Physiology lecture on Ophthalmology 2024 (PDF)

‫بسم هللا الرحمن الرحيم‬ Physiology of the eye Dr.Mohamed Abdel Aziz Barakat Professor Of Physiology Physiology of the eye Vision  Objectives  The students should know and understand the functional structure of the eye.  The students should know and understand functions of...

‫بسم هللا الرحمن الرحيم‬ Physiology of the eye Dr.Mohamed Abdel Aziz Barakat Professor Of Physiology Physiology of the eye Vision  Objectives  The students should know and understand the functional structure of the eye.  The students should know and understand functions of cornea and corneal reflex.  The students should know and understand the components and functions of aqueous humor.  Structure of the eye -The adult eye is a spherical in shape with a diameter about 2.5 cm. -It is formed of The outer coat a) The Cornea:-forms the anterior 1/6 and it is transparent. b) The Sclera: -forming the posterior 5/6 of the outer coat. The middle coat vascular layer (uveal tract)is formed of a)The ciliary body b) Iris. c)The Choroid - lies posteriorly between the sclera and the retina. -It is pigmented and highly vascular layer which is important for the support and nourishment of the inner layers of the retina. The inner coat is formed of the retina  External protection of the eye Bonny orbit Eye brows -They help in protection of the eye ball from -direct sun rays. -sweat. Eye lids -They protect from excessive light. -They protect from foreign bodies. -They contineous blinking leading to spread of tear over the cornea. Types of eye lid movements : -Spontaneous ,subconscious (20 times /min.) -Voluntary -Involuntary (reflex) with–corneo-conjunctival reflex. (Afferent 5th.cr N) -bright light (Afferent 2nd cr.N ) -auditory impulse (Afferent 8th.cr.N)  Lacrimal gland -Site :Upper lateral part of the orbit -Nerve supply :facial nerve. -Function :Tear formation Contents :-H2O (98.2%) -Solids-inorganic-Na+ - Cl- ,HCO3- -organic –glucose -lysozymes. -IgA Characters of tear - Isotonic fluid - pH is 7.4 Refractive index=1.33 -*Drainage of lacirmal secretion: lacrimal canaliculi the puncti on the eye lid margins to the lacrimal sac, then , into the naso-lacrimal duct which opens into the inferior meatus of nasal cavity.  Tear *Composition of the tear film: -It is formed of 3 layers from outside inwards: 1-Lipid layer (1%):-it is the outer most layer, -secreted by Meibomian glands, -is formed mainly of fatty acids. -It prevents evaporation of the underlying layers. 2-Water layer (90%): -it is the central layer, -It is secreted by the lacrimal glands. -It contains water, NaCl (0.9%), HCO3-, glucose, proteins and lysozymes. 3-Mucous layer (9%):-it is the inner most layer, -It is formed by goblet cells of the conjunctiva. -It makes the corneal and conjunctival surface hydrophilic, -It makes tears spread homogeneously.  *Functions of Tear -Cleansing: -It removes the cells, foreign bodies and irritant substances. -Protection of the cornea by lysozymes and IgA -Nutritive: -It dissolves O2 which supplies the cornea. -Lubricant:-It moisten and facilitates the movement of the lids over the cornea. -Healing effects: -It supplies the corneal wound with WBCs from blood by diffusion. -Optimizing corneal optic property:-The tear fills the minute irregularities of the corneal surface,to become regular. NB:Vitamin A deficiency leads to Xerophthalmia(no tear ) may lead to corneal opacity and blindness  The outer layer 1-The Cornea -It forms the anterior 1/6 th of the outer layer of the eye. *Structure 5 layers from outside-inwards 1-The surface epithelium 2- Bowman’s membrane:-It is an elastic membrane. -It acts as a protective barrier against trauma and bacteria. 3-The substantia propria:-It is formed of regular collagen fibrils. 4-Descemet membrane :membrane of collagen fibers. 5- The endothelial cells contain Na+-K+ ATPase, carbonic anhydrase and Na+-H+ exchanger that maintains low Na+ concentration in substantia propria.  Functions of the cornea 1-It is transparent structure that allows light entry into the eye. 2-It is the most important refractive medium in the eye , acting as a convex lens (with -refractive power = 42 D). -refractive index =1.38. NB:Light refraction mainly on the air-cornea interface 3-Its regular curvature leads to formation of sharp retinal images. 4-Protective function : Corneal reflex.  Corneal reflex -Importance :It is a protective superficial reflex. -Touching the cornea of one eye by a foreign body (cotton) causes reflex closure of both eyes. Pathway of corneal reflex -Receptors : Free nerve endings. -Afferent :Ophthalmic division of trigeminal nerve to trigeminal ganglion to tigeminal nucleus in pons. -Center: Superior colliculus.Brain stem -Efferent: Facial nerves on both sides. -Effector: Orbicularis occuli muscles. -Response : Closure of the eye lids on both sides (bilateral response). *  2-The Sclera -It forms the posterior 5/6 of the outer coat. -It is as opaque hard fibrous tissue covered anteriorly by the conjunctiva. -The cornea fits in the sclera as a watch glass. -The limbus is the junction between cornea and sclera.  Middle layer The Ciliary body Structure -The ciliary body is triangular in shape, -It extends anteriorly from the iris, and posteriorly from ora-serrata (anterior border of the retinal attachment). Functionally it is divided into: Contractile part Secretory part: Ciliary muscles for Ciliary procsses for accommodation of near vision. formation of aqueous humor.  *Contractile part of Ciliary body (ciliary muscles) 1-Longitudinal fibers antagonize the pull of the choroid on the ciliary muscle. 2-Radial fibers pull of the tension fibers to maintain the right angle of suspensory ligaments. 3-Circular fibers:Their contraction decreases the tension of the suspensory ligaments on the lens capsule, the lens becomes more convex on its anterior surface leading to increase in its diopteric power. ** Secretory part of ciliary body , forms aqueous humor : -It is formed of ciliary processes. -The inner non pigmented layer :contains Na+-K+ ATPase, responsible for active Na+ transport to the intercellular spaces followed by H2O passively. -The outer pigmented layer : Absorbs light and prevents its reflection.  The Fluid System of the Eye “The Intraocular Fluid” -The eye is filled with intraocular fluid, which maintains sufficient pressure in the eyeball to keep it distended. IO fluid Aqueous humor Infront of the lens Vitreous humor -The vitreous humor is a gelatinous mass and it fills between the posterior surface of the lens and the retina.  *The Aqueous Humor -Definition : -It is a clear watery transparent fluid that fills the anterior and posterior chambers. *Composition -H2O -Solids :-Inorganic :-Cations: Na+ -Anions :Hco3-, Cl-, lactate and -Organic :- small amount of glucose and protein. Mechanism of formation of aqueous humor -The aqueous humor is secreted continuously at the rate of about 2ml per day. -It is formed by the cells of the ciliary processes by the following mechanisms : 1-Active secretion mechanism : -Non pigmented epithelial cells contain Na+-K+ ATPase, actively pumps Na+ followed by Cl- & water passively into the posterior chamber. -Also , carbonic anhydrase helps the secretion of HCO3- to the intercellular spaces. (CO2 + H2O H2CO3 HCO3- + H+ ). 2- Passive mechanism : -Ultrafiltration of Na , Cl, and glucose across the concentration gradient from capillaries , followed by water diffusion.  Circulation and drainage of aqueous humor -From posterior chamber through the pupil to the anterior chamber, then to the irido-corneal angle, where it is drained through: -Episcleral outflow (90%): spaces of fontana to canal of Schlemm to the episcleral veins. -Uveo-scleral outflow ( 10%): It passes through: a-Ciliary body to be absorbed by the blood vessels. b-Scleral wall to the perioccular orbital tissues. Regulation of Intraocular Pressure -Intraocular pressure remains constant in the normal eye, (15 mm Hg ±2 mm Hg ) by the balance between the rate of formation and the rate of absorption. - IOP is higher in the morning than in the afternoon with maximal difference of 5 mmHg. (IOP) is measured clinically by the "tonometer“  Functions of Aqueous humor 1-It supplies nutrition and drainage of waste products to avascular structures of the eye (cornea and lens). 2-It acts as a refractive medium(R.I=1.33). 3-It maintains the normal intraocular pressure (IOP). IOP is important for:-Focusing mechanisms of the eye (the tension of suspensory ligament). -Keeping intra-ocular structures in position (iris, lens and retina). -Keeping the normal spherical shape of the eye. Functions of IOP 1-It maintains the spheroid shape of the eye, for proper optical function of the eye. 2-It exerts two forces on the lens for normal focusing mechanism a-A stretching force on the lens. b-A pressure flattening force on the lens surface. These 2 forces maintain the lens flat during rest.  Glucoma -Definition: -It is a pathological condition in which IOP is increased above normal level. - Increase in rate of formation or -decrease in rate of absorption than normal. Types of glucoma 1-Open angle glucoma: due to increased formation of aqueous. -In this type,the angle of anterior chamber is open ( root of the iris is away from the angle). Causes:a. Primary open angle glucoma due to unknown cause. b. Secondary open angle glaucoma due to ocular diseases. 2-Closed angle glucoma: - Closure of the angle by the root of the iris, impairing the drainage of the aqueous.  *Complications of glucoma: 1-Failure of accommodation to near vision. 2-cataract. 3-Severe pain (headache). 4-Atrophy of the retina due to pressure on blood vessels resulting in blindness 5- causing increased cupping of the optic disc Glucoma is one of the most frequent causes of blindness due to IOP on retinal vessels, retina and optic nerve. *Treatment: 1-Aim to reduce the formation of aquous humor by diamox (inhibits carbonic anhydrase enzyme). 2-to improve the drainage of the fluid out of the eye: by pilocarpine (pupilloconstriction). NB:Sympathomimetics or parasympatholytic (atropine) are contra-indicated (They reduce drainage by pupil-dilation that compresses the canal of schlemm ). ‫الحمد هلل‬