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Unformatted text preview: Vision Accessory Structures of the Eye eyebrow pupil iris eyelash uppereyelid (palpebra) medial commissure lateral commissure palpebral fissure next slide lacrimal caruncle lower eyelid (palpebra conjunctiva (over sclera) eyelids (palpebrae) shade eyes, protect, spread lubricating secretions over the eyeball tarsal plate thick fold of c.t. that gives form and support to the eye lids palpebral fissure space between the tarsal (Meibomian) gland row of upper and lower eye lid elongated sabaceouis glands, embedded in tarsal plate that secrets a fluid that helps keep the eyelids form adhering to each other (chalazion – inflammation of) lateral and medial commissure angles of the palpebral fissure hordeolum (sty) infection of pilosabaceous units or meibomian glands lacrimal caruncle contains oil and conjunctiva thin protective mucous membrane composed of strat, sq. epi with goblet cells, palbebral and bulbar conjuctiva. blood shot eyes dilation and congest of bulbar conjuctival vessels sweat glands, source of whitish material at the medial commisure photo 1 photo 2 Lacrimal Apparatus • group of structures that produce and drains lacrimal fluid or tears – lacrimal gland secrete lacrimal fluid into 612 excretory lacrimal ducts that empty tears onto the surface of the conjunctiva of the upper lid. – tears pass medially over the anterior surface of the eyeball to enter two openings called the lacrimal puncta, then to the lacrimal canals, which lead into the lacrimal sac and then into the nasolacrimal duct. Lacrimal Apparatus lacrimal puncta Anatomy of the Eyeball • about 1” diameter and only 1/6 exposed • Consist of three layers – fibrous tunic the superficial coat of the eyeball – vascular tunic the middle layer – retina the inner layer Fibrous Tunic • the superficial coat (outer layer) of the eyeball • is avascular and consist of – anterior cornea – posterior sclera Fibrous Tunic: Cornea Fibrous Tunic: Cornea • Because of its curvature, most of the bending of the light rays (refraction) occurs at the cornea. • Outer surface consist of nonkeratinized st. sq. epi. • Middle coat consist of collagen fibers and fibroblast • Inner surface is simple squamous epithelium Fibrous Tunic: Sclera • the white of the eye • a layer of dense c.t., mostly collagen fibers and fibroblast • covers the entire eyeball except the cornea • gives the eyeball shape, makes it more rigid, and protects its inner parts • scleral venous sinus (canal of Schlemm) an opening located at the junction of the sclera and cornea that drains fluid (aqueous humor). Vascular Tunic (Uvea) • Inner layer of the eyeball • Has three layers – choroid – ciliary body – iris Vascular Tunic: Choriod • highly vascularized • is the posterior portion of the vascular tunic that lines most of the internal surface of the sclera Vascular Tunic: Ciliary Body • choroid becomes the ciliary body toward the anterior portion of the vascular tunic • extends from the ora serrata (the jagged anterior margin of the retina) to the point just posterior to the junction of the sclera and cornea Vascular Tunic: Ciliary Body • consist of the – ciliary processes protrusions or folds on the internal surface of the ciliary body • contain blood vessels and secrete aqueous humor – zonular fibers (suspensory ligaments extends from the ciliary processes to the to the lens – ciliary muscle circular band of smooth muscle that contracts and relaxes the alter the shape of the lens to adapt it for near or far vision Iris Vascular Tunic: Iris • flat donut shaped colored portion of the eyeball • suspended between the cornea and the lens • consist of circular and radial smooth muscle fibers • functions to regulate the amount of eye entering the pupil (the hole in the center of the iris) Vascular Tunic: Iris • Bright light causes the stimulation of the parasympathetic neurons causing contraction of the circular fibers (sphincter pupillae) resulting in pupillary constriction • Dim light stimulates sympathetic neurons, causing contraction of the radial muscles (dilator pupillae), increasing pupil size (dilation) Retina • third inner coat of the eyeball • lines the posterior ¾ of the eyeball • the only surface in the body where blood vessels can be viewed directly Retina • Optic disc the site where the optic nerve and retinal artery and vein enter and exit the eye – central retinal artery branch of the ophthalmic artery • branches of the central retinal artery fan out to nourish the anterior surface of the retina Retina: Optic Disc Retina Consist of pigmented layer and neural layer Retina: Pigmented Layer • a sheet of melanin containing epithelial cells located between the choriod and neural part • Melanin in the choriod and in the pigmented layer absorbs stray light rays, which prevents reflection and scattering of light within the eyeball, allowing for sharp images Retina: Neural Layer • three layers of retinal neurons – photoreceptor layer – bipolar cell layer – ganglion cell layer • layers are separated by the outer (plexiform) and inner (plexiform) synaptic zones Retina: Photoreceptors • two types – Rods aprox. 120 million, low light threshold, good for dim light, no color, shades of gray – Cones aprox. 6 million, higher light threshold, color vision, difficult seeing in dim light, most vision experience mediated by cones, the loss causes visual legal blindness Retina • macula lutea exact center of posterior retina (visual axis of the eye) • central fovea small depression in the center of the macula lutea that contains only cones Lens • behind the pupil and iris • composed of a proteins arranged in onion skin rows called crystallins • focuses light on the retina Lens • divides the internal eye in to – anterior cavity contains watery fluid called aqueous humor that nourishes the lens and cornea – Posterior cavity Lens: Funtion Focuses the light on the retina by changing its shape. Anterior Cavity • divided into two chambers: anterior chamber and posterior chamber • contains watery fluid called aqueous humor that nourishes the lens and cornea • circulates between the ciliary body capillaries and scleral venous sinus (canal of Schlemm) • intraocular pressure provided by aqueous humor and vitreous body – maintains the shape of the eyeball and prevents the eyeball from collapsing. Posterior Cavity • posterior cavity contains vitreous body, a jellylike substance that contributes to intraocular pressure that hold the retina flush against the choroid so that the retina has an even surface for the reception of clear images • unlike the aqueous humor, it does not undergo constant replacement • contains phagocytic cells that remove debris • hyaloid canal a narrow channel that was occupied by the hyaloid artery in the fetus Image Formation • Involves – the refraction or bending of light by the lens and cornea. • 75% by cornea • 25% by the lens – accomodation the change in the shape of the lens thereby varying refraction . – constriction or narrowing of the pupil by the constriction and dilation of the iris according to the amount of light available. The light from objects closer have to be bent more that the light from objects farther away. The Normal Eye Emmetropia Myopia (nearsightedness) If you are shortsighted, the focusing power of your eyes is too strong. Objects at a distance appear blurred and close up vision is clear. Hyperopia (farsightedness) If you are longsighted, the focusing power of your eyes are too weak. You will be able to view objects more comfortably at a distance than close up (less than 6 feet). Astigmatism Astigmatism is caused by an unevenly shaped eyeball Presbyopia (old age vision) The lens loses elasticity and thus its ability to accommodate. Therefore, older people cannot read print at the same close range as can youngsters Cataract Clouding of the normally clear lens of the eye. Convergence The medial movement of the eyes so that both are directed at the object as it moves closer to the face. Photoreceptors & Photopigments • Rods and cones are named for the different appearance of each’s outer segment. • photopigments are integral proteins in the plasma membrane of the outer segment • the outer segment are sloughed off and renewed very rapid pace • sloughed off tips are phagocytized by pigment epithelial cells Photoreceptors & Photopigments Steps in visual transduction • absorption of light by photopigment (a colored protein that undergoes structural changes when it absorbs light). •rhodopsin pigment in rods •cone photopigment ( photopsin or iodopsin) three types, one for each of the three types of cones •All photopigments contain two parts • opsin four kinds, one for each color and one for rhodopsin (absorbs blue to green) •cones that absorb longwavelength light (red) •cones that absorb middlewavelength light (green) •cones that absorb shortwavelength light (blue) •retinal a gycoprotein derivative of vitamin A •light absorbing part • large quantities of vitamin A stored in pigmented cells need for the regeneration process in rods Other Cells of the Retina • Bipolar cells – bipolar cells synapse with rods and cones and ganglion cells. – beginning of signal convergence grouping of input signals • Ganglion cells largest neurons of the retina – axons form the optic nerve – some cells absorb light and are involved with light intensity and the control of pupillary diameter • Horizontal and amacrine cell involved in enhancing perception of contrast, the edges of objects, and changes in light intensity Electrical response of photoreceptors to illumination. • In darkness, CNG (cyclic nucleotide gated) channels in the plasma membrane of photoreceptor outer segments are kept open by cGMP and the cell depolarizes (left). •Upon illumination, CNG channels close and the photoreceptor hyperpolarizes (right). • light causes a change in the retinal molecule which disassociates from opsin (bleaching) resulting in activation of opsin. Deactivation of cGMP Closes Na+ Channels Chalazion A chalazion is a generally painless, slowly enlarging nodule on the eyelid formed by inflammation of the meibomian glands. Hordeolum (Sty) A hordeolum (ie, stye) is a localized infection or inflammation of the eyelid margin involving hair follicles of the eyelashes (ie, external hordeolum) or meibomian glands (ie, internal hordeolum). ...
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- Spring '06