HEARIN G AN D EQ U IL IB RIU M - Weeblyfkep-james.weebly.com/uploads/1/2/7/6/12761858/hearing_and_equil… · HEARIN G AN D EQ U IL IB RIU M O B J E C T I V E S ... inner ear from

620 CHAPTER 17 • THE SPECIAL SENSES

l4 Each optic tract consists of crossed and uncrossed axonsthat project from the optic chiasm to the thalamus onone side.

l5 Axon collaterals (branches) of the retinal ganglion cellsproject to the midbrain,where they participate in neuralcircuits that govern constriction of the pupils in responseto light and coordination of head and eye movements.Collaterals also extend to the suprachiasmatic nucleus of thehypothalamus,which establishes patterns of sleep and otheractivities that occur on a circadian or daily schedule inresponse to intervals of light and darkness.

l6 The axons of thalamic neurons form the optic radiations asthey project from the thalamus to the primary visual area ofthe cortex on the same side.

Although we have just described the visual pathway as a sin-gle pathway,visual signals are thought to be processed byat least three separate systems in the cerebral cortex,each withits own function. One system processes information related tothe shape of objects,another system processes informationregarding color of objects,and a third system processes informa-tion about movement,location,and spatial organization.

C H E C K P O I N T

6 . W h a t is th e fu n c tio n o f th e la c rim a l a p p a ra tu s ?

7 . W h a t ty p e s o f c e lls m a k e u p th e n e u ra l la y e r a n d th e

p ig m e n te d la y e r o f th e re tin a ?

8 . Ho w d o p h o to p ig m e n ts re s p o n d to lig h t a n d re c o v e r in

d a rk n e s s ?

9 . Ho w d o re c e p to r p o te n tia ls a ris e in p h o to re c e p to rs ?

1 0 . B y w h a t p a th w a y d o n e rv e im p u ls e s trig g e re d b y a n

o b je c t in th e n a s a l h a lf o f th e v is u a l fie ld o f th e le ft e y e

re a c h th e p rim a ry v is u a l a re a o f th e c o rte x ?

HEARI N G AN D EQ U I L I B RI U MO B J E C T I V E S

• D e s c rib e th e a n a to m y o f th e s tru c tu re s in th e th re e m a in

re g io n s o f th e e a r.

• Lis t th e m a jo r e v e n ts in th e p h y s io lo g y o f h e a rin g .

• Id e n tify th e re c e p to r o rg a n s fo r e q u ilib riu m , a n d d e s c rib e

h o w th e y fu n c tio n .

• D e s c rib e th e a u d ito ry a n d e q u ilib riu m p a th w a y s .

The ear is an engineering marvel because its sensory receptorscan transduce sound vibrations with amplitudes as small as thediameter of an atom of gold (0.3nm) into electrical signals1000 times faster than photoreceptors can respond to light.Besides receptors for sound waves,the ear also contains recep-tors for equilibrium.

A n a t o m y o f t h e E a r

The ear is divided into three main regions:(1) the external ear,which collects sound waves and channels them inward; (2) the

d

d

middle ear,which conveys sound vibrations to the oval window;and (3) the internal ear,which houses the receptors for hearingand equilibrium.

External (Outer) Ear

The external (outer) ear consists of the auricle,external audi-tory canal,and eardrum (Figure 17.18). The auricle (pinna) isa flap of elastic cartilage shaped like the flared end of a trumpetand covered by skin. The rim of the auricle is the helix;theinferior portion is the lobule.Ligaments and muscles attachthe auricle to the head. The external auditory canal (audit-5

hearing) is a curved tube about 2.5cm (1in.) long that lies inthe temporal bone and leads to the eardrum. The tympanic

membrane (tim-PAN-ik; tympan-5 a drum) or eardrum is athin,semitransparent partition between the external auditorycanal and middle ear. The tympanic membrane is covered byepidermis and lined by simple cuboidal epithelium. Betweenthe epithelial layers is connective tissue composed of collagen,elastic fibers,and fibroblasts. Tearing of the tympanic membraneis called a perforated eardrum.It may be due to pressure froma cotton swab,trauma,or a middle ear infection,and usuallyheals within a month. The tympanic membrane may be exam-ined directly by an otoscope (oto-5 ear;-skopeo 5 to view),aviewing instrument that illuminates and magnifies the externalauditory canal and tympanic membrane.

Near the exterior opening, the external auditory canalcontains a few hairs and specialized sweat glands called cerumi-

nous glands(se-ROO-mi-nus) that secrete earwax or cerumen

(se-ROO-men). The combination of hairs and cerumen helpsprevent dust and foreign objects from entering the ear. Cerumenalso prevents damage to the delicate skin of the external earcanal by water and insects. Cerumen usually dries up and fallsout of the ear canal. However,some people produce a largeamount of cerumen,which can become impacted and can muffleincoming sounds. The treatment for impacted cerumen is usu-ally periodic ear irrigation or removal of wax with a blunt instru-ment by trained medical personnel.

Middle Ear

The middle ear is a small,air-filled cavity in the petrous portionof the temporal bone that is lined by epithelium (Figure 17.19 onpage 622). It is separated from the external ear by the tympanicmembrane and from the internal ear by a thin bony partition thatcontains two small membrane-covered openings:the oval windowand the round window. Extending across the middle ear and at-tached to it by ligaments are the three smallest bones in the body,the auditory ossicles(OS-si-kuls),which are connected by sy-novial joints. The bones,named for their shapes,are the malleus,incus,and stapes— commonly called the hammer,anvil,and stir-rup,respectively. The “handle”of the malleus(MAL-e-us) at-taches to the internal surface of the tympanic membrane. The headof the malleus articulates with the body of the incus. The incus

(ING-kus),the middle bone in the series,articulates with the headof the stapes. The base or footplate of the stapes(STA-pez) fitsinto the oval window.Directly below the oval window is another

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HEARING AND EQUILIBRIUM 621

opening,the round window,which is enclosed by a membranecalled the secondary tympanic membrane.

Besides the ligaments,two tiny skeletal muscles also attach tothe ossicles (Figure 17.19). The tensor tympani (TIM-pan-e-)muscle,which is supplied by the mandibular branch of thetrigeminal (V) nerve,limits movement and increases tension onthe eardrum to prevent damage to the inner ear from loud noises.The stapedius(sta-PE-de-us) muscle,which is supplied by thefacial (VII) nerve,is the smallest skeletal muscle in the humanbody. By dampening large vibrations of the stapes due to loudnoises,it protects the oval window,but it also decreases the sen-sitivity of hearing. For this reason,paralysis of the stapediusmuscle is associated with hyperacusia (abnormally sensitivehearing). Because it takes a fraction of a second for the tensortympani and stapedius muscles to contract,they can protect theinner ear from prolonged loud noises,but not from brief onessuch as a gunshot.

The anterior wall of the middle ear contains an opening thatleads directly into the auditory (pharyngotympanic) tube,

commonly known as the eustachian tube.The auditory tube,which consists of both bone and elastic cartilage,connectsthe middle ear with the nasopharynx (superior portion of thethroat). It is normally closed at its medial (pharyngeal) end.During swallowing and yawning,it opens,allowing air to enteror leave the middle ear until the pressure in the middle earequals the atmospheric pressure. Most of us have experiencedour ears popping as the pressures equalize. W hen the pressuresare balanced,the tympanic membrane vibrates freely as soundwaves strike it. If the pressure is not equalized,intense pain,hearing impairment,ringing in the ears,and vertigo could de-velop. The auditory tube also is a route for pathogens to travelfrom the nose and throat to the middle ear,causing the mostcommon type of ear infection (see otitis media in the Disorderssection at the end of this chapter).

External ear

Frontalplane

Temporal boneInternal auditory canal

Round window (covered bysecondary tympanic membrane)

Semicircular canal

IncusMalleus

Vestibulocochlear(VIII) nerve:

Vestibular branch

Cochlear branch

To nasopharynx

Auditory tube

Stapes inoval window

Helix

Lobule

Frontal section through the right side of the skullshowing the three principal regions of the ear

Auricle

Tympanic membraneExternal auditory canal

Cochlea

Cerumen

Elasticcartilage

Internal ear

Middle ear

F ig u r e 17 .18 Anatomy of the ear.(See Tortora,A Photographic Atlas of the Human Body,Second Edition,Figure 9.4a.)

T h e e a r h a s th re e p rin c ip a l re g io n s : th e e x te rn a l (o u te r) e a r, th e m id d le e a r, a n d th e in te rn a l (in n e r) e a r.

To which structure of the external ear does the malleus of the middle ear attach??

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Internal (Inner) Ear

The internal (inner) ear is also called the labyrinth (LAB-i-rinth) because of its complicated series of canals (Figure 17.20).Structurally,it consists of two main divisions:an outer bonylabyrinth that encloses an inner membranous labyrinth.The bony labyrinthis a series of cavities in the petrous portionof the temporal bone divided into three areas:(1) the semicircu-lar canals and (2) the vestibule,both of which contain receptorsfor equilibrium,and (3) the cochlea,which contains receptorsfor hearing. The bony labyrinth is lined with periosteum andcontains perilymph.This fluid,which is chemically similar tocerebrospinal fluid,surrounds the membranous labyrinth,a se-ries of epithelial sacs and tubes inside the bony labyrinth thathave the same general form as the bony labyrinth. The epithelialmembranous labyrinth contains endolymph.The level of potas-sium ions (K1) in endolymph is unusually high for an extracel-lular fluid,and potassium ions play a role in the generation ofauditory signals (described shortly).

The vestibule (VES-ti-bul) is the oval central portion of thebony labyrinth. The membranous labyrinth in the vestibuleconsists of two sacs called the utricle (U-tri-kl 5 little bag) andthe saccule (SAK-ul 5 little sac),which are connected by a

small duct. Projecting superiorly and posteriorly from thevestibule are the three bony semicircular canals,each of whichlies at approximately right angles to the other two. Based ontheir positions,they are named the anterior,posterior,and lateralsemicircular canals. The anterior and posterior semicircularcanals are vertically oriented; the lateral one is horizontallyoriented. At one end of each canal is a swollen enlargementcalled the ampulla (am-PUL-la 5 saclike duct). The portions ofthe membranous labyrinth that lie inside the bony semicircularcanals are called the semicircular ducts.These structures con-nect with the utricle of the vestibule.

The vestibular branch of the vestibulocochlear (VIII) nerveconsists of ampullary,utricular,and saccular nerves.Thesenerves contain both first-order sensory neurons and motorneurons that synapse with receptors for equilibrium. The first-order sensory neurons carry sensory information from thereceptors,and the motor neurons carry feedback signals to thereceptors,apparently to modify their sensitivity. Cell bodies ofthe sensory neurons are located in the vestibular ganglia (seeFigure 17.21b).

Anterior to the vestibule is the cochlea (KOK-le-a 5 snail-shaped),a bony spiral canal (Figure 17.21a on pages 624–625)


Auditoryossicles

Auditorytube

Superior ligamentof malleus

Incus

Facial (VII) nerve

Stapes in oval window(fenestra vestibuli)

Round window(fenestra cochlea)

Auditory tubeMiddle ear

Tympanic membrane

External auditory canal

LATERALMEDIAL

Anterior ligamentof malleus (cut)

Lateral ligamentof malleus

Malleus

Frontal section showing location of auditory ossicles

Tensor tympanimuscle

Posterior ligament of incus

Stapedius muscle

External ear

Internal ear

Middle ear

W h a t s t r u c t u r e s s e p a r a t e t h e m id d le e a r f r o m t h e in t e r n a l e a r ??

F ig u r e 1 7 .1 9 The right middle ear and the auditory ossicles.(See Tortora,A Photographic Atlas of the Human Body,Second Edition,Figure 3.14.)

Co m m o n n a m e s fo r th e m a lle u s , in c u s , a n d s ta p e s a re th e h a m m e r, a n v il, a n d s tirru p , re s p e c tiv e ly .


Semicircular canals(contain semicircularducts):

Posterior

Anterior

Lateral

LATERAL

Ampulla ofsemicircular duct

Stapes inoval window

Round window

Bony labyrinth(contains perilymph)

Membranous labyrinth(contains endolymph)

Ampulla of semicircular canal

Oval window

Vestibule

Utricle

Saccule

Cochlea

Cochlearduct

MEDIAL

External ear

Internal ear

Middle ear

Components of the right internal ear

that resembles a snail’s shell and makes almost three turnsaround a central bonycore called the modiolus(mo-DI-o-lus;Figure 17.21b). Sections through the cochlea reveal that it is di-vided into three channels:cochlear duct,scala vestibuli,andscala tympani (Figure 17.21a–c). The cochlear duct (scala me-

dia) is a continuation of the membranous labyrinth into thecochlea; it is filled with endolymph. The channel above thecochlear duct is the scala vestibuli,which ends at the oval win-dow. The channel below is the scala tympani,which ends at theround window. Both the scala vestibuli and scala tympani arepart of the bony labyrinth of the cochlea; therefore,these cham-bers are filled with perilymph. The scala vestibuli and scala tym-pani are completely separated by the cochlear duct,except for anopening at the apex of the cochlea,the helicotrema (hel-i-ko-TRE-ma;Figure 17.22b). The cochlea adjoins the wall of thevestibule,into which the scala vestibuli opens. The perilymph inthe vestibule is continuous with that of the scala vestibuli.

The vestibular membrane separates the cochlear ductfrom the scala vestibuli,and the basilar membrane sepa-rates the cochlear duct from the scala tympani. Resting on thebasilar membrane is the spiral organ or organ of Corti(Fig-ure 17.21c,d). The spiral organ is a coiled sheet of epithelial

cells,including supporting cells and about 16,000 hair cells,

which are the receptors for hearing. There are two groups of haircells:The inner hair cells are arranged in a single row whereasthe outer hair cells are arranged in three rows. At the apical tipof each hair cell are 40–80 stereocilia that extend into the en-dolymph of the cochlear duct. Despite their name,stereocilia areactually long,hairlike microvilli arranged in several rows ofgraded height.

At their basal ends,inner and outer hair cells synapse bothwith first-order sensory neurons and with motor neurons fromthe cochlear branch of the vestibulocochlear (VIII) nerve. Cellbodies of the sensory neurons are located in the spiral

ganglion (Figure 17.21b,c). Although outer hair cells outnum-ber them by 3 to 1,the inner hair cells synapse with 90–95% ofthe first-order sensory neurons in the cochlear nerve that relayauditory information to the brain. By contrast,90% of themotor neurons in the cochlear nerve synapse with outer haircells. The tectorial membrane (tector-5 covering),a flexiblegelatinous membrane,covers the hair cells of the spiral organ(Figure 17.21d). In fact,the ends of the stereocilia of the haircells are embedded in the tectorial membrane while the bodiesof the hair cells rest on the basilar membrane.


F ig u r e 1 7 .20 The right internal ear.The outer,cream-colored area is part of the bony labyrinth; the inner,pink-colored area is the membranous labyrinth.

T h e b o n y la b y rin th c o n ta in s p e rily m p h , a n d th e m e m b ra n o u s la b y rin th c o n ta in s e n d o ly m p h .

What are the names of the two sacs that lie in the membranous labyrinth of the vestibule??

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F ig u r e 1 7 .21 Semicircular canals,vestibule,and cochlea of the right ear. Note that the cochlea makes nearly three complete turns.

Th e th re e c h a n n e ls in th e c o c h le a a re th e s c a la v e s tib u li, th e s c a la ty m p a n i, a n d th e c o c h le a r d u c t.

(a) Sections through the cochlea

(b) Components of the vestibulocochlear nerve (cranial nerve VIII)

Utricle

Stapes in oval window

Cochlear duct

Saccule

Scala vestibuli

Scala vestibuli

Cochlea

MEDIAL

Scala tympani

Scala tympani

LATERAL

Vestibular membrane

Basilar membrane

Secondary tympanicmembrane in round window

Transmission of sound waves fromscala vestibuli to scala tympani by way of helicotrema

Cochlear duct

Vestibular ganglia

Utricular nerve

Spiral organ(organ of Corti)Ampullary nerves

Saccular nerves

Spiral ganglion

Stapes in oval window

Helicotrema

Scala vestibuli

Scala tympani

Cochlear duct Round window

MEDIALLATERAL

Vestibulocochlear (VIII) nerve:

Vestibular branch

Cochlear branch

Spiral ganglion

Modiolus


(e) Histology of the spiral organ (organ of Corti)

140xLM

Cochlear duct

Tectorial membrane

Inner hair cell

Basilar membrane

Outer hair cells

Supporting cells

Scala tympani

(d) Enlargement of spiral organ (organ of Corti)

Inner hair cell

Sensory and motor fibersin cochlear branch ofvestibulocochlear (VIII) nerve

Basilar membrane

Supporting cells

Tectorial membrane

Hair bundle

Outer hair cell

Cells liningscala tympani

(c) Section through one turn of the cochlea

Cochlear branch ofvestibulocochlear(VIII) nerve

Scala tympani(contains perilymph)

Basilar membrane

Cochlear duct(contains endolymph)

Tectorial membrane

Spiral organ(organ of Corti)

Spiral ganglion

Scala vestibuli(contains perilymph)

Vestibular membrane

What are the three subdivisions of the bony labyrinth??



The Nature of Sound Waves

In order to understand the physiology of hearing,it is necessaryto learn something about its input,which occurs in the formof sound waves. Sound wavesare alternating high- and low-pressure regions traveling in the same direction through somemedium (such as air). They originate from a vibrating object inmuch the same way that ripples arise and travel over the surfaceof a pond when you toss a stone into it. The frequency ofa sound vibration is its pitch.The higher the frequency of vibra-tion,the higher is the pitch. The sounds heard most acutely bythe human ear are those from sources that vibrate at frequenciesbetween 500 and 5000hertz (Hz; 1Hz5 1 cycle per second).The entire audible range extends from 20 to 20,000Hz. Soundsof speech primarily contain frequencies between 100 and3000Hz,and the “high C”sung by a soprano has a dominantfrequency at 1048Hz. The sounds from a jet plane several milesaway range from 20 to 100Hz.

The larger the intensity (size or amplitude) of the vibration,the louderis the sound. Sound intensity is measured in unitscalled decibels (dB).An increase of one decibel represents atenfold increase in sound intensity. The hearing threshold— thepoint at which an average young adult can just distinguish soundfrom silence— is defined as 0dB at 1000Hz. Rustling leaveshave a decibel level of 15; whispered speech,30; normal conver-sation,60; a vacuum cleaner,75; shouting,80; and a nearbymotorcycle or jackhammer,90. Sound becomes uncomfortableto a normal ear at about 120dB,and painful above 140dB.

membrane to vibrate back and forth. The distance it moves,which is very small,depends on the intensity and frequencyof the sound waves. The eardrum vibrates slowly in re-sponse to low-frequency (low-pitched) sounds and rapidlyin response to high-frequency (high-pitched) sounds.

l3 The central area of the eardrum connects to the malleus,which also starts to vibrate. The vibration is transmittedfrom the malleus to the incus and then to the stapes.

l4 As the stapes moves back and forth,it pushes the membraneof the oval window in and out. The oval window vibratesabout 20 times more vigorously than the eardrum becausethe ossicles efficiently transmit small vibrations spread overa large surface area (eardrum) into larger vibrations of asmaller surface (oval window).

l5 The movement of the oval window sets up fluid pressurewaves in the perilymph of the cochlea. As the oval windowbulges inward,it pushes on the perilymph of the scalavestibuli.

l6 Pressure waves are transmitted from the scala vestibuli tothe scala tympani and eventually to the round window,caus-ing it to bulge outward into the middle ear. (See l9 in the fig-ure.)

l7 As the pressure waves deform the walls of the scalavestibuli and scala tympani,they also push the vestibularmembrane back and forth,creating pressure waves in theendolymph inside the cochlear duct.

l8 The pressure waves in the endolymph cause the basilarmembrane to vibrate,which moves the hair cells of thespiral organ against the tectorial membrane. This leads tobending of the hair cell stereocilia,which produces receptorpotentials that ultimately lead to the generation of nerve im-pulses.

Sound waves of various frequencies cause certain regions ofthe basilar membrane to vibrate more intensely than otherregions. Each segment of the basilar membrane is “tuned”fora particular pitch. Because the membrane is narrower and stifferat the base of the cochlea (portion closer to the oval window),high-frequency (high-pitched) sounds near 20,000Hz inducemaximal vibrations in this region. Toward the apex of thecochlea near the helicotrema,the basilar membrane is wider andmore flexible; low-frequency (low-pitched) sounds near 20Hzcause maximal vibration of the basilar membrane there. Asnoted previously,loudness is determined by the intensity ofsound waves. High-intensity sound waves cause larger vibrationsof the basilar membrane,which leads to a higher frequency ofnerve impulses reaching the brain. Louder sounds also maystimulate a larger number of hair cells.

The hair cells transduce mechanical vibrations into electricalsignals. As the basilar membrane vibrates,the hair bundles at theapex of the hair cell bend back and forth and slide against oneanother. A tip link protein connects the tip of each stereociliumto a mechanically gated ion channel called the transduction

6 2 6 CHAPTER 17 • THE SPECIAL SENSES

• C L I N I C A L C O NNE C T I O N L oud Sounds and H air C el lD am ag e

Exposure to loud music and the engine roar of jet planes, revved-up mo-

torcycles, lawn mowers, and vacuum cleaners damages hair cells of the

cochlea. Because prolonged noise exposure causes hearing loss, em-

ployers in the United States must require workers to use hearing protec-

tors when occupational noise levels exceed 90 dB. Rock concerts and

even inexpensive headphones can easily produce sounds over 110 dB.

Continued exposure to high-intensity sounds is one cause of deafness,

a significant or total hearing loss. The louder the sounds, the more

rapid is the hearing loss. Deafness usually begins with loss of sensitiv-

ity for high-pitched sounds. If you are listening to music through head-

phones and bystanders can hear it, the dB level is in the damaging

range. Most people fail to notice their progressive hearing loss until de-

struction is extensive and they begin having difficulty understanding

speech. Wearing earplugs with a noise-reduction rating of 30 dB while

engaging in noisy activities can protect the sensitivity of your ears. •

P hy siolog y of H earing

The following events are involved in hearing (Figure 17.22):

l1 The auricle directs sound waves into the external auditorycanal.

l2 W hen sound waves strike the tympanic membrane,the alter-nating high- and low-pressure of the air causes the tympanic



channel in its taller stereocilium neighbor. As the stereociliabend in the direction of the taller stereocilia,the tip links tugon the transduction channels and open them. These channelsallow cations in the endolymph,primarily K1,to enter the haircell cytosol. As cations enter,they produce a depolarizing recep-tor potential. Depolarization quickly spreads along the plasmamembrane and opens voltage-gated Ca21 channels in the baseof the hair cell. The resulting inflow of Ca21 triggers exocytosisof synaptic vesicles containing a neurotransmitter,which isprobably glutamate. As more neurotransmitter is released,thefrequency of nerve impulses in the first-order sensory neuronsthat synapse with the base of the hair cell increases. Bendingof the stereocilia in the opposite direction closes thetransduction channels,allows hyperpolarization to occur,and re-duces neurotransmitter release from the hair cells. This de-creases the frequency of nerve impulses in the sensory neurons.

Besides its role in detecting sounds,the cochlea has thesurprising ability to produce sounds. These usually inaudiblesounds,called otoacoustic emissions,can be picked up byplacing a sensitive microphone next to the eardrum. They arecaused by vibrations of the outer hair cells that occur in response

to sound waves and to signals from motor neurons. As theydepolarize and repolarize,the outer hair cells rapidly shortenand lengthen. This vibratory behavior appears to change thestiffness of the tectorial membrane and is thought to enhancethe movement of the basilar membrane,which amplifies theresponses of the inner hair cells. At the same time,the outerhair cell vibrations set up a traveling wave that goes backtoward the stapes and leaves the ear as an otoacousticemission. Detection of these inner ear–produced sounds is a fast,inexpensive,and noninvasive way to screen newborns for hear-ing defects. In deaf babies,otoacoustic emissions are not pro-duced or are greatly reduced in size.

T h e A u d it o r y P a t h w a y

Bending of the stereocilia of the hair cells of the spiral organcauses the release of a neurotransmitter (probably glutamate),which generates nerve impulses in the sensory neurons that in-nervate the hair cells. The cell bodies of the sensory neuronsare located in the spiral ganglia.Nerve impulses pass along

F ig u r e 1 7.22 Events in the stimulation of auditory receptors in the right ear.The numbers correspond to the events listed in the text. The cochlea has been uncoiled to more easily visualize the transmission of sound waves and their distortion of the vestibular and basilar membranes of the cochlear duct.

Ha ir c e lls o f th e s p ira l o rg a n (o rg a n o f C o rti) c o n v e rt a m e c h a n ic a l v ib ra tio n (s tim u lu s ) in to a n e le c tric a l s ig n a l (re c e p to r p o te n tia l).

Scalavestibuli

Cochlear duct(contains endolymph)

Scalatympani

Perilymph

Basilarmembrane

Cochlea

Sound waves

HelicotremaStapes vibratingin oval window

Malleus Incus

External auditorycanal

Tympanicmembrane

Secondary tympanicmembrane vibratingin round window Auditory tube

Vestibular membrane

Middle ear

Tectorial membrane

Spiral organ(organ of Corti)

1 2

3

4

5

6

8

9

7

8

Which part of the basilar membrane vibrates most vigorously in response to high-frequency (high-pitched) sounds??


Medial geniculatenucleus in thalamus

Cochlear branch of vestibulocochlear(VIII) nerve

Cerebellum

Superior olivary nucleus in pons

Lateral menisci

Inferior colliculus in midbrain

Primary auditory areain cerebral cortex

Cochlear nuclei in medulla oblongata


P h y s io lo g y o f E q u ilib r iu m

There are two types of equilibrium (balance). Static equilibrium

refers to the maintenance of the position of the body (mainly thehead) relative to the force of gravity. Body movements that stim-

the axons of these neurons, which form the cochlear branch ofthe vestibulocochlear (VIII) nerve (Figure 17.23). These axonssynapse with neurons in the cochlear nuclei in the medulla ob-longata on the same side. Some of the axons from the cochlearnuclei decussate (cross over) in the medulla, ascend in a tractcalled the lateral meniscus on the opposite side, and terminatein the inferior colliculus in the midbrain. Other axons from thecochlear nuclei end in the superior olivary nucleus in the ponson each side. Slight differences in the timing of nerve impulsesarriving from the two ears at the superior olivary nuclei allow usto locate the source of a sound. Axons from the superior olivarynuclei also ascend in the lateral meniscus tracts on both sidesand end in the inferior colliculi. From each inferior colliculus,nerve impulses are conveyed to the medial geniculate nucleus

in the thalamus and finally to the primary auditory area of thecerebral cortex in the temporal lobe of the cerebrum (see areas41 and 42 in Figure 14.15on page 519). Because many auditoryaxons decussate in the medulla while others remain on the sameside, the right and left primary auditory areas receive nerve im-pulses from both ears.

• C L I N I C A L C O N N E C T I O N C o c h l e a r I m p l a n t s

A c o c h le a r im p la n t is a device that translates sounds into electrical sig-

nals that can be interpreted by the brain. Such a device is useful for

people with deafness that is caused by damage to hair cells in the

cochlea. The external parts of a cochlear implant consist of (1) a micro-

phone worn around the ear that picks up sound waves, (2) a sound

processor, which may be placed in a shirt pocket, that converts sound

waves into electrical signals, and (3) a transmitter, worn behind the ear,

which receives signals from the sound processor and passes them to an

internal receiver. The internal parts of a cochlear implant are the (1) in-

ternal receiver, which relays signals to (2) electrodes implanted in the

cochlea, where they trigger nerve impulses in sensory neurons in the

cochlear branch of the vestibulocochlear (VIII) nerve. These artificially

induced nerve impulses propagate over their normal pathways to the

brain. The perceived sounds are crude compared to normal hearing, but

they provide a sense of rhythm and loudness; information about certain

noises, such as those made by telephones and automobiles; and the

pitch and cadence of speech. Some patients hear well enough with

a cochlear implant to use the telephone. •

F ig u r e 1 7 .23 The auditory pathway.

F ro m h a ir c e lls o f th e c o c h le a , a u d ito ry in fo rm a tio n is c o n v e y e d a lo n g th ec o c h le a r b ra n c h o f th e v e s tib u lo c o c h le a r(V III) n e rv e a n d th e n to th e b ra in s te m ,th a la m u s , a n d c e re b ra l c o rte x .

W h a t is t h e f u n c t io n o f t h e s u p e r io r o liv a r y n u c le u s o f t h e p o n s ??


Hair cell

Supporting cell

Vestibular branches of vestibulocochlear (VIII) nerve

(a) Overall structure of a section of the macula

Location of utricleand saccule(contain maculae)

Otolithicmembrane

Otoliths Hair bundle

Saccule

Utricle

Key:

Sensory fiber

Motor fiber

Supportingcell

Hair cells

Stereocilia

Hair bundle:

Kinocilium

Otolithicmembrane

Otoliths

(c) Position of macula with head upright (left) and tilted forward (right)

Head upright Head tilted forward

(b) Details of two hair cells

Hair cellOtolithsOtolithicmembrane

Force ofgravity


ulate the receptors for static equilibrium include tilting the headand linearacceleration or deceleration, such as when the body isbeing moved in an elevator or in a car that is speeding up orslowing down. Dynamic equilibrium is the maintenance ofbody position (mainly the head) in response to rotationalaccel-eration or deceleration. Collectively, the receptor organs forequilibrium are called the vestibular apparatus(ves-TIB-u-lar);these include the saccule, utricle, and semicircular ducts.

Otolithic Organs: Saccule and UtricleThe walls of both the utricle and the saccule contain a small,thickened region called a macula (MAK-u-la; Figure 17.24). Thetwo maculae (plural) (MAK-u-le-), which are perpendicular toone another, are the receptors for static equilibrium. They providesensory information on the position of the head in space andare essential for maintaining appropriate posture and balance.The maculae also detect linear acceleration and deceleration.

F ig u r e 1 7 .24 Location and structure of receptors in the maculae of the right ear.Both first-order sensory neurons (blue)and motor neurons (red) synapse with the hair cells.

T h e m o v e m e n t o f s te re o c ilia in itia te s

d e p o la riz in g re c e p to r p o te n tia ls .

With which type of equilibrium are the maculae associated??


Cupula

Hair cell

Crista

Supporting cell

Semicircular duct

Ampulla

Location of ampullae of semicircular ducts (contain cristae)

Ampullary nerve

(a) Details of a crista

Hair bundle

Key:

Sensory fiber

Motor fiber


The maculae consist of two kinds of cells:hair cells,whichare the sensory receptors, and supporting cells.Hair cells haveon their surface 40–80 stereocilia (which are actually microvilli)of graduated height, plus one kinocilium, a conventional ciliumanchored firmly to its basal body and extending beyond thelongest stereocilium. As in the cochlea, the stereocilia are con-nected by tip links. Collectively, the stereocilia and kinociliumare called a hair bundle.Scattered among the hair cells arecolumnar supporting cells that probably secrete the thick, gelati-nous, glycoprotein layer, called the otolithic membrane,thatrests on the hair cells. A layer of dense calcium carbonate crys-tals, called otoliths(oto-5 ear;-liths5 stones), extends overthe entire surface of the otolithic membrane.

Because the otolithic membrane sits on top of the macula, ifyou tilt your head forward, the otolithic membrane (and theotoliths as well) is pulled by gravity. It slides “downhill”overthe hair cells in the direction of the tilt, bending the hair bundles.However, if you are sitting upright in a car that suddenly jerksforward, the otolithic membrane lags behind the head move-ment, pulls on the hair bundles, and makes them bend in theother direction. Bending of the hair bundles in one direction

stretches the tip links, which pull open transduction channels,producing depolarizing receptor potentials; bending in the oppo-site direction closes the transduction channels and produces hy-perpolarization.

As the hair cells depolarize and repolarize, they releaseneurotransmitter at a faster or slower rate. The hair cells synapsewith first-order sensory neurons in the vestibular branch ofthe vestibulocochlear (VIII) nerve (see Figure 17.21b). Theseneurons fire impulses at a slow or rapid pace depending onthe amount of neurotransmitter present. Motor neurons alsosynapse with the hair cells and sensory neurons. Evidently, themotor neurons regulate the sensitivity of the hair cells andsensory neurons.

Semicircular Ducts

The three semicircular ducts function in dynamic equilibrium.The ducts lie at right angles to one another in three planes(Figure 17.25):The two vertical ducts are the anterior and poste-rior semicircular ducts, and the horizontal one is the lateralsemicircular duct (see also Figure 17.20). This positioning per-mits detection of rotational acceleration or deceleration. In the

F ig u r e 1 7 .2 5 Location and structure of the semicircular ducts of the right ear.Both first-order sensory neurons (blue) and motorneurons (red) synapse with the hair cells. The ampullary nerves are branches of the vestibular division of thevestibulocochlear (VIII) nerve.

Th e p o s itio n s o f th e s e m ic irc u la r d u c ts p e rm it d e te c tio n o f ro ta tio n a l m o v e m e n ts .


Ampulla

As the head rotatesin one direction, cupulais dragged throughendolymph and bentin opposite direction

Ampullary nerve

Head rotatingHead in still position

(b) Position of a cupula with the head in the still position (left) and when the head rotates (right)

Cupula

ampulla,the dilated portion of each duct, is a small elevationcalled the crista (KRIS-ta 5 crest; plural is cristae). Each cristacontains a group of hair cellsand supporting cells.Coveringthe crista is a mass of gelatinous material called the cupula (KU-pu-la). W hen you move your head, the attached semicircularducts and hair cells move with it. The endolymph within theampulla, however, is not attached and lags behind. As the mov-ing hair cells drag along the stationary endolymph, the hairbundles bend. Bending of the hair bundles produces receptorpotentials. In turn, the receptor potentials lead to nerve impulsesthat pass along the vestibular branch of the vestibulocochlear(VIII) nerve.

Equilibrium Pathways

Bending of hair cells in the semicircular ducts, utricle, or sacculecauses the release of a neurotransmitter (probably glutamate),which generates nerve impulses in the sensory neurons that in-nervate the hair cells. The cell bodies of sensory neurons are lo-cated in the vestibular ganglia.Nerve impulses pass along the

axons of these neurons, which form the vestibular branch of thevestibulocochlear (VIII) nerve (Figure 17.26). Most of these ax-ons synapse with sensory neurons in vestibular nuclei,the ma-jor integrating centers for equilibrium, in the medulla oblongataand pons. The vestibular nuclei also receive input from the eyesand somatic receptors, especially proprioceptors in the neckmuscles that indicate the position of the head. The remaining ax-ons enter the cerebellum through the inferior cerebellar pedun-cles (see Figure 14.8b on page 509). Bidirectional pathwaysconnect the cerebellum and vestibular nuclei.

The vestibular nuclei integrate information from vestibular,visual, and somatic receptors and then send commands to:(1)the nuclei of cranial nerves— oculomotor (III), trochlear (IV),and abducens (VI)— that control coupled movements of the eyeswith those of the head to help maintain focus on the visual field;(2) nuclei of the accessory (XI) nerves to help control head andneck movements to assist in maintaining equilibrium; (3) thevestibulospinal tract, which conveys impulses down the spinalcord to maintain muscle tone in skeletal muscles to help main-tain equilibrium; and (4) the ventral posterior nucleus in the thal-amus and then to the vestibular area in the parietal lobe of the

HEARING AND EQUILIBRIUM 6 3 1

With which type of equilibrium are the semicircular ducts associated??


Trochlear nerve(IV) motor nucleus

Oculomotor nerve(III) nucleus

Vestibular area in cerebral cortex

Cerebellum

Spinal cord

Abducens nerve(V) motor nucleus

Accessory nerve (XI) nucleus

Vestibulospinal tract

Vestibular

nuclei

Vestibular ganglion

Ventral posteriornucleus in thalamus

Vestibular branch of vestibulocochlear (VIII) nerve

cerebral cortex (which is part of the primary somatosensoryarea; see areas 1, 2, and 3 in Figure 14.15on page 519) to pro-vide us with the conscious awareness of the position and move-ments of the head.

Table 17.2 summarizes the structures of the ear related tohearing and equilibrium.

C H E C K P O I N T

11. How are sound waves transmitted from the auricle to thespiral organ of Corti?

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12 . How do hair cells in the cochlea and vestib ular apparatustransduce mechanical vib rations into electrical signals?

13 . W hat is the pathway for auditory impulses from thecochlea to the cereb ral cortex ?

14 . Compare the function of the maculae in maintainingstatic eq uilib rium with the role of the cristae inmaintaining dy namic eq uilib rium.

15 . W hat is the role of vestib ular input to the cereb ellum?16 . D escrib e the eq uilib rium pathway s.

6 3 2 CHAPTER 17 • T HE S P E CIA L S E N S E S

F ig u r e 1 7 .2 6 The equilibrium pathway.

F rom hair cells of the semicircular ducts, utricle, and saccule, vestib ular information is convey ed along the vestib ularb ranch of the vestib ulocochlear (V III) nerve and then to the b rain stem, cereb ellum, thalamus, and cereb ral cortex .

W h e r e a r e t h e v e s t ib u la r n u c le i lo c a t e d ??


DEVELOPMENT OF THEEY ES A ND EA R S

O B J E C T I V E

• Describe the development of the eyes and the ears.

Ey e s

The eyesbegin to develop about 22 days after fertilization whenthe ectoderm of the lateral walls of the prosencephalon (fore-brain) bulges out to form a pair of shallow grooves called the

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optic grooves(Figure 17.27a). W ithin a few days, as the neuraltube is closing, the optic grooves enlarge and grow toward thesurface ectoderm and become known as the optic vesicles

(Figure 17.27b). W hen the optic vesicles reach the surface ecto-derm, the surface ectoderm thickens to form the lens placodes.

In addition, the distal portions of the optic vesicles invaginate(Figure 17.27c), forming the optic cups;they remain attached tothe prosencephalon by narrow, hollow proximal structures calledoptic stalks(Figure 17.27d).

The lens placodes also invaginate and develop into lens vesi-cles that sit in the optic cups. The lens vesicles eventually

DE V E L O P M E N T O F T H E E Y E S A N D E A R S 6 3 3

Auricle

Tympanicmembrane

External auditorycanal

TA BL E 1 7 . 2

Summary of Structures of the Ear

R EG ION S OF TH E EA R A N D K EY S TR U CTU R ES F U N CTION

Ex t e r n a l ( o u t e r ) e a r Auricle (pinna):Collects sound waves.

External auditory canal (meatus):Directs sound waves to the eardrum.

Tympanic membrane (eardrum):Sound waves cause it to vibrate, which in turn causes the

malleus to vibrate.

Mid d le e a r Auditory ossicles:Transmit and amplify vibrations from tympanic membrane to oval window.

Auditory (eustachian) tube:Equalizes air pressure on both sides of the tympanic membrane.

In t e r n a l ( in n e r ) e a r Cochlea:Contains a series of fluids, channels, and membranes that transmit vibrations to the

spiral organ (organ of Corti), the organ of hearing; hair cells in the spiral organ produce receptor

potentials, which elicit nerve impulses in the cochlear branch of the vestibulocochlear (VIII)

nerve.

Vestibular apparatus:Includes semicircular ducts, utricle, and saccule, which generate nerve

impulses that propagate along the vestibular branch of the vestibulocochlear (VIII) nerve.

Semicircular ducts:Contain cristae, site of hair cells for dynamic equilibrium (maintenance

of body position, mainly the head, in response to rotational acceleration and deceleration).

Utricle:Contains macula, site of hair cells for static equilibrium (maintenance of body posi-

tion, mainly the head, relative to the force of gravity).

Saccule:Contains macula, site of hair cells for static equilibrium (maintenance of body posi-

tion, mainly the head, relative to the force of gravity).

Auditorytube

Auditoryossicles

Semicircularducts

Utricle

Cochlea

Saccule


develop into the lenses.Blood is supplied to the developinglenses (and retina) by the hyaloid arteries. These arteries gainaccess to the developing eyes through a groove on the inferiorsurface of the optic cup and optic stalk called the choroid

fissure.As the lenses mature, part of the hyaloid arteriesthat pass through the vitreous chamber degenerate; the remain-ing portions of the hyaloid arteries become the central reti-nal arteries.


W all of prosencephalon

(forebrain)

Optic stalk

Outer layer

Mesenchyme

W all ofprosencephalon

Prosencephalon

Opticvesicles

Choroid fissure

Opticgrooves

Optic cup:

Surfaceectoderm

Lens placode andoptic vesicleinvaginating

(a) About 22 days

Lens

placode

(b) About 28 days (c) About 31 days

Inner layer

Lens vesicle

Mesenchyme

(d) About 32 days

Hyaloid

artery

Heart prominence

External view, about 28-day embryo

Lens placode

Prosencephalon (forebrain)

Otic placode

F ig u r e 1 7 .2 7 Development of the eyes.

Th e e y e s b e g in to d e v e lo p a b o u t 2 2 d a y s a fte r

fe rtiliz a tio n fro m e c to d e rm

o f th e p ro s e n c e p h a lo n .

W h ic h s t r u c t u r e g iv e s r is e t o t h e n e u r a l a n d p ig m e n t e d la y e r s o f t h e r e t in a ??

The inner wall of the optic cup forms the neural layerofthe retina, while the outer layer forms the pigmented layerof theretina. Axons from the neural layer grow through the optic stalkto the brain, converting the optic stalk to the optic (II) nerve.Although myelination of the optic nerves begins late in fetal life,it is not completed until the tenth week after birth.

The anterior portion of the optic cup forms the epithelium ofthe ciliary body, iris, and circular and radial musclesof the iris.The connective tissue of the ciliary body, ciliary muscle, andzonular fibersof the lens develop from mesenchyme around theanterior portion of the optic cup.

Mesenchyme surrounding the optic cup and optic stalkdifferentiates into an inner layer that gives rise to the choroidand an outer layer that develops into the sclera and part ofthe cornea.The remainder of the cornea is derived fromsurface ectoderm.

The anterior chamberdevelops from a cavity that forms inthe mesenchyme between the iris and cornea; the posteriorchamberdevelops from a cavity that forms in the mesenchymebetween the iris and lens.

Some mesenchyme around the developing eye enters theoptic cup through the choroid fissure. This mesenchyme occu-pies the space between the lens and retina and differentiates intoa delicate network of fibers. Later the spaces between the fibersfill with a jellylike substance, thus forming the vitreous bodyinthe vitreous chamber.


Mesenchyme

Otic pit

Otic placode Neural tube

forming

Notochord

Endoderm

Pharynx

W all of

rhombencephalon

(hindbrain)

Invaginating

otic placode

(a) About 22 days (b) About 24 days

Otic vesicle

(c) About 27 days (d) About 32 days

Lens placode

Heart prominence

Rhombencephalon

(hindbrain)

Otic placode

Pharyngeal arches

External view, about 28-day embryo

Pharyngeal clefts

4 3 2 1

The eyelidsform from surface ectoderm and mesenchyme.The upper and lower eyelids meet and fuse at about eight weeksof development and remain closed until about 26 weeks ofdevelopment.

Ears

The first portion of the ear to develop is the internal ear.It begins to form about 22 days after fertilization as a thicken-ing of the surface ectoderm, called otic placodes (Figure17.28a), that appear on either side of the rhombencephalon(hindbrain). The otic placodes invaginate quickly (Figure17.28b) to form the otic pits(Figure 17.28c). Next, the oticpits pinch off from the surface ectoderm to form the otic vesi-

cles within the mesenchyme of the head (Figure 17.28d).During later development, the otic vesicles will form thestructures associated with the membranous labyrinth of the

internal ear. Mesenchyme around the otic vesicles producescartilage that later ossifies to form the bone associated withthe bony labyrinthof the internal ear.

The middle eardevelops from a structure called the first pha-

ryngeal (branchial) pouch,an endoderm-lined outgrowth ofthe primitive pharynx (see the inset in Figure 17.28). The pha-ryngeal pouches are discussed in detail in Chapter 29 onpage 1149. The auditory ossicles develop from the first and sec-ond pharyngeal arches.

The external ear develops from the first pharyngeal cleft,anendoderm-lined groove between the first and second pharyngealarches (see the inset in Figure 17.28). The pharyngeal clefts arediscussed in detail in Chapter 29 on page 1149.

C H E C K P O I N T

17. How do the origins of the eyes and ears differ?

d

D E V E L O P M E N T O F T HE E Y E S A N D E A R S 6 3 5

F ig u r e 1 7 .2 8 Development of the ears.

T he first p arts of the ears to dev elop are the internal ears, whic h b egin to form ab ou t 2 2 days after

fertiliz ation as thic k enings of su rfac e ec toderm .

How do the three parts of the ear differ in origin??


AGING AND THE SPECIAL SENSESO B J E C T I V E

• Describe the age-related changes that occur in the eyesand ears.

Most people do not experience any problems with the senses ofsmell and taste until about age 50. This is due to a gradual lossof olfactory receptors and gustatory receptor cells coupled withtheir slower rate of replacement as we age.

Several age-related changes occur in the eyes. As notedearlier,the lens loses some of its elasticity and thus cannotchange shape as easily,resulting in presbyopia (see page 614).Cataracts (loss of transparency of the lenses) also occur withaging (see below). In old age,the sclera (“white” of the eye) becomes thick and rigid and develops a yellowish orbrownish coloration due to many years of exposure to ultravioletlight,wind,and dust. The sclera may also develop randomsplotches of pigment,especially in people with dark complex-ions. The iris fades or develops irregular pigment. The musclesthat regulate the size of the pupil weaken with age and the pupilsbecome smaller,react more slowly to light,and,dilate moreslowly in the dark. For these reasons,elderly people find thatobjects are not as bright,their eyes may adjust more slowlywhen going outdoors,and they have problems going from

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brightly lit to darkly lit places. Some diseases of the retina aremore likely to occur in old age,including age-related maculardisease (see page 610) and detached retina (see page 610). A disorder called glaucoma (see below) develops in the eyes of aging people as a result of the buildup of aqueous humor.Tear production and the number of mucous cells in the conjunc-tiva may decrease with age,resulting in dry eyes. The eyelidslose their elasticity,becoming baggy and wrinkled. The amountof fat around the orbits may decrease,causing the eyeballs tosink into the orbits. Finally,as we age the sharpness of visiondecreases,color and depth perception are reduced,and “vitrealfloaters”increase.

By about age 60,around 25% of individuals experience a no-ticeable hearing loss,especially for higher-pitched sounds. Theage-associated progressive loss of hearing in both ears is calledpresbycusis(pres9-bı-KU-sis;presby-5 old;-acou 5 hearing;-sis 5 condition). It may be related to damaged and lost haircells in the spiral organ or degeneration of the nerve pathway forhearing. Tinnitus (ringing in the ears)and vestibular imbalancealso occur more frequently in the elderly.

C H E C K P O I N T

1 8 . W hat changes in the eyes and ears are related to theaging p rocess, and how do they tak e p lace?

d

6 3 6 CHAPTER 1 7 • T H E S P E C IA L S E N S E S

D I S OR D E R S : H OM EOS TA TIC I M BA L A N CES

Ca t a r a c t s

A com m on cause of blindness is a loss of transp arency of the lensk now n as a c a t a r a c t (C A T -a-rak t 5 w aterfall). T he lens becom es cloudy(less transp arent) due to changes in the structure of the lens p roteins.C ataracts often occur w ith aging but m ay also be caused by injury, ex -cessiv e ex p osure to ultrav iolet rays, certain m edications (such aslong-term use of steroids), or com p lications of other diseases (for ex -am p le, diabetes). P eop le w ho sm ok e also hav e increased risk ofdev elop ing cataracts. F ortunately, sight can usually be restored by sur-gical rem ov al of the old lens and im p lantation of a new artificial one.

G la u c o m a

G la u c o m a (glaw -K O -m a) is the m ost com m on cause of blindness inthe U nited S tates, afflicting about 2% of the p op ulation ov er age 40.G laucom a is an abnorm ally high intraocular p ressure due to a buildupof aq ueous hum or w ithin the anterior cav ity. T he fluid com p resses thelens into the v itreous body and p uts p ressure on the neurons of theretina. P ersistent p ressure results in a p rogression from m ild v isualim p airm ent to irrev ersible destruction of neurons of the retina, dam -age to the op tic nerv e, and blindness. G laucom a is p ainless, and theother eye com p ensates largely, so a p erson m ay ex p erience consider-able retinal dam age and loss of v ision before the condition is diag-nosed. B ecause glaucom a occurs m ore often w ith adv ancing age, reg-ular m easurem ent of intraocular p ressure is an increasingly im p ortantp art of an eye ex am as p eop le grow older. R isk factors include race(black s are m ore suscep tible), increasing age, fam ily history, and p asteye injuries and disorders.

D e a f n e s s

D e a f n e s s is significant or total hearing loss. S e n s o r in e u r a l d e a f n e s s

is caused by either im p airm ent of hair cells in the cochlea or dam -age of the cochlear branch of the v estibulocochlear ( V III) nerv e. T histyp e of deafness m ay be caused by atherosclerosis, w hich reducesblood sup p ly to the ears; by rep eated ex p osure to loud noise, w hichdestroys hair cells of the sp iral organ; and/or by certain drugs suchas asp irin and strep tom ycin. Co n d u c t io n d e a f n e s s is caused by im -p airm ent of the ex ternal and m iddle ear m echanism s for transm ittingsounds to the cochlea. C auses of conduction deafness include oto-sclerosis, the dep osition of new bone around the ov al w indow ; im -p acted cerum en; injury to the eardrum ; and aging, w hich often re-sults in thick ening of the eardrum and stiffening of the joints of theauditory ossicles. A hearing test called Weber’s test is used to dis-tinguish betw een sensorineural and conduction deafness. In the test,the stem of a v ibrating fork is held to the forehead. In p eop le w ithnorm al hearing, the sound is heard eq ually in both ears. If thesound is heard best in the affected ear, the deafness is p robably ofthe conduction typ e; if the sound is heard best in the norm al ear, itis p robably of the sensorineural typ e.

M é n iè r e ’s D is e a s e

M é n iè r e ’s d is e a s e (m en9-e -A R Z ) results from an increased am ount ofendolym p h that enlarges the m em branous labyrinth. A m ong the sym p -tom s are fluctuating hearing loss (caused by distortion of the basilarm em brane of the cochlea) and roaring tinnitus (ringing). S p inning or

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MEDICAL TERMINOLOGY 637

M E D I C A L T E R M I N O L O G Y

Ageusia (a -GU -s e -a ; a- 5 w ith ou t; -geusis 5 ta s te ) Los s of th e s e n s eof ta s te .

Amblyopia (a m 9-b le -O -p e -a ; ambly- 5 d u ll or d im ) Te rm u s e d tod e s c rib e th e los s of v is ion in a n oth e rw is e n orm a l e y e th a t,b e c a u s e of m u s c le im b a la n c e , c a n n ot foc u s in s y n c h ron y w ith th eoth e r e y e . S om e tim e s c a lle d “ w a n d e rin g e y e b a ll” or a “ la z y e y e .”

Anosmia (a n -OZ -m e -a ; a- 5 w ith ou t; osmi 5 s m e ll, od or) Tota l la c k ofth e s e n s e of s m e ll.

Barotrauma (b a r9-o -TRAW -m a ; baros- 5 w e ig h t) Da m a g e or p a in ,m a in ly a ffe c tin g th e m id d le e a r, a s a re s u lt of p re s s u re c h a n g e s . Itoc c u rs w h e n p re s s u re on th e ou te r s id e of th e ty m p a n ic m e m b ra n eis h ig h e r th a n on th e in n e r s id e , for e x a m p le , w h e n fly in g in a na irp la n e or d iv in g . S w a llow in g or h old in g y ou r n os e a n d e x h a lin gw ith y ou r m ou th c los e d u s u a lly op e n s th e a u d itory tu b e s , a llow in ga ir in to th e m id d le e a r to e q u a liz e th e p re s s u re .

Blepharitis (b le f-a -RI -tis ; blephar- 5 e y e lid ; -itis 5 in fla m m a tion of ) Anin fla m m a tion of th e e y e lid .

Conjunctivitis (pinkeye) An in fla m m a tion of th e c on ju n c tiv a ; w h e n c a u s e db y b a c te ria s u c h a s p n e u m ococ c i, s ta p h y lococ c i, or Hemophilus

influenzae, it is v e ry c on ta g iou s a n d m ore c om m on in c h ild re n .Con ju n c tiv itis m a y a ls o b e c a u s e d b y irrita n ts , s u c h a s d u s t, s m ok e ,or p ollu ta n ts in th e a ir, in w h ic h c a s e it is n ot c on ta g iou s .

Corneal abrasion (K OR-n e -a l a -B RA -z h u n ) A s c ra tc h on th e s u rfa c e ofth e c orn e a , for e x a m p le , from a s p e c k of d irt or d a m a g e d c on ta c tle n s e s . S y m p tom s in c lu d e p a in , re d n e s s , w a te rin g , b lu rry v is ion ,s e n s itiv ity to b rig h t lig h t, a n d fre q u e n t b lin k in g .

Corneal transplant A p roc e d u re in w h ic h a d e fe c tiv e c orn e a isre m ov e d a n d a d on or c orn e a of s im ila r d ia m e te r is s e w n in . It isth e m os t c om m on a n d m os t s u c c e s s fu l tra n s p la n t op e ra tion .S in c e th e c orn e a is a v a s c u la r, a n tib od ie s in th e b lood th a t m ig h tc a u s e re je c tion d o n ot e n te r th e tra n s p la n te d tis s u e , a n dre je c tion ra re ly oc c u rs . Th e s h orta g e of d on or c orn e a s h a s b e e np a rtia lly ov e rc om e b y th e d e v e lop m e n t of a rtific ia l c orn e a s m a d eof p la s tic .

Diabetic retinopathy (re t-i-NOP -a -th e ; retino- 5 re tin a ; -pathos 5

s u ffe rin g ) De g e n e ra tiv e d is e a s e of th e re tin a d u e to d ia b e te sm e llitu s , in w h ic h b lood v e s s e ls in th e re tin a a re d a m a g e d or n e won e s g row a n d in te rfe re w ith v is ion .

Exotropia (e k 9-s o -TRO-p e -a ; ex- 5 ou t; -tropia 5 tu rn in g ) Tu rn in gou tw a rd of th e e y e s .

Keratitis (k e r9-a -TI -tis ; kerat- 5 c orn e a ) An in fla m m a tion or in fe c tionof th e c orn e a .

Miosis (m ı -O -s is ) Con s tric tion of th e p u p il.

Motion sickness P a le n e s s , re s tle s s n e s s , n a u s e a , w e a k n e s s , d iz z in e s s ,a n d m a la is e th a t m a y p rog re s s to v om itin g c a u s e d b y in c re a s e d

a c tiv ity of th e s e m ic irc u la r c a n a ls . It oc c u rs d u rin g m otion , fore x a m p le , in a c a r, on a b oa t, on a tra in , or in a n a irp la n e . U s u a lly ,w h e n m otion s top s , s y m p tom s im p rov e . Ov e r-th e -c ou n te r m e d ic a -tion s s u c h a s m e c liz in e ( B on in e ®) or d im e n h y d rin a te (Dra m a m in e ®)c a n b e ta k e n b e fore e m b a rk in g on a trip . A p re s c rip tion s k in p a tc hth a t c on ta in s s c op ola m in e ( Tra n s d e rm S c op ® ) c a n a ls o b e ta k e nb e fore s y m p tom s oc c u r.

Mydriasis (m i-DRI -a -s is ) Dila tion of th e p u p il.

Nystagmus (n is -TAG-m u s ; nystagm- 5 n od d in g or d row s y ) A ra p idin v olu n ta ry m ov e m e n t of th e e y e b a lls , p os s ib ly c a u s e d b y ad is e a s e of th e c e n tra l n e rv ou s s y s te m . It is a s s oc ia te d w ithc on d ition s th a t c a u s e v e rtig o.

Otalgia (o -TAL-je -a ; oto- 5 e a r; -algia 5 p a in ) Ea ra c h e .

Photophobia (fo 9-to -F O -b e -a ; photo- 5 lig h t; -phobia 5 fe a r) Ab n orm a l v is u a l in tole ra n c e to lig h t.

Ptosis (TO -s is 5 fa ll) F a llin g or d roop in g of th e e y e lid (or s lip p a g e ofa n y org a n b e low its n orm a l p os ition ).

Retinoblastoma (re t-i-n o -b la s -TO -m a ; -oma 5 tu m or) A tu m or a ris in gfrom im m a tu re re tin a l c e lls ; it a c c ou n ts for 2% of c h ild h oodc a n c e rs .

Scotoma (s k o -TO -m a 5 d a rk n e s s ) An a re a of re d u c e d or los t v is ion inth e v is u a l fie ld .

Strabismus (s tra -B IZ -m u s ; strabismos 5 s q u in tin g ) Mis a lig n m e n t ofth e e y e b a lls s o th a t th e e y e s d o n ot m ov e in u n is on w h e n v ie w in ga n ob je c t; th e a ffe c te d e y e tu rn s e ith e r m e d ia lly or la te ra lly w ithre s p e c t to th e n orm a l e y e a n d th e re s u lt is d ou b le v is ion (d ip lop ia ).It m a y b e c a u s e d b y p h y s ic a l tra u m a , v a s c u la r in ju rie s , or tu m orsof th e e x trin s ic e y e m u s c le or th e oc u lom otor (III), troc h le a r (IV ),or a b d u c e n s (V I) c ra n ia l n e rv e s .

Tinnitus (ti-NI -tu s ) A rin g in g , roa rin g , or c lic k in g in th e e a rs .

Tonometer (to -NOM-e -te r; tono- 5 te n s ion or p re s s u re ; -metron 5

m e a s u re ) An in s tru m e n t for m e a s u rin g p re s s u re , e s p e c ia llyin tra oc u la r p re s s u re .

Trachoma (tra -K O -m a ) A s e riou s form of c on ju n c tiv itis a n d th e g re a te s t s in g le c a u s e of b lin d n e s s in th e w orld . It is c a u s e d b y th eb a c te riu m Chlamydia trachomatis. Th e d is e a s e p rod u c e s a ne x c e s s iv e g row th of s u b c on ju n c tiv a l tis s u e a n d in v a s ion of b loodv e s s e ls in to th e c orn e a , w h ic h p rog re s s e s u n til th e e n tire c orn e ais op a q u e .

Vertigo ( V ER-ti-g o 5 d iz z in e s s ) A s e n s a tion of s p in n in g or m ov e m e n tin w h ic h th e w orld s e e m s to re v olv e or th e p e rs on s e e m s tore v olv e in s p a c e , ofte n a s s oc ia te d w ith n a u s e a a n d , in s om e c a s e s ,v om itin g . It m a y b e c a u s e d b y a rth ritis of th e n e c k or a n in fe c tionof th e v e s tib u la r a p p a ra tu s .

w h irlin g v e rtig o (d iz z in e s s ) is c h a ra c te ris tic of Mé n iè re ’s d is e a s e .Alm os t tota l d e s tru c tion of h e a rin g m a y oc c u r ov e r a p e riod of y e a rs .

O t it is M e d ia

O t it is m e d ia is a n a c u te in fe c tion of th e m id d le e a r c a u s e d m a in ly b yb a c te ria a n d a s s oc ia te d w ith in fe c tion s of th e n os e a n d th roa t.S y m p tom s in c lu d e p a in , m a la is e , fe v e r, a n d a re d d e n in g a n d ou tw a rdb u lg in g of th e e a rd ru m , w h ic h m a y ru p tu re u n le s s p rom p t tre a tm e n t isre c e iv e d . ( Th is m a y in v olv e d ra in in g p u s from th e m id d le e a r.)

B a c te ria p a s s in g in to th e a u d itory tu b e from th e n a s op h a ry n x a re th ep rim a ry c a u s e of m id d le e a r in fe c tion s . Ch ild re n a re m ore s u s c e p tib leth a n a d u lts to m id d le e a r in fe c tion s b e c a u s e th e ir a u d itory tu b e s a rea lm os t h oriz on ta l, w h ic h d e c re a s e s d ra in a g e . If otitis m e d ia oc c u rsfre q u e n tly , a s u rg ic a l p roc e d u re c a lle d t y m p a n o t o m y (tim 9-p a -NOT-o -m e ; tympano- 5 d ru m ; -tome 5 in c is ion ) is ofte n e m p loy e d . Th isc on s is ts of th e in s e rtion of a s m a ll tu b e in to th e e a rd ru m to p rov id e ap a th w a y for th e d ra in a g e of flu id from th e m id d le e a r.



S T U D Y O U T L I N E

11.Horizontal cells transmit inhibitory signals to bipolar cells; bipolar

or amacrine cells transmit excitatory signals to ganglion cells,

which depolarize and initiate nerve impulses.

12.Impulses from ganglion cells are conveyed into the optic (II) nerve,

through the optic chiasm and optic tract,to the thalamus. From the

thalamus,impulses for vision propagate to the cerebral cortex

(occipital lobe). Axon collaterals of retinal ganglion cells extend to

the midbrain and hypothalamus.

Hearing and Equilibrium (p. 620)

1.The external (outer) ear consists of the auricle,external auditory

canal,and tympanic membrane (eardrum).

2.The middle ear consists of the auditory tube,ossicles,oval win-

dow,and round window.

3.The internal (inner) ear consists of the bony labyrinth and

membranous labyrinth. The internal ear contains the spiral organ

(organ of Corti),the organ of hearing.

4.Sound waves enter the external auditory canal,strike the tympanic

membrane,pass through the ossicles,strike the oval window,set

up waves in the perilymph,strike the vestibular membrane and

scala tympani,increase pressure in the endolymph,vibrate the

basilar membrane,and stimulate hair bundles on the spiral organ

(organ of Corti).

5.Hair cells convert mechanical vibrations into a receptor potential,

which releases neurotransmitter that can initiate nerve impulses in

first-order sensory neurons.

6.Sensory axons in the cochlear branch of the vestibulocochlear

(VIII) nerve terminate in the medulla oblongata. Auditory signals

then pass to the inferior colliculus,thalamus,and temporal lobes of

the cerebral cortex.

7.Static equilibrium is the orientation of the body relative to the pull

of gravity. The maculae of the utricle and saccule are the sense

organs of static equilibrium. Body movements that stimulate the

receptors for static equilibrium include tilting the head and linear

acceleration or deceleration.

8.Dynamic equilibrium is the maintenance of body position in response

to rotational acceleration or deceleration. The cristae in the semicircu-

lar ducts are the main sense organs of dynamic equilibrium.

9.Most vestibular branch axons of the vestibulocochlear nerve enter

the brain stem and terminate in the medulla and pons; other axons

enter the cerebellum.

Development of the Eyes and Ears (p. 633)

1.The eyes begin their development about 22 days after fertilization

from ectoderm of the lateral walls of the prosencephalon (forebrain).

2.The ears begin their development about 22 days after fertilization

from a thickening of ectoderm on either side of the rhomben-

cephalon (hindbrain). The sequence of development of the ear is

internal ear,middle ear,and external ear.

Aging and the Special Senses (p. 636)

1.Most people do not experience problems with the senses of smell

and taste until about age 50.

2.Among the age-related changes to the eyes are presbyopia,

cataracts,difficulty adjusting to light,macular disease,glaucoma,

dry eyes,and decreased sharpness of vision.

3.W ith age there is a progressive loss of hearing and tinnitus occurs

more frequently.

Olfaction:Sense of Smell (p. 599)

1.The receptors for olfaction,which are bipolar neurons,are in the

nasal epithelium along with olfactory glands,which produce

mucus that dissolves odorants.

2.In olfactory reception,a generator potential develops and triggers

one or more nerve impulses.

3.The threshold of smell is low, and adaptation to odors

occurs quickly.

4.Axons of olfactory receptors form the olfactory (I) nerves,which

convey nerve impulses to the olfactory bulbs,olfactory tracts,

limbic system,and cerebral cortex (temporal and frontal lobes).

Gustation:Sensation of Taste (p. 602)

1.The receptors for gustation,the gustatory receptor cells,are located

in taste buds.

2.Dissolved chemicals,called tastants,stimulate gustatory recep-

tor cells by flowing through ion channels in the plasma mem-

brane or by binding to receptors attached to G proteins in the

membrane.

3.Receptor potentials developed in gustatory receptor cells cause the

release of neurotransmitter,which can generate nerve impulses in

first-order sensory neurons.

4.The threshold varies with the taste involved,and adaptation to taste

occurs quickly.

5.Gustatory receptor cells trigger nerve impulses in cranial nerves

VII,IX,and X. Taste signals then pass to the medulla oblongata,

thalamus,and cerebral cortex (parietal lobe).

Vision (p. 604)

1.Accessory structures of the eyes include the eyebrows,eyelids,

eyelashes,lacrimal apparatus,and extrinsic eye muscles.

2.The lacrimal apparatus consists of structures that produce and

drain tears.

3.The eye is constructed of three layers:(a) fibrous tunic (sclera and

cornea),(b) vascular tunic (choroid,ciliary body,and iris),and

(c)retina.

4.The retina consists of a pigmented layer and a neural layer that

includes a photoreceptor layer,bipolar cell layer,ganglion cell

layer,horizontal cells,and amacrine cells.

5.The anterior cavity contains aqueous humor; the vitreous chamber

contains the vitreous body.

6.Image formation on the retina involves refraction of light rays by

the cornea and lens,which focus an inverted image on the fovea

centralis of the retina.

7.For viewing close objects,the lens increases its curvature (accom-

modation) and the pupil constricts to prevent light rays from

entering the eye through the periphery of the lens.

8.The near point of vision is the minimum distance from the eye at

which an object can be clearly focused with maximum accom-

modation.

9.In convergence,the eyeballs move medially so they are both

directed toward an object being viewed.

10.The first step in vision is the absorption of light by photopigments

in rods and cones and isomerization of cis-retinal. Receptor

potentials in rods and cones decrease the release of inhibitory

neurotransmitter,which induces graded potentials in bipolar cells

and horizontal cells.


SELF-QUIZ QUESTIONS 639

S E L F - Q U I Z Q U E S T I O N S

Fill in the blanks in the following statements.

1.The five primary taste sensations are , , ,,and .

2. equilibrium refers to the maintenance of the position of thebody relative to the force of gravity; equilibrium refers tothe maintenance of body position in response to rotational acceler-ation or deceleration.

Indicate whether the following statements are true or false.

3.Of all of the special senses,only smell and taste sensations projectboth to higher cortical areas and to the limbic system.

4.The ability to change the curvature of the lens for near vision isconvergence.

Choose the one best answer to the following questions.

5.W hich of the following are true? (1) The sites of olfactory trans-duction are the olfactory hairs. (2) The olfactory bulbs transmitimpulses to the temporal lobe of the brain. (3) The axons of olfac-tory receptors pass through the olfactory foramina in the cribri-form plate of the ethmoid bone. (4) The olfactory nerves are bun-dles of axons that terminate in the olfactory tracts. (5) W ithin theolfactory bulbs,the first-order neurons synapse with the second-order neurons. (a) 1,2,and 4 (b) 2,3,4,and 5 (c) 1,2,3,4,and 5 (d) 1,3,and 5 (e) 1,2,3,and 5

6.W hich of the following statements is incorrect?(a) Olfactory receptors respond to the chemical stimulation of an

odorant molecule by producing a receptor potential. (b) Basal stem cells continually produce new olfactory receptors. (c) Adaptation to odors is rapid and occurs in both olfactory

receptors and the CNS. (d) Production of nasal mucus by olfactory glands serves to

moisten the olfactory epithelium and dissolve odorants. (e) The orbitofrontal area is an important region for odor

identification and discrimination.

7.W hich of the following statements is incorrect?(a) Taste is a chemical sense. (b) The receptors for taste sensations are found in taste buds located

on the tongue,the soft palate,the pharynx,and the epiglottis. (c) Gustatory hairs are the sites of taste transduction. (d) The threshold for bitter substances is the highest. (e) Complete adaptation to taste can occur in 1 to 5 minutes.

8.W hen viewing an object close to your eyes,which of the followingare required for proper image formation on the retina? (1) in-creased curvature of the lens,(2) contraction of the ciliary muscle,(3) divergence of the eyeballs,(4) refraction of light at the anteriorand posterior surfaces of the cornea,(5) constriction of the pupilby contraction of the extrinsic eye muscles. (a) 1,2,3,4,and 5 (b) 1,2,and 4 (c) 1,2,3,and 4 (d) 2,4,and 5 (e) 2,3,and 4

9.W hich of the following are mismatched?(a) fungiform papillae:scattered over the entire tongue’s surface. (b) filiform papillae:contain taste buds in early childhood. (c) vallate papillae:each houses 100–300 taste buds. (d) foliate papillae:located in trenches on the lateral margins of

the tongue. (e) fungiform papillae:each houses about five taste buds.

10.Place in order the structures involved in the visual pathway. (a) optic tract (b) ganglion cells (c) cornea (d) lens (e) bipolar cells (f) optic nerve(g) visual cortex (h) vitreous body (i) optic chiasm( j) aqueous humor (k) pupil (l) photoreceptors (m) thalamus

11.W hich of the following statements is incorrect?(a) Retinal is the light-absorbing portion of all visual photo-

pigments.(b) The only photopigment in rods is rhodopsin,but three different

cone photopigments are present in the retina. (c) Retinal is a derivative of vitamin C. (d) Color vision results from different colors of light selectively

activating different cone photopigments. (e) Bleaching and regeneration of the photopigments account for

much but not all of the sensitivity changes during light anddark adaptation.

12.W hich of the following is the correct sequence for the auditorypathway? (a) external auditory canal,tympanic membrane,auditory ossicles,

oval window,cochlea and spiral organ (b) tympanic membrane,external auditory canal,auditory ossicles,

cochlea and spiral organ,round window(c) auditory ossicles,tympanic membrane,cochlea and spiral

organ,round window,oval window,external auditory canal(d) auricle,tympanic membrane,round window,cochlea and spiral

organ,oval window(e) external auditory canal,tympanic membrane,auditory ossicles,

internal auditory canal,spiral organ,oval window

13.Match the following:(a) upper and lower eyelids; shade the

eyes during sleep,spread lubricatingsecretions over the eyeballs

(b)produces and drains tears(c) arch transversely above the eyeballs

and help protect the eyeballs fromforeign objects,perspiration,and thedirect rays of the sun

(d)move the eyeball medially,laterally,superiorly,or inferiorly

(e) a thick fold of connective tissue thatgives form and support to the eyelids

(f) modified sebaceous glands;secretion helps keep eyelids fromadhering to one another

(g)project from the border of eacheyelid; help protect the eyeballsfrom foreign objects,perspiration,and direct rays of the sun

(h)a thin,protective mucous membranethat lines the inner aspect of theeyelids and passes from the eyelidsonto the surface of the eyeball,where it covers the sclera

(1)palpebrae

(2)tarsal or

Meibomian

glands

(3)conjunctiva

(4)eyelashes

(5)lacrimal

apparatus

(6)extrinsic eye

muscles

(7)eyebrows

(8)tarsal plate


14.Match the following:(a) lines most of the internal surface

of the sclera; provides nutrientsto the posterior surface of theretina

(b)colored portion of the eyeball;regulates the amount of light en-tering the posterior part of theeyeball

(c) innermost layer of the eyeball;beginning of the visual pathway;contains rods and cones

(d)biconvex transparent structurethat fine tunes focusing of lightrays for clear vision

(e) transparent part of the eyeballthat covers the iris; helps focuslight

(f) circular band of smooth musclethat alters the shape of the lensfor near or far vision

(g)site where the optic nerve exitsthe eyeball; the blind spot

(h)watery fluid in the anterior cavitythat helps nourish the lens andcornea; helps maintain shape ofthe eyeball

(i) the hole in the center of the iris(j) jellylike substance in the vitreous

chamber that helps prevent theeyeball from collapsing andholds the retina flush against theinternal portions of the eyeball

(k)white of the eye; gives shape tothe eyeball,makes it more rigid,protects its inner parts

(l) avascular superficial layer of theeyeball; includes cornea andsclera

(m)small depression in the center ofthe macula lutea that containsonly cone photoreceptors and isthe area of highest visual acuity

(n)contain blood capillaries that se-crete aqueous humor; attach tosuspensory ligaments of lens

(o)middle,vascularized layer of theeyeball; includes choroid,ciliarybody,and iris

15.Match the following:(a) partition between external

auditory canal and middle ear (b)oval central portion of the bony

labyrinth; contains utricle andsaccule

(c) receptor for static equilibrium;also contributes to some aspectsof dynamic equilibrium; consistsof hair cells and supporting cells

(d)contains hair cells which are thereceptors for hearing

(e) ear bones:malleus,incus,stapes(f) the pressure equalization tube

that connects the middle ear tothe nasopharynx

(g)contains the spiral organ(h)fluid found within the membra-

nous labyrinth; pressure waves inthis fluid cause vibration of thebasilar membrane

(i) receptor organs for equilibrium;the saccule,utricle,and semicir-cular canals

(j) swollen enlargement in semicir-cular canals; contains structuresinvolved in dynamic equilibrium

(k)opening between the middle earand internal ear; is enclosed by amembrane called the secondarytympanic membrane

(l) the flap of elastic cartilage cov-ered by skin that captures soundwaves; the pinna

(m)fluid found inside bony labyrinth;bulging of the oval windowcauses pressure waves in thisfluid

(n)opening between the middle andinner ear; receives base of stapes


(1)cornea

(2)sclera

(3)choroid

(4)ciliary

processes

(5)ciliary muscle

(6)iris

(7)pupil

(8)uvea

(9)retina

(10)optic disc

(11)fibrous tunic

(12)fovea centralis

(13)aqueous humor

(14)lens

(15)vitreous body

(1)auricle

(2)tympanic

membrane

(3)auditory

ossicles

(4)vestibular

apparatus

(5)ampulla

(6)cochlea

(7)perilymph

(8)oval window

(9)round window

(10)auditory or

eustachian tube

(11)vestibule

(12)endolymph

(13)spiral organ

(14)macula


ANSWERS TO FIGURE QUESTIONS 641

C R I T I C A L T H I N K I N G Q U E S T I O N S

1. Mario has experienced damage to his facial nerve. How would thisaffect his special senses?

2. The shift nurse brings ailing eighty-year-old Gertrude her dinner.As Gertrude eats a small amount of her food,she comments thatshe isn’t hungry and that “hospital food just doesn’t taste good!”The nurse gives Gertrude a menu so she can choose her morningbreakfast. Gertrude complains that she is having trouble readingthe menu and asks the nurse to read it to her. As the nurse begins

to read,Gertrude loudly asks her to “speak up and turn off thebuzzing.”W hat does the nurse know about aging and the specialsenses that help to explain Gertrude’s comments?

3. As you help your neighbor put drops in her six-year-old daughter’seyes,the daughter states,“That medicine tastes bad.”How do youexplain to the neighbor how her daughter can “taste”the eyedrops?

17.1An olfactory receptor has a life span of about one month.17.2Olfactory transduction occurs in the olfactory hairs of an olfac-

tory receptor.17.3Basal cells develop into gustatory receptor cells.17.4Visible light that has a wavelength of 700 nm is red.17.5The conjunctiva is continuous with the inner lining of the eye-

lids.17.6Lacrimal fluid,or tears,is a watery solution containing salts,

some mucus,and lysozyme that protects,cleans,lubricates,andmoistens the eyeball.

17.7The fibrous tunic consists of the cornea and sclera; the vasculartunic consists of the choroid,ciliary body,and iris.

17.8The parasympathetic division of the ANS causes pupillary con-striction; the sympathetic division causes pupillary dilation.

17.9An ophthalmoscopic examination of the blood vessels of the eyecan reveal evidence of hypertension,diabetes mellitus,cataracts,and age-related macular disease.

17.10The two types of photoreceptors are rods and cones. Rods pro-vide black-and-white vision in dim light; cones provide high vi-sual acuity and color vision in bright light.

17.11After its secretion by the ciliary process,aqueous humor flowsinto the posterior chamber,around the iris,into the anteriorchamber,and out of the eyeball through the scleral venous sinus.

17.12During accommodation the ciliary muscle contracts,causing thezonular fibers to slacken. The lens then becomes more convex,increasing its focusing power.

17.13Presbyopia is the loss of lens elasticity that occurs with aging.17.14Both rods and cones transduce light into receptor potentials,use

a photopigment embedded in outer segment discs or folds,andrelease neurotransmitter at synapses with bipolar cells and hori-zontal cells.

17.15The conversion of cis-retinal to trans-retinal is called isomeriza-tion.

17.16Cyclic GMP is the ligand that opens Na+ channels in photore-ceptors,causing the dark current to flow.

17.17Light rays from an object in the temporal half of the visual fieldfall on the nasal half of the retina.

17.18The malleus of the middle ear is attached to the eardrum,whichis part of the external ear.

17.19The oval and round windows separate the middle ear from theinternal ear.

17.20The two sacs in the membranous labyrinth of the vestibule arethe utricle and saccule.

17.21The three subdivisions of the bony labyrinth are the semicircularcanals,vestibule,and cochlea.

17.22The region of the basilar membrane close to the oval and roundwindows vibrates most vigorously in response to high-frequencysounds.

17.23The superior olivary nucleus of the pons is the part of the audi-tory pathway that allows a person to locate the source of asound.

17.24The maculae are associated with static equilibrium; they providesensory information about the position of the head in space.

17.25The semicircular ducts are associated with dynamic equilibrium.17.26The vestibular nuclei are located in the medulla and the pons.17.27The optic cup forms the neural and pigmented layers of the optic

part of the retina.17.28The internal ear develops from surface ectoderm,the middle ear

develops from pharyngeal pouches,and the external ear devel-ops from a pharyngeal cleft.

A N S W E R S T O F I G U R E Q U E S T I O N S?


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