Eye and Visual Pathway PDF

0  Eye and Visual Pathway The eyeball, the peripheral organ of vision, is situated in the orbit. Coats of the Eyeball 1. Fibrous Coat It’s the outer layer and made up of the sclera (posteriorly) and the cornea (anteriorly). - Sclera Opaque and...

0  Eye and Visual Pathway The eyeball, the peripheral organ of vision, is situated in the orbit. Coats of the Eyeball 1. Fibrous Coat It’s the outer layer and made up of the sclera (posteriorly) and the cornea (anteriorly). - Sclera Opaque and white structure. Posteriorly, it is pierced by the optic nerve (C.N 2). o It’s fused with the dural sheath of this nerve. o The lamina cribrosa is the area of the sclera that is pierced by the nerve fibers of the optic nerve. ▪ It is also pierced by the ciliary arteries and nerves and their associated veins, the venae vorticosae. It is directly continuous in front with the cornea at the corneoscleral junction (limbus). - Cornea It is transparent, and It is the most important refractive medium of the eye. It is in contact posteriorly with the aqueous humor. The cornea is avascular and devoid of lymphatic drainage. o It is nourished by diffusion from the aqueous humor and from the capillaries at its edge. Its nerve Supply is by the long ciliary nerves from the ophthalmic division of the trigeminal nerve. 2. Vascular Pigmented Coat (Uvea) It’s the middle layer and consists from post. to ant. of the choroid, the ciliary body, and the iris. - Choroid It is composed of an outer pigmented layer and an inner, highly vascular layer. o Its blood vessels provide nutrients to the posterior surface of the retina. o The choroid also contains melanocytes that produce the pigment melanin. ▪ Melanin absorbs stray light rays, thus 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. 1  - Ciliary Body It is continuous posteriorly with the choroid, and anteriorly it lies behind the peripheral margin of the iris. It is composed of: (1) ciliary ring, (2) ciliary processes, and (3) ciliary muscle. o Ciliary ring is the posterior part of the body, and its surface has shallow grooves, the ciliary striae. o Ciliary processes are radially arranged folds (ridges). ▪ They contain blood capillaries that secrete aqueous humor. ▪ Extending from it: the zonular fibers (suspensory ligaments) that attach to the lens. o Ciliary muscle is a smooth muscle composed of meridional and circular fibers. ▪ Meridional fibers run backward from the region of the corneoscleral junction to the ciliary processes. ▪ Circular fibers are fewer in number and lie internal to the meridianal fibers. Nerve supply o Ciliary muscle is supplied by the parasympathetic fibers from the oculomotor nerve (CN 3). ▪ After synapsing in the ciliary ganglion, the postganglionic fibers pass forward to the eyeball in the short ciliary nerves. Action o Contraction of the ciliary muscle pulls the ciliary body forward. ▪ This relieves the tension in the suspensory ligament, and the lens becomes more convex (globular). ▪ This increases the refractive power of the lens during accommodation. - Iris (colored portion of the eyeball) The iris is a thin, contractile (smooth muscle), pigmented diaphragm. o It has a central aperture: the pupil. It is suspended in the aqueous humor between the cornea and the lens. Its periphery is attached to the anterior surface of the ciliary body. It divides the space between the cornea and the lens (anterior cavity) into: o Anterior chamber (between cornea and iris). o Posterior chamber (between iris and lens). Its muscle fibers are involuntary (smooth) and consist of: o Circular fibers form the sphincter pupillae ▪ Arranged around the margin of the pupil. o Radial fibers form the dilator pupillae ▪ Consist of a thin sheet of radial fibers that lie close to the posterior surface. 2  Nerve supply o Sphincter pupillae is supplied by parasympathetic fibers from the oculomotor nerve. ▪ After synapsing in the ciliary ganglion, the postganglionic fibers pass forward to the eyeball in the short ciliary nerves. o Dilator pupillae is supplied by sympathetic fibers ▪ It pass forward to the eyeball in the long ciliary nerves (branch from ophthalmic of trigeminal). Action o The sphincter pupillae constricts the pupil in the presence of bright light or by parasympathetic activity. ▪ It also constricts the pupil during accommodation. o The dilator pupillae dilates the pupil in the presence of dim light (low intensity) or by sympathetic activity. 3. Nervous Coat: The Retina It’s the inner layer and consists of an outer pigmented layer and an inner nervous layer. o Its outer surface is in contact with the choroid. o Its inner surface is in contact with the vitreous body. Its posterior three quarters (neural retina) is the receptor organ. o It contains rods and cones. o Its anterior edge forms a wavy ring, the ora serrata. ▪ Where the nervous layer (neural retina) end. o At the center of the posterior part of the retina is macula lutea. ▪ It’s an oval, yellowish area. ▪ The area of the retina for the most distinct vision. ▪ Macula lutea has a central depression, the fovea centralis.  Fovea centralis contains cones only.  It is the area of highest visual acuity (or resolution). o The optic nerve leaves the retina to the medial (nasal) side of the macula lutea by the optic disc. ▪ It is depressed at its center, where it is pierced by the central artery of the retina (and vein). ▪ In optic disc, rods and cones are completely absent.  it is insensitive to light → the blind spot. Its anterior quarter is nonreceptive. o It consists of pigment cells, with a deeper layer of columnar epithelium. ▪ It covers the posterior surfaces of ciliary processes and the back of the iris. Neural Layer of the retina The neural layer has three layers of retinal neurons. 1. Rods and cones o Specialized photoreceptor neurons in the retina. 2. Bipolar neurons o Connect the rods and cones to the ganglion cells. 3. Ganglion cells o The axons of the ganglion cells converge toward the optic disc and exit as the optic (II) nerve. NOTE: The myelin sheath of the optic nerve is produced by oligodendrocytes. ➔ On ophthalmoscopic examination, the optic disc is seen medial (nasal) to macula lutea which has fovea centralis. 3 Contents of the Eyeball (refractive media) - The eyeball has 3 supportive (refractive) media: 1. Aqueous Humor A clear fluid that fills the anterior and posterior chambers of the eyeball. o Secretions from the capillaries in the ciliary processes. o It’s drained away through the spaces at the iridocorneal angle into the canal of Schlemm. ▪ Canal of Schlemm is also called the scleral venous sinus. ▪ Obstruction to the draining of the aqueous humor results in a rise in intraocular pressure called glaucoma. Functions: a. To support the wall of the eyeball by exerting internal pressure and thus maintaining its optical shape. b. It also nourishes the cornea and the lens and removes the products of metabolism. ▪ These functions are important because the cornea and the lens do not possess a blood supply. c. It’s a refractive medium. 2. Vitreous Body It is a transparent gel. o It fills the eyeball behind the lens ▪ It’s in the posterior cavity (vitreous chamber). o It can’t be replaced like the aqueous humor. The hyaloid canal is a narrow channel that runs through the vitreous body from the optic disc to the posterior surface of the lens. o In the fetus, it is filled by the hyaloid artery, which disappears before birth. Functions: a) It contributes to the magnifying power of the eye (refractive medium). b) It supports the posterior surface of the lens. c) It assists in holding the neural part of the retina against the pigmented part of the retina. ▪ Prevent detachment of the retina. 3. Lens It is a transparent, biconvex structure enclosed in a capsule. It is situated behind the iris and in front of the vitreous body and is encircled by the ciliary processes. o Its circumference, is attached to the ciliary processes of the ciliary body by the suspensory ligament. It consists of: o An elastic capsule o A cuboidal epithelium (in the anterior surface of the lens) o Lens fibers Function: o It helps focus images on the retina. ▪ It can adapt for near or far vision (accommodation). 4  Orbit The orbit is a pyramidal cavity, and it contains several openings: Opening in the orbital cavity Structures inside Orbital opening 1/6 of the eyeball protrudes through the base Supraorbital notch (supraorbital foramen) Supraorbital nerve and blood vessels. Infraorbital groove and canal Infraorbital nerve (a continuation of the maxillary nerve) and blood vessels. Nasolacrimal canal Transmits the nasolacrimal duct and communicates with the inferior meatus of the nose Anterior & posterior ethmoidal foramina Anterior and posterior ethmoid nerves Zygomaticotemporal & zygomaticofacial foramina Zygomaticotemporal and zygomaticofacial nerves Inferior orbital fissure Maxillary nerve (V2) and its infraorbital and zygomatic branches, the inferior ophthalmic vein, and sympathetic nerves Superior orbital fissure Oculomotor (III), trochlear (IV), ophthalmic (V1), abducens (VI) nerves and the superior and inferior ophthalmic veins. Optic canal Optic nerve (II) and the ophthalmic artery. Blood and Lymph Vessels Rich network of ophthalmic blood vessels & No lymph vessels in the orbital cavity. Ophthalmic Artery Branch of the internal carotid artery Enters the orbit through the optic canal with the optic nerve Crosses the optic nerve to reach the medial wall of the orbit. Gives off numerous branches that accompany the nerves in the orbital cavity. Forms a notable facial anastomosis with external carotid artery branches: o The angular artery (the terminal end of the facial artery) o The frontal branch (branch of the superficial temporal artery). - Branches of ophthalmic artery: The central artery of the retina o Small branch that pierces the meningeal sheaths of the optic nerve to gain entrance to the nerve. o Runs in the substance of the optic nerve and enters the eyeball at the center of the optic disc. The muscular branches. o Supply the extraocular muscles. The ciliary arteries o Supply the eyeball o Divided into anterior and posterior groups. The lacrimal artery o Supply the lacrimal gland. The supratrochlear and supraorbital arteries o Supply the skin of the forehead. Ophthalmic Veins 1. The superior ophthalmic vein It forms from the union of the supraorbital and supratrochlear veins. It communicates with the facial vein at the medial angle of the eye 2. The inferior ophthalmic vein It communicates by the inf. orbital fissure with the pterygoid venous plexus ➔ Both ophthalmic veins pass posteriorly through the superior orbital fissure o Both drain into the cavernous sinus 5 Extrinsic Ocular (Extraocular) Muscles 6 skeletal muscles run from the bony walls of the orbit to insertions on the exterior of the eyeball. o The 4 rectus muscles: medial, lateral, superior, and inferior. ▪ Arise from a common tendinous ring surrounding the optic foramen. o The 2 oblique muscles: superior oblique and inferior oblique. All of them supplied by oculomotor (III) nerve, except: o Superior oblique by trochlear (IV) nerve [SO4]. o Lateral rectus by abducens (VI) nerve [LR6]. - action (change the position of the pupil): 1. Superior rectus ▪ Raises cornea upwards and medially. 2. Inferior rectus ▪ Depresses cornea downward and medially 3. Medial rectus ▪ Rotates eyeball so that cornea points medially (Adduction) 4. Lateral rectus ▪ Rotates eyeball so that cornea points laterally (Abduction). 5. Superior oblique ▪ Depresses cornea downward and rotates eyeball laterally. 6. Inferior oblique ▪ Raises cornea upward and rotates eyeball laterally. NOTE: To examine the function of superior and inferior recti muscles, the patient must first look lateral. Optic Nerve (C.N. II) Origin of the Optic Nerve Fibers of the optic nerve are the axons of the cells in the ganglionic layer of the retina. o That converge on the optic disc and exit from the eye, about 3 or 4 mm to the nasal side of its center, as the optic nerve. Fibers of the optic nerve are myelinated, but the sheaths are formed from oligodendrocytes. o Since the optic nerve is comparable to a tract within the central nervous system. The optic nerve leaves the orbital cavity through the optic canal. o And unites with the optic nerve of the opposite side to form the optic chiasma. 6 Optic Chiasma It is situated at the junction of the anterior wall and floor of the third ventricle. The fibers from the nasal (medial) half of each retina decussate and enter the optic tract of the opposite side. The fibers from the temporal (lateral) half of each retina pass posteriorly in the optic tract of the same side. Optic Tract It emerges from the optic chiasma and passes posterolaterally around the cerebral peduncle. Most of the fibers now terminate by synapsing with nerve cells in the lateral geniculate body of the thalamus. o 4 neurons conduct visual impulses to the visual cortex. ▪ Rods and cones → bipolar neurons → ganglionic cells → neurons in LGB (its axons pass to cerebral cortex). NOTE: A few of the fibers pass to the pretectal nucleus and the superior colliculus of the midbrain and are concerned with light reflexes. Optic Radiation (geniculocalcarine tract) Its fibers are the axons of the neuron cells of the lateral geniculate body. The tract passes posteriorly through the retrolenticular part of the internal capsule o It terminates in the primary visual cortex (striate cortex/ area 17). ▪ Which occupies the upper (cuneus gyrus) and lower (lingual gyrus) lips of the calcarine sulcus. ▪ On the medial surface of the cerebral hemisphere. o The visual association cortex (area 18 & 19) is responsible for recognition of objects and perception of color. Binocular Vision In binocular vision, right and left fields of vision are projected on portions of both retinae. Image of the visual field is inverted )‫ (مقلوبة ومعكوسة‬on the retina o The upper part of visual field is reflected on the lower part of the retina o The right side of visual field is reflected on the left side of the retinae. Image of an object in the right visual field is projected on: ▪ The nasal (medial) half of the right retina. ▪ The temporal (lateral) half of the left retina. o In optic chiasma, the axons from these two retinal halves are combined to form the left optic tract. ▪ Axons from the nasal half of the right retina decussates to the left side. o Left optic tract synapse with left lateral geniculate body neurons. ▪ It project its axons to the visual cortex of the left hemisphere (to complete the right field of vision). The right visual field project on the left visual cortex. The left visual field project on the right visual cortex. Upper field of vision project on Lower retinal quadrants → the lower wall (lingual gyrus) of the calcarine sulcus. lower field of vision project on Upper retinal quadrants → the upper wall (cuneus gyrus) of the calcarine sulcus. 7