Anatomy of eye

Anatomy of eye

1. ANATOMY OF EYE and DRUG ABSORPTION Presented by Sneha A. Chavan Department of pharmaceutics M pharmacy 1 st year II semester 1

2. Introduction The adult eyeball measures about 2.5 cm (1 in.) in diameter. Of its total surface area, only the anterior one-sixth is exposed; the remainder is recessed and protected by the orbit, into which it fits . Anatomically , the wall of the eyeball consists of three layers: 2

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4. The External Structure of the Eye This includes the Eyelashes Eyelids Cornea Sclera Conjunctiva Tear Gland Iris Pupi l 4

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6. Eyelashes Short , stiff hairs Protect the eyeballs from foreign objects, perspiration, and the direct rays of the sun. 6

7. Eyelids ( P alpebrae) Function :- shade the eyes during sleep, protect the eyes from excessive light and foreign objects, and spread lubricating secretions over the eyeballs The space between the upper and lower eyelids that exposes the eyeball is the palpebral fissure . Its angles are known as the lateral commissure , which is narrower and closer to the temporal bone, and the medial commissure , which is broader and nearer the nasal bone. In the medial commissure is a small, reddish elevation, the lacrimal caruncle , which contains sebaceous (oil ) glands and sudoriferous (sweat) glands. The whitish material that sometimes collects in the medial commissure comes from these glands. 7

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9. lacrimal glands (lacrim - tears) This secretes tears (tears contain salts and a bacterial enzyme lysozyme ) to Wash out foreign particles or chemical irritants. Lubricate the conjunctiva which reduces friction between the eyelids and eyeball. Prevent the eye from drying out. The tear film allows atmospheric oxygen to dissolve and diffuse into the cornea . 9

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11. Fibrous Tunic The fibrous tunic is the superficial layer of the eyeball and consists of the anterior cornea and posterior sclera. CORNEA corneal diameter is about 11.7 mm with radius of curvature of anterior surface is about 7.8 mm Thickness is 0.5 – 0.7 mm and thicker in center than limbus. Clear like glass transparent coat that covers the colored iris. Has no blood vessels in it. This five-layered membrane is continuous with the sclera. Because it is curved, the cornea helps focus light onto the retina. The cornea consists of collagen fibers and fibroblasts. 11

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13. SCLERA ( SCLER- HARD), the “white” of the eye, is a layer of dense connective tissue made up mostly of collagen fibers and fibroblasts. The sclera covers the entire eyeball except the cornea; it gives shape to the eyeball, makes it more rigid, protects its inner parts, and serves as a site of attachment for the extrinsic eye muscles. At the junction of the sclera and cornea is an opening known as the scleral venous sinus (canal of Schlemm). A fluid called aqueous humor. 13

14. Conjunctiva known as mucus membrane. This is a transparent membrane that covers the sclera and lines the inner surface of the eyelids. It also secretes mucous and a small volume of tears for the eye to be moist. 14

15. Iris Pigmented layer of muscular tissue which gives the eye its color. It consists of melanocytes and circular and radial smooth muscle fibers. The amount of melanin in the iris determines the eye color. The eyes appear brown to black when the iris contains a large amount of melanin. Blue when its melanin concentration is very low, and green when its melanin concentration is moderate. The iris has two sets of  involuntary muscles: the circular and the radial muscles which control the diameter of the pupil and hence, the amount of light that enters the eye. A principal function of the iris is to regulate the amount of light entering the eyeball through the pupil. 15

16. Pupil The Pupil is an opening at the center of the Iris. It permits light to enter the eye . Responses of the pupil to light of varying brightness. 16

17. The Internal Structure of the Eye Includes Choroid Ciliary body Lens Suspensory ligament Retina Optic Nerve 17

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19. Choroid The highly vascularized choroid which is the posterior portion of the vascular tunic and Black-pigmented layer under the sclera that prevents the internal reflection of light rays. Filled with blood capillaries, it is rich in blood vessels that bring oxygen and nutrients to nourish the eyeball. The choroid is modified to form the iris and the ciliary body at the front of the eye. Also contains melanocytes that produce the pigment melanin, which causes this layer to appear dark brown in color. Melanin in the choroid absorbs stray light rays, which prevents reflection and scattering of light within the eyeball. As a result, the image cast on the retina by the cornea and lens remains sharp and clear. 19

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21. Ciliary Body In the anterior portion of the vascular tunic Like the choroid, the ciliary body appears dark brown in color because it contains melanin-producing melanocytes. In addition, the cil iary body consists of ciliary processes and ciliary muscle. The ciliary processes are protrusions or folds on the internal surface of the ciliary body. They contain blood capillaries that secrete aqueous humor. Extending from the ciliary process are zonular fibers (suspensory ligaments) that attach to the lens. The fibers consist of thin, hollow fibrils that resemble elastic connective tissue fibers . The ciliary muscle is a circular band of smooth muscle. Contraction or relaxation of the ciliary muscle changes the tightness of the zonular fibers, which alters the shape of the lens, adapting it for near or far vision. 21

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25. Lens The Lens is a transparent, round, biconvex structure. It is elastic so that its curvative can be changed to adjust its refractive or focusing power. It's responsible for focusing light onto the Retina. Behind the pupil and iris, within the cavity of the eyeball, is the lens. Perfectly transparent and lacks blood vessels. Held in position by encircling zonular fibers, which attach to the ciliary processes. The lens helps focus images on the retina to facilitate clear vision. lens divides the interior of the eyeball into two cavities: the anterior cavity and vitreous chamber. 25

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27. Anterior cavity The space anterior to the lens — consists of two chambers. The anterior chamber lies between the cornea and the iris. The posterior chamber lies behind the iris and in front of the zonular fibers and lens . Both chambers of the anterior cavity are filled with aqueous humor ( aqua = water ), a transparent watery fluid that nourishes the lens and cornea. Aqueous humor continually filters out of blood capillaries in the ciliary processes of the ciliary body and enters the posterior chamber. It then flows forward between the iris and the lens, through the pupil , and into the anterior chamber. From the anterior chamber, aqueous humor drains into the scleral venous sinus (canal of Schlemm ) and then into the blood. Normally , aqueous humor is completely replaced about every 90 minutes. 27

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30. Vitreous Chamber The larger posterior cavity of the eyeball is the vitreous chamber , which lies between the lens and the retina. Within the vitreous chamber is the vitreous body, a transparent jellylike substance that holds the retina flush against the choroid, giving the retina an even surface for the reception of clear images. It occupies about four-fifths of the eyeball. The vitreous body does not undergo constant replacement. It is formed during embryonic life and consists of mostly water plus collagen fibers and hyaluronic acid. The vitreous body also contains phagocytic cells that remove debris, keeping this part of the eye clear for unobstructed vision. 30

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32. Cont… The pressure in the eye, called intraocular pressure, is produced mainly by the aqueous humor and partly by the vitreous body ; normally it is about 16 mmHg (millimeters of mercury). The intraocular pressure maintains the shape of the eyeball and prevents it from collapsing. Puncture wounds to the eyeball may cause the loss of aqueous humor and the vitreous body. This in turn causes a decrease in intraocular pressure, a detached retina, and in some cases blindness. 32

33. Suspensory ligament A tissue that joins the lens to the ciliary body. This hold the Lens in place . 33

34. Retina The third and inner layer of the eyeball, the retina, lines the posterior three-quarters of the eyeball. light sensitive layer of the eyeball, on which images are formed. They contain  Photoreceptors  which are light-sensitive cells.  The surface of the retina is the only place in the body where blood vessels can be viewed directly and examined for pathological changes, such as those that occur with hypertension, diabetes mellitus, cataracts, and age-related macular disease And are visible through an ophthalmoscope . 34

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36. Photoreceptors Photoreceptors are specialized cells that begin the process by which light rays are ultimately converted to nerve impulses. Each retina has about 6 million cones and 120 million rods . 36

37. Photoreceptors Rods  are responsible to detect light in  dim light  (black and white) enabling night vision. They contain a pigment in rods called  Rhodospin  which is light sensitive. If exposed to bright light then it will be bleached. Rhodospin enables a person to see in the dark.        Cones  are responsible to detect  colored light . There are green, blue and red cones. Red is used to respond to long wavelengths. Blue is used to respond to short wavelengths and Green is used to respond to medium lengths.   Cones are closely packed to allow  the perception of details but cannot work well in dim light. 37

38. Blind Spot The point in the Retina where the optic nerve exits the eye. There are no photoreceptors which makes this Blind Spot insensitive to light.  38

39. Fovea A pit in the Retina where images are  focused . It has a high density of cones but it has no   rods. The Fovea permits detailed vision. It is concentrated with  photoreceptors 39

40. Optic Nerve The Optic Nerve is located at the back of the eye. It transmits nerve impulses from photoreceptors to the brain . 40

41. Blood ocular barrier 41

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44. Absorption of drug in eye 44

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47. Precorneal constraints 47

48. Cont… 1. Spillage of drug by overflow Normal volume of tear 7µl. (under normal condition) If blinking doesn’t occur the human eye can accommodate 30 µl without spillage from palpebral fissure. With an estimated drop volume of 50 µl Due to overflow 70% of administered dose is expelled. If blinking occur, then the residual volume left is 10 µl. ( 90% OF DOSE IS EXPELLED ) 2. Dilution of drug by tear turnover Normal human Tear turnover is approx. 16% per minute. Which is stimulated by factor drug entity, pH, tonicity of dosage form. 48

49. Cont… 3. Nasolacrimal drainage / systemic drug absorption Absorbed across nasal mucosa and git 4. Conjunctival absorption 5. Enzymatic metabolism Occur in Precorneal space or in the cornea Loss of drug entity that posses labile bond. Reduced bioavaibility of instilled dose To overcome this, frequent dose of drug at high conc. a re recommended. 49

50. Trans corneal penetration Trans corneal penetration is affected by corneal barrier, physicochemical property of drug Active ion transport system present at cornea Corneal barrier Corneal epithelial is barrier to drug absorption into eye. Corneal epithelial is more permeable compared to stratum corneum of skin. But relatively impermeable when compared to epithelial tissue of nasal, intestinal, bronchial, tracheal. Protective barrier to ion transport and foreign molecule. 50

51. Cont.. Corneal stroma is highly HYDROPHILLIC tissue containing mostly water. Drug with molecular size upto 500000 can diffuse in normal stroma. This corneal stroma is rate limiting barrier for ocular absorption of lipophilic drug. 51

52. Physicochemical properties of drug Main route for drug to penetrate across corneal epithelial Transcellular pathway (lipophilic drug penetrate through) Paracellular pathway (hydrophilic drug penetrate through) Lipophilicity, solubility, molecular size and shape, charge and degree of ionization affect route and rate penetration in cornea. Esterase, peptidase, proteases enzymes present in ocular tissue including cornea. So, drug metabolized during or after absorption. Ocular prodrug is converted into active parent drug by enzymatic transformation of corneal epithelial. 52

53. Ion transport In addition to, Passive corneal diffusion of ion via paracellular pathway, ions can be actively transported across corneal epithelium and endothelium. Corneal epithelial contains ionic channel that selective for cation over anion. Also contain anion channel in apical membrane and conductive for potassium channel in basal cell. Sodium penetrate from tears into epithelium via passive diffusion, but it is actively transported from epithelium to the stroma. 53

54. Non corneal absorption Route involve drug penetration across bulbar conjunctiva and underlying sclera into uveal tract and vitrous humor. Important for hydrophilic and large molecule e.g., insulin and p- aminoclonidine (poor corneal permeability) Tight junction of superficial conjunctival epithelium main barrier for drug penetration. Limiting molecular size for conjunctival penetration is between 20000 and 40000. Ocularly applied drugs penetrate across the sclera through perivascular spaces through the aqueous media of gel like mucopolysaccharide or through empty space within collagen network. Sclera is more permeable in comparison to cornea e.g., sucrose, inulin, PEG, β blocker. 54

55. Reference Principles of ANATOMY & PHYSIOLOGY Gerard J. Tortora. Bryan Derrickson, 13th Edition, John Wiley & Sons, Inc . Remington Essential of Pharmaceutics, edited by Linda Felton, page no. 908 - 911 http:// thevisibleworld.weebly.com/the-internal-structure.html http:// thevisibleworld.weebly.com/the-external-structure.html 55

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