THE SPECIAL SENSES PART 2THE SPECIAL SENSES PART 2 SMELL, TASTESMELL, TASTE

Chemical SensesChemical Senses

Taste and smell (olfaction) Taste and smell (olfaction) Their chemoreceptors respond to chemicals in aqueous solutionaqueous solution

Sense of SmellSense of SmellThe organ of smell—olfactory epithelium in the roof of the nasal cavity Olfactory receptor cells—bipolar neurons with radiating lf t iliolfactory cilia

Bundles of axons of olfactory receptor cells form the filaments of the olfactory nerve (cranial nerve I)of the olfactory nerve (cranial nerve I)Supporting cells surround and cushion olfactory receptor cells Basal cells lie at the base of the epitheliumBasal cells lie at the base of the epithelium

Olfactoryepithelium

Olf t t t

epithelium

Olfactory tractOlfactory bulb

Nasalconchae

Figure 15.21a

(a) Route ofinhaled air

Mitral cell (output cell)Olfactory Mitral cell (output cell)Olfactorytract

Cribriform plate of ethmoid boneOlfactory bulbGlomeruli

Filaments of olfactory nerve

Cribriform plate of ethmoid bone

Lamina propria connective tissueOlfactorygland

Olfactory

Basal cell

S ti ll

Axon

Olfactory receptor cellOlfactoryepithelium

Supporting cellDendriteOlfactory ciliaMucus

Figure 15.21a

Route of inhaled aircontaining odor molecules(b)

Physiology of SmellPhysiology of SmellDissolved odorants bind to receptor proteins in the p polfactory cilium membranesA G protein mechanism is activated, which produces A G protein mechanism is activated, which produces cAMP as a second messengercAMP opens Na+ and Ca2+ channels, causing cAMP opens Na and Ca channels, causing depolarization of the receptor membrane that then triggers an action potentialgg p

Olfactory PathwayOlfactory Pathway

Olfactory receptor cells synapse with mitral cells in Olfactory receptor cells synapse with mitral cells in glomeruli of the olfactory bulbsMitral cells amplify refine and relay signals along Mitral cells amplify, refine, and relay signals along the olfactory tracts to the:

fOlfactory cortex Hypothalamus, amygdala, and limbic system

1 Od t bi d1 Odorant bindsto its receptor.

Odorant Adenylate cyclaseG protein (Golf)

O

Receptor

OpencAMP-gated

cation channelGDP

2 Receptoractivates G

3 G proteinactivates

4 Adenylatecyclase converts

5 cAMP opens a cation channel allowing

Figure 15.22

protein (Golf). adenylate cyclase.

ATP to cAMP. Na+ and Ca2+ influx and causing depolarization.

Sense of TasteSense of Taste

Receptor organs are taste buds Receptor organs are taste buds Found on the tongue

On the tops of fungiform papillae On the tops of fungiform papillae On the side walls of foliate papillae and circumvallate (vallate) papillae( ) p p

Epiglottis

Palatine tonsilLingual tonsil

Foliate papillae

Fungiform papillae

Figure 15.23a

(a) Taste buds are associated with fungiform,foliate, and circumvallate (vallate) papillae.

g p p

Circumvallate papilla

Taste bud

Figure 15.23b

(b) Enlarged section of acircumvallate papilla.

Structure of a Taste BudStructure of a Taste Bud

Flask shapedFlask shaped50–100 epithelial cells:

B l ll d i ll Basal cells—dynamic stem cells Gustatory cells—taste cells

Microvilli (gustatory hairs) project through a taste pore to the surface of the epithelium

ConnectiveG t t

Taste fibersof cranial

tissue Gustatoryhair

nerve

Gustatory(taste) cells

Tastepore

Stratifiedsquamousepithelium

f t

Basalcells

of tongue

Figure 15.23c

(c) Enlarged view of a taste bud.

Taste SensationsTaste SensationsThere are five basic taste sensations1. Sweet—sugars, saccharin, alcohol, and some amino

acids2. Sour—hydrogen ions3. Salt—metal ions4. Bitter—alkaloids such as quinine and nicotine5. Umami—amino acids glutamate and aspartateg p

Physiology of TastePhysiology of TasteIn order to be tasted, a chemical:In order to be tasted, a chemical:

Must be dissolved in salivaMust contact gustatory hairsMust contact gustatory hairs

Binding of the food chemical (tastant)D l i th t t ll b i l f Depolarizes the taste cell membrane, causing release of neurotransmitterInitiates a generator potential that elicits an action potentialInitiates a generator potential that elicits an action potential

Taste TransductionTaste TransductionThe stimulus energy of taste causes gustatory cell The stimulus energy of taste causes gustatory cell depolarization by:

Na+ influx in salty tastes (directly causes depolarization)Na influx in salty tastes (directly causes depolarization)H+ in sour tastes (by opening cation channels)G protein gustducin in sweet bitter and umami tastes (leads G protein gustducin in sweet, bitter, and umami tastes (leads to release of Ca2+ from intracellular stores, which causes opening of cation channels in the plasma membrane)

Gustatory PathwayGustatory Pathway

Cranial nerves VII and IX carry impulses from taste Cranial nerves VII and IX carry impulses from taste buds to the solitary nucleus of the medullaImpulses then travel to the thalamus and from there Impulses then travel to the thalamus and from there fibers branch to the:

Gustatory cortex in the insulaHypothalamus and limbic system (appreciation of taste)

Gustatory cortex(i i l )(in insula)

Thalamic nucleus(ventral posteromedial(ventral posteromedialnucleus) Pons

Solitary nucleus inmedulla oblongatag

Facial nerve (VII)Glossopharyngealnerve (IX)

Vagus nerve (X)

( )

Figure 15.24

Influence of Other Sensations on TasteInfluence of Other Sensations on Taste

Taste is 80% smellTaste is 80% smellThermoreceptors, mechanoreceptors, nociceptors in the mouth also influence tastesthe mouth also influence tastesTemperature and texture enhance or detract from taste

The Ear: Hearing and BalanceThe Ear: Hearing and Balance

Three parts of the earThree parts of the ear1. External (outer) ear2 Middle ear (tympanic cavity)2. Middle ear (tympanic cavity)3. Internal (inner) ear

The Ear: Hearing and BalanceThe Ear: Hearing and BalanceExternal ear and middle ear are involved with hearingInternal ear (labyrinth) functions in both hearing and Internal ear (labyrinth) functions in both hearing and equilibriumReceptors for hearing and balance Receptors for hearing and balance

Respond to separate stimuliAre activated independentlyAre activated independently

Externalear

Middleear

Internal ear(labyrinth)

Auricle(pinna)

ear

Helix

Externalacoustic

LobulePharyngotympanicTympanic

Figure 15.25a

acousticmeatus(a) The three regions of the ear

y g y p(auditory) tube

y pmembrane

External EarExternal Ear

The auricle (pinna) is composed of:The auricle (pinna) is composed of:Helix (rim)Lobule (earlobe)Lobule (earlobe)

External acoustic meatus (auditory canal)Short, curved tube lined with skin bearing hairs, sebaceous glands, and ceruminous glands

External EarExternal Ear

Tympanic membrane (eardrum)Tympanic membrane (eardrum)Boundary between external and middle earsConnective tissue membrane that vibrates in response to Connective tissue membrane that vibrates in response to soundT f d t th b f th iddl Transfers sound energy to the bones of the middle ear

Middle Ear Middle Ear

A small, air-filled, mucosa-lined cavity in the A small, air filled, mucosa lined cavity in the temporal bone

Flanked laterally by the eardrumFlanked laterally by the eardrumFlanked medially by bony wall containing the oval (vestibular) and round (cochlear) windows(vestibular) and round (cochlear) windows

Middle Ear Middle Ear

Epitympanic recess—superior portion of the middle Epitympanic recess superior portion of the middle earPharyngotympanic (auditory) tube connects the Pharyngotympanic (auditory) tube—connects the middle ear to the nasopharynx

Equalizes pressure in the middle ear cavity with the external air pressure

SemicircularOval window(deep to stapes)Entrance to mastoidantrum in the epitympanic recess

Semicircularcanals

(deep to stapes)

Auditoryossicles

p y p

VestibularIncu(anvil)

Malleus(hammer)

Stapes

Vestibule

ossicles

Tympanic membraneCochlea

Cochlearnervenerve

( )Stapes(stirrup)

Round window

Figure 15.25b

Pharyngotympanic(auditory) tube(b) Middle and internal ear

Ear OssiclesEar OssiclesThree small bones in tympanic cavity: the malleus, incus, y p y , ,and stapes

Suspended by ligaments and joined by synovial jointsTransmit vibratory motion of the eardrum to the oval windowT i d di l fl i l i Tensor tympani and stapedius muscles contract reflexively in response to loud sounds to prevent damage to the hearing receptors p

MalleusSuperior IncusEpitympanic

recessp

Anterior

Lateralrecess

View

Pharyngotym-panic tube

T T i St St di

Figure 15.26

Tensortympanimuscle

Tympanicmembrane(medial view)

Stapes Stapediusmuscle

Internal EarInternal Ear

Bony labyrinthBony labyrinthTortuous channels in the temporal boneThree parts vestibule semicircular canals and cochlea Three parts: vestibule, semicircular canals, and cochlea

Filled with perilymphSeries of membranous sacs within the bony labyrinthFilled with a potassium-rich endolymph

Superior vestibular ganglion

SemicircularTemporalbone

Superior vestibular ganglionInferior vestibular ganglion

Anterior

ducts insemicircularcanals

Posterior

Facial nerveVestibularnerve

PosteriorLateral

Cristae ampullaresin the membranous

CochlearnerveMaculaein the membranous

ampullaeUtricle investibuleSaccule investibule Stapes in

MaculaeSpiral organ(of Corti)Cochlearductin cochlea

Figure 15.27

vestibule Stapes inoval window

Roundwindow

VestibuleVestibuleCentral egg-shaped cavity of the bony labyrinthC gg p v y y yContains two membranous sacs1 Saccule is continuous with the cochlear duct1. Saccule is continuous with the cochlear duct2. Utricle is continuous with the semicircular canals

These sacsThese sacsHouse equilibrium receptor regions (maculae)R d i d h i h i i f h h dRespond to gravity and changes in the position of the head

Semicircular CanalsSemicircular CanalsThree canals (anterior, lateral, and posterior) that each define two-thirds of a circleMembranous semicircular ducts line each canal and

i t ith th t i lcommunicate with the utricleAmpulla of each canal houses equilibrium receptor region called the crista ampullarisregion called the crista ampullarisReceptors respond to angular (rotational) movements of the head

Superior vestibular ganglion

SemicircularTemporalbone

Superior vestibular ganglionInferior vestibular ganglion

Anterior

ducts insemicircularcanals

Posterior

Facial nerveVestibularnerve

PosteriorLateral

Cristae ampullaresin the membranous

CochlearnerveMaculaein the membranous

ampullaeUtricle investibuleSaccule investibule Stapes in

MaculaeSpiral organ(of Corti)Cochlearductin cochlea

Figure 15.27

vestibule Stapes inoval window

Roundwindow

The CochleaThe Cochlea

A spiral, conical, bony chamberA spiral, conical, bony chamberExtends from the vestibuleCoils around a bony pillar (modiolus)Coils around a bony pillar (modiolus)Contains the cochlear duct, which houses the spiral

( f C ti) d d t th hl organ (of Corti) and ends at the cochlear apex

The CochleaThe CochleaThe cavity of the cochlea is divided into three chambersv y v

Scala vestibuli—abuts the oval window, contains perilymphScala media (cochlear duct)—contains endolymphScala media (cochlear duct) contains endolymphScala tympani—terminates at the round window; contains perilymphp y p

The scalae tympani and vestibuli are continuous with each other at the helicotrema (apex)( p )

The CochleaThe CochleaThe “roof” of the cochlear duct is the vestibular membraneThe “floor” of the cochlear duct is composed of:

The bony spiral laminaThe basilar membrane, which supports the organ of C tiCorti

The cochlear branch of nerve VIII runs from the organ of Corti to the brain organ of Corti to the brain

Modiolus Cochlear nerve,division of thevestibulocochlearnerve (VIII)Snerve (VIII)Spiral ganglionOsseous spiral laminaVestibular membrane

(a) Helicotrema

Cochlear duct(scala media)

Figure 15.28a

(a)

Vestibular membrane Osseous spiral lamina

Cochlear duct(scala media;

Tectorial membraneScalavestibuli( t i

Spiralganglion( ;

containsendolymph)

(containsperilymph)

Strial i

Scala tympaniSpiral organ(of Corti)

vascularis

y p(containsperilymph)

Basilarmembrane

( )

Figure 15.28b

(b)

Tectorial membrane Inner hair cell

Outer hair cellsHairs (stereocilia) Afferent nerve

fibers

Fib f

Supporting cells

Fibers ofcochlearnerve

Basilar

Figure 15.28c

(c)Basilarmembrane

InnerInnerhaircell

Outerhair

llcell

Figure 15.28d

(d)

Properties of SoundProperties of Sound

Sound is Sound is A pressure disturbance (alternating areas of high and low pressure) produced by a vibrating objectlow pressure) produced by a vibrating object

A sound waveM d ll dMoves outward in all directionsIs illustrated as an S-shaped curve or sine wave

Area ofhigh pressure(compressedmolecules)Area oflow pressure( f ti )

Wavelengthss

ure

CrestTrough

Di t

(rarefaction)

Air

pre

s

Distance AmplitudeA struck tuning fork alternately compresses

and rarefies the air molecules around it, creating alternate zones of high and

A

creating alternate zones of high and low pressure.

(b) Sound waves

Figure 15.29

(b) Sou d a esradiate outward in all directions.

Properties of Sound WavesProperties of Sound WavesFrequencyq y

The number of waves that pass a given point in a given time

WavelengthThe distance between two consecutive crests

AmplitudeThe height of the crestsThe height of the crests

Properties of SoundProperties of SoundPitch

Perception of different frequenciesNormal range is from 20–20,000 HzNormal range is from 20 20,000 HzThe higher the frequency, the higher the pitch

LoudnessLoudnessSubjective interpretation of sound intensityNormal range is 0–120 decibels (dB)Normal range is 0–120 decibels (dB)

High frequency (short wavelength) = high pitchLow frequency (long wavelength) = low pitch

essu

re

Time (s)(a) Frequency is perceived as pitch.

Pre

High amplitude = loudLow amplitude = soft

eP

ress

ure

Figure 15.30

(b) Amplitude (size or intensity) is perceived as loudness.Time (s)

Transmission of Sound to the Internal EarTransmission of Sound to the Internal Ear

Sound waves vibrate the tympanic membraneSound waves vibrate the tympanic membraneOssicles vibrate and amplify the pressure at the oval windowoval windowPressure waves move through perilymph of the scala vestibuli

Transmission of Sound to the Internal EarTransmission of Sound to the Internal Ear

Waves with frequencies below the threshold of qhearing travel through the helicotrema and scali tympani to the round windowSounds in the hearing range go through the cochlear duct, vibrating the basilar membrane at a , gspecific location, according to the frequency of the sound

Malleus IncusAuditory ossicles

Stapes Cochlear nerve

Scala tympani

p

Ovalwindow

Scala vestibuliHelicotrema

Cochlear nerve

Cochlear ductBasilarmembrane

32

1

Sounds with frequencies

1 Sound waves vibrate 3 Pressure waves created by

below hearing travel through the helicotrema and do not excite hair cells.Sounds in the hearing range go through the cochlear duct,

Roundwindow

Tympanicmembrane(a) Route of sound waves through the ear

Figure 15.31a

1 Sound waves vibratethe tympanic membrane. 2 Auditory ossicles vibrate.Pressure is amplified.

3 Pressure waves created bythe stapes pushing on the oval window move through fluid in the scala vestibuli.

g g ,vibrating the basilar membrane and deflecting hairs on inner hair cells.

Resonance of the Basilar MembraneResonance of the Basilar Membrane

Fibers that span the width of the basilar membrane Fibers that span the width of the basilar membrane are short and stiff near oval window, and resonate in response to high-frequency pressure wavesin response to high frequency pressure waves.Longer fibers near the apex resonate with lower-f frequency pressure waves

Basilar membraneBasilar membrane

High-frequency sounds displaceHigh-frequency sounds displacethe basilar membrane near the base.

Fibers of basilar membraneMedium-frequency sounds displacethe basilar membrane near the middle. Base

(short,stiff

Apex(long,floppy

(b) Different sound frequencies cross thebasilar membrane at different locations

Low-frequency sounds displace thebasilar membrane near the apex.

stifffibers)

Frequency (Hz)

floppyfibers)

Figure 15.31b

basilar membrane at different locations.

Excitation of Hair Cells in the Spiral OrganOrgan

Cells of the spiral organCells of the spiral organSupporting cellsCochlear hair cellsCochlear hair cells

One row of inner hair cellsThree rows of outer hair cellsThree rows of outer hair cells

Afferent fibers of the cochlear nerve coil about the b f h i llbases of hair cells

Tectorial membrane Inner hair cell

Outer hair cellsHairs (stereocilia) Afferent nerve

fibers

f

Supporting cells

Fibers ofcochlearnerve

Basilar

nerve

Figure 15.28c

(c)Basilarmembrane

Excitation of Hair Cells in the Spiral OrganOrgan

The stereociliaProtrude into the endolymphEnmeshed in the gel-like tectorial membraneEnmeshed in the gel like tectorial membrane

Bending stereociliaOpens mechanically gated ion channelsOpens mechanically gated ion channelsInward K+ and Ca2+ current causes a graded potential and the release of neurotransmitter glutamatethe release of neurotransmitter glutamate

Cochlear fibers transmit impulses to the brain

Auditory Pathways to the BrainAuditory Pathways to the BrainImpulses from the cochlea pass via the spiral ganglion to p p v p g gthe cochlear nuclei of the medullaFrom there, impulses are sent to theFrom there, impulses are sent to the

Superior olivary nucleus Inferior colliculus (auditory reflex center)Inferior colliculus (auditory reflex center)

From there, impulses pass to the auditory cortex via the thalamusthalamusAuditory pathways decussate so that both cortices receive input from both ears

Medial geniculategnucleus of thalamus

Primary auditorycortex in temporal lobeInferior colliculusLateral lemniscusSuperior olivary nucleus Midbrainp y(pons-medulla junction)

Medulla

MidbrainCochlear nuclei

Spiral ganglion of cochlear nerveVestibulocochlear nerve

MedullaVibrations

Vibrations

Figure 15.33

Spiral organ (of Corti)Bipolar cell

Auditory ProcessingAuditory Processing

Impulses from specific hair cells are interpreted as Impulses from specific hair cells are interpreted as specific pitchesLoudness is detected by increased numbers of Loudness is detected by increased numbers of action potentials that result when the hair cells

i l d fl tiexperience larger deflectionsLocalization of sound depends on relative intensity and relative timing of sound waves reaching both ears

Homeostatic Imbalances of HearingHomeostatic Imbalances of HearingConduction deafness

Blocked sound conduction to the fluids of the internal ear

Can result from impacted earwax, perforated eardrum, or otosclerosis of the ossicles

Sensorineural deafnessDamage to the neural structures at any point from the

hl h i ll h di i l llcochlear hair cells to the auditory cortical cells

Homeostatic Imbalances of Hearing Homeostatic Imbalances of Hearing Tinnitus: ringing or clicking sound in the ears in the g g gabsence of auditory stimuli

Due to cochlear nerve degeneration, inflammation of g ,middle or internal ears, side effects of aspirin

Meniere’s syndrome: labyrinth disorder that affects y ythe cochlea and the semicircular canals

Causes vertigo, nausea, and vomiting

Equilibrium and OrientationEquilibrium and Orientation

Vestibular apparatus consists of the equilibrium Vestibular apparatus consists of the equilibrium receptors in the semicircular canals and vestibule

Vestibular receptors monitor static equilibriumVestibular receptors monitor static equilibriumSemicircular canal receptors monitor dynamic equilibriumequilibrium

MaculaeMaculaeSensory receptors for static equilibrium S y p qOne in each saccule wall and one in each utricle wallMonitor the position of the head in space necessary for Monitor the position of the head in space, necessary for control of postureRespond to linear acceleration forces but not rotationRespond to linear acceleration forces, but not rotationContain supporting cells and hair cellsStereocilia and kinocilia are embedded in the otolithic membrane studded with otoliths (tiny CaCO3 stones)

OtolithsKinocilium Ot lithi

Hair bundle

KinociliumStereocilia

Otolithicmembrane

Macula ofsaccule

Macula ofutricle

saccu e

Hair cellsSupporting

Figure 15.34

Vestibularnerve fibers

pp gcells

MaculaeMaculae

Maculae in the utricle respond to horizontal Maculae in the utricle respond to horizontal movements and tilting the head side to sideMaculae in the saccule respond to vertical Maculae in the saccule respond to vertical movements

Activating Maculae ReceptorsActivating Maculae Receptors

Bending of hairs in the direction of the kinociliaBending of hairs in the direction of the kinociliaDepolarizes hair cells Increases the amount of neurotransmitter release and Increases the amount of neurotransmitter release and increases the frequency of action potentials generated in the vestibular nervein the vestibular nerve

Activating Maculae ReceptorsActivating Maculae Receptors

Bending in the opposite directionBending in the opposite directionHyperpolarizes vestibular nerve fibersReduces the rate of impulse generation Reduces the rate of impulse generation

Thus the brain is informed of the changing position fof the head

Otolithic membraneKinocilium

St iliStereocilia

H l i tiReceptorpotentialNerve impulsesgenerated in

When hairs bend towardthe kinocilium, the hair

When hairs bend awayfrom the kinocilium, the

DepolarizationHyperpolarization

generated investibular fiber

the kinocilium, the hair cell depolarizes, exciting the nerve fiber, which generates more frequent action potentials.

from the kinocilium, the hair cell hyperpolarizes, inhibiting the nerve fiber, and decreasing the action potential frequency.

Figure 15.35

p p q y

Crista Ampullaris (Crista)Crista Ampullaris (Crista)Sensory receptor for dynamic equilibriumy p y q

One in the ampulla of each semicircular canalMajor stimuli are rotatory movementsMajor stimuli are rotatory movements

Each crista has support cells and hair cells that extend into a gel-like mass called the cupulaextend into a gel like mass called the cupulaDendrites of vestibular nerve fibers encircle the base of the hair cellsbase of the hair cells

CristaE d l h

Cupula

Hair bundle (kinociliumpl s stereocilia)

ampullaris Endolymph

Fibers of vestibular nerve

plus stereocilia)Hair cell

SupportingMembranouslabyrinth

CristaampullarisFibers of vestibular nerve cell

y

Cupula

(a) Anatomy of a crista ampullaris in asemicircular canal

p

(b) Scanning electron

Figure 15.36a–b

( ) gmicrograph of acrista ampullaris(200x)

Activating Crista Ampullaris ReceptorsActivating Crista Ampullaris Receptors

Cristae respond to changes in velocity of rotatory Cristae respond to changes in velocity of rotatory movements of the headBending of hairs in the cristae causesBending of hairs in the cristae causes

Depolarizations, and rapid impulses reach the brain at f t ta faster rate

Activating Crista Ampullaris ReceptorsActivating Crista Ampullaris Receptors

Bending of hairs in the opposite direction causesBending of hairs in the opposite direction causesHyperpolarizations, and fewer impulses reach the brain

Th th b i i i f d f t ti l t Thus the brain is informed of rotational movements of the head

Section of

Fibers ofvestibular

ampulla,filled withendolymphCupula Flow of endolymph

nerve

At rest, the cupula standsupright.(c) Movement of the

cupula during

During rotational acceleration,endolymph moves inside thesemicircular canals in thedirection opposite the rotation

As rotational movementslows, endolymph keepsmoving in the directionof the rotation, bending

Figure 15.36c

p grotationalaccelerationand deceleration

pp(it lags behind due to inertia).Endolymph flow bends thecupula and excites the haircells

, gthe cupula in theopposite direction fromacceleration andinhibiting the hair cells

Equilibrium Pathway to the BrainEquilibrium Pathway to the BrainPathways are complex and poorly tracedy p p yImpulses travel to the vestibular nuclei in the brain stem or the cerebellum, both of which receive other inputthe cerebellum, both of which receive other inputThree modes of input for balance and orientation

Vestibular receptorsVestibular receptorsVisual receptorsSomatic receptorsSomatic receptors

Input: Information about the body’s position in space comesfrom three main sources and is fed into two major processing

i th t l t

Visualreceptors

Somatic receptors(from skin, muscle

and joints)

areas in the central nervous system.

Vestibularreceptors

Cerebellum

and joints)

Vestibularnuclei

(i b i t )

O l t t l S i l t t l

(in brain stem)Central nervoussystem processing

Oculomotor control(cranial nerve nuclei

III, IV, VI)

(eye movements)

Spinal motor control(cranial nerve XI nuclei

and vestibulospinal tracts)

(neck movements)

Figure 15.37

( y ) ( )Output: Fast reflexive control of the muscles serving the eyeand neck, limb, and trunk are provided by the outputs of thecentral nervous system.

Developmental AspectsDevelopmental AspectsAll special senses are functional at birthpChemical senses—few problems occur until the fourth decade, when these senses begin to declineVision—optic vesicles protrude from the diencephalon during the fourth week of development

Vesicles indent to form optic cups; their stalks form optic nervesL t th l f f t dLater, the lens forms from ectoderm

Developmental AspectsDevelopmental AspectsVision is not fully functional at birthBabies are hyperopic, see only gray tones, and eye movements are uncoordinatedDepth perception and color vision is well developed by age Depth perception and color vision is well developed by age fiveEmmetropic eyes are developed by year sixWith age

The lens loses clarity, dilator muscles are less efficient, and visual acuity is drastically decreased by age 70acuity is drastically decreased by age 70

Developmental AspectsDevelopmental AspectsEar development begins in the three-week embryoInner ears develop from otic placodes, which invaginate into the otic pit and otic vesicleTh i i l b h b l b i h d The otic vesicle becomes the membranous labyrinth, and the surrounding mesenchyme becomes the bony labyrinthMiddle ear structures develop from the pharyngeal Middle ear structures develop from the pharyngeal pouchesThe branchial groove develops into outer ear structures