Chapter 17: The Special Senses

Transcription

1 Chapter 17: The Special Senses The Big Idea: The Special Senses and Hmestasis Sensry rgans have special receptrs that allw us t smell, taste, see, hear, and maintain equilibrium r balance Infrmatin cnveyed frm these receptrs t the CNS is used t help maintain hmestasis A sensatin is the cnscius r subcnscius awareness f an internal r external stimulus Receptrs fr the special senses f smell, taste, visin, hearing, and equilibrium are anatmically distinct frm ne anther and are cncentrated in specific lcatins in the head There are specific afferent pathways and translatin sites in the brain fr infrmatin assembled frm these special senses 17.1: Olfactin: The Sense f Smell Smell and taste are chemical sensatins that arise frm the interactin f mlecules with smell r taste receptrs; t be detected, the mlecules must be disslved Gustatin and lfactin wrk tgether, but lfactin is much strnger/mre sensitive Because impulses fr smell and taste prpagate t the limbic system, certain drs and tastes can evke strng emtinal respnses f a fld f memries Anatmy f Olfactry Receptrs There are millin f these receptrs in the nse that respnd t drant mlecules It is estimated that humans can recgnize abut 10,000 different drs The lfactry epithelium is lcated in the superir part f the nasal cavity cvering the surface f the cribrifrm plate and extending alng the superir nasal cncha The lfactry epithelium cnsists f 3 kinds f cells The lfactry receptrs are the first- rder neurns f the lfactry pathway Each receptr is a biplar neurn with cilia (called lfactry hairs) that respnd t inhaled chemicals Chemicals that have an dr and can stimulate lfactry hairs are called drants Supprting cells are clumnar epithelial cells f the mucus membrane lining the nse They prvide supprt, nurishment, and electrical insulatin fr the lfactry receptrs; als helps t detxify chemicals that cme in cntact with the lfactry epithelium Basal cells are stem cells that cnstantly replace lfactry receptrs; lcated between the bases f the supprting cells Olfactry (Bwman s) Glands, fund within the CT that supprts the lfactry epithelium, prduce mucus that is carried t the surface f the epithelium by ducts Mistens the epithelium and disslves drants fr transductin Supprting cells and lfactry glands are innervated by the facial nerve (VII), which can be stimulated by certain chemicals Impulses in these nerves will stimulate the lacrimal glands in the eyes and the nasal mucus glands (ex. tears and runny nse after smelling ammnia)

2 Physilgy f Olfactin A generatr ptential (deplarizatin) develps and triggers ne r mre nerve impulses When an drant binds t the receptr f an lfactry hair it initiates a cascade f intracellular events thrugh a G- prtein and a 2 nd messenger Prductin f camp pening f Na + channels inflw f Na + generatr ptential generatin f nerve impulse and prpagatin alng an axn f the lfactry receptr Odr Threshlds & Adaptatin The lfactry apparatus can detect abut 10,000 different drs, ften in cncentratins as lw as 1/25 billinth f a milligram per milliliter f air Adaptatin (decreasing sensitivity) ccurs rapidly Adapt by abut 50% after the first secnd f stimulatin, but adapt slwly after that Cmplete insensitivity ccurs abut a minute after expsure The Olfactry Pathway On each side f the nse, 40 bundles f unmyelinated axns f lfactry receptrs extend thrugh 20 lfactry framina in the cribifrm plate f the ethmid bne The bundles f axns cllectively frm the right and left lfactry nerves (I) Terminate in the brain in paired masses f gray matter called the lfactry bulbs; lcated belw the frntal lbe and lateral t the crista galli f the ethmid bne Once generated, nerve impulses travel thrugh the tw lfactry nerves lfactry bulbs lfactry tract primary lfactry area in the tempral lbe f the crtex The primary lfactry area is where cnscius awareness f smell begins Olfactin is the nly sensry system that has direct crtical prjectins withut first ging thrugh relay statins in the thalamus Other axns f the lfactry tract prject t the limbic system and hypthalamus; accunts fr emtinal and memry- evked respnses t drs Pathways als extend t the frntal lbe; an imprtant regin fr dr identificatin is the rtibfrntal area; peple with damage in this area have difficulty discriminating drs 17.2: Gustatin: The Sense f Taste Taste, r gustatin, is als a chemical sense Only five primary tastes can be distinguished: sur, sweet, bitter, salty, and umami ( meaty r savry ) Umami is believed t arise frm taste receptrs that are stimulated by mnsdium glutamate (MSG), a substance naturally present in many fds and added t thers as a flavr enhancer All ther flavrs, such as chclate, pepper, and cffee, are cmbinatins f the five primary tastes, plus accmpanying lfactry and tactile (tuch) sensatins Odrs frm fd can pass upward frm the muth int the nasal cavity, where they stimulate lfactry receptrs Because lfactin is mre sensitive than taste, a given cncentratin f fd may stimulate the lfactry system When yu have a cld/allergies, lfactin is blcked, nt taste Anatmy f Taste Buds & Papillae We have nearly 10,000 taste buds lcated n the tngue, sft palate, pharynx, and larynx Each taste bud is cmpsed f abut 50 gustatry receptr cells, surrunded by a number f supprting cells Basal cells lcated near the CT base multiply and differentiate, first t becme the supprting cells arund the bud, then the gustatry receptr cells inside the taste bud A single, lng micrvillus, called a gustatry hair, prjects frm each receptr cell t the surface thrugh the taste pre Each gustatry receptr cell has a lifespan f abut 10 days At their base, the gustatry receptr cells synapse with dendrites f first- rder neurns that frm the first part f the gustatry pathway The dendrites f each first- rder neurn branch prfusely and cntact many gustatry receptr cells in several taste buds Taste buds are fund in 3 different types f papillae (elevatins n the tngue which prvide a rugh texture and increase surface area)

3 Abut 12 very large vallate papillae frm a rw at the back f the tngue (each huses taste buds) Fungifrm papillae are mushrm- shaped and are scattered ver the entire surface f the tngue (cntaining abut 5 taste buds each) Fliate papillae are lcated in small trenches n the lateral margins f the tngue, but mst f their taste buds degenerate in early childhd In additin, the entire surface f the tngue has filifrm papillae that cntain tactile receptrs but n taste buds They increase frictin between the tngue and fd, making it easier t mve fd in the ral cavity Physilgy f Gustatin Tastants are chemicals that stimulate gustatry receptr cells When disslved in saliva, it can make cntact with the plasma membrane f the gustatry hairs, which are the sites f taste transductin The result is a receptr ptential fr different tastants Fr salty fds, sdium ins enter gustatry receptrs via sdium channels in the plasma membrane accumulatin f sdium leads t deplarizatin and the release f a neurtransmitter In sur fds, hydrgen ins flw int gustatry receptrs via hydrgen in channels leads t deplarizatin and the release f a neurtransmitter Fr sweet, bitter, and umami, tastants bind t receptrs linked t G prteins activates secnd messengers in the gustatry receptr cell release neurtransmitter Different tastes arise frm the activatin f different grups f taste neurns Taste Threshlds and Adaptatin The threshld fr taste varies fr each f the primary tastes We are mst sensitive t bitter substances, such as quinine Because pisnus substances are ften bitter, this high sensitivity may have a prtective functin The threshld fr sur substances is smewhat higher, fllwed by salty and sweet substances Cmplete adaptatin t a specific taste can ccur in 1 5 minutes f cntinuus stimulatin The Gustatry Pathway Three cranial nerves cntain axns f the first- rder gustatry neurns that innervate the taste buds The facial (VII) nerve serves taste buds in the anterir 2/3 f the tngue The glsspharyngeal (IX) nerve serves taste buds in the psterir 1/3 f the tngue The vagus (X) nerve serves taste buds in the thrat and epiglttis Nerve impulses prpagate alng these cranial nerves t the gustatry nucleus in the medulla blngata Frm there, axns carrying taste signals prject t the hypthalamus, limbic system, and thalamus Taste signals arrive at the primary gustatry area at the base f the smatsensry crtex in the parietal lbe and give rise t the cnscius perceptin f taste

4 17.3: Visin Our visual perceptin is dependent n the eye, its accessry structures, the ptic tracts, and the 1 visual crtex and it s assciatin areas Visin (the act f seeing) is pssible because f phtreceptrs that are able t catch phtns f EM radiatin in the nm wavelengths what we perceive as visual light Accessry Structures f the Eye The eyeball is abut 2.5 cm in diameter, with nly abut 16% f it viewable by just lking at a persn The accessry structures f the eye are the extracular muscles, palpebra, eyebrws, eyelashes, cnjunctiva, and the lacrimal glands and ducts The upper and lwer palpebrae are the eyelids, with the fissure being the space between them Functin is t shade the eyes during sleep, prtect the eyes frm excessive light and freign bjects, and spread lubricating secretins ver the eyeballs The upper eyelid is mre mvable than the lwer eyelid; the levatr palpebrae superiris muscles that raise the upper eyelid The lacrimal caruncle cntains sebaceus glands and sudriferus glands The tarsal plate is a thick fld f CT that gives frm and supprt t the eyelids Tarsal r Meibmian glands secrete a fluid that helps keep the eyelids frm adhering t ne anther The cnjunctiva is a clear mucus membrane that cvers the white (avascular) part f the eye Palpebral cnjuctiva lines the inner eyelids; bulbar cnjunctiva passes frm the eyelids nt the surface f the eyeball, where it cvers the sclera but nt the crnea Dilatin and cngestin f the bld vessels f the bulbar cnjunctiva causes bldsht eyes The eyelashes prtect the brder f each eyelid The eyebrws help prtect the eyeballs frm freign bjects, perspiratin, and direct rays f the sun Infectin f sebaceus ciliary glands (at the base f hair fllicles) causes sty eye The lacrimal apparatus is a grup f structures that prduces and drains lacrimal fluid (tears) The lacrimal glands are each abut the size an almnd, situated superlateral t the eyeball Leading frm the lacrimal glands are 6 t 12 excretry lacrimal ducts Tears (lacrimal fluid) run frm the lacrimal glands, int the excretry lacrimal ducts, nt the surface f the cnjunctiva; ver the surface f the eyeball sme lacrimal fluid als evaprates Tears are a watery slutin cntaining salts, mucus, and lyszyme Prtects, cleans, lubricates, and mistens the eyeball; each lacrimal gland prduces abut 1mL f lacrimal fluid a day Tears drain int the lacrimal puncta, which are tw penings n the nasal side f the extreme edge f the eyeball Superir and inferir lacrimal canals empty the tears int the naslacrimal sac and naslacrimal duct The right and left sided naslacrimal ducts empty int each side f the nse Watery eyes ccur when lacrimal fluid builds up, as when smething bstructs the naslacrimal ducts fr instance Blcked naslacrimal ducts can be caused by an inflammatin f the nasal mucsa, such as a cld Over prductin f lacrimal fluid ccurs in respnse t parasympathetic stimulatin, caused by an emtinal respnse (crying), and tears spill ver the edges f the eyelids and drain int the nasal cavity (causing nasal stuffiness)

5 The eyes sit in depressins called rbits, which help t prtect the eyes, stabilizes them in a 3- D space, and anchrs them t muscles that prduce mvement The extrinsic eye muscles extend frm the walls f the bny rbit t the sclera f the eye and are surrunded by perirbital fat These muscles can mve the eye in almst every directin Superir rectus, inferir rectus, lateral rectus, medial rectus, superir blique, inferir blique Supplied by cranial nerves III, IV, and VI Neural circuits in the brain stem and cerebellum synchrnize mvements f the eyes Anatmy f the Eye The wall f the eyeball cnsists f three layers r tunics The fibrus tunic is the uter layer and is cmpsed f the sclera ( white f the eye) and the crnea (the transparent epithelium the prtects the frnt f the eye) Even thugh yu can t easily see it, the crnea is a very imprtant structure in the uter avascular fibrus tunic It s cmpsed f a transparent epithelium that cvers the anterir eye and helps fcus light nt the retina The uter surface is stratified squamus epithelium; the middle layer cnsists f cllagen and fibrblasts; and the inner surface is cmpsed f simple squamus epithelium Since the central part f the crnea receives xygen frm the utside air, cntact lenses that are wrn fr lng perids f time must be permeable The sclera has a high amunt f cllagen fibers and fibrblasts and frms the tugh, white part f the eye The sclera gives the eye it s shape, makes it mre rigid, prtects the inner anatmical parts, and serves as a site f attachment fr the extrinsic eye muscles The vascular tunic r uvea is the middle layer and is cmpsed f the chrid, the ciliary bdy, and the iris The chrid frms the majr vascular prtin that lines the internal surface f the sclera It als cntains melancytes that prduce the pigment melanin; the melanin absrbs stray light rays, which prevents reflectin and scattering f light within the eyeball; as a result, the image cast n the retina by the crnea remains sharp and clear The ciliary bdy cnsists f the ciliary prcesses that secrete aqueus humr, and the ciliary muscle that changes the shape f the lens t adapt t near and far visin The iris is the clred prtin f the eyeball cnsisting f circular and radial smth muscle fibers It is suspended between the crnea and the lens, and is attached at the uter margin t ciliary prcesses The amunt f melanin in the iris determines the eye clr The primary functin f the iris is t regulate the amunt f light entering the eyeball thrugh the pupil Autnmic reflexes regulate pupil diameter in respnse t light levels; high levels cnstrictin, lw levels dilatin The nervus tunic is the inner retinal layer The retina cnsist f a layer f melanin pigmented epithelium that allws light t be absrbed rather than scattered

6 The exact center f the retina is called the macula lutea, and in its center is a small depressin called the central fvea (r fvea centralis) There are n rds r nerve cells in the fvea, nly a high cncentratin f cnes - this gives us the sharp central visin necessary in any activity where detail is f primary imprtance The retina can be viewed thrugh the pupil using an phthalmscpe, allwing direct inspectin f the retinal vessels fr any pathlgical changes This is the nly place in the bdy where arterial vessels can be s viewed (withut pening the bdy) The ptic disc is where the ptic nerve and retinal vessels enter and exit the eyeball Its existence creates a necessary defect n the retina an area where there are n cnes r rds Bilateral visin, and saccade (invluntary, quick) muscle mvements allw ur brain t crrect fr this blind spt, and mst are nt even aware they have ne The retina cnsists f tw types f phtreceptr cells, rds and cnes Rds are abundant in the periphery f the retina whereas cnes are fund mre frequently in the central areas Each eye cntains 120 millin rd- shaped phtreceptrs that are adapted fr a lw light threshld (high sensitivity) - they prduce lw reslutin, black and white images Lss f rds with age makes it difficult t drive at night Cne- shaped phtreceptrs functin in bright light t prduce high reslutin clr images They exist in three varieties, crrespnding t the type f pigment they cntain: red, green r blue The phtpigments are cncentrated in the uter segment f the receptr, while the inner segment cntains the nucleus and rganelles The lens is an avascular refractry structure situated psterir t the pupil and iris It cnsists f a capsule with crystallin prteins arranged in layers, and like the crnea, the lens is transparent It attaches t the ciliary muscle f the ciliary bdy by suspensry ligaments that fine tune the fcusing f light n the retina The lens divides the eyeball int tw cavities: an anterir cavity anterir t the lens, and a psterir cavity (vitreus chamber) behind the lens The anterir cavity is further divided at the level f the iris int anterir and psterir chambers (bth filled with aqueus humr) The much larger psterir cavity f the eyeball (vitreus chamber) lies between the lens and the retina Within the vitreus chamber is the vitreus bdy, a transparent jellylike substance that hlds the retina flush against the chrid, giving the retina an even surface fr the receptin f clear images Occasinally, cllectins f debris called vitreal flaters cast shadws n the retina and create a spt in ur field f visin (they are usually harmless and d nt require treatment) The eye requires a cnstant bath in a nurishing fluid t deliver enugh O 2 t supprt the avascular lens and crnea It als needs fluid t help inflate the walls f the eyeball (maintain a cnstant intracular pressure IOP) and supprt the vitreus bdy This need is accmplished thrugh the prductin f aqueus humr, which flws thrugh the anterir cavity f the eye and is replaced every 90 minutes Aqueus humr is prduced at the ciliary bdy and flws first thrugh the psterir chamber (f the anterir cavity f the eye) Traveling alng the psterir surface f the iris it passes thrugh the pupil t enter the anterir chamber It prceeds alng the anterir surface f the iris until it is reabsrbed int the scleral venus sinus (canal f Schlemm) and returned t the venus system Any srt f blckage t aqueus humr flw, r verprductin at the ciliary bdy, may result in an increase f pressure inside the eye a cnditin called glaucma If nt treated, glaucma can lead t a degeneratin f eye functin The vitreus bdy (humr) als cntributes t maintain prper intracular pressure as it hlds the retina against the chrid The vitreus humr, hwever, is nly frmed during embrylgical develpment and is nt replaced

7 As we age, shrinkage f the vitreus bdy may lead t a detachment f the retina frm the chrid Image Frmatin Nrmal image frmatin depends n refractin f light waves, accmmdatin f the lens, cnstrictin f the pupil, and cnvergence f the tw eyes Refractin is the prcess f bending light rays Bth the crnea and the lens refract light rays, and bth must be functining in rder t prperly fcus light nt the right spt n the retina t prduce clear visin Since the crnea has a fixed shape, its fcal length is als fixed; and its ability t refract light is likewise fixed In rder t fcus light that has already been bent by the crnea the lens must change shape the amunt depending n the type f light rays we are trying t see An increase in the curvature f the lens fr near visin is called accmmdatin The near pint f visin is the minimum distance frm the eye that an bject can be clearly fcused - abut 4 in (a distance that increases with age due t a lss f elasticity in the lens) Cnvergence Cnvergence is the inward mvement f the eyes s that bth are directed at the bject being viewed - becming a little crss- eyed when viewing things clse up The nearer the bject, the greater the degree f cnvergence needed t maintain bincular visin The crdinated actin f the extrinsic eye muscles brings abut cnvergence Cnvergence helps us maintain ur bincular visin and see in three dimensins With nearsightedness (mypia), nly clse bjects can be seen clearly Light rays cming in frm distant bjects are naturally fcused in frnt f the retina and appear blurry Crrectin invlves the use f a cncave (negative) lens

8 With farsightedness (hyperpia), nly distant bjects can be seen clearly: Light rays cming in frm nearer bjects are naturally fcused behind the retina Crrectin invlves the use f a cnvex (psitive) lens Abnrmal refractive capabilities f the eye are the result f a misshapen eyeball (usually t lng r t shrt), r because the lens becmes stiff (usually with age) Crrectins are accmplished using either a psitive (cnvex) r negative (cncave) lens (eyeglasses, cntacts, r lens replacements) Physilgy f Visin Once light waves have been successfully fcused n the retina, the infrmatin stred in that electrmagnetic energy must be changed by phtpigments in the phtreceptrs int signals ur brain can interpret - a prcess called visual transductin The single type f phtpigment in rds is rhdpsin, whereas there are 3 different cne phtpigments Clr visin results frm different clrs f light selectively activating the different cne phtpigments The first step in visual transductin is absrptin f light by a phtpigment, a clred prtein that underges structural changes when it absrbs light in the uter segment f a phtreceptr Light absrptin initiates a series f events that lead t the prductin f a receptr ptential (number 4 in the diagram) All phtpigments assciated with visin cntain tw parts: a glycprtein knwn as psin and a derivative f vitamin A called retinal Althugh there are 4 different psins, retinal is the light- absrbing part f all visual phtpigments T simplify the prcess we can say that there is a cyclical bleaching and regeneratin f phtpigment Bleaching is a term describing a cnfrmatinal change in the retinal mlecule in respnse t light In darkness, retinal has a bent shape called cis- retinal Absrptin f a phtn f light causes it t straighten int the trans- retinal frm in a prcess called ismerizatin Trans- retinal cmpletely separates frm the psin; since the final prducts lk clrless, this part f the cycle is called bleaching f phtpigment An enzyme cnverts trans- retinal cis- retinal The cis- retinal regenerates the phtpigment In daylight, regeneratin f rhdpsin cannt keep up with the bleaching prcess, s rds cntribute little t daylight visin In cntrast, cne phtpigments regenerate rapidly enugh that sme f the cis frm is always present, even in very bright light As a cnsequence, light adaptatin (frm dark cnditins t light cnditins) happens in secnds; dark adaptatin (frm light t dark) takes minutes t ccur (up t 40 minutes t fully adapt) Mst frms f clr blindness, an inherited inability t distinguish between certain clrs, result frm the absence r deficiency f ne f the three types f cnes Mst cmmn type is red- green clr blindness in which red cnes r green cnes are missing Prlnged vitamin A deficiency and the resulting belw- nrmal amunt f rhdpsin may cause night blindness r nyctalpia, an inability t see well at lw light levels

9 The Visual Pathway The graded ptentials generated by the phtreceptrs underg cnsiderable prcessing at synapses amng the varius types f neurns in the retina (hrizntal cells, biplar cells, and amacrine cells)- certain features f visual input are enhanced while thers are discarded Overall, cnvergence pre- dminates as 126 millin pht- receptrs impinge n nly1 millin ganglin cells The axns f retinal ganglin cells prvide utput that travels back twards the light, exiting the eyeball as the ptic nerve, which emerges frm the vitreus surface f the retina The axns then pass thrugh a crssver pint called the ptic chiasm Sme axns crss t the ppsite side, while thers remain uncrssed Once thrugh the ptic chiasm the axns enter the brain matter as the ptic tracts (mst terminate in thalamus) Here they synapse with neurns that prject t the 1 visual crtex in the ccipital lbes

10 17.4: Hearing & Equilibrium Auditin, the prcess f hearing (which the ability t perceive sunds), is accmplished by the rgans f the ear The ear is an engineering marvel because its sensry receptrs can transduce sund vibratins with amplitudes as small as the diameter f an atm f gld int electrical signals 1000 times faster than the eye can respnd t light The ear als cntains receptrs fr equilibrium Anatmy f the Ear The ear has 3 principle regins The external ear, which uses air t cllect and channel sund waves The middle ear, which uses a bny system t amplify sund vibratins The internal ear, which generates actin ptentials t transmit sund and balance infrmatin t the brain The anatmy f the external ear includes The auricle (pinna), a flap f elastic cartilage cvered by skin and cntaining ceruminus glands A curved 1 lng external auditry canal situated in the tempral bne leading frm the meatus t the tympanic membrane (TM r ear drum) which separates the uter ear frm the cavity f the middle ear The middle ear is an air- filled cavity in the tempral bne It is lined with epithelium and cntains 3 auditry ssicles (bnes) The stapes (stirrup) The incus (anvil) The handle f the malleus (hammer) attaches t the TM Tw small skeletal muscles (the tensr tympani and stapedius) attach t the ssicle and dampen vibratins t prevent damage frm sudden, lud sunds The Eustachian (auditry) tube cnnects the middle ear with the naspharynx (upper prtin f the thrat) It cnsists f bne and hyaline cartilage and is nrmally passively cllapsed It pens t equalize pressures n each side f the TM(allwing it t vibrate freely) The internal ear (inner ear) is als called the labyrinth because f its cmplicated series f canals Structurally, it cnsists f tw main divisins: an uter bny labyrinth that enclses an inner membranus labyrinth The bny labyrinth is sculpted ut f the petrus part f the tempral bne, and divided int three areas: (1) the semicircular canals, (2) the vestibule, and (3) the cchlea The vestibule is the middle part f the bny labyrinth The membranus labyrinth in the vestibule cnsists f tw sacs called the utricle and the saccule The three semicircular canalsare abve the vestibule, each ending in a swllen enlargement called the ampulla (fr dynamic equilibrium) The snail shaped cchlea cntains the hearing apparatus Tw types f fluid (perilymph and endlymph) fill its 3 different internal channels: The scala vestibuli, scala tympani, and cchlear duct Perilymph transmits the vibratins cming frm the stapes in the val windw up and arund the scala vestibuli, and then back dwn and arund the scala tympani causing the endlymph in the cchlear duct t vibrate Pressure waves in the endlymph cause the basilar membrane f the cchlear duct t vibrate, mving the hair cells f the spiral rgan f Crti against an verhanging flexible gelatinus membrane called the tectrial membrane

11 The Nature f Sund Waves Sund waves are alternating high- and lw- pressure regins traveling in the same directin thrugh sme medium The frequency f a sunds vibratin is its pitch Mst sunds: 500 t 5000 Hz Audible range: 20 t 20,000 Hz The larger the intensity (amplitude), the luder the sund Measured in decibels Physilgy f Hearing & The Auditry Pathway Mvements f the hair cells in cntact with the tectrial membrane transduce mechanical vibratins int electrical signals which generate nerve impulses alng the cchlear branch f CN VIII The cell bdies f the sensry neurns are lcated in the spiral ganglia Nerve impulses pass alng the axns f these neurns, which frm the cchlear branch f the vestibulcchlear (VIII) nerve The nerve impulses fllw CN VIII en rute t the medulla, pns, midbrain, and thalamus, and finally t the primary auditry crtex in the tempral lbe Slight differences in the timing f nerve impulses arriving frm the tw ears at the superir livary nuclei in the pns allw us t lcate the surce f a sund

12 Physilgy f Equilibrium Equilibrium is anther functin f the inner ear - cntrlled by the vestibular apparatus (the saccule and utricle f the vestibule, and the 3 semicircular canals) Static equilibrium refers t a state f balance relative t the frce f gravity Static equilibrium is cntrlled by the sensry hairs within the macula f the utricle and saccule An tlithic membrane, studded with dense calcium carbnate crystals (tliths), respnds t gravity when head psitin is changed This mvement pens transductin channels in the hair cells, prducing lcal ptentials which summate t frm nerve AP Dynamic equilibrium invlves the maintenance f balance during sudden mvements Dynamic equilibrium is cntrlled by the sensry hairs within the ampulla f the semicircular canals Within each ampulla is a small elevatin called the crista Each crista cntains hair cells and supprting cells cvered by gelatinus material called the cupula With mvement, the endlymph within the ampulla lags behind the mving cupla, causing a difference in the inertial frces the hair bundle f the cupla bends and nerve impulses are generated Equilibrium Pathways Once generated, nerve impulse travel up the vestibular branch f CN VIII Mst f these axns synapse in the majr integrating centers fr equilibrium, in the medulla and pns, which als receive input frm the eyes and prpriceptrs Ascending neurns cntinue t the primary auditry area in the parietal lbe t prvide us with cnscius awareness f the psitin and mvements f the head and limbs

13 17.5: Develpment f the Eyes and Ears Eyes The eyes begin t develp abut 22 days after fertilizatin when the ectderm f the lateral walls f the prsencephaln (frebrain) bulges ut t frm the ptic grves Ears The ears begin t develp abut 22 days after fertilizatin frm a thickening f ectderm n either side if the rhmbencephaln (hindbrain) Internal ear middle ear external ear 17.6: Aging & the Special Senses Mst peple d nt experience any prblems with the senses f smell and taste until abut age 50 Gradual lss f receptr cells and slwer rate f regeneratin The lens f the eye lses elasticity and cannt change shape easily; the sclera becmes thick and rigid and becmes disclred; the iris fades; and the muscles that regulate pupil size weaken and react mre slwly t light and dark Sme diseases f the retina are mre likely t ccur; cataracts frm; tear prductin may decrease and lead t dry eyes; eyelids may lse elasticity, becming wrinkled and baggy; the amunt f fat arund the rbits may decrease, causing the eyeballs t sink int the rbits Sharpness f visin decreases, clr and depth perceptin and reduced, and vitreal flaters increase By age 60, abut 25% f peple experience a decrease in hearing, especially fr higher- pitch sunds May be related t damaged and lst hair cells in the spiral rgan r degeneratin f the nerve pathway fr hearing