UNIT 3 - CHAPTER 12: NERVOUS SYSTEM III: SENSES. 1. Differentiate between general senses and special senses.

LEARNING OUTCOMES: 12.1 Intrductin UNIT 3 - CHAPTER 12: NERVOUS SYSTEM III: SENSES 1. Differentiate between general senses and special senses. 12.2 Receptrs, Sensatin, and Perceptin 2. Name the five types f receptrs and state the functin f each. 3. Explain hw receptrs stimulate sensry impulses. 4. Explain sensatin prductin and adaptatin. 12.3 General Senses 5. Describe the differences amng receptrs assciated with the senses f tuch, pressure, temperature, and pain. 6. Describe hw the sensatin f pain is prduced. 7. Explain the imprtance f stretch receptrs in muscles and tendns. 12.4 Special Senses 8. Explain the relatinship between the senses f smell and taste. 9. Describe hw the sensatins f smell and taste are prduced and interpreted. 10. Name the parts f the ear and explain the functin f each part. 11. Distinguish between static and dynamic equilibrium. 12. Describe the rles f the accessry rgans t the eye. 13. Name the parts f the eye and explain the functin f each part. 14. Explain hw the eye refracts light. 15. Explain hw the brain perceives depth and distance. 16. Describe the visual nerve pathways. 12-1

UNIT 3 - CHAPTER 12: NERVOUS SYSTEM III: SENSES LEARNING OUTCOMES: 12.5 Life-Span Changes 17. Describe aging-assciated changes that diminish the senses. 12-2

12.1 INTRODUCTION UNIT 3 - CHAPTER 12: NERVOUS SYSTEM III: SENSES A. Sensry receptrs are sensitive t envirnmental changes and initiate impulses t the brain and spinal crd. B. Senses are divided int tw grups, general (smatic) senses and special senses. 1. Receptrs assciated with skin, muscles, jints, and viscera prvide general (smatic) senses. 2. Special senses are senses whse sensry receptrs are lcated in large, cmplex rgans in the head. a. The five special senses are visin, hearing, equilibrium, taste, and smell. 12.2 RECEPTORS, SENSATION, AND PERCEPTION A. Receptr Types = specialized structures at the end f peripheral nerves that respnd t stimuli; can be classified accrding t their lcatin in the bdy, stimulus type, and structure. 1. Classificatin by Stimulus Type: a. Chemreceptrs respnd t changes in chemical cncentratins. b. Nciceptrs respnd t extreme (harmful) stimuli by prducing the sensatin f pain (i.e. all types under extreme stimuli). c. Thermreceptrs are sensitive t temperature change. d. Mechanreceptrs respnd t changes in pressure (i.e. tuch, pressure, vibratins, stretch). e. Phtreceptrs (in retina f eye) respnd t light energy. B. Sensry Impulses All senses wrk in basically the same fashin. Sensry receptrs cllect infrmatin frm the envirnment, and if stimulated t threshld, stimulate sensry neurns (PNS) t send a message t the brain (CNS). There the cerebral crtex frms a perceptin, a persn's particular view f the stimulus. C. Sensatins and Perceptin 1. Sensatin = the cnscius r uncnscius awareness f external r internal stimuli. 2. Perceptin = the cnscius awareness and interpretatin f sensatins. a. Prjectin. At the same time a sensatin frms, the cerebral crtex interprets it t seem t cme frm the receptrs being stimulated. The brain prjects a sensatin back t its apparent surce. Prjectin allws a persn t pinpint the regin f stimulatin. 3. See Table 12.1, page 445 t distinguish between sensatin and perceptin. D. Sensry Adaptatin 1. The prcess by which a sensry receptr becmes less stimulated fllwing cntinuus stimuli. 2. All sensry receptrs, except nciceptrs, adapt t cntinuus stimuli (i.e. underg sensry adaptatin). 12-3

12.3 GENERAL SENSES UNIT 3 - CHAPTER 12: NERVOUS SYSTEM III: SENSES A. Intrductin 1. Sensry receptrs are sensitive t envirnmental changes and initiate impulses t the brain and spinal crd. 2. Senses are divided int tw grups, general (smatic) senses and special senses. a. In this sectin we will study receptrs assciated with skin, muscles, jints, and viscera which prvide general (smatic) senses. b. In the next sectin, we will study special senses, which are senses whse sensry receptrs are lcated in large, cmplex rgans in the head. The five special senses are visin, hearing, equilibrium, taste, and smell. B. Three grups f General Senses: 1. Exterceptive Senses: a. detect changes at the bdy's surface tuch pressure temperature 2. Viscerceptive Senses: a. detect changes in viscera nly pain will be discussed here 3. Prpriceptive Senses: a. detect changes in muscles, tendns, and bdy psitin C. Tuch and Pressure Senses: See Fig 12.1, page 447. 1. Emply three types f receptrs: a. Free nerve endings (naked dendritic) in epithelium, CT b. Tactile (Meissner's) Crpuscles are encapsulated dendritic endings. surrunded by CT wrapping mechanreceptrs detect light tuch abundant in the hairless prtins f skin (i.e. lips, fingertips, palms, sles, nipples, external genitalia) c. Lamellated (Pacinian) Crpuscles are als encapsulated dendritic endings. surrunded by CT wrapping mechanreceptrs detect heavy pressure abundant in deep subcutaneus tissues f hands, feet, penis, clitris, urethra, breasts 12-4

12.3 GENERAL SENSES UNIT 3 - CHAPTER 12: NERVOUS SYSTEM III: SENSES D. Temperature Senses 1. Tw types that respnd t temperature change a. Warm receptrs sensitive t temps abve 25 C (77 F) unrespnsive at temps abve 45 C (113 F). * Pain receptrs are als triggered as this temperature appraches, prducing a burning sensatin. b. Cld receptrs sensitive t temps between 10 C (50 F) and 20 C (68 F) belw 10 C, pain receptrs are triggered prducing a freezing sensatin. 2. Bth underg rapid sensry adaptatin! E. Sense f Pain 1. Intrductin Free nerve endings are the receptrs that detect pain. They are widely distributed thrughut the skin and internal tissues, with the exceptin f the nervus tissue f the brain. 2. Pain Receptrs (Nciceptrs) a. functin t prtect against further tissue damage b. Many stimuli may trigger them (i.e. temperature, pressure, chemreceptrs). c. generally d nt adapt t cntinual stimuli 3. Visceral Pain: See Fig 12.2, page 448. a. Pain receptrs are the nly visceral receptrs that prduce sensatins. b. Stretch and/r chemreceptrs are stimulated by pressure and/r a change in chemical levels. c. may feel as if it s cming frm anther area f the bdy = referred pain. may derive frm cmmn nerve pathways. See Fig 12.3, page 449. * See bx n page 447 re: genetic differences and sensitivity t pain. * See bx n page 448 re: neurpathic pain. 12-5

UNIT 3 - CHAPTER 12: NERVOUS SYSTEM III: SENSES 12.3 GENERAL SENSES E. Sense f Pain (cntinued) 4. Pain Pathways: a. Acute pain fibers ccurs rapidly (0.1 sec) is nt felt in deep tissues sharp, fast, pricking pain cnducted n myelinated fibers ceases when stimulus is remved artificial OTC pain relief usually adequate b. Chrnic pain fibers begins slwly and increases in intensity ver a perid f several secnds r minutes dull, aching, burning, thrbbing pain can ccur anywhere cnducted n unmyelinated fibers may cntinue after stimulus is remved Natural pain relief (see belw) r narctics are needed. 5. Regulatin f Pain Impulses a. Inapprpriate pain = when pain sensatins are nt warning f impending tissue damage b. Analgesics are used t reduce inapprpriate pain. natural pain relief (prduced by CNS) Neurpeptides 1. enkephalins 2. sertnin 3. endrphins Inhibit pain nerve pathways in the spinal crd Stp pain signal frm reaching brain = n perceptin. * See Clinical Applicatin 12.1, page 451 re: Treating Pain. 12-6

UNIT 3 - CHAPTER 12: NERVOUS SYSTEM III: SENSES 12.3 GENERAL SENSES E. Sense f Pain (cntinued) 5. Regulatin f Pain Impulses b. Analgesics Artificial pain relievers OTC drugs blck frmatin f prstaglandins, which stimulate nciceptrs: 1. Aspirin (acetylsalicylic acid) 2. Tylenl (acetaminphen) 3. Mtrin (ibuprfen) Narctics mimic natural pain relief by blcking nerve impulses 1. mrphine 2. vicdin 3. demerl c. Surgery may be necessary: Crdtmy = severing the sensry nerve Rhiztmy = cutting f spinal drsal (sensry) nerve rts. * See bx n page 450 re: cannabinids that may relieve pain and a synthetic frm f marijuana, marinl used t treat nausea and vmiting in patients receiving cancer chemtherapy. F. Prpriceptin 1. Intrductin Stretch receptrs are prpriceptrs that send infrmatin t the spinal crd and brain cncerning the length and tensin f muscles. There are tw main types: a. Muscle Spindles: See Fig 12.4a, page 450. lcated in skeletal muscles near their junctin with tendns This sensry receptr is stimulated when the skeletal muscle relaxes and therefre the spindle is stretched. Actin prduced is called the "stretch reflex". helps maintain the desired psitin f a limb despite ther frces tending t mve it b. Glgi tendn rgans: See Fig 12.4b, page 450. fund in tendns clse t their muscle attachment Each is cnnected t a set f skeletal muscle fibers and is innervated by a sensry neurn. These receptrs have a high threshld and are stimulated by increased tensin. stimulate a reflex with an ppsite effect as abve help maintain psture and prevent tearing f tendns G. Visceral Senses include lamellated crpuscles and free nerve endings in internal rgans. These receptrs cnvey the sense f fullness after a meal, r discmfrt f intestinal gas, and the pain that signals a heart attack. * See Table 12.2, page 452, Receptrs Assciated with General Senses. 12-7

I. Intrductin A. SPECIAL SENSES are senses whse sensry receptrs are lcated in large, cmplex rgans in the head. B. The five special senses are 1. visin 2. hearing 3. equilibrium 4. taste 5. smell II. Sense f Smell = Olfactin See Fig 12.5 and Fig 12.6, page 453. A. Olfactry Receptrs: 1. chemreceptrs that are lcated in the upper nasal cavity a. sensitive prtin is cilia-like dendrites n biplar neurns b. chemicals must be disslved in slutin t be detected c. underg rapid sensry adaptatin B. Olfactry Organs = lfactry epithelium in upper nasal cavity f nse (n the superir nasal cnchae). C. Olfactry Pathways: 1. Primary Neurn = Olfactry receptr cell. a. Axns pass thrugh cribrifrm plate f ethmid b. synapse in Olfactry bulb 2. Secndary Neurn in lfactry bulb (CN I) a. Axns reach t cerebral crtex b. d nt pass thrugh thalamus D. Olfactry Stimulatin 1. Thery is smell stimulates many receptrs. 2. Certain cmbinatins = specific smells. 3. Smetimes sniffing is needed t bring drant mlecules up t lfactry epithelium. 4. Olfactry receptrs are cntinually replaced by stem cells. * See bx n page 453 re: canine s excellent sense f smell. 12-8

III. Sense f Taste (Gustatin) A. Organ = taste buds n tngue. See Fig 12.7 and Fig 12.8, page 455. B. Taste Receptrs = 1. Chemreceptrs that are lcated in taste buds C. Taste Sensatins a. Sensitive prtin is a "taste hair" which prtrudes ut f a "taste pre", which is an pening in a "taste cell", which makes up the "taste bud". b. Chemicals must be disslved in saliva t be detected. c. underg rapid sensry adaptatin 1. Mst taste buds are psterir, near the base f the tngue. 2. The rest f the taste buds f the five primary tastes prvide sensatins based n lcatin: a. sweet = tip f tngue b. sur = lateral tngue c. salt = perimeter f tngue d. bitter = psterir tngue e. umami = thrughut 3. Taste varies frm persn t persn. D. Taste Pathways: 1. Three pathways: a. CN VII anterir 2/3 f tngue b. CN IX psterir 1/3 f tngue c. CN X walls f ral cavity and pharynx 2. Once chemreceptrs in these areas are stimulated, a gustatry impulse passes t the a. medulla, b. thalamus, and is directed t the c. gustatry crtex within parietal lbe. * See Clinical Applicatin 12.2, page 452 re: Mixed up Senses (Synesthesia). * See Clinical Applicatin 12.3, page 456 re: Smell and Taste Disrders. 12-9

IV. Sense f Hearing A. Intrductin: The rgan f hearing is the Spiral Organ (Organ f Crti), which is present in the cchlea f the inner ear. The sensry receptrs are called mechanreceptrs. Once these mechanreceptrs are stimulated t threshld, the impulse travels n the cchlear branch f the vestibulcchlear (CN VIII) nerve, which leads t the primary auditry crtex (tempral crtex) f the cerebrum. B. Ear Structure: See 12.9, page 457. 1. Outer (External) Ear: a. Auricle = uter ear (elastic cartilage). Functin = cllectin f sund waves. b. External acustic (auditry) meatus = ear canal. Functin = starts vibratins f sund waves and directs them tward tympanic membrane. 2. Middle Ear: Functin = t amplify and cncentrate sund waves. a. Tympanic membrane = eardrum. * Tympanic (attenuatin) Reflex = prtective mechanism fr hearing mechanreceptrs. Lud nises cause tw muscles assciated with the tympanic membrane t cntract. This decreases amplificatin effect f ssicles (see belw). b. Tympanic cavity = air-filled space behind eardrum; separates uter frm inner ear. c. Auditry ssicles = 3 tiny bnes in middle ear: See Fig 12.10, page 458. Malleus (hammer) is cnnected t tympanic membrane. Incus (anvil) cnnects malleus t stapes. Stapes (stirrup) cnnects incus t the * Oval windw = the entrance t inner ear. d. Auditry (Eustachian) Tube = passageway which cnnects middle ear t naspharynx (thrat). Functin = t equalize pressure n bth sides f the tympanic membrane, which is necessary fr prper hearing. * See bx n page 456 re: ear wax. * See bx n page 458 re: tympanic reflex. * See bx n page 459 re: titis media. 12-10

IV. Sense f Hearing B. Ear Structure (cntinued) 3. Inner (Internal) Ear: See Fig 12.11, page 460. a. The inner ear cnsists f a cmplex system f intercmmunicating chambers and tubes called a labyrinth. Actually, tw labyrinths cmpse the inner ear: Osseus labyrinth = bny canal in tempral bne. Membranus labyrinth = membrane within sseus labyrinth. b. Tw types f fluid fill the spaces in the labyrinths: Perilymph fills the space between the sseus and membranus labyrinth. Endlymph fills the membranus labyrinth. c. The inner ear labyrinth can further be divided int three regins (cchlea, vestibule and semi-circular canals), each with a specific functin: Cchlea = snail shaped prtin. Functin = sense f hearing. Semicircular canals = three rings in different planes. Functin = dynamic equilibrium. Vestibule = area between cchlea and semi-circular canals. Functin = static equilibrium. d. The sseus labyrinth f the cchlea can be divided int tw cmpartments: See Fig 12.12, page 461 and Fig 12.13, page 462. Scala vestibuli = upper cmpartment which extends frm val windw t apex. Scala tympani = lwer cmpartment which extends frm apex t rund windw. * Bth cmpartments are filled with perilymph. e. Between the tw bny cmpartments, we find the membranus labyrinth = cchlear duct. The cchlear duct is filled with endlymph. 12-11

IV. Sense f Hearing B. Ear Structure (cntinued) 3. Inner (Internal) Ear: See Fig 12.11, page 460. f. There are membranes that separate the cchlear duct frm the bny cmpartments: Vestibular membrane separates the cchlear duct frm the scala vestibuli. Basilar membrane separates the cchlear duct frm the scala tympani. g. The mechanreceptrs respnsible fr the sense f hearing are cntained in the Spiral Organ (Organ f Crti) = 16,000 hearing receptr cells lcated n the basilar membrane. See Fig 12.14 page 463. The receptr cells are called "hair cells". The hair cells are cvered by the tectrial membrane, which lies ver them like a rf. C. Auditry Pathways: See Table 12.4 and Fig 12.16, page 464. Pathway f sund waves frm utside t the Spiral Organ. 1. auricle 2. external acustic (auditry) meatus 3. tympanic membrane 4. malleus 5. incus 6. stapes 7. val windw 8. perilymph f scala vestibuli 9. endlymph f cchlear duct 10. hair cells in Spiral Organ (Organ f Crti). Once these mechanreceptrs are stimulated t threshld, a sensry impulse is triggered and then travels n the 11. cchlear branch f vestibulcchlear nerve (CN VIII) t the 12. thalamus fr directin t the 13. primary auditry crtex (tempral lbes) f cerebrum fr interpretatin. * See Clinical Applicatin 12.4, Getting a Cchlear Implant, page 459. * See Fig 12.15, page 463, Receptrs in regins f the cchlear duct sense different frequencies f vibratin, expressed in cycles per secnd (cps). * See Clinical Applicatin 12.5, pages 465 cncerning Hearing Lss. 12-12

V. Sense f Equilibrium A. Static Equilibrium functins t sense the psitin f the head and help us maintain psture while mtinless. 1. The vestibule f the inner ear cntains the tw membranus chambers respnsible fr static equilibrium. See Fig 12.17, page 465. a. The utricle cmmunicates with the semi-circular canals. b. The saccule cmmunicates with the cchlear duct. c. Each f these chambers cntains a macula = rgan f static equilibrium. Fig 12.18, page 466. The macula is cmpsed f "hair cells" (see Fig 12.19, page 466) that are in cntact with a jelly-like fluid cntaining calcium carbnate crystals (tlith). When the head is mved, the gelatin sags due t gravity and the hair cells bend. This triggers a sensry impulse, which travels n the vestibular branch f the VC nerve t the pns, which directs the impulse t the cerebellum fr interpretatin. B. Dynamic Equilibrium functins t prevent lss f balance during rapid head r bdy mvement. 1. The three semicircular canals cntain the rgan respnsible fr dynamic equilibrium. See Fig 12.17, page 465. a. Each semicircular canal ends in an enlargement called an ampulla. See Fig 12.17, page 465. b. Each ampulla huses a sensry rgan fr dynamic equilibrium called the crista ampullaris, which cntains a patch f "hair cells" in a mass f gelatin. Fig 12.20, page 467. c. When the head is mved, the gelatin stays put due t inertia, causing the hair cells t bend. This triggers a sensry impulse, which travels n the vestibular branch f the VC nerve t the pns, which directs the impulse t the cerebellum fr interpretatin. Fig 12.21, page 467. * See bx n page 468 cncerning mtin sickness. 12-13

VI. Sense f Sight: Visin A. Intrductin: The rgan f visin is the retina f the eye. The sensry receptrs are called phtreceptrs. When phtreceptrs are stimulated t threshld, impulses travel within the ptic nerve (CN II) t the visual (ccipital) crtex fr interpretatin. B. Visual Accessry Organs: 1. Eyelids = prtective shield fr the eyeball. See Fig 12.22, page 469. a. Cnjunctiva= inner lining f eyelid; red prtin arund eye. * See bx n page 468 cncerning a frm f cnjunctivitis, pink eye. 2. Lacrimal apparatus = tear secretin and distributin. See Fig 12.23, page 469. a. Lacrimal gland = tear secretin, lcated n upper lateral surface. Tears cntain an enzyme called lyszyme, which functins as an anti-bacterial agent. b. Naslacrimal duct = duct which carries tears int nasal cavity fr drainage. 3. Extrinsic muscles hld eyeball in rbital cavity and allw fr eye mvement. (Name the cranial nerves that innervate these muscles). See Fig 12.24, page 470. a. superir rectus muscle b. inferir rectus muscle c. lateral rectus muscle d. medial rectus muscle e. inferir blique muscle f. superir blique muscle * See Table 12.5, page 470, Muscles Assciated with the Eyelids and Eyes. * See bx n page 470 re: duble visin. 12-14

VI. Sense f Sight: Visin C. Structure f the Eye: See Fig 12.25, page 471. The eye is cmpsed f three distinct layers r tunics: 1. The Outer Tunic (fibrus) = prtectin. a. Crnea = transparent anterir prtin. Functin: fcuses (75%) incming light rays. See bx n page 471 n crnea transplant. b. Sclera = white psterir prtin, which is cntinuus with eyeball, except where the ptic nerve and bld vessels pierce thrugh it in the back f eye. Functins: 1. prtectin 2. attachment (f eye muscles) 2. The Middle Tunic (vascular; uveal layer)= nurishment, and mre... a. Chrid cat = membrane jined lsely t sclera cntaining many bld vessels t nurish the tissues f the eye. b. Ciliary bdy = anterir extensin frm chrid cat, which is cmpsed f 2 parts: See Fig 12.28, page 472. Ciliary muscles which cntrl the shape f the lens (i.e. Accmmdatin, see belw) Ciliary prcesses which are lcated n the periphery f the lens. 1. Suspensry ligaments extend frm the ciliary prcesses n the lens t the ciliary muscles (i.e. they cnnect abve structures), and functin t hld the lens in place. * Accmmdatin = the prcess by which the lens changes shape t fcus n clse bjects. Fig 12.29, page 473. 1. The lens is respnsible (with crnea) fr fcusing incming light rays. 2. If light rays are entering the eye frm a distant bject, the lens is flat. 3. When we fcus n a clse bject, the ciliary muscles cntract, relaxing the suspensry ligaments. Accrdingly, the lens thickens allwing us t fcus. 12-15

VI. Sense f Sight: Visin C. Structure f the Eye: See Fig 12.25, page 471. 2. The Middle Tunic (vascular) c. Iris = clred ring arund pupil. thin diaphragm muscle lies between crnea and lens The iris separates the anterir cavity f the eye int an anterir chamber and psterir chamber. See Fig 12.26, page 472. The entire anterir cavity is filled with aqueus humr, which helps nurish the anterir prtins f the eye, and maintains the shape f the anterir eye. See Fig 12.30, page 473. 3. The Inner Tunic (nervus, sensry) a. Retina = inner lining f the eyeball; site f phtreceptrs.. A picture f the retina can be taken with a camera attached t an phthalmscpe as seen in Fig 12.34, page 475. The ptic disc is the lcatin n the retina where nerve fibers leave the eye and jin with the ptic nerve. The central artery and vein als pass thrugh this disc. N phtreceptrs are present in the area f the ptic disc = blind spt. See Fig 12.34, page 475. The psterir cavity f the eye is ccupied by the lens, ciliary bdy, and the retina. The psterir cavity is filled with vitreus humr, which is a jelly-like fluid, which maintains the spherical shape f the eyeball, and literally keeps the retina attached. See Fig 12.25, page 471. * See Table 12.6, page 476, fr a summary f the Layers f the Eye. 12-16

VI. Sense f Sight: Visin D. Light Refractin: 1. Intrductin: Incming light rays are refracted (bent) nt the retina due t the cnvex surface f bth the crnea and the lens. See Fig 12.36, page 476 and Fig 12.37, page 478. 2. Pathway f Light Thrugh Eye: a. crnea b. aqueus humr c. lens E. Visual Receptrs d. vitreus humr e. phtreceptrs in retina. * Once the rds and/r cnes are stimulated t threshld, a sensry impulse is then carried int the brain (see full pathway belw). 1. There are tw types f visual receptrs (phtreceptrs) in the retina: See Fig 12.32, page 474 and Fig 12.38, page 479. a. Cnes = phtreceptrs fr clr visin and prduce sharp images. b. Rds = phtreceptrs fr night visin and prduce silhuettes f images. c. Macula lutea yellw spt f mstly cnes. d. Fvea centralis depressin f ALL cnes and prduces sharpest visin. * See bx n page 479 re: detached retina. 12-17

VI. Sense f Sight: Visin F. Visual Pigments: See Fig 12.38, page 479. 1. fund in membrane sacs in rds and cnes 2. sensitive t light energy 3. Rds rhdpsin: See Fig 12.39, page 480. a. cntains prtein (psin) attached t pigment (retinal) b. light causes retinal t change shape releasing it frm psin c. chain reactin f events results in clsing f Na+ channels d. resulting hyperplarizatin slws the tnic firing f AP s e. dark adapted all psin and retinal is tgether, therefre rds are VERY sensitive, visin pssible even in dark f. light adapted mst psin and retinal decmpses cnes take ver sharp, clr visin results 4. Cnes Idpsins a. similar t rhdpsin b. 3 types f pigment erythrlabe = red cnes chlrlabe = blue cnes cyanlabe = green cnes c. cmbinatin f stimulatin = different clrs * See intrductin n page 444, The Wrld Withut Clr. * See bx n page 478 re: Albinism. * See bx n page 480 re: t little vitamin A and night blindness. G. Sterescpic Visin: See Fig 12.40, page 481. 1. prduced because humans have bincular visin 2. Each eye prduces a slightly different image fr the brain t interpret. 3. Visual crtex puts tgether as 3-D image. H. Visual Pathways Once the rds and/r cnes are stimulated, a sensry impulse is carried n the: 1. ptic nerve (CN II) which crsses at the 2. ptic chiasma frming ptic tracts that carry the impulse t the 3. thalamus (lateral geniculate bdy) fr directin t the 4. primary visual crtex (ccipital lbe) fr interpretatin. * See Clinical Applicatin 12.6, page 477 re: Refractive Disrders. 12-18

12.5 LIFE-SPAN CHANGES Diminished senses are ften ne f the first nticeable signs f aging. By the 50 s, smell and taste diminishes due t ansmia, a lss f taste receptrs. A. Hearing lss can be attributed t decades f cumulative damage t: 1. Spiral Organ. a. By age 60, 25% f ppulatin has hearing lss. b. By ages 65-74, 33% f ppulatin has hearing lss. c. By age 85, 50% f ppulatin has hearing lss. In additin t damage t Spiral rgan, hearing lss can be due t: 2. degeneratin f pathways t brain 3. tinnitus B. Visual prblems include: 1. dry eyes a. itching, burning, and diminished visin 2. flaters and light flashes a. clumping f vitreus humr and vitreus humr pulling away frm retina, respectively 3. presbypia = inability t read small print a. lss f elasticity f lens (i.e. diminished accmmdatin) 4. glaucma a. rate f aqueus humr frmatin exceeds the rate f remval causing increased pressure 5. cataracts a. caused by accumulatin n lens b. lens becmes cludy and paque causing a yellw tint 6. retinal detachment 7. macular degeneratin. CHAPTER SUMMARY see pages 482-484. CHAPTER ASSESSMENTS see pages 485-486. INTEGRATIVE ASSESSMENTS/ CRITICAL THINKING - see page 486. 12-19