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Transcript
Chapter 8 SPECIAL SENSES Special senses Touch (discussed in ch. 7) , taste, smell, sight and hearing 5th sense is equilibrium (receptors in the ear) Special sense receptors Can be large, complex sensory organs (eyes and ears) Can be localized clusters of receptors (taste buds and olfactory equilibrium) Anatomy of the eye adult eye is a sphere approximately 1 inch in diameter Only 1/6 of the anterior surface is seen The other 5/6 of the eye is enclosed and protected by fat and the bony orbit External and accessory structures Eyelids protect the eye anteriorly; eyelids meet at the medial and lateral canthus Eyelashes project from the border of each eyelid Meibomian glands (sebaceous glands) produce an oily secreti0n to lubricate the eye Ciliary glands (modified sweat glands) lie between the eyelashes Conjuctiva (membrane) lines the eyelids and covers part of the outer surface of the eyeball; secretes mucus for lubrication External and accessory structures Lacrimal apparatus consists of the lacrimal glands and ducts (drain lacrimal secretions into the nasal cavity) Lacrimal glands (above lateral side of each eye) release dilute salt solution (tears) onto the anterior surface of the eye Tears flush across the eye into lacrimal canals medially, then into the lacrimal sac, and then to the nasolacrimal duct to the nasal cavity External and accessory structures Lacrimal secretions have antibodies and lysozyme – an enzyme that kills bacteria Secretions cleanse and protect the eye surface as well as moistens and lubricates Lacrimal secretions increase, they spill over the eyelids and fill the nasal cavities External and accessory structures Extrinsic eye muscles are attached to the outer eye surface Produce gross eye movements and make it possible to “follow” an object Internal structures Eyeball is a hollow sphere Wall is composed of three tunics (layers) Interior is filled with fluid (humors) that maintain shape Lens is the focusing apparatus Tunics of the eyeball Sclera – outermost tunic Thick, white connective tissue Also called the fibrous tunic “white of the eye” Central anterior portion is modified to be clear (this is the cornea) so that light can enter Well supplied with nerves (mostly pain fibers) Most exposed part Vulnerable to damage Can be transplanted without rejection by the recipient’s immune system Tunics of the eyeball Middle tunic is the choroid Blood-rich and nutritive Contains dark pigment (which prevents light from scattering inside the eye) Anteriorly it’s modified into ciliary bodies that are attached to the lens and the iris Pigmented iris has a round opening, pupil, for light to pass Iris is made of circularly and radially arranged smooth muscle fibers (to control size of pupil) Tunics of the eyeball Retina is the innermost (sensory) tunic Contains millions of receptor cells (rods and cones) Rods / cones are distributed over the entire retina except where the optic nerve leaves the eyeball (optic disc or blind spot) Rods and cones are photoreceptors since they respond to light Electrical signals go from rods / cones to bipolar cells then to ganglion cells before entering the optic nerve to go to the optic cortex Result is vision Rods and cones Rods are most dense at the periphery of the retina Cones are most dense at the center of the retina Fovea centralis is lateral to each blind spot (this contains only cones) is the point of sharpest vision Cones Three varieties Each type is sensitive to a particular wavelength (one responds to blue light, one responds to green light, and red cones respond to red and green lights) If more than one type is stimulated, intermediate colors are “seen” If all are stimulated at once, white is “seen” Lens Focuses the light onto the retina A flexible, biconvex structure Held upright by a suspensory ligament attached to the ciliary body Lens Divides the eye into 2 parts Anterior (aqueous) segment is anterior to the lens Contains clear, watery aqueous humor Continuously secreted by the choroid Maintains pressure inside the eye Provides nutrients for lens and cornea (they lack blood supply) Reabsorbed into venous blood through scleral venous sinus Lens Posterior (vitreous) segment is posterior to the lens Maintains pressure inside the eye Prevents the eyeball from collapsing Vision Disorders Myopia (nearsightedness) Elongated eyeball shape Lens focuses on objects in front of the retina (instead of upon it) Distant objects are blurry Snellen chart is used to diagnose myopia Corrective lenses or laser surgery corrects vision Vision Disorders Hyperopia (farsightedness) Distance from lens to retina is shortened Eyeball is more flattened Light focuses behind the retina (instead of upon it) Objects in the distance are clear, close objects are blurry Corrective lenses “fix” vision Vision Disorders Presbyopia (age-related farsightedness) Onset occurs between ages 40-45 Lens becomes stiff and discolored Blurring of up-close objects Impedes ability to read Vision Disorders Astigmatism Irregular curvature of cornea or lens Blurred vision Corrective lenses can partially or completely correct it Vision Disorders Amblyopia (lazy eye) Appears during childhood One eye is more dominant; the other eye has poor vision Treatment includes covering the good and strengthening the muscles of the “lazy eye” Vision Disorders Diplopia (double vision) One eye is misaligned and produces 2 images Treatment includes a patch, corrective lenses, or surgery Vision Disorders Strabismus (crossed eyes) One eye drifts in different directions Malfunctioning extrinsic eye muscles Corrected with exercise, corrective lenses, or surgery Vision Disorders Night blindness Rods in retina do NOT function properly Usually associated with aging Cataracts Lens becomes hard and opaque Due to age Causes vision to become hazy Eventually causes blindness Treatment includes surgical removal of the “old” lens and transplanting in a new lens Glaucoma Drainage of aqueous humor is blocked Fluid puts pressure on the retina and optic nerve Causes pain and blindness Symptoms include “halos” around lights, headaches, blurred vision Treatment includes eyedrops to aid aqueous humor drainage or surgical enlargement of the drainage canal Macular degeneration Progressive loss of central vision Peripheral vision is unaffected Dry macular degeneration Thinning of retina Do not completely lose vision Wet macular degeneration Leakage of small blood vessels Can be corrected with meds / surgery Diabetic retinopathy Damage to the retina due to diabetes (high blood sugar) Swelling and leaking of blood vessels that supply the retina See red spots Corrected with surgery Light refraction Light passes from one substance to another substance with a different density, rays are bent or refracted Light rays are refracted as they encounter the cornea, aqueous humor, lens, and vitreous humor Refractive powers of the cornea and humors are constant Light refraction Refractive powers of the lens can change depending on its shape Greater lens convexity the greater the refraction Less lens convexity (more flat) less the refraction Pathway of light Human eye is “set” for distance vision Light from distant sources approaches the eye as parallel rays so NO change in lens shape is needed to focus light on the retina At close range, light from close objects scatters, so the lens must bulge (be more convex) for light to be focused on the retina Bulging of lens occurs when the ciliary bodies contract Ability to “focus” on close objects is called accommodation Pathway to the brain Axons from retina are bundled together and leave the posterior eye as the optic nerve Nerve fibers from each eye cross over each other at the optic chiasma Resulting fiber tracts are called optic tracts Contain fibers from the lateral side of the eye on the same side and the medial side of the opposite eye Synapse with neurons in the thalamus to form the optic radiation which runs to the occipital lobe Visual fields Each eye has a different view Visual fields overlap Humans have binocular vision “two-eyed” vision Gives depth perception or “3-D” vision Two slightly different views are fused into one image Eye reflexes Internal eye muscles controlled by the Autonomic nervous system Can alter lens curvature Controls pupil size Protects from bright light (photopupillary reflex) by constricting pupils so photoreceptors are not damaged Accommodation pupillary reflex constricts pupils to all0w more acute vision of close objects Eye reflexes External eye muscles also controlled by ANS Extrinsic muscles control eye movements and allow the “following” of objects Cause convergence (move eyes medially to view close objects) which is controlled by cranial nerves III, IV, and VI Reading Requires both sets of eye muscles Lens must bulge and pupils must constrict for focusing close Extrinsic muscles must converge the eyes and move them to follow printed lines So long periods of reading cause tiring of the eyes and may result in eyestrain The ear Sound vibrations move fluid to stimulate hearing receptors Movements of the head disturb fluids around the balance organs Mechanoreceptors allow us to hear a wide range of sounds but also keep the nervous system up-to-date on position and movement of the head Receptors for hearing and balance react independently of each other Anatomy of the ear Three parts Outer ear Hearing only Middle ear Hearing only Inner ear Hearing Balance Anatomy of the ear: Outer Pinna “ear” Collects and directs sound waves in most animals (lost in humans) External auditory canal Short tube carved into temporal bone 1 inch long x ¼ inch wide Lined with ceruminous glands that secrete cerumin (wax) Ends at the eardrum, which separates the outer ear from middle ear Anatomy of the ear: middle Tympanic cavity- air filled chamber in the temporal bone Flanked laterally by eardrum and medially by a bony wall with the oval window and the round window Auditory tube runs obliquely downward to link middle ear with throat Usually flattened Yawning or swallowing can open it to equalize pressure Equalizing pressure allows eardrum to vibrate freely In infants, auditory tube is more horizontal Anatomy of the ear: middle Contains 3 ossciles (bones) that transmit vibrations of the eardrum to fluids in the inner ear Bones are named for shape Hammer (malleus) Anvil (incus) Stirrup (stapes) Eardrum moves, hammer moves with it and transfers vibration to anvil Anvil passes the movement to the stirrup which presses on the oval window Oval window sets fluids of inner ear into motion which excites hearing receptors Anatomy of the ear: inner Maze of bony chambers: osseous labyrinth Deep in the temporal bone, behind the eye socket Filled with fluid (perilymph); suspends the membranous labyrinth which contains the endolymph Three subdivisions Cochlea Vestibule Semicircular canals Mechanism of hearing Hearing receptors (hair cells) are located inside the organ of Corti (inside the cochlea) Vibrations reaching the oval window activates the fluids in the inner ear Receptor cells on the basilar membrane in the organ of Corti are stimulated when the “hairs” are moved by the tectorial membrane that lies over them Hairs cells transmit impulses along the cochlear nerve (cranial nerve VIII) to the auditory cortex in the temporal lobe where the sound is interpreted Mechanisms of equilibrium This is a response to the movement of the head Vestibular apparatus (equilibrium receptors) are divided into 2 arms Static equilibrium Dynamic equilibrium Static equilibrium Vestibule contains sacs of receptors called maculae Each macula is a patch of receptor cells with “hairs” embedded in the otolithic membrane (a jelly-like material) containing otoliths (small pieces of calcium salts) Head movements cause otoliths to “roll” in response to gravity, this pulls the gel, slides a plate over the “hairs” to bend them which sends impulses down vestibular nerve to the cerebellum which receives info about the position of the head This helps keep the head erect and indicates “up and down” Dynamic equilibrium Receptors found in semicircular canals Respond to angular or rotatory movements of the head Each semicircular canal contains a receptor region: crista ampullaris (hair cells covered with a gel cap or cupula) When the head moves in angular direction, endolymph in the canal lags and moves in the opposite direction, pushing cupula opposite the body’s movement Hair cells are stimulated and send impulses up the vestibular nerve to the cerebellum Dynamic equilibrium When angular motion stops, endolymph flows in opposite direction and reverses cupula’s movement; hair cells reduce the rate of firing Causes a reverse movement sensation Deafness Hearing loss or impairment Two kinds of deafness Conduction deafness Something interferes with the conduction of sound vibrations to the inner ear Caused by mechanical problems such as earwax, fusion of ossicles, ruptured eardrum otitis media Sensorineural deafness Degeneration or damage to receptor cells in organ of Corti, cochlear nerve, or neurons of auditory cortex Caused by listening to very loud sounds Due to problems with nervous system structures Equilibrium problems Cause nausea, dizziness, problems with balance Impulses from vestibular apparatus “disagree” with the visual input May have jerky or rolling eye movements Meniere’s syndrome is serious problem with inner ear May be caused by heart disease, degeneration of cranial nerve VIII, pressure on inner ear Results in progressive deafness and vertigo (sensation of spinning) Chemical senses Chemoreceptors are used for taste and olfaction Respond to chemicals in solution Olfactory receptors respond to a wider range of chemicals Olfaction and taste compliment each other Olfactory receptors Thousands located in a very tiny area in the roof of each nasal cavity Olfactory receptor cells are neurons having olfactory hairs (cilia) that protrude from the epithelium and are bathed by mucus When stimulated, receptors send impulses along the olfactory nerve to the olfactory cortex of the brain Olfactory receptors Since olfaction is closely related to the limbic system (emotions), “smells” are long-lasting and are part of our emotions and memories They adapt quickly when exposed to an unchanging stimulus (ex: woman will stop smelling her own perfume minutes after it is applied) Olfactory disorders Anosmias result from head injuries, aftereffects of nasal cavity inflammation, or aging Most cases caused by zinc deficiency; can be corrected with supplements Taste buds Specific receptors for sense of taste Widely scattered in the oral cavity 10,000 taste buds, most of which are located on the tongue (some on the soft palate, some on inner surface of cheeks) Tongue Dorsal surface covered with papillae of 3 types Filiform Fungiform Circumvallate Taste buds located on the sides of the fungiform and circumvallate papillae Gustatory cells respond to chemicals dissolved in saliva (epithelial cells surrounded by supporting cells in the taste bud) Tongue Gustatory hairs protrude through the taste pore; when stimulated, impulses are transmitted to the brain Cranial nerves VII, IX, and X carry taste impulses to the gustatory cortex Facial nerve VII serves the anterior tongue Glossopharyngeal and vagus nerves serve the other taste bud areas Taste sensations 4 basic taste sensations that correspond to a specific type of taste bud Sweet receptors – respond to sugar, saccharine, some amino acids Sour receptors – respond to hydrogen ions or acidity Bitter receptors – respond to alkaloids Salty receptors – respond to metal ions in solution Taste sensations Tip of tongue – sensitive to salty and sweet Sides of tongue – sensitive to sour Back of tongue – sensitive to bitter Taste sensations Homeostatic values to tastes “sweets” satisfy a need for carbohydrates and minerals (and some amino acids) Sour (citrus) have high levels of vitamin C Dislike for bitter is protective – many natural poisons or spoiled foods are bitter to the taste Taste depends heavily on olfaction; if sense of smell is inhibited, taste will be altered Development Eyes are formed by the 4th week of embryo development All senses are functional at birth Eyes are not fully functional at birth Eyeballs continue to enlarge until age 8-9 Lens grows throughout life Infants have foreshortened eyeballs are are hyperopic Infants only see gray tones and eye movements are uncoordinated Tear ducts are inoperable until 2 weeks of age Development 5 months of age Infant can focus on close objects and follow movement Visual acuity is poor 5 years of age Color vision is developed Visual acuity is improved to 20/30 School age to middle age Hyperopia is replaced with emmetropia until age 40 Presbyopia sets in with middle age resulting from decreased lens elasticity; decreases close vision Lacrimal glands are less active Development After middle age Lens loses clarity Dilator muscles of iris are less efficient keeping pupils constricted Decreased light reaching retina; visual acuity is decreased by age 70 Elderly are susceptible to glaucoma and cataracts Diabetes and heart disease can also lead to poor vision and blindness Development Newborns Can hear after first cry Responses to sound are strictly reflexive Can turn toward noises by 3-4 months of age Critical listening begins with toddlers as they start to vocalize Development Infants - adults Few problems affect hearing except otitis (ear infections) Elderly adults Deterioration of the organ of Corti Cannot hear high-pitched sounds (presbycusis); sensorineural deafness Sometimes the ossicles fuse which inhibits sound conduction Hearing aides help to alleviate the problems