Anatomy and Physiology of the Eye

1. Anatomy and Physiology of the Eye Optometry 8110

2. Course Data • Class meets on Mondays from 9:00-9:50, Tuesdays from 10:00-10:50, and Fridays from 10:00-10:50 in classroom GB, Marillac Hall • Laboratories are on Tuesdays: Group B from 1:00-2:50, and Group A from 3:00-4:50 in Room #214 of the Marillac Classroom Building • Laboratories begin on Tuesday, February 6 • Recommended text is Clinical Anatomy of the Visual System, 2nd Edition, 2005, Elsevier Butterworth Heinemann

3. Course Data • Material will be presented and tested in eight units (Units 6, 7 and 8 will be tested only on the final exam) • There will be a laboratory practical exam on Tuesday, March 6, and a final practical exam on Tuesday, April 10

4. Grading Policy • The final course grade will be calculated as follows: • Five Unit Exams 54% of final grade • Laboratory 22% of final grade • Final Exam 24% of final grade • Trivia points—there will be a trivia contest in both lab sections. Each lab group will be divided into four trivia teams for the semester. Trivia questions will be asked based on the laboratory done that day • The winning team will be awarded 25 grade points, the 2nd-place team 20 points, the 3rd-place team 15 points, and the last-place team 10 points

5. Visual System Overview of Ocular Anatomy

6. The Eyeball • The eyeball lies in the anterior part of the orbit and occupies only about 20% of the orbital space • It is surrounded by a mass of muscles, vessels, nerves, and fascia, which is surrounded by orbital fat • The eyeball consists of two different-sized spheres fused together: the cornea and the sclera • Cornea: anterior 1/6, greater curvature, lesser radius of curvature of about 7.8 mm • Sclera: posterior 5/6, flatter, radius of curvature about 12 mm

7. The Eyeball • The two spheres come together at the external scleral sulcus • Inside the eye is the internal scleral sulcus and it is here that the transitional zone, the limbus, is found • Eyeball dimensions: • Anterior-posterior = 24 mm • Vertical = 23 mm • Transverse = 23.5 mm

9. Stucture of the Eyeball • Three concentric layers or “tunics” form the eye: the fibrous tunic, the uveal tunic, and the neural tunic

10. Fibrous Tunic • Collagenous tissue and has two parts: the sclera and the cornea • It is the outer layer that contains the intraocularpressure (IOP) and preserves the critical spacing of the optical elements • The cornea is transparent and provides the greatest amount of refraction of all the optical elements of the eye, about 40 diopters of the 60 diopter eye • Elliptical, with the greatest diameter horizontally • Astigmatism is asphericity of the cornea

12. Fibrous Tunic II • The sclera is the opaque, white part of the outer eye • It is tough, nontransparent and viscoelastic with an outer episcleral layer and an inner stromal layer • It provides shape to the globe and a place for the extraocular muscles to attach • Its thickness varies from 1.0 mm at the posterior pole to 0.3 mm just behind the rectus muscle insertions • Its elasticity changes with age because of collagen tissue changes • The sclera is thin at birth and is bluish; with age it can appear yellow due to fatty deposits; it can also appear yellow in cases of liver disease due to metabolic waste buildup

13. Uveal Tunic • The uveal tunic is the middle layer of the eye and consists of the iris, ciliary body, and the choroid • Its function is primarily nutritive, although its structures also include muscles that control pupil size and accommodation, and include the epithelial cells that produce the aqueous humor that accounts for the IOP

15. Choroid • The choroid is primarily vascular and is the most posterior portion of the uveal tract • It also contains pigmented cells, which absorb photons (light quanta) not absorbed by the visual pigment, thereby, reducing stray light and degradation of the retinal image • Its combination of pigment and a bed of vessels allows the choroid to dissipate the heat generated by photons not absorbed by the photopigment molecules in the rods and cones • A main function of the choroid is to supply blood to the outer avascular layers of the retina (too many vessels in the retina would block light)

16. Ciliary Body • This structure lies just anterior to the choroid, just behind the limbus (region in which the cornea changes into the sclera) • It is subdivided into the: • Posterior pars plana – forms some of the vitreous • Anterior pars plicata – responsible for the formation of the aqueous humor and consists of 80, or so, ciliary processes • The ciliary body contains the ciliary muscle, and when the muscle fibers contract the tension on the zonular fibers, connecting the ciliary body to the crystalline lens capsule, is reduced and the lens front and back surfaces increase in curvature, that is, accommodation occurs

17. Ciliary Body II • The ciliary epithelium that covers the pars plicata, particularly the nonpigmented layer, produces the aqueous that fills the anterior and posterior chambers of the eye and regulates the IOP • The aqueous also provides nutrients to and carries waste from the avascular cornea and lens

19. Iris • Most anterior portion of the uveal tract and is inwardly deflected from the fibrous tunic • Separates the anterior and posterior chambers • The lens and its suspensory ligament lie immediately behind the iris • It forms a variable diaphragm called the pupil, which is the aperture stop of the eye’s optical system • It controls the light reaching the retina (retinal illumination) and improves the retinal image quality • The iris sphincter and dilator muscles control the size of the pupil

20. Neural Tunic • This tunic is the retina, the innermost, photosensitive layer of the eye, which is embryologically an extension of the brain • It features over 100 million photoreceptors and 70 to 100, or so, different types of neurons, based on anatomical, physiological, or histochemical differences • The retina’s primary purposes are: • Photoreception • Phototransduction • Peripheral neural encoding • Information transfer

21. Neural Tunic II • The retina transmits information to the brain over about a million optic nerve axons • Its circuitry is arranged so as to identity, quantify, and encode “change” in the retinal image • The change may be spatial, temporal, or spatiotemporal (a moving contour) • Signals in neighboring retinal neurons are compared (spatial change) or the signals over time at the same neuron or collections of neurons are compared (temporal change)

23. Crystalline Lens • The avascular lens is enclosed in the lens capsule that is attached by the ciliary zonular fibers to the ciliary body (to the valleys between the ciliary processes) • It lies immediately behind the iris and the centralized pupil • The lens does not have as much refractive power as the cornea, because the refractive index difference between the lens and the aqueous and vitreous is not as great as the refractive index between air and the cornea • The lens is the eye’s dynamic optical element, as it increases its power for near vision

25. Chambers of the Eye • The anterior chamber lies in front of the iris and behind the cornea, and laterally, it is bordered by the ciliary body and the drainage apparatus • The posterior chamber is an irregular narrow space between the posterior surface of the iris and the anterior surface of the lens and vitreous • Both the anterior and posterior chambers are filled with aqueous formed by the epithelium on the ciliary processes • Aqueous humor is produced in the posterior chamber, passes through the pupil into the anterior chamber, and leaves the eye at the drainage angle (limbus)

26. Anterior chamber Posterior chamber Vitreal chamber

27. Chambers of the Eye II • The aqueous leaves the eye through the trabecular meshwork, the canal of Schlemm, and the scleral veins • The vitreal chamber is bounded anteriorly by the lens, laterally by the ciliary body and the retina, and posteriorly by the retina • It is filled with a gel-like vitreous, which is 98% water in a collagen matrix • Anteriorly, the vitreous has a depression called the patellar fossa in which the posterior lens surface is located

28. Cornea The Principle Refracting Component of the Eye

29. Introduction The cornea is an intriguing structure, because, as a component of the refracting apparatus of the eye, it must be transparent, which requires a very regulated metabolism, and yet, in the interests of tissue transparency, it cannot have a direct blood supply. In addition it forms part of the outer fibrous wall of the eye and is in a position to be easily damaged. While aspects of their structure are similar, the cornea and the sclera seem quite different.

30. Corneal Dimensions • Oval from the front: 12 mm horizontal and 11 mm vertical • Circular from the rear: 11.7 mm, anterior vertical overlap • Corneal profile is elliptical – flattens in the periphery • Central radius of anterior surface curvature is about 7.8 mm • Radius of the posterior surface is 6.5 mm • Central corneal thickness is 0.53 mm • Peripheral corneal thickness is 0.71 mm • All the above are approximations

32. Astigmatism • Astigmatism is that refractive condition in which light rays coming from a point source are not imaged as a point – unequal refraction by different meridians • Regular astigmatism: longest radius of curvature and the shortest lie 90 apart • With-the-rule astigmatism: the steepest curvature lies in the vertical meridian • Against-the-rule astigmatism: horizontal steepest • Irregular astigmatism: meridians corresponding to the greatest difference are not 90 apart

33. Corneal Components • The cornea contains a large amount of collagen, a common protein found all through the body • The collagen in the cornea and sclera exists in conjunction with polysaccharide molecules called glycosaminoglycans (GAGs) • The main ones in the cornea and sclera are keratan sulphate and dermatan sulphate • A large number of these will bind one end to core proteins, the triple helix of three chains of amino acids, forming a complex structure known as a proteoglycan

34. Corneal Components • The long, ropelike core protein acquires a dense fuzz of GAGs along its length • The GAGs are not compact and these molecules have a strong negative charge, which attracts positively charged ions, like Na+, and with this cation comes water, which is drawn into the molecular network • Water and the GAGs form a gel-like matrix around the collagen fibrils

35. Corneal Components • Proper spacing of the collagen fibrils is crucial to corneal transparency, so the water content in the proteoglycan matrix must be controlled within narrow limits to maintain spacing • The attraction of water is much greater than is optimal for corneal transparency, so anatomical and physiological characteristics of the cornea are geared to preventing the cornea from imbibing and retaining as much water as its structure and chemical properties demand

36. Corneal Layers • Epithelium – 50 m • Bowman’s layer -- 14 m • Stroma – 500 m • Descemet’s membrane – 5 m • Endothelium – 5 m

38. Epithelial Layer • The entire epithelial layer is 5 to 7 cells thick • Surface cells – 2 cells thick • Wing cells – 2-3 cells thick • Basal cells – 1 cell thick

40. Schwann cell covering lost

41. Surface Layer • The surface layer is two cells thick and is composed of nonkeratinized squamous cells with a flattened nucleus • The plasma membrane of the surface cells is thought to secrete a glycocalyx that adjoins the mucous layer of the tears • There are many projections from the surface cells into the tear fluid – microvilli (fingerlike) and microplicae (ridgelike) • Tight junctions (zonula occludens) join the surface cells along their lateral walls and provide a barrier to intercellular movement of substances from the tear layer and prevent the uptake of excess water from the tear film • With time the surface cells are sloughed off and are replaced from the layers below

42. Wing Cells • The wing cells have winglike lateral processes, and have convex anterior surfaces and concave posterior surfaces • These cells fit over the rounded tops of the basal cells • Desmosomes and gap junctions join these cells together • Desmosomes also connect the wing cells and the basal cells

43. Basal Cells • The basal cell layer is the innermost epithelial layer and is a single layer of columnar cells • The rounded apical surface of the cells lie under the wing cells, and the flat basal surface is attached to a basement membrane (basal lamina) secreted by the basal cells • The bases of the cells are attached by hemidesmosomes with anchoring fibrils passing through Bowman’s layer into anchoring plaques in the stroma

44. Basal Cells 2 • Desmosomes and gap junctions (less numerous than in the wing cells) also connect the basal cells to one another • The basal layer is the germinal layer where mitosis occurs

45. Epithelial Replacement • Epithelial surface cells are constantly being sloughed off • Cell proliferation (mitosis) occurs in the basal cells, and basal cells move up to become wing cells, and wing cells move up to become surface cells • There are apoptotic changes in the cell during this process • Stem cells are located in a 0.5-1.0 mm band around the periphery of the cornea and are the source of renewal of the basal cells, necessary because the rate of sloughing is greater than the rate of basal cell mitosis • Basal cells migrate from the corneal periphery toward the corneal center

46. Epithelial Replacement 2 • Turnover time for the entire corneal epithelium is about 7-10 days • Minor abrasions heal within hours and larger ones may heal overnight • However, if the basement membrane is damaged, complete healing with replacement of the basement membrane and hemidesmosomes may take months • Despite the normal sloughing of cells, the barrier function of the epithelium is maintained by tight junctions, located only in the superficial surface cells

48. Epithelial Wound Healing • Mitosis stops • Basal cells at the wound’s edge retract and lose hemidesmosomes (wound becomes slightly larger) • Centripetal slide occurs – cells’ size increases and there is ameboid migration (filopodia) • The wound is covered (temporary tight junctions) • The cells temporarily anchor • Mitosis resumes until normal epithelial thickness is reached • Surface tight junctions reform • Cells are permanently re-anchored to Bowman’s layer

50. Recurrent Corneal Erosion • Recurrent corneal erosion is a condition in which the corneal epithelium sloughs off either continually or periodically • The underlying cause is either poor attachment between the epithelial basal cells and the basement membrane, or poor attachment between the basement membrane and Bowman’s layer • Aging can also contribute to corneal erosion, as the basement membrane thickens rendering the anchoring fibrils too short