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Transcript
Basic Ocular Anatomy Aqueous humor Cornea! Canal of Schlemm Iris! Ciliary muscle Features of the eye Angle Lens Sclera Vitreous humor Choroid Pigment epithelium Retina Macula Optic nerve Fovea Horizontal cross section of the human eye. VA = visual axis AP = anterior pole PP = posterior pole Lam. = lamina crib. = cribrosa Med. = medial Three Ocular Layers A.! Outer layer •! •! Sclera Cornea B.! Middle layer (uvea) •! •! •! Iris Ciliary body Choroid C.! Inner layer •! Retina Sclera & Cornea Sclera •! Consists largely of collagen. •! Provides support and protection, and maintains shape of eye. Cornea •! Transparent anterior part of eye, the most powerful optical component of the eye. •! Lacks blood vessels, gets oxygen directly from the air and the aqueous humor. •! Very sensitive nerve endings, responds rapidly to injury. Uvea: Iris Uvea •! Highly vascularized, provides nutrition to various elements of the eye. Iris •! The colored part of the eye. •! Controls pupil size -> regulates the amount of light entering the eye. •! Influences sharpness of retinal image. Choroid Detail The Outer Retina is served via the choroidal vessels Uvea: Ciliary Body & Choroid Ciliary body •! Produces aqueous humor (function: nourishes the cornea and lens). •! Ciliary muscle plays a major role in accommodation (change in lens shape to focus at distance or near). Choroid •! Provides nourishment to the retina. Other Ocular Structures •! •! •! •! •! Crystalline lens Vitreous humor Canal of Schlemm Fovea Optic Nerve Crystalline Lens •! Provides ~1/3 of the power of the eye. •! Accommodation: changes its shape to focus at different distances. •! Loses its ability to change shape easily with age -> presbyopia. •! With age, lens becomes less transparent -> cataract. Vitreous Humor •! Consists primarily of collagen and hyaluronic acid and has a gel-like structure. •! Provides structural support to the eye and helps nourishes the retina. •! With age, may liquefy -> floaters. Canal of Schlemm •! Provides drainage for the aqueous humor. •! Located at the angle of the eye (where the iris inserts into the ciliary body). •! Production and drainage of aqueous humor help maintain the intraocular pressure (IOP). Horizontal section through the optic nerve and macula optic disc fovea choroid retina sclera Optic Nerve •! Axons of the ganglion cells (leaving the eye). •! Optic nerve head, or optic disc: the part of the fundus where the bundle of ganglion cells exits the eye. •! No photoreceptors in optic disc, therefore no perception of light -> physiological blind spot. Fundus Photograph vein optic disc artery fovea Retinal Blood Vessels Retinal vessels imaged with fluorescein angiography Note the absence of blood vessels in the fovea. fovea From Livingston,1995 Retina •! A sheet of neural tissue, ~0.2 to 0.4 mm thick. •! 5 classes of neurons: photoreceptors, bipolar cells, ganglion cells, horizontal cells and amacrine cells. Each of these classes has subtypes, as well. Anatomic Subdivisions of the Macula perifovea foveola parafovea Fovea centralis capillary free zone umbo From Gass,1997 Section through the center of the fovea Outer Plexiform Lay. photoreceptors Ganglion Cell Layer Inner Plexiform Lay. Inner Nuc. Layer Ext. Limit. Memb. Outer Nuc. Layer Distribution of Macular Pigment (monkey fovea) The dark regions in blue light that are absent in green light are areas of high macular pigment density -- most dense along the fibers of Henle. Fovea •! Specialized area of the retina that subserves most acute vision. •! Foveal pit: neural elements of inner retina piled up on the side of pit. •! Part of the macula (area centralis). •! Macula contains a pigment that screens out blue light, thus giving the macula a yellow tinge. pigment epith. outer segments inner segments outer nuclear lay. outer plexiform lay. Retinal Cross Section rods & cones outer limiting memb. photoreceptor terminals inner nuclear lay. inner plexiform lay. ganglion cell lay. nerve fiber lay. inner limiting memb. Vertical section through a human retina (1.25 mm from the fovea). From Dowling, 1987 Retinal Wiring Abbreviations Rb = rod bipolar AIIa = AII amacrine Am = amacrine cell M = midget gc = ganglion cell Pa = parasol gc on = ON off = OFF F = flat I = invaginating d = diffuse cb = cone bipolar Hz = horizontal cell From Livingston,1995 CHOROID! Retinal Wiring L I G H T! Photoreceptors •! 2 classes of photoreceptors: rods and cones. •! Rods: night-time vision, very sensitive at dim light level, no rods in the fovea. •! Cones: daytime vision, not as sensitive as rods but work well in bright light, most densely packed in the fovea. Three cone types provide trichromatic (color) vision. Simplified Retinal Wiring Diagram Photons are absorbed in photoreceptor Electrical potential changes in the membrane cause a release of neurotransmitter into “synapse” Bipolar cell communicates signals to Ganglion Cell Ganglion Cell generates an “Action Potential” that travels up the Optic Nerve to the brain.