Ophtha - Anatomy And Physiology Of The Eye 2007

Anatomy and Physiology of the Eye Dr. Silvestre A. Pascual Jr. Department of Ophthalmology

Out li ne 1. THE GLOBE 2. THE OUTER COAT • The Cornea    

The Corneal Epithelium The Corneal Stroma The Descemet’s Membrane The Endothelium

• The Sclera • The Corneoscleral Limbus

Out li ne 1. THE MIDDLE COAT • • •

The Iris The Ciliary Body The Choroid

2. THE INNER COAT • • • • • •

The Retina The Photoreceptor The Modulator Cells The Transmitter Cells The Supporting Cells Regions of the Retina

Out li ne 1. CHAMBERS OF THE EYE • •

Aqueous Humor Vitreous Humor

2. LENS 3. ZONULES

THE GLOBE 

Occupies the front half of the orbit

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Only the anterior third is exposed consisting of the cornea and sclera

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The exposed sclera is covered by the bulbar conjunctiva

THE GLOBE  Has three main layers or coats: o Outer: cornea, sclera, corneoscleral limbus o Middle: iris, ciliary body, choroids o Inner: retinal pigment epithelium and retina

THE GLOBE  Has 3 chambers: o Anterior Chamber (w/ aqueous humor)

o Posterior Chamber (w/ aqueous humor)

o Vitreous Chamber (w/ vitreous humor)

 Contains a crystalline lens located immediately behind the iris

THE GLOBE AP length of 22-26 mm (ave.24mm) Circumference 69-81mm (ave.75mm at the equator)

Volume of the globe = about 6ml

THE OUTER COAT

Composed of tough fibrous tissues shaped as segments of 2 spheres: o Transparent cornea (anterior 1/6) o White opaque sclera (posterior 5/6)

The junction of the cornea and sclera known as the corneoscleral limbus contains the Trabecular Meshwork and Canal of Schlemm Focusing light and protecting contents of the globe

The Cornea  Has an average radius of curvature of 7.5mm  Functions as the MAIN refracting structure of the eye  Composed mostly of collagen fibers (transparent) regular both in size and arrangement

The Cornea  It is thinnest at the center (0.5mm) and thickest at the periphery (1.0mm)

The Cornea  Has 5 layers: o o o o

Epithelium Bowman’s layer Stroma Descemet’s membrane o Endothelium

The Cor neal Epit hel ium  Composed of stratified squamous epithelium, 5-6 layers  Epithelial cells form in the deepest layer, become progressively flatter and are shed 7 days later Basal columnar cells

Wing cells

Stratified squamous cells

The Cor neal Epit hel ium  Basement membrane of the basal cells are adherent to the Bowman’s layer, the acellular anterior portion of the stroma, resistant to passage foreign bodies or infective organisms

The Cor neal St roma  Form 90% of the corneal thickness  Contain regularly shaped and sized lamellae of collagen fibers secreted by interspersed keratocytes

The Des cemet’s Membr ane  The basement membrane of the endothelial cells of the cornea  Terminates abruptly in the periphery of the cornea forming SCHWALBE’S line, the anterior border of the Trabecular Meshwork

The Endo the lium  Single layer of mesothelium, the apices of which are in direct contact with aqueous humor.  Responsible for deturgesence of the stroma  Does not regenerate in adult humans

The Sclera  Dense fibrous collagenous structure comprising the posterior 5/6 of the outer coat  Is thickest at the region surrounding the optic nerve and thinnest immediately posterior to muscle insertion

The Sclera The posterior scleral foramen through which the optic nerve exits is cone shaped and bridged by a sieve like structure, the lamina cribrosa Has three layers: the episclera, stroma,

The Sclera  The episclera is the outermost layer of moderately vascularized connective tissue. It is attenuated posterior to the equator, making it relatively avascular  The scleral stroma is composed of collagen fibers irregular in size and arrangement  The lamina fusca is the innermost layer made of fine collagen

Which is Cornea? Sc le ra?

The Corneoscleral Limbus  Is a transitional zone about 1.5mm in diameter  Inner part of corneoscleral limbus is the Trabecular Meshwork which is where aqueous humor is drained

The Corneoscleral Limbus

 Also contain the Canal of Schlemm, an oval channel line by a single layer of endothelium that drain aqueous humor from Trabecular Meshwork.  25-35 collector channels connect with the Canal of Schlemm which drain the aqueous humor to the anterior ciliary and episcleral veins

THE MIDDLE COAT  Composed of the iris, ciliary body and choroid  Iris limits the amount of light entering the eye  Ciliary body produces aqueous and the accomodation of the lens  Choroid provides the blood supply of the outer half of the retina

The Iris  The iris is a diaphragm that lies in front of the lens and ciliary body  The anterior surface is divided by the collarette into a central pupillary zone and a peripheral ciliary zone  Composed of an anterior stroma and a posterior pigmented epithelium

The Iris  The anterior stroma has 2 layers: o Anterior border layer composed of loose collagen tissue densely packed with pigmented and nonpigmented cells. o Stroma proper consists of bundles of collagen fibrils . It has more elastic tissues and fewer pigmented cells

The Iris  The pigmented epithelium has 2 layers of cells densely packed with melanin: o The iris sphincter muscles and dilator muscles are located between the iris pigmented epithelium and the stroma proper

The Iris  Blood supply - is formed by the 2 long posterior ciliary arteries and 7 anterior ciliary arteries - at the iris root is found the major arterial circle of the iris - radial arteries travel the iris and form the minor arterial circle at the area of the collarette

The Ciliary Body  Is a ring of tissue about 6mm wide located posterior to the iris root  Has 2 parts: o The uveal portion -is composed of 3 layers of smooth muscles: longitudinal, radial and circular -function is mostly for accommodation

The Ciliary Body o The epithelial portion -is divided into: • pars plicata - forms the anterior 2mm and consists of 60-70 folds on ciliary processes. Each ciliary process is made of an outer nonpigmented epithelium and an inner pigmented epithelium. Secretion of aqueous humor is the main function of the nonpigmented epithelium. • pars plana - forms the posterior 4mm of the epithelial portion

The Choroid

 Is the vascular sheet that provides the blood supply of the retinal pigment epithelium and outer half of the retina  Extends from ora serrata to the optic nerve  Composed of 3 layers of blood vessels: the outer vascular layer of Haller, middle vascular layer of Sattler and Choriocapillaris  They are bounded by fibrous tissues: the Lamina Fusca and Bruch’s membrane

THE INNER COAT  The Retina  Receptor and Processor of images  Innermost film that receives, modulates and transmits light impulses to form images

The Retina  Its anterior border is the ora serrata and extends posteriorly up to the optic nerve  Composed of the retinal pigment epithelium and the sensory retina

The Retina  The retinal pigment epithelium is a single layer of hexagonal cells rich in melanin pigment which functions to trap light

The Retina  The sensory retina is made of layers of cells composed of receptor neurons and glial cells to receive, modulate and transmit images to the brain

The Phot ore cep tors: Rods & Con es  Specialized receptors which receives images  2 cell types: rods for dark vision and cones for light vision as well as color vision  Macular lutea (only cones are found) – temporal to the optic disc

The M odu lator Cell s  Light received by photoreceptors are then processed by modulator cells  3 types of cells: bipolar, horizontal and amacrine cells

The M odu lator Cell s  Bipolar cell dendrites attach to photoreceptors and the axons synapse with ganglion cells

The M odu lator Cellcellsdendrites  Horizontal attach to several adjacent photoreceptors and the axons synapse with several photoreceptors in a distant part of the retina and some to bipolar cells. Horizontal cells act as condensers collecting impulses from groups of photoreceptors, integrate and trigger a visual impulse

The M odu lator Cell s  Amacrine cells processes are oriented in the opposite direction in terms of light transmission. Their processes attach to groups of ganglion cells and bipolar cells. They are believed to inhibit integration of visual impulse

The Tran smi tte r Cell s: Gan on Cell s  Transmitgli the visual impulses from bipolar and amacrine cells to the midbrain  Physiologically, they transmit either visual impulses to the midbrain (LGB) or afferent impulses of the light reflex (IC)  Axons of the ganglion cells form the nerve fiber layer

The Tran smi tte r Cell s: Gan gli on Cell s  The nerve fibers from the ganglion cells converge to exit the eye through the lamina cribrosa as the optic nerve  Intraocularly, it is viewed as the vertically oval shaped optic disc  At the center of the disc is a depression known as the optic cup

The Supp ort ing Cell s: Müll er C ells  Müller cells are large astrocytes that mechanically support the retina  Also known as retinal “nurse cells” as they are believed to provide the retina enzymes used for glycolysis

Regions of the Retina  Ora serrata  Central retina (macular lutea)  Fovea centralis  Peripheral retina

CHAMBERS OF THE EYE  Anterior chamber –aqueous humor  Posterior chamber –aqueous humor  Vitreous chamber – vitreous humor

Aqueous Humor  Fluid that fills the anterior and posterior chamber  Provides nutrients to the avascular tissues  Secreted by the nonpigmented epithelium of the ciliary body and drained through the Trabecular meshwork  Volume is about 0.2mL

Vitreous Humor

 Fills the vitreous cavity  Attached firmly to the periphery at the ora serrata and posteriorly to the rim of the optic nerve and the central retina  Mainly composed of salts, protein, hyaluronic acid and 98% water  Act as a refracting medium and a volume filler of the eye

LENS  Crystalline lens is a grossly transparent, biconvex tissue  More curved posteriorly (radius of curvature of about 6mm) than anteriorly  Refracting surface with a refracting power of about 1/2 of the cornea

LENS  Composed of: lens capsule, cells or lens fibers with their nuclei at the lens equator and those with their nuclei under the anterior capsule, lens cortex, and lens nucleus  Held in place by zonules of Zinn

LENS  Lens capsule – envelopes the entire lens; composed of thick anterior capsule & thin posterior capsule  Lens nucleus – innermost, central region composed of lens fibers that are no longer attached to the posterior capsule  Lens cortex – composed of lens fibers that have lost their nuclei near the equator of the lens & their posterior processes attached to the posterior capsule

ZONULES  Suspensory ligament of the lens  Supports the lens in position and connects the lens to the ciliary muscle  Composed of fine fibrils of modified collagenous tissue

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