anatomy and physiology of the visual system minor course for dentistry

anatomy and physiology of the visual system minor course for dentistry

1. Anatomy and physiology of the visual system Dr Kumale T.

2. Introduction Vision: one of sense organs 2 eyes Each can function independently Coordinates to provide us a wide field of vision Functions of the eye Visual acuity Visual field Depth perception Colour vision Contrast discrimination

3. Basic anatomy Eye ball: globular structure AP diameter 24mm Horizontal diameter 23.5mm Vertical diameter 23mm Volume 6.5ml Weight 7gm

4. Basic anatomy 3 coats: Outer coat Cornea Sclera Middle coat / uvea Iris Ciliary body Choroid Inner coat / retina

5. Outer coat: The cornea Epithelium (approx. 40 μ m) Basement membrane (approx. 1 μ m) Bowman’s layer (approx. 8–14 μm ) Corneal stroma (approx. 450 μ m) Descemet’s membrane (approx. 5–10 μm) Corneal endoth elium (approx. 4 μm ) Anterior 1/6 th part Transparent Chief refractive lens of the eye 43 diopters 5 histologic layers

6. Outer coat: The cornea New layer: pre- escemet’s membrane layer Dua’s layer Functions of the cornea It acts as major refracting medium It protects the intraocular contents Plays role in absorption of topically applied drugs Wound repair after anterior segment surgery or trauma

7. Outer coat: The sclera Outer fibrous coat Opaque & shines as the white of the eye

8. Function of sclera Retina and choroid maintained in the correct optical shape by sclera It provide rigid insertion for the extraocular muscles Stress and strain are overcome by the disposition of fibrous bands of sclera

9. Limbus The junction of cornea & sclera Important Surgical landmark The anterior limbal border The mid limbal line The posterior limbal border

10. Middle coat: The Uvea Densely pigmented, vascular tunic of the eye. S andwiched between the corneoscleral shell externally and the retina internally. It is relatively firmly adherent to the sclera at the scleral spur anteriorly and at the optic nerve posteriorly and in between at the exit points of the four vortex veins.

11. Middle coat: The Uvea It described in three anatomic sections the iris the ciliary body and the choroid .

12. Function of the uveal tract To supply nutrition to the eye . Provides sustenance for the retinal rods and cones through the choriocapillaris and for the lens through the aqueous . Assist in the refractive action of the eye. May affect aqueous outflow

13. The Iris Anterior most part Acts like a diaphragm to control the rays of light Central opening: pupil 2 types of muscles Sphincter pupillae Dilator pupillae

14. The ciliary body Intermediate part Triangular Two sections T he anterior 2 mm (the pars plicata ) Secretory – aqueous formation Lodges ciliary muscle - accommodation The posterior 4 mm (the pars plana ) Flatter Merges with choroid & retina at ora serrata

15. The choroid Posterior part Thin,spongy , pigmented, vascular lamina L ocated b/n the sclera and the retina, extending from the ora serrata to the optic nerve 3 vascular layers

16. Function of choroid To supply nutrition to the rod and cone layer of the retina. Also have a role in regulating intraocular pressure. In acting as a heat diffuser to protect the photoreceptors from the heating effect of absorbed light, particularly at the macula.

17. The inner coat: Retina The eye functions like a camera The rays from an object of fixation Focused by cornea & lens Focused on to the retina Consists of two kinds of photoreceptors. Cones- color & bright light sensitive cells that are found toward the center Rods- sensitive to dim lights that are found peripherally antennae of visual system (react with light) light energy is transformed into a visual perception.

18. The inner coat: Retina Macula: central part of the retina Point of sharpest vision and color vision About 1.5 mm in diameter and is located two disc diameter temporal to optic disc It appears darker than the rest of retina Fovea centralis : centeremost point The area with maximum concentration of cones (sensory end organs)

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20. The inner coat: Retina Consists of 3types of cell & their synapse arranged as RPE Layer of rods and cons ELM ONL OPL INL IPL GCL NFL ILM 20

21. Blood supply of the retina Choriocapillaries Central retinal artery Cilioretinal artery(30%) Outer 4 layers: RPE, layer of rods & cones, ELM & ONL---- from choriocapillaries 6 inner layers: OPL, INL, IPL, GCL, NFL & ILM--- from CRA 21

22. Blind spot Is an area of complete blindness in the visual field. Anatomically it corresponds to optic nerve head, which is located nasally Measure 1.5 mm in diameter At this point, there are no photoreceptors.

23. The visual pathway The image formed on the retina is inverted This image is transmitted through the optic nerve Optic nerve exists 12 to 15 degrees nasal to the fovea and continues as visual pathway Visual cortex/occipital cortex: visual images are interpreted in association with inputs from parietal & temporal cortical areas

24. The chambers of the globe Has three ocular chambers Anterior Chamber Posterior Chamber Vitreous Space

25. Anterior Chamber The space between cornea and iris Filled with aqueous humor Anterior Chamber angle: formed by iris root connective tissue in front of the ciliary body(CBB) The trabecular meshwork up to schwalbe`s line

26. Aqueous out flow system It includes The trabecular meshwork Schlemm`s canal Collector channels Aqueous veins The episcleral veins

27. Posterior chamber Limited anteriorly and laterally by the posterior iris surface and ciliary body Posterior by lens & vitreous body Aqueous produced by ciliary body passes through this chamber to the pupil then to the anterior chamber

28. The Vitreous humour Transparent, inert, colorless, hydrophilic Gel like but contains 99% water Viscosity is twice that of water b/s of Mucopolysaccharide hyaluronic acid Bounded anteriorly by lens and ciliary body, posteriorly by retina Monday, 4/4/2011 Vitreous humour anatomy, RI seminar 29

29. The Vitreous humour Weight - 4 g & volume - 4 ml 4/5 of the globe Three parts Hyaloid Membrane Cortical Vitreous Medullary Vitreous Function: to act as intervening medium in the light pathway b/n the lens and retina and also gives the shape of the eye. 30

30. The lens Transparent, biconvex, crystalline structure Placed b/n iris & vitreous in saucer-shaped depression(patellar fossa ) Held in place by suspensory ligaments ( zonules ) running from the ciliary body and inserting into the superficial lens capsule around the equator Refractive power: 17 dioptres Ability to change its curvature to provide accommodation

31. The lens Accommodative power varies with age 14-16 D at birth 7-8 D at 25 years of age 1-2 D at 50 years. Color :- colorless at infants and young adults yellowish (after about 30 years) to amber color develops with age(old).

32. The lens Unique feature Transparent No blood or nerve supply. Has higher protein content than other body tissues. Continues to grow through out life, new in the top, old compressed to ward the centre The only solid structure inside the eye Has biconvex shape Epithelial cells are not shedding type Has three anatomical parts: capsule, cortex, nucleus

33. The protection of the eye Orbit Eyelid Conjunctiva Lacrimal apparatus Lacrimal gland Tear excretory system

34. The Orbit Bony protective structure A space which cushions the eyeball with orbital fat & other tissues The orbital walls are composed of seven bones: ethmoid frontal Lacrimal maxillary Palatine Sphenoid zygomatic. 35

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37. The Orbit Provides communication with the middle cranial fossa Lies below the anterior cranial fossa & above the maxillary sinus Medially nasal cavity separates the 2 orbits Paranasal sinuses lie adjacent to the floor, medial wall & anterior portion of the orbital roof

38. Extra ocular muscles (EOM) The marvel of the eyes is enhance by the precise & accurate ocular motility 6 EOM are responsible for ocular motility Medial rectus Lateral rectus Superior rectus Inferior rectus Superior oblique Inferior oblique

39. Extra ocular muscles (EOM)

40. Extra ocular muscles (EOM) Fine striate muscles Lying in the space b/n the conjunctiva & sclera They run from the apex of the orbit except inferior oblique (comes from the anterior nasal part of the orbital wall) They hold the two eyeballs in alignment with each other & in reference to head & neck changes as well as to the body movements

41. ANNULUS OF ZINN All four rectus muscles arise directly from the annulus. the levator & SO muscles are not attached to the annulus arising instead above the SR muscle and the lesser wing/body of the sphenoid, respectively 42

42. SURGICAL SPACES 43

43. PARANASAL SINUSES The orbit is surrounded on 3 sides by the paranasal sinuses 44

44. CAVERNOUS SINUS The r/n b/n the CS & the orbit is often misunderstood. a venous space located directly behind the SOF serves as a conduit for most of the motor, sensory, and autonomic nerves that supply the orbit. It is also the major venous drainage for the orbit 45

45. Eyelid Mobile tissue curtains placed in front of the eye balls Function Act as shutters protecting the eyes from injuries & excessive light Spreading the tear film over the cornea & conjunctiva Maintain proper positioning of the globe Important contribution to the facial features

46. Eyelid Extent : The upper eyelid from the eyebrow down ward to end in a free margin w/c forms the superior boundary of the palpebral fissure The lower lid below merge in to the skin of the cheek Position : in primary gaze the upper eyelid covers about 1/6 th of the cornea & the lower eyelid just touches the cornea

47. Eyelid Eyelids divided into 7 structural layers: Skin & subcutaneous tissue Muscles of protraction Orbital septum Orbital fat Muscles of retraction Tarsus Conjunctiva

48. Eyelid Skin: has three important features Thinnest, more elastic and mobile than skin else where in the body Little or no subcutaneous fat under the skin makes it a good source of skin graft Has an extremely good blood supply that is why wound heals well and quickly

49. Eyelid Canthi : the 2 eyelids meet at medial & lateral angles ( inner & outer canthi ) Lateral canthus : 5-7mm from lateral orbital margin & lies in contact with the eyeball Forms an acute angle of about 60 ₀ with eyes wide open & 30-40 ₀ with eyes open normally Medial canthus : rounded & separated from the globe by tear lake (lacus lacrimalis ) There is caruncle & plica semilunaris

50. Eyelid Eyelid margin: opposing margins are nearly flat 2mm width Divided into 2 parts by lacrimal papilla (a small elevation present on the medial side, contains lacrimal punctum in its center) Lacrimal portion : medial portion, extending from punctum to medial canthal angle,rounded & devoid of lashes or glands Ciliary portion : consists of a rounded anterior border, a sharp posterior border & an inter-marginal strip b/n the 2 borders Gray line : marks junction of skin & conjunctiva divides the inter marginal strip into an anterior strip bearing lashes & a posterior strip containing openings of meibomian glands arranged in a row & a lipid strip

51. Eyelid Eyelashes : arranged in 2-3 rows In upper lid 100-150 in number, directed forward, upward & backward In lower lid 50-75 in number, directed forward, downward & backward Size 20-120 μ m in diameter, length 6-12mm Taper through out their length to end in fine sharp point Darker than scalp hair, remain through out life Life span 3-4 months Lash follicles pass obliquely in the lids & lack erectors Each lash follicle - surrounded by a dense plexus of Vs & Ns

52. Eyelid The sebaceous glands of Zeis & glands of Moll Unbranched, convulated tubular apocrine glands empty into the infundibulum of each piliary canal Palpebral aperture or fissure Elliptical space b/n upper & lower eyelid margins At birth 18-21mm horizontally, 8mm vertically In adults 28-30mm horizontally, 9-11mm vertically In Caucasians the x- axis is horizontal in 50% of population, in the rest lateral canthus 2mm higher than the medial Great elevation of the lateral canthus– mongoloid slant Lateral canthus lower than the medial– antimongoloid slant

53. Eyelid Muscles Orbicularis oculi muscle: Important for closure of eye lid Innervated by facial (7 th cranial) nerve Levator Palpebrae : Elevator of eye lid Innervated by Oculomotor (3 rd cranial) nerve Muller’s muscle: Help to retract the upper eye lid Innervated by cervical sympathetic nerve Tarsal plates Are composed of dense fibrous tissue Keep the eye lids rigid and firm Contain meibomian glands, which open at lid margin, and makes oily secretion that forms a part of corneal tear film

54. Conjunctiva It is a thin mucous membrane which lines the inner surface of the eye lid and outer surface of the eye ball. Function of the conjunctiva: Protect the cornea: During opening and closure of the eyelids, it lubricates the cornea with tears. Protects the exposed parts of the eye from infection because it contains lymphocytes and macrophages to fight infections Mucin from goblet cells has wetting effect of tear film The conjunctival epithelium is continuous with the corneal epithelium at the limbus Contains many small islands of lymphoid tissue especially in the fornix

55. Conjunctiva Three parts: Tarsal Conjunctiva The part lining in the inner aspect of the eye lid Firmly attached to the underlying tarsal plate Bulbar Conjunctiva The part lining the eye ball Loosely attached to the underlying sclera Fornix Part in which the tarsal and bulbar conjunctivas are continuous.

56. L acrimal gland Tear secreting organ grayer & pinker than the surrounding yellow adipose tissue 2 parts Main lacrimal gland Orbital part Palebral part Accessory lacrimal glands Glands of Krause Glands of Wolfring Rudimentary accessory lacrimal glands 57

57. Tear excretory system Tears, secreted from the lacrimal gland in the superolateral region of the conjunctiva spread over the surface of the globe by Eyelid movement & Gravity collect in the lacrimal lake at the medial aspect of the conjunctival fornix. pass by capillary attraction into the lacrimal puncta & canaliculi 58

58. By lacrimal pump mechanism & gravity, fluid passes into the common canaliculus & the lacrimal sac continues downward into the NLD, emptying into inferior meatus of the nose under the inferior turbinate bone. 59

59. Tearfilm The tear forms a thin film of fluid on the surface of the conjunctiva and cornea, Vital for the health, and transparency of the cornea. Outer part( Lipid layer)- oily secretion from meibomian and Zeis gland Middle part(Aqueous layer)-Water from Lacrimal gland and accessory lacrimal glands of Krause and Wolfring . Inner part( Mucin layer)-Mucus from goblet cells of the conjunctiva Function of tear film provides moist environment for the surface epithelial cells of the conjunctiva and cornea Along with the lids, it washes away debris Transport metabolic products (oxygen, carbon dioxide) to and from the surface cells Antimicrobial actions Provides a smooth refracting surface over the cornea

60. Innervations of the eye Motor nerve Oculomotor (CN III) Innervate- medial rectus, superior rectus, inferior rectus, & inferior oblique. Trochlear (CN IV) nerve- innervates superior oblique Abducent (CN VI) nerve- innervates lateral rectus Facial nerve (CN VII ) - innervates orbicularis oculi muscle Sensory nerve Ophthalmic branch of trigeminal nerve is the sensory nerve of the globe & adnexa and has three branches -frontal, lacrimal , nasociliary Optic nerve (CNII) - responsible for vision Autonomic nerves Sympathetic nerve - supplies Muller's muscles and dilator pupillae Parasympathetic comes via oculomotor and innervates the ciliary muscle and sphincter pupillae

61. Oculomotor nerve 62

62. Trochlear nerve 63

63. Abducent nerve 64

64. Facial nerve 65

65. Ophthalmic division of trigeminal nerve 66

66. Trigeminal nerve 67

67. Ciliary ganglion & its roots 68

68. Parasympathetic pathway 69

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70. Sympathetic pathway 71

71. Blood supply to the eye The eye is supplied by anastomosing vessels from internal and external carotid arteries Blood flow to the eye comes mostly from the ophthalmic artery. The ophthalmic artery is the first tributary of ICA branching off as the ICA turns to pierce the dura and emerge from the cavernous sinus .

72. Blood supply to the eye enters the orbit through the optic canal & lies inferolateral to the optic nerve. As it turns medially in the orbit, it gives off its major branches. central retinal artery (CRA) the medial and lateral posterior ciliary arteries the muscular branches

73. Blood supply to the eye Retina Inner layer gets blood from central retinal artery Uvea Supplied by ciliary circulation, medial and lateral posterior ciliary arteries Eye lid Gets its blood supply from facial and ophthalmic arteries.

74. Blood supply to the eye The venous drainage of the eye generally follows the arterial supply The choroid is drained through the vortex vein system Four vortex veins, one in each of the four quadrants

75. Blood supply to the eye The vortex veins drain into the superior and inferior orbital veins. The superior orbital vein then drains into the cavernous sinus the inferior orbital vein drains into the pterygoid plexus through the inferior orbital fissure Collateralization between the superior and inferior orbital veins usually exists.

76. Blood supply to the eye The iris and ciliary body area are drained mostly through the anterior ciliary veins, part of the vortex vein system. Some venous drainage of these areas is anterior, however, by way of episcleral veins into the external carotid system.

77. Lymphatic drainage There are no lymphatic vessels inside the globe The lymphatic drainage of The medial eye lid is to sub mandibular lymph node The lateral eyelid is to the superficial preauricular lymph nodes and then to deeper cervical lymph nodes