The Eye and Vision.doc

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Anatomy and Physiology
Jacqueline Carnegie

The Eye and Vision -vision is the dominant sense -70% of all sensory receptors in the body are in the eyes -half of the cerebral cortex is involved in some aspect of visual processing -accessory structures protect the eye or aid its function: 1) Eyebrows -short, coarse hairs that overlie the supraorbital margins of the skull -help to shade the eyes from sunlight -prevent perspiration fro reaching the eyes -contraction of the orbicularis muscle depresses the eyebrow -contraction of the corrugator muscle moves the eyebrow medially 2) Eyelids -known as the palpebrae -protect the eyes anteriorly -eyelids are separated by the palpebral fissure and meet at the medial and lateral commisures -tarsal plates support it internally and anchor the orbicularis oculi and the levator palpebrae superioris muscles that run within the eyelid -the upper eyelid is more motile because the levator palpebrae superioris raises it to open the eye -eyelashes are richly innervated and triggers reflex blinking -tarsal glands are embedded in the tarsal plates and produce an oily secretion that lubricates the eyelid and the eye and prevents the eyelids from sticking together 3) Conjuctiva -a transparent mucous membrane that lines the eyelids (palpebral conjuctiva) and folds back over the anterior surface of the eyeball (bulbar conjuctiva) -the bulbar conjunctiva covers only the white of the eye and not the cornea -functions to produce a lubricating mucous that prevents the eyes form drying out -conjuctivitis is the inflammation of the conjucta and results in reddened, irritated eyes 4) Lacrimal Apparatus -consists of the lacrimal gland and the ducts that drain excess lacrimal secretions into the nasal cavity -the lacrimal gland lies in the orbit above the lateral end of the eye -it continuously releases a dilute solution called lacrimal secretion (tears) -blinking spreads the tears downward and across the eyeball to the medial commisure, where they enter the paired lacrimal canaliculi via the lacrimal puncta (the red dot in the corner of your eye) -the tears drain into the lacrimal sac and then into the nasolacrimal duct, which empties into the nasal cavity at the inferior nasal meatus -lacrimal fluid contains mucus, antibodies, and lysozyme which is an enzyme that destroys bacteria -it helps cleanse and protect the eye surface, as well as moistening and lubricating it Extrinsic Eye Muscles -six strap-like extrinsic eye muscles control the movement of the eyeball -allows the eyes to follow a moving object and help maintain the shape of the eyeball and hold it in orbit -the 4 rectus muscles originate from the common tendinous ring (annular ring) located at the back of the orbit and run straight to their insertion on the eyeball -movements are indicated by their names: superior, inferior, lateral, and medial rectus muscles -the 2 oblique muscles move the eye in the vertical plane -the superior oblique muscle depresses the eye and turns it laterally -the inferior oblique muscle elevates the eye and turns it laterally -extrinsic eye muscles are the most precisely and rapidly controlled skeletal muscles in the body -reflected by their high axon-to-muscle fibre ration Structure of the Eyeball -the eyeball is an irregular sphere with an anterior pole and a posterior pole -its wall is composed of a fibrous layer, a vascular layer, and inner layers -the internal cavity is filled with fluids known as humors that help to maintain its shape -anterior portion = aqueous humor -forms and drains continuously -supplies nutrients and oxygen to the lens and cornea and carries away metabolic wastes -posterior portion = vitreous humor -helps transmit light, support the posterior surface of the lens and hold the neural retina firmly against the pigmented layer, and contributes to intraocular pressure (helps counteract the pulling force of the extrinsic eye muslces) -the lens acts as the adjustable focusing apparatus of the eye and is supported vertically within the eyeball, dividing it into anterior and posterior segments Layers Forming the Wall of the Eyeball 1) Fibrous Layer -outermost coat of the eye -made up of the sclera (white of the eye) and the cornea (lets light enter the eye and is a major part of the light bending apparatus of the eye) 2) Vascular Layer -middle coat of the eyeball -known as the uvea -made up of the choroid, ciliary body, and the iris -the choroid is blood-vessel rich and supplies nutrition to all layers of the eyeball -contains brown pigment (produced by melanocytes) that helps absorb light, preventing it from scattering and reflecting within the eye -the ciliary body has ciliary muscles that control lens shape -the iris is the visible colored part of the eye and lies between the cornea and the lens -it has a round central opening known as the pupil which allows light to enter the eye -the iris is made up of 2 smooth muscle layers that act as a reflexively activated diaphragm to vary pupil size -in close vision and bright light, the sphincter pupillae (circular muscles) contract and the pupil constricts -in distant vision and dim light, the dilator pupillae (radial muscles) contract and the pupil dilates, allowing more light to enter the eye -pupillary dilation is controlled by sympathetic fibres -pupillary constriction is controlled by parasympathetic fibres -the iris contains only brown pigment; the amount of brown pigment determines eye color 3) Inner Layer (Retina) -two-layered -contains pigmented epithelial cells which makes up the outer layer -helps absorb light and prevent it from scattering -act as phagocytes to remove dead or damaged photoreceptor cells and store vitamin A needed by photoreceptor cells -contains the neural layer which makes up the inner layer -only the neural layer plays a direct role in vision -composed of photoreceptors, bipolar cells, and ganglion cells (from posterior to anterior) -signals are produced in response to light and spread from the photoreceptors to the bipolar cells and then to the innermost ganglion cells, where action potentials are generated -ganglion axon cells run along the inner surface of the retina and leave the eye as the optic nerve -the optic nerve exits at the optic disc which lacks photoreceptors and is known as the blind spot -the brain "fills in" the information in order to deal with the absence of information -the retina contains photoreceptors that transduce / convert light energy, other neurons involved in the processing of light stimuli, and glia -rod cells are dim-light and peripheral vision receptors -more sensitive to light than cones and best suited for night vision -more numerous than cones -used for peripheral vision -absorb all wavelengths of visible light -perceived input is in gray tones only -the sum of all visual input feeds into a single ganglion cell -produces fuzzy, indistinct images since the visual cortex does not know which rod (out of the large number that influence a ganglion cell) -cone cells operate in bright light and provide precise, color vision -found in the macula lutea -concentrated in the fovea centralis -need bright light for activation and have low sensitivity -each cone in the fovea has a straight-through pathway to a ganglion cell -this accounts for detailed, high-resolution views of very small areas of the visual field provided by cones -because rods are absent from the foveae and cones do not respond to low-intensity light, we see dimly lit objects best when we do not look directly at them, and we recognize them when they move The Lens -biconvex, transparent, flexible structure -can change shape to allow precise focusing of light on the retina -avascular -made up of the lens epithelium and the lens fibres -the lens epithelium is confined to the anterior lens surface -consists of cuboidal cells that differentiate into the lens fibres -lens fibres make up the bulk of the lens -contains transparent, precisely folded proteins called crystallins that form the body of the lens -the lens enlarges throughout life, becoming denser, more convex, and less elastic; this impairs its ability to focus light properly Physiology of Vision -electromagnetic radiation includes all energy waves, from long radio waves to very short gamma waves / X-rays -our eyes respond to the visible light portion of the spectrum -red waves are the longest waves and have the lowest energy -violet waves are the shortest and have the highest energy -objects have color because they absorb some wavelengths and reflect others -different cone receptor cells in the retina respond to different wavelengths of the visible spectrum -refraction involves bending of light rays as it meets the surface of a different medium at an oblique angle rather than a right angle Focusing Light on the Retina -light sequentially passes through the cornea, aqueous humor, the lens, the vitreous humor, and then passes through the entire thickness of the neural layer to excite the photoreceptors -light is refracted 3 times: 1) as it enters the cornea 2) entering the lens 3) leaving the lens -the lens curvature and elasticity can be actively changed to allow fine focusing of the image Focusing for Distant Vision -eyes are best adapted for distant vision -the far point of vision is the distance beyond which no change in lens shape (accommodation) is needed for focusing -for a normal (emmetropic) eye, the far point is 20 feet -during distant vision, ciliary muscles are completely relaxed and the lens is stretched flat -ciliary muscles are relaxed when sympathetic input to them increases and para
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