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Chapter 15

ANP1106 Chapter 15: Vision

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Anatomy and Physiology
Jackie Carnegie

ANP1106 C Dr. Jackie Carnegie & Dr. W Staines 1. Special Sense: Vision 03.21.2017- 03.23.2017 P. 549-569, 496-497, 499 Accessory Structures of the Eye Eyebrows: short coarse hairs that overlie the supraorbital margins; function to shade the eye from sunlight and preventing perspiration from reaching the eye  Orbicularis muscle – depresses the eyebrows; Corrugator muscles – move the eyebrows medially Palperbrae (eyelids): thin skin-covered folds supported internally by CT sheets called tarsal plates; functions to protect the eye anteriorly  Palpebral fissure separates eyelids and meet at medial and lateral angles of the eye  Levator palpebrae superioris gives the upper eyelid mobility, anchored by tarsal plates Conjunctiva: Transparent mucous membrane that lines the eyelids as the palpebral conjunctiva and covers eyeball as ocular conjunctiva (covers only white, not the cornea); functions to produce lubricating mucus that prevents the eye from drying out  Conjunctivitis is an inflammation of the conjunctiva resulting with red and irritated eyes; pink eye is a conjunctival infection caused by bacteria/viruses Lacrimal Apparatus: consists of lacrimal gland (secrete tears) and ducts that drain lacrimal secretions into nasal cavity  enter eye via superolateral excretory ducts (lacrimal canaliculi), exit the eye medially via lacrimal punctum, drain into lacrimal sac then into nasolacrimal duct which empties into nasal cavity  Tears contains mucus, antibodies, and lysozyme that destroy bacteria o clean and protect eye surface as it moistens and lubricates it Structure of Eyeball – A slightly irregular hollow sphere with anterior and posterior poles  The wall is composed of three tunics (layers) – fibrous, vascular (blood supply), and inner sensory  The internal cavity is filled with fluids called humors that help maintain its shape  The lens separates the internal cavity into anterior and posterior segments o aqueous anterior and vitrius posterior 1) Fibrous Layer – outermost coat of eyeball, composed of dense avascular CT; two regions: sclera: posterior portion making bulk of fibrous layer, glistening white and opaque (white of the eye)  tendon-like; protects & shapes eyeball & provide sturdy anchoring site for extrinsic eye muscles  connected to optic nerve and continuous with dura mater of brain Cornea: anterior sixth of fibrous layer, transparent, bulges anteriorly from its junction with the sclera  forms a window to allow light to enter (major light-bending apparatus of eye)  well supplied with nerve endings (most are pain receptors) o when cornea is touched – blinking and tearing reflexes occur  most exposed part of the eye (easily damaged) but can regenerate and repair really well  avascular and beyond reach of immune system – the only body tissue that can be transplanted from one person to another with little risk of rejection 2) Vascular Layer – middle coat of eye, pigmented layer with 3 regions: choroid: blood-vessel-rich, dark brown membrane in posterior aspect  blood vessels nourish the whole eye; brown pigment produced by melanocytes, helps absorb light preventing it from scattering and reflections within the eye  incomplete where optic nerve leaves the eye ciliary body: choroid become ciliary body anteriorly. thick ring of tissue encircling the lens to hold it up  consist of smooth muscle bundles – ciliary muscles that control lens shape  posterior surface has radiating folds – ciliary processes that secrete fluid to fill cavity of anterior segment of eye  ciliary zonule (suspensory ligament) extends from ciliary processes to the lens iris: visible colored part of eye, most anterior portion of vascular layer, btwn cornea and lens and continuous with ciliary body posteriorly  pupil is the round central opening that allows light to enter  made of two smooth muscle that allow it to act reflexively to vary pupil size  in close vision and bright light – sphincter pupialle contract and pupil constricts; parasympathetic  in distant vision and dim light – dilator papillae contract and pupil dilates allowing more light; sympathetic o also dilate when something is appealing, response to fear or during problem solving 3) Inner Layer: The Retina – contains millions of photoreceptors that transducer (convert) light energy, neurons involved in light processing, and glia; Two layers: i. pigmented layer: outer layer, single-cell-thick lining, abuts choroid, and extends anteriorly to cover ciliary body and posterior face of the iris; pigment cells absorb light and prevent it from scattering (like choroid)  also act as phagocytes in photoreceptor cell renewal & store vit A needed by photoreceptor cells ii. neural layer: transparent inner layer extending anteriorly to posterior margin of ciliary body – junction is ora serrata (smoother margin); composed of 3 types of cells – photoreceptors, bipolar cells, and ganglion cells  signals are produced from light and spread from photoreceptors to bipolar cells to ganglion cells where APs are generated  ganglion cells run along inner surface of retina and leave posterior aspect of eye as the thick optic nerve  Photoreceptors: Rods and Cones o Rods – sensitive to dim light (suited for night vision), detect black and white, and are used for peripheral vision; more numerous than cones and more sensitive  absorb all wavelengths of visible light, perceived input in gray tones only  Sum of visual input from many rods feeds into a single ganglion cell, results in fuzzy and indistinct images o cones – respond to bright light, have high-acuity (resolution) colour vision; they’re found in macula lutea and are concentrated in the fovea centralis  each cone synapses with a single ganglion cell, bision is detailed and has high resolution optic disc: where optic nerve leaves the eye, blind spot b/c it lacks photoreceptors Macula lutea: ovular region lateral to blind spot on posterior pole with a pit in its center – fove centralis; allow light to pass directly to photoreceptors instead of through retinal layers – greatly enhance visual acuity  fovea contains only cones; macula lutea contains mostly cones; cone density declines gradually from edge of macula twd retina periphery where its mostly rods  sufficient cone density in fovea allow for detailed colour vision Lens – biconvex, transparent, flexible, avascular structure that changes shape to precisely focus light onto retina; composed of epithelium and lens fibers  Lens epithelium: anterior cells that differentiate into lens fibers  Lens fibers: cells filled with transparent protein crystalin that form the body of the lens o With age, lens gets more compact and dense, and loses its elasticity – gradually impair its ability to focus light properly Extrinsic Eye Muscles – 6 straplike extrinsic eye muscles enable the eye to following moving objects and maintain the shape of the eyeball  4 rectus muscles originate from the annular ring – Lateral rectus, inferior rectus, superior rectus, medial rectus  2 oblique muscles move eye vertically – inferior and superior oblique muscle Light and Optics electromagnietic radiation: is energy waves from short gamma rays to long radio waves  Our eyes respond to the visible spectrum (Small portion of the spectrum) which travels in the form of waves and its wavelengths range from 400-700nm  Different cone receptor cells in the retina respond to different wavelengths of the visible spectrum Focusing Light on Retina – light passes into the eye and moves sequentially through cornea, aqueous humor, lens, vitreous humor, and then through entire neural layer of the retina to excite the photoreceptors abutting from the pigmented layer (inverted image)  Light is refracted 3 times – entering cornea, entering the lens, and leaving the lens  The lens curvature and shape allow for fine focusing of an image; lens changes its focus (with tiny muscles) as you move closer to the object Focusing for Distant Vision – light from a distance needs a little adjustment of lens shape for proper focusing  Far point of vision is the distance beyond which the lens doesn’t need to change space to focus o normal emmetropic eye, far point = 6m (20ft), focal point on retina  sphincter ciliary muscles are completely relaxed, lens is flat at lowest refractory power Focusing for close vision – light diverges as it approaches the eyes – focal point farther from lens; 3 processes occur simultaneously:  Accommodation of lens – ciliary muscles contract, pulling ciliary body anteriorly twd pupil to release tension in ciliary zonule; lens bulges providing shorter length needed to focus the image on the retina (lens changed shape to increase refractory power) o parasympathetic fibers of oculomotor nerves control contraction of ciliary muscles  Constriction of pupils – sphincter papillae muscles of iris reduce pupil size - the pupillary reflex (mediated by parasympathetic fibers of oculomotor nerves) prevent divergent light rays from entering the eye  Convergence of eyeballs – controlled by somatic motor nerves of the oculomotor nerves; medial rotation of the eyeballs (by medial rectus muscles) twd the object being viewed; cross-eyed Homeostatic Imbalances:  myopic eye: nearsighted, uncorrected focal point is in front of retina (eye ball too long), concave lens moves focal point further back  Hyperopic eye: farsighted, uncorrected focal point is behind retina (eyeball too short), correct with convex lens to move focal point fwd Pathway of Light  Light enters the eye and passes through the back wall of the orbit which is covered with photosensitive sensory organ called the retina  The true photosensitive layer of the retina is the photoreceptor cell layer and light has to pass through two neuronal but his has little effect on light
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