Week 7: Orbit and Eye Anatomy PDF

Week 7 Orbit and eye How the Eye Works Bony Orbit Walls: ● ● ● ● Superior - orbital plate of frontal, lesser wing of sphenoid Inferior - orbital surface of body maxilla, orbital surface of zygomatic bone, orbital process of palatine Lateral - orbital surfaces of zygomatic bone and greater wing...

Week 7 Orbit and eye How the Eye Works Bony Orbit Walls: ● ● ● ● Superior - orbital plate of frontal, lesser wing of sphenoid Inferior - orbital surface of body maxilla, orbital surface of zygomatic bone, orbital process of palatine Lateral - orbital surfaces of zygomatic bone and greater wing of sphenoid Medial - frontal process of maxilla, lacrimal bone, orbital plate of ethmoid, small part of body of sphenoid Bony Orbit Communications: ● ● Optic canal - opening between two roots of lesser wing that transmits optic nerve and ophthalmic artery Superior orbital fissure - opening between lesser and greater wings. It transmits the superior ophthalmic vein, oculomotor, trochlear, and abducens nerves and branches of ophthalmic nerve (frontal, lacrimal, and nasociliary) Bony Orbit Communications: ● ● Inferior orbital fissure - opening between greater wing and maxilla transmits infraorbital nerve and vessels, zygomatic nerve and veins, communicating inferior ophthalmic vein, and pterygoid venous plexus. Supraorbital notch (or foramen) located at junction of medial and middle thirds of superior orbital margin and transmits supraorbital nerve and vessels Bony Orbit Communications: ● ● ● Infraorbital foramen - located 7 8mm below inferior orbital margin. Is the opening of infraorbital canal onto face. Nasolacrimal canal - downward continuation of fossa for lacrimal sac that communicates with inferior nasal meatus. It houses the nasolacrimal duct. Anterior and posterior ethmoidal foramina - located along frontoethmoidal suture. Transmits anterior and posterior ethmoidal nerves and vessels. Eyeball Consists of 3 concentric layers (coats/tunics) 1. 2. 3. Fibrous layer Vascular layer Inner layer (retina) Eyeball Two parts of the fibrous layer: 1. 2. Sclera Cornea Sclera - the white of the eye, firm, smooth, opaque fibrous cup which forms posterior ⅚ of fibrous layer and is continuous in the front with the cornea at the corneoscleral junction. Cornea - transparent, anterior ⅙ of fibrous layer that represents a segment of a smaller sphere (compared to the sclera). It bulges from the sclera and has no blood vessels. It receives sensory nerve supply from ophthalmic nerve. Eyeball Three parts of the vascular layer: 1. 2. 3. Choroid Ciliary body Iris Choroid - thin, vascular membrane immediately deep to the sclera that consists of a dense capillary plexus and small arteries and veins held together by connective tissue with abundant melanocytes Ciliary Body ● ● Raised area in anterior part of vascular layer Wedge-shaped in cross-section, inwardly projecting angle of the wedge is directed toward the lens and is connected to it by fibers of the suspensory ligament of the lens (zonular fibers) Eyeball Ciliary body (cont.): ● ● ● Greatest bult is contributed by ciliary muscle. It is the muscle of accommodation. When shifting gaze from distant to near, ciliary muscle contracts and its contraction reduces tension of suspensory ligament of the lens. Under less tension, because of its natural elasticity, the lens increases its curvature and becomes thicker in the A-P orientation. This, in turn, increases refractive power of the lens for vision of close objects. The opposite happens when shifting gaze from a near point to a distant one. Ciliary processes - internal projections of ciliary body. Its covering of epithelium secretes aqueous humor, which fills the space between the cornea and lens. Eyeball Iris: ● ● Pigmented diaphragm with central aperture (pupil) Contains two smooth muscles that control pupillary size ○ ○ ● ● Sphincter (constrictor) pupillae muscle - formed by circular fibers surrounding margin of pupil, serves to decrease pupillary size Dilator pupillae muscle - formed by fibers that radiate from pupillary margin toward outer circumference of iris, serves to increase pupillary size Ciliary and sphincter pupillae muscles receive parasympathetic innervation via the oculomotor nerve Dilator pupillae muscle receives sympathetic innervation from superior cervical ganglion Eyeball Inner layer (retina): ● ● ● Optic part is the posterior portion of the retina, which covers the inner aspect of the choroid and contains photoreceptors (rods and cones). It ends anteriorly at a jagged border a short distance behind the ciliary body The nonvisual (ciliary and iridial) part is the thin, anterior part of the retina which lines the internal aspect of the ciliary body and posterior surface of the iris, but has no nervous elements Eyeball Inner layer (cont.): ● ● At the posterior pole of the eyeball, the retina has a yellowish oval area (macula lutea) with a central depression (fovea centralis). Fovea centralis is the part of the retina that provides maximal visual acuity and contains only cones. ○ ● Peripherally from the fovea capitis, there are less cones and more rods Optic disc - located medial to macula lutea and is the area where axons of ganglionic cells converge to form the optic disc ○ ○ Contains no photoreceptors (blind spot) Branches of central artery and vein of retina spread out from center of optic disc over inner surface of retina Optic Nerve ● ● ● ● Begins at optic disc (medial to posterior pole of eyeball) and is formed by axons of ganglion cells of retina Runs posterior and medial in orbit to leave orbit and enters middle cranial fossa via optic canal Surrounded by sheaths of pia mater, arachnoid, and dura mater, which fuse with sclera of eyeball Subarachnoid space with its CSF extends from middle cranial fossa around optic nerve as far as the eyeball. A rise in pressure of CSF within the cranial cavity (increased intracranial pressure) is transmitted to the eyeball. This causes swelling of the optic disc (papilledema). Eyeball Innervation: ● ● ● ● Trigeminal, ophthalmic division or CNⅤ V1 - sensory to upper ⅓ of face Optic or CNⅡ - special sense/sight Extraocular motor innervation - CNⅢ, Ⅳ, and Ⅵ Autonomic (parasympathetic) through CNⅢ Arteries of the Orbit Ophthalmic artery: ● Branch of internal carotid artery that enters orbit via the optic canal, inferolateral to the optic nerve. It passes over the optic nerve toward the medial wall of the orbit and then anteriorly, parallel to the medial wall to end by dividing into two terminal branches: the supratrochlear and dorsal nasal arteries. It gives off the following branches: ○ Central artery of retina - pierces optic nerve with meningeal sheaths and passes forward in center of nerve, accompanied by the central vein of the retina that enters the eyeball at the center of the optic disc. Its branches spread over the internal surface of the retina, which can be studied using an ophthalmoscope. Arteries of the Orbit Ophthalmic artery branches: ● ● ● ● ● ● ● Posterior ciliary arteries - supply eyeball Muscular branches - supply extraocular muscles that branch to rectus muscles and continue as anterior ciliary arteries, which supply the eyeball Lacrimal artery - usually the largest branch of ophthalmic artery that accompanies the lacrimal nerve and gives off branches that accompany the zygomaticofacial and zygomaticotemporal nerves Supraorbital artery - accompanies supraorbital nerve Anterior and posterior ethmoidal arteries - accompany anterior and posterior ethmoidal nerves Supratrochlear artery - accompanies supratrochlear nerve Dorsal nasal artery - accompanies infratrochlear nerve Ophthalmic Veins ● Superior ophthalmic vein ○ ○ ○ ● Communicates anteriorly with facial vein Usually joined by inferior ophthalmic vein Passes through superior orbital fissure to drain into cavernous sinus Inferior ophthalmic vein ○ ○ ○ Communicates anteriorly with facial vein Communicates with pterygoid venous plexus via veins that pass through inferior orbital fissure Usually joins superior ophthalmic vein, but may drain directly into cavernous sinus Lacrimal Apparatus ● Lacrimal gland - located in superolateral part of the orbit, just behind the superior orbital margin and is divided by the tendon of the levator palpebrae superioris into superior (orbital) and inferior (palpebral) parts. Its ducts are ~12 in number and open into the superior conjunctival fornix ○ ● ● Tears move across eyeball toward the lacrimal lake (triangular area at medial angle of eye) and are distributed in a uniform layer by blinking, and from the lacrimal lake tears are drained by the lacrimal canaliculi Lacrimal canaliculi (superior and inferior) - located in medial part of margins of eyelids. They begin in two small orifices (lacrimal pores/puncta) and end in the lacrimal sac Lacrimal sac - upper dilated part of nasolacrimal duct is the fossa for the lacrimal sac. In the anterior part of the medial wall of the orbit and inferiorly it becomes continuous with the nasolacrimal duct that opens into the inferior nasal meatus Eyeball The space between the cornea and lens is divided by the iris into two compartments 1. 2. Anterior chamber (between cornea and iris) Posterior chamber (between iris and lens) The anterior and posterior chambers communicate with each other via pupil and contain aqueous humor ● ● Aqueous humor is secreted by the epithelium of ciliary process and passes from posterior chamber to anterior chamber via the pupil. They drain at irido-corneal angle into scleral venous sinus (canal of Schlemm) Vitreous chamber - space located posterior to the lens filled with transparent semi-gelatinous material (vitreous humor/body) Clinical Considerations Glaucoma - condition characterized by increased intraocular pressure due to excessive amount of aqueous humor. This is usually caused by drainage obstruction or overproduction. If left untreated, it can cause progressive blindness. Glaucoma Explained Clinical Considerations Conjunctivitis - aka, pink eye. Inflammation or infection of the conjunctiva. It may be due to allergies or bacterial/viral infections. Extraocular Muscles 7 in number: 6 move the eyeball and the 7th elevates the upper eyelid 1. 2. 3. 4. 5. 6. 7. Superior rectus Inferior rectus Medial rectus Lateral rectus Superior oblique Inferior oblique Levator palpebrae superioris Extraocular Muscles Rectus muscles: ● ● ● ● Named according to their orientation with respect to the eyeball Originate from a common tendinous ring which surrounds the optic canal and inferior medial part of the superior orbital fissure Each muscle inserts into sclera, about 6mm behind corneoscleral junction. Nerve supply - abducens nerve supplies the lateral rectus, superior oblique supplies trochlear nerve, oculomotor nerve supplies remaining three rectus muscles (superior, inferior, and medial). Extraocular Muscles Anatomical Movements vs Cardinal Positions of Gaze Eyeball Movements ● ● ● ● ● ● Eye abduction - lateral rectus (abducens nerve) Eye adduction - medial rectus (oculomotor nerve) Elevation of adducted eye - superior rectus (oculomotor nerve) Depression of adducted eye - inferior rectus (oculomotor nerve) Elevation of abducted eye - inferior oblique (oculomotor nerve) Depression of abducted eye - superior oblique (trochlear nerve)