, flicker
Color vision
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ORGANISATION OF THE VISUAL PATHWAYS

71. The Primary Visual Cortex It is also called Broadman’s area 17 or VI
The Visual Cortex
The Primary Visual Cortex
It is also called Broadman’s area 17 or VI
It receives point-to-point representation from the lateral geniculate body
It processes input in terms of orientation, edges etc
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72. Medial view of the human right cerebral hemisphere showing projection of the retina on the occipital cortex around the calcarine fissure.
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73. Other Cortical Areas Concerned with Vision
The primary visual cortex projects to several other areas of the occipital cortex and the brain.
These areas are as listed below.
V2,V3, VP – Continued processing, larger visual fields
V3A – Motion
V4v – Unknown
MT/V5 – Motion, control of movement
LO – Recognition of objects
V7 – Unknown
V8 – Color vision
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74. Some of the main areas to which the primary visual cortex (V1) projects in the human brain. Lateral and medial views
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75. Pupillary light reflex Consensual light reflex Accommodation reflex
Visual Reflexes
Pupillary light reflex
Consensual light reflex
Accommodation reflex
Pupillary (Direct) light reflex
When bright light shines into one eye, it results in pupillary constriction in the affected eye.
Pathway: retinal cells → pretectal nucleus → Edinger-Westiphal nucleus → occulomotor nerves → pupillary constriction.
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76. Consensual light reflex
When bright light is shines into one eye, it results in pupillary constriction in both eyes.
Pathway: retinal cells → Edinger-Westiphal nucleus → occulomotor nerves (of both eyes) → pupillary constriction of both eyes.
The pupils of both eyes are constricted by the contraction of their pupillary circular muscle (sphincter pupillae).
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77. Pupillary constriction
Accommodation Reflex
Accommodation reflex is the reaction of both eyes when they are focused on a near object.
It consists of:
Pupillary constriction
The contraction of the ciliary muscles in order to relax the pull on the suspensory ligament, increasing the curvature of the lens, thus allowing focusing on the retina.
The convergence of the eyeballs in order to enable same part of the object to be focused simultaneously on same part of both retinae.
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78. Accommodation. The solid lines represent the shape of the lens, iris, and ciliary body at rest, and the dashed lines represent the shape during accommodation.
Friday, August 24, 2018

79. Focus and Accommodation
Emmetropia: Normal resting condition of lens
Far vision: 20 feet or more from eye
Near vision: Closer than 20 feet
Accommodation
Pupil constriction
Convergence

80. Decline in the amplitude of accommodation in humans with advancing age
Decline in the amplitude of accommodation in humans with advancing age. The different symbols identify data from different studies
Friday, August 24, 2018

81. The Retina Provides black backdrop for increasing visual acuity
Sensory retina and pigmented retina
Photoreceptors
Rods: Noncolor vision
Rhodopsin reduction: Light adaptation
Rhodopsin production: Dark adaptation
Cones: Color vision

82. Photoreceptive Mechanism of the Eye
Cones are for daylight and colour vision.
They have high threshold and are therefore used for bright light.
Rods are for monochromatic and twilight or night vision.
They have low threshold (scotopic vision).
The visual pigment in the rods in rhodopsin derived from the proteins - opsin and retinal, the aldehyde form of vitamin A or retinol.
Light rays cause decomposition of rhodopsin leading to the development of an action potential.
Friday, August 24, 2018

83. Rhodopsin Cycle

84. Range of luminance to which the human eye responds, with the receptive mechanisms involved.
Friday, August 24, 2018

85. The pigment for cones is not well-known.
The cone pigments are for red sensitivity (erythrolabe), green sensitivity (chlorolabe) and blue sensitivity (cyanolabe).
Rhodopsin = Scotopsin (an Opsin) and retinal.
Retinal exists as cis-retinal in the dark.
When light falls on the retinal cell, cis-retinal is converted to all-trans-retinal.
Friday, August 24, 2018

86. This mechanism involves cyclic GMP gated ion channels.
This leads to the formation of intermediates including metarhodopsin II, which then initiates the development of an action potential in the photoreceptor cell via closure of Na+ channels hyperpolarisation
There is decreased transmitter release and excitation of bipolar and ganglion cells causing impulse transmission to the brain.
This mechanism involves cyclic GMP gated ion channels.
Deficiency of vitamin A in the diet will result in night-blindness (nyctalopia) since it is a component of rhodopsin.
Friday, August 24, 2018

87. Sensory Receptor Cells

88. Rod Cell Hyperpolarization

89. Effect of light on current flow in visual receptors
Effect of light on current flow in visual receptors. In the dark, Na+ channels in the outer segment are held open by cGMP. Light leads to increased conversion of cGMP to 5'-GMP, and some of the channels close. This produces hyperpolarisation of the synaptic terminal of the photoreceptor.
Friday, August 24, 2018

90. Initial steps in phototransduction in rods
Initial steps in phototransduction in rods. Light activates rhodopsin, which activates transducin to bind GTP. This activates phosphodiesterase, which catalyzes the conversion of cGMP to 5'-GMP. The resulting decrease in the cytoplasmic cGMP concentration causes cGMP-gated ion channels to close.
Friday, August 24, 2018

91. Sequence of events involved in phototransduction in rods and cones
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92. Equal stimulation of the three cones leads to a white colour.
Colour Vision
Young-Helmboltz or trichromatic theory proposes three types of cones response to light of 3 different wavelengths/colours i.e. 450 nm (Blue), 550 nm (Green) and 600 nm (Red).
The colour perceived is a combination of the degree to which each of these cones or primary colours are stimulated.
Equal stimulation of the three cones leads to a white colour.
Friday, August 24, 2018

93. Absorption spectra of the three cone pigments in the human retina
Absorption spectra of the three cone pigments in the human retina. The S pigment that peaks at 440 nm senses blue, and the M pigment that peaks at 535 nm senses green. The remaining L pigment peaks in the yellow portion of the spectrum, at 565 nm, but its spectrum extends far enough into the long wavelengths to sense red
Friday, August 24, 2018

94. These are dichromats and monochromats.
Colour blindness can occur when any or all (total colour blindness) of the three cone types are absent.
These are dichromats and monochromats.
Trichromats have all the 3 cones and so have normal colour vision.
Monochromats are completely colour blind.
They only have one cone type and see only black and white or grey.
Friday, August 24, 2018

95. Blue cone loss (tritanopia) is rare.
Dichromats have two cone types losing either that to Red (protanopia) or Green (deuteranopia).
They confuse red or green and perceive colours in shade of yellow and blue.
Blue cone loss (tritanopia) is rare.
Colour blindness is a sex-linked recessive gene and therefore is more common in males.
Colour blindness is tested by Ishihara charts.
Friday, August 24, 2018

96. Friday, August 24, 2018

97. There are two components:
Dark Adaptation
When the eye is exposed to poor light it adapts and becomes increasingly sensitive to the dark resulting in increased visual acuity.
There are two components:
An initial rapid adaptation within 5 minutes though with poor detail.
This is accomplished by the cones.
Friday, August 24, 2018

98. This results to an increase in acuity and is due to the Rods.
Then follows a more prolonged adaptation which is about 60% of the total adaptation.
This results to an increase in acuity and is due to the Rods.
Hence the rods contribute a quantitatively greater proportion to visual adaptations in the dark, due to their higher content of photopigment.
Friday, August 24, 2018

99. Dark adaptation. The curve shows the change in the intensity of a stimulus necessary to just excite the retina in dim light as a function of the time the observer has been in the dark.
Friday, August 24, 2018

100. EFFECTS OF AGING ON THE SPECIAL SENSES
Slight loss in ability to detect odors
Decreased sense of taste
Lenses of eyes lose flexibility
Development of cataracts, macular degeneration, glaucoma, diabetic retinopathy
Decline in visual acuity and color perception