Gross Anatomy of the Globe Extraocular Muscles of the Eye PDF

GROSS ANATOMY OF THE GLOBE EXTRAOCULAR MUSCLES OF THE EYE Anna Chiambiro & Naboth Nemaire  EMBRYOLOGY OF THE EYE The eye is an outgrowth from brain ,started as optic vesicle connected to forebrain by optic stalk. The eye is derived from : Ectoderm (a) neur...

GROSS ANATOMY OF THE GLOBE EXTRAOCULAR MUSCLES OF THE EYE Anna Chiambiro & Naboth Nemaire  EMBRYOLOGY OF THE EYE The eye is an outgrowth from brain ,started as optic vesicle connected to forebrain by optic stalk. The eye is derived from : Ectoderm (a) neuroectoderm of the forebrain (b)surface ectoderm of the head mesoderm Neural crest cells  Neuroectoderm -Neural retina & Retinal pigmented epithelium (risk of detachment) Iris epithelium Conjunctiva epithelium Surface ectoderm- Lens and corneal epithelium Mesoderm- Extraocular muscles Vascular coat of the eye Neural ectoderm Everything else During embryogenesis the lens and vitreous are supplied by hyaloid vessels that disappears at late stages of embryogenesis.  DEVELOPMENT OF THE EYE AFTER BIRTH At birth, the eye is relatively large in relation to the rest of the body. The eye reaches full size by the age of 8 years. Visual acuity developed until the age of 20 years. The lens continues to enlarge throughout the life. The iris has a bluish color due to little or no pigment on the anterior surface. During early infant life, the cornea & sclera can be stretched by raised IOP → enlargement of the eye. Normal axial of the eye is 21-24mm (2.4cm) - Elevated intraocular pressure with onset in the first year of life it called congenital glaucoma. - Eye consist of globe (eyeball, extraocular muscles) and adnexa (lacrimal gland and sac).  GROSS ANATOMY OF THE GLOBE Refers to the external and internal structures of the eyeball which are visible without the aid of a microscope The structures include : Sclera Cornea Iris Pupil Lens Vitreous body Choroid Retina Optic nerve Aqueous humor Anterior chamber Posterior Chamber Macula Optic disc Conjunctiva SCLERA  SCLERA is the opaque, fibrous, protective outer layer of the eye containing mainly collagen and some crucial elastic fiber. The sclera forms the posterior five-sixths of the connective tissue coat of the human eyeball The sclera is perforated by many nerves and vessels passing through the posterior scleral foramen, the hole that is formed by the optic nerve At the optic disc, the outer two-thirds of the sclera continues with the dura mater (outer coat of the brain) via the dural sheath of the optic nerve It is continuous with the dura mater and the cornea, and maintains the shape of the eyeball It offers resistance to internal and external forces provides an attachment for the extraocular muscle insertions.  CORNEA dome-shaped transparent membrane about 12 mm (0.5 inch) in diameter that covers the front part of the eye Comprises 1/6 of the anterior surface of the eyeball Makes up 2/3 of the refractive power of the eye It protects the pupil, the iris, and the inside of the eye from penetration by foreign bodies The cornea consists of 5 layers : epithelium Bowmans layer Stroma Descemets membrane endothelium 5 LAYERS OF THE CORNEA  Epithelium Layer: This is the surface layer of cells. They provide barrier function and a smooth surface for the tear film. It consists of non-keratinized stratified squamous epithelial cells around 50 microns thick, and five to seven-cells deep. It is continuous with the epithelium of the bulbar conjunctiva at the limbus and acts as the main barrier to infection of the cornea. The most superficial layer of the 2-3 layers of squamous cells exhibit surface microvillae and microplicae , small finger like projections that serve as an adhesion site for the tear film. It also increases the surface area of epithelial cells to aid oxygen and carbon dioxide exchange. Tight junctions between epithelial cells act as a permeability barrier for the cornea.  BOWMANS LAYER Bowman's layer is approximately 10-14 microns thick. It is a cellular and composed of irregularly arranged collagen fibrils that have been embedded in a mucoprotein ground substance. Bowman's layer is a cellular, and contains collagen fibrils of small diameter that, unlike the corneal stroma, are not ordered into bundles. Although Bowman’s layer is sometimes referred to as a "membrane" it is more correctly considered a transition zone to the stroma. This layer is tough, and keeps the cornea from swelling forward which means when the cornea swells, it must do so backwards into the anterior chamber.  STROMA The stroma forms about 90% of the total corneal thickness and It consists of thick dense collagen. The arrangement of collagen fibrils within the corneal stroma is unique and facilitates transparency of the cornea. Each collagen fibril is20-25nm in diameter and run parallel to neighbouring fibrils. the spacing between each fibril is highly ordered. Groups of fibrils are called lamellae. The stroma consists of 200-250 flattened lamellae of collagen fibrils that are embedded in glycosaminoglycans and that run across the entire cornea. This laminated structure gives the cornea greater strength. Within the stroma, lamellae run at different angles to one another, although within each lamellae, the collagen fibrils run parallel to one another. Transparency for the cornea depends on the ordered arrangement of lamellae and the similarity of the collagen diameter. This highly ordered arrangement contrasts with that of the scleral stroma where the collagen fibrils differ in diameter and density  DESCEMET'S MEMBRANE Descemet's Membrane is a 10 micron thick basement membrane produced continuously throughout life by the corneal endothelium. It displays strong elastic properties and terminates abruptly at the limbus. This terminus is visible clinically using a method called gonioscopy. One of the leading needs for cornea transplant is from a dystrophy of Descemet’s layer called Fuch’s dystrophy.  ENDOTHELIUM The endothelium is the inner most layer of the cornea that faces the anterior chamber. It is a single layer of interdigitating flattened hexagonal cells whose basal surface rests on Descemet's membrane.It is continuous with the endothelial cells that line the trabecular meshwork. Corneal endothelial cells are derived embryologically from neural crest cells and have a very limited capacity for proliferation. Their density gradually drops from 5000cell/mm2 in the central cornea at birth to 1500- 2000/mm2 by middle age. The endothelium plays an essential role in transparency by maintaining corneal hydration and thickness. These functions depend on the barrier and fluid transport systems that reside within endothelial cells. The walls of the hexagonal endothelial cells have many ridges that interdigitate with their neighbour.  ENDOTHELIUM This, together with a series of macula occludens rather than zonula occludens, located near the apical surface, results in a barrier that is slightly leaky. As a consequence, large molecules, nutrients like glucose and amino acids from the aqueous humour can pass across the endothelium into more anterior layers of the cornea. The endothelium is crucial for maintaining corneal hydration and thickness. This is achieved by a rich collection of metabolic pumps, such as Na+-K+-ATPase, that actively pump ions including sodium into the aqueous humour. Water passes down its concentration gradient from the cornea into the aqueous. Channels called aquaporins are also expressed on the surface of endothelial cells, and provide an additional route for fluid movement out of the cornea. The endothelium is rich in mitochondria and cellular organelles reflecting the high metabolic needs of these pumps and transporters. The hexagonal shape of the corneal endothelium provides a means for ensuring that the entire cornea is covered without leaving gaps. There are unique contacts between adjacent cells that inhibit cellular proliferation. In addition, when cells are lost, remaining cells migrate to fill the gaps, resulting in cells of varying size or polymegathism.  ANTERIOR CHAMBER DEPTH The anterior chamber (AC) is the aqueous humor-filled space inside the eye between the iris and the cornea's innermost surface, the endothelium. The depth of the anterior chamber of the eye varies between 1.5 and 4.0 mm, averaging 3.0 mm. It tends to become shallower at older age and in eyes with hypermetropia (far sightedness). As depth decreases below 2.5 mm, the risk for angle closure glaucoma increases. crucial for the production and circulation of aqueous humor, a clear fluid that provides nutrients and oxygen to the avascular structures in the anterior segment of the eye, such as the cornea and lens. The depth of the anterior chamber contributes to maintaining the intraocular pressure within the eye. Proper balance of aqueous humor production and outflow, which is influenced by the anterior chamber depth, helps to regulate the pressure inside the eye.  IRIS The iris is the colored part of the eye that surrounds the pupil. It is a circular, thin structure that constricts or dilates to control the amount of light entering the eye. The iris can adjust the size of the pupil to control how much light reaches the retina, depending on the lighting conditions. The iris helps protect the delicate structures inside the eye by regulating the amount of light and harmful UV radiation that enters the eye.  PUPIL Is the opening at the center of the iris. The iris adjusts the size of the pupil and controls the amount of light that can enter an eye In bright conditions , the pupil constricts to reduce the amount of light entering the eye. In dark conditions the pupil dilates to allow more light in improving visibility  LENS The lens is a clear part of the eye behind the iris that helps to focus light , or an image on the retina. The lens is flexible , elastic structure made up primarily of the proteins and water arranged in a specific way to allow light to pass through. With age the proteins in the lens can clump together and cause clouding of the lens , a condition known as cataract.  RETINA Is the thin layer of tissue located at the back of the eye. It consists of several layers including photoreceptor cells [rods and cones] , nerve cells and supporting cells It receives the light that the lens has focused , convert it into neural signals and these signals to the brain for visual recognition Macula is specialized area in the retina that is responsible sharp , central vision Fovea contains high concentration of cones and is crucial for detailed color vision  EXTRAOCULAR MUSCLES Are muscles that pass between the eyeball and the orbit , and controls the movement and stabilization of the eyeball in relation to the orbit.  Superior rectus  Inferior rectus  Superior oblique  Inferior oblique  Superior tarsal muscle  Inferior tarsal muscle EXTRAOCULAR MUSCLES  OCULAR MOVEMENTS Abductions-an upward /lateral movement of the eye Addiction –an inward /medial movement the eye Elevation –an upward movement of the eye Depression –an downward movement of the eye Intorsion –an inward (nosal) rotation of the eye Exortosion –An outward (temporal) rotation of the eye