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enters the cranium, diverges from the facial nerve, and enters the solitary tract in the medulla. A
somatosensory branch innervating the pinna joins the facial nerve in the canal. The GLN runs just
underneaththepharyngealepitheliumtoinnervatefoliateorcircumvallatepapillae.TheGLNformsthe
petrosalganglion,andtheVNformsthenodosalganglion.Thelattertwoenterthecraniumtothesolitary
tract.In humans, all three taste nerves enter thesolitary tract before terminating in the NTS, while in
monkeystheIMNandGLNdonot.
25
Fig.13.4Courseofperipheraltaste nerves.FN,facial nerve; Ggl,geniculateganglion; Ndgl,nodosalganglion;Pgl,petrosalganglion;SGP,
superficial greater petrosal nerve;tm, tympanicmembrane;Slgl, semilunar ganglion;SLN,superiorlaryngeal nerve;GLN, glossopharyngeal
nerve;CT,chordatympani.
TheIMN,GLN,andsupralaryngealnervescontainlargeandsmallsomatosensoryfibers.TheIMNand
GLNalso containautonomic nerve fibersinnervatingsalivaryglands. The conductionvelocity of taste
fibersissimilartothatofmyelinatedorunmyelinatedfibers.
26
ResponseProfileofTasteNerveFibers:NeuralTasteInformation
Singletastefibersareresponsivetoasingleorafewbasictastantsdependingonthespeciesofanimal.In
monkeysandmice,singletastefibersgenerallyrespondtoasinglebasictaste,butinratsandcatsthey
respondtomultiplebasictastes.Theresponsivenessoftastefiberstovarioustastantsisoftenexpressed
intermsofthetastantthat produces thelargestresponseof thefouror fivebasictastes(best-stimulus
category).
27
Whentastantsarearrangedintheorderofsweet,salt,sour,andbitteralongtheabscissa,and
responsemagnitudes areplottedontheordinate, theresponseprofileofsingle tastefibershasasingle
peakineachbest-stimuluscategoryintheCTNandGLN.
27
Inmonkeys,single CTNfibers aremostresponsivetosweet,salty,or soursubstances,butrarelyto
bitter or umami substances, whereas single GLN fibers best respond to sweet, bitter, and umami
substances (Fig. 13.5).
28–30
In rats, umami mixtures evoke synergistic effects in the CTN (the
sweetresponsive fibers inparticular
31
) butnot intheGLN.In rodents, thewholeSGP bundle is more
responsive to sweet substances than theCTN.
32
However, single-fiber analysis does not confirmthis
finding.Mostsinglefibersinthesupralaryngealnerveareresponsivetowaterinrabbitsorrats,andthis
responseisinhibitedbyadditionofions.
33
TheCTNofprimatesalsoyieldsresponsestowater.
34
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Each of the taste nerves innervates different regions of the oral cavity, conveying various
characteristicsoftastesignals.Theytaketheirowncoursestothecentralnervoussystemwherethey
maybedamagedduringsurgicalprocedures,e.g.,surgeryofthemiddleear.
CentralPathways:TasteRelayNucleusandRelatedStructures
NucleusoftheSolitaryTract
Anatomy
The NTS is located at the dorsal medulla. The anterior portion lies ventral to the dorsal vestibular
nucleus and the posterior portion fuses with the contralateral counterpart to make the nucleus
communicans near the obex, the posterior end of the rhomboid fossa. In monkeys, the NTS is
histologically subdivided into the lateral and medial subnucleus.
25
The anterior portion, made of the
lateralsubnucleus,isthefirstrelayofthecentralgustatorypathway.TheIMNenterstheNTSposteriorto
the entryofthefacialnerve, followedbythe GLN, andtheVNmostposteriorly.
25
TheIMNandGLN
terminateatthelateralsubnucleus,whilethe VN terminatesatbothsubnuclei.
25
Thelateralsubnucleus
probably contains taste neurons. The human NTS is subdivided into ten subnuclei, among which the
interstitial subnucleuscorrespondstothelateral subnucleusinmonkey.
25
Somatosensorycomponentsof
tastenerves,includingthefacialnerveinnervatingthepinna,terminateatthenucleusofthespinaltractof
thetrigeminus.
EfferentConnections
In monkeys, neuronsintheanterior NTSsendaxons ipsilaterallythrough the central tegmental tractto
terminatedirectlyinthethalamictasterelay, a parvicellularpartoftheventromedial posteriornucleus
(VPMpc)(Fig.13.6).
25
No projection is seen to the parabrachial nucleus from the lateral subnucleus, but from the medial
subnucleus receiving visceral afferents.
25
Thus, in subhuman primates, the parabrachial nucleus is
probablynotinvolvedintastesensation.Inrats,therostralportionoftheNTSprojectstothereticular
formationinthemedulla,whichisinvolvedintheingestionreflexandmastication.
35
Noreporthasbeen
publishedonpathwaystothethalamusthroughthereticularformation.
PhysiologicalFeatures
Tasteneurons have been recorded from theNTSinmacaque monkeys.
36
However,detailed studyhas
been limited to rats, in which taste neurons are found among neurons responding to tactile or high-
thresholdmechanical stimulation. Tasteneuronsrespondbesttosweet,salty,orsourtastants,asinthe
CTN,butrarelytobittertastants,withsomeresponsivetomechanicalstimulation.
37
Tasteneuronsfrom
the anterior tonguerespondbest tosalt,butthose from both thetongue andthenasoincisorduct inthe
Fig.13.5a–dResponsesofsinglechordatympanifibers.NaCl, sodiumchloride;HCl, hydrochloricacid.(Reproducedwith kindpermission
fromSatoetal28fora,bandd;andSatoetal29forc.)
aSucrose-bestfiber.
bQuinine-bestfiber.
cResponseprofileoftotal50fibers.
dResponseprofilesofafewsinglefibersineachbest-stimuluscategory.
palaterespondbesttosweettastants.
25,38
Tasteneuronswithareceptivefield(RF)attherostralpartof
theoralcavitytendtobelocatedinthemorerostralportionofthenucleus.Tasteneuronsrespondingto
saltyorsourtastantsoftenhaveRFsintheanteriortongue,andthosetosweettastantshaveRFsinboth
theanteriortongueandthenasoincisorduct,inwhichmanytastebudsarefoundinrodents.
38
Sometaste
neuronshaveRFsintheposteriortongueorsoftpalate.
38
Sometasteneuronsrespondtoseveralolfactorystimuli
39
inrats,probablybywayoftrigeminalnerve
innervationofthenasalcavity.
TheVentralPosteromedialNucleusoftheThalamus,aParvicellularPart
Anatomy
The VPMpcliesbetweentheVPMproperandthemedial lemniscusattheventralposteriorpartofthe
thalamus,andispackedwithsmall-sizedcells.Thegustatorypartis themedial half,
25
comprisingthe
secondrelayofthegustatorypathwayinprimates.
AfferentConnections
Inmonkeys,theVPMpcreceivesafferentsipsilaterallyfromthelateralsubnucleusoftheNTS,andalso
ipsilaterallyfromtheparabrachialnucleus.
25
Fig.13.6Gustatorypathwayfromthesolitarytractnucleus(NTS)toa parvocellularpartofthe ventromedial posteriornucleus (VPMpc).
Thecentral gustatory pathwaystartsfromthelateral subnucleusoftheNTSandgoesupthroughthecentraltegmentalpathtotheVPMpc,
which iscontrastto the visceralpathwaythatgoesupfromthemedial subnucleusofthe NTStothe parabrachial nucleus(PBN). AbdN,
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abducence nucleus; bc, brachium conjunctivum; CG, central gray; cp, cerebral peduncle; FN, facial nerve; IO, inferior olive; LC, locus
coeruleus; LGB,lateralgeniculatebody; lNTS, lateralsubnucleusoftheNTS;MGB, medialgeniculatebody; ml,mediallemniscus;mNTS,
medialsubnucleusoftheNTS;PUL,pulvinarnucleus;PV,principalnucleusofthetrigeminus;py,pyramidaltract;SC,superiorcolliculus;SN,
substantia nigra;sptV, spinaltractofthetrigeminus;VPL,ventralposterolateralthalamic nucleus;VPMpc, parvocellular partoftheventral
posteromedialthalamicnucleus;VesN,vestibularnucleus.
TheVPMpcisreciprocallyconnectedwiththereticularformationofthalamusinrats,
40
andisfound in
specificnucleiinthethalamusinotheranimals.Thegustatorypartofthethalamicreticularformationis
ventraltothecaudalendofthesomatosensorypart,rostraltoboththevisualandauditoryparts.
40
Inrats,thegustatorycortexandVPMpcarethoughttoformthethalamocorticalreverberatingcircuits
involvingthereticularformationandcontaininginhibitoryneurons.
41
EfferentConnections
NeuronsinthegustatorypartoftheVPMpcprojecttotwogustatorycortices,attheexposedpartofthe
frontaloperculumandattherostralendoftheSylviansulcusinmonkeys.
25
PhysiologicalFeatures
In humans and monkeys, taste responses are recorded from the VPMpc.
25
Detailed studies have been
carried out in rats.
25,36,42
Thalamic neurons are spontaneously active, but yield a small response to
tastants.TheirRFsarelargerthanthoseofNTSneurons,coveringthewholetongueoranteriortongue.
42
ThecentraltastepathwayfromtheNTS,thefirstrelay,totheVPMpcrunsdirectlyandipsilaterally
through the central tegmental path. Then the VPMpc sends axons to the two primary gustatory
cortices:thefrontalextentofArea3,andAreaGwithintheSylviansulcus.
GustatoryCorticesinSubhumanPrimates
PrimaryCortex
AnatomyandAfferentConnections
Theprimarygustatorycorticesinmonkeyshavebeenidentifiedbytheevokedpotentialmethod
43,44
and
bytracermethods.
25,44
Twoprimarygustatorycorticesarerecognized:(1)TheprecentralextensionofArea3(precentral
Area3)towhichtheCTN,LN,andGLNprojectbilaterallywithipsilateralpredominance;
43,44
and
(2)AreaG
45
attherostralendintheinsideoftheSylviansulcustowhichtheCTNandGLN,butnot
theLN,projectipsilaterally.
AreaGliesatthe transitionbetween Areas1 and2atthe frontaloperculumand granularinsula(Fig.
13.7),
44,45
andis the granular area, characterized with well-developed outer and inner granular layers
(layersIIandIV)andwithGennaristriaatlayerIV.
44,45
CytochromeoxidaseactivitydistinguishesArea
Gfromthesurroundingareas.
44
StudiesusingtracershowtheprojectionsoftheVPMpctobothAreaG
andprecentralArea3.
25
TheaxonsofVPMpcneuronsmaybebifurcatedtoprojecttobothareas.
43
Areas
1and2mayalsobeincludedintheprimarygustatoryareas,asthedepositionofalargeconcentrationof
tracers in the VPMpc has identified terminals in Areas 1 and 2 (Pritchard, personal communication,
September1994).
EfferentConnections
The precentralArea3projectstothe precentraloperculum(PrCO)andArea12,thelateralpartofthe
orbitofrontal cortex,
46,48
whereas Area G projects to Area 12 and the orbitofrontal operculum(OFO)
(Fig. 13.7).
47,48
In rats, the gustatory cortex is connected to the contralateral counterpart through the
corpuscallosum.
49
PhysiologicalFeatures
AreasGand3contain manymechanoreceptiveneurons,suchaschewingmovement–relatedneuronsor
tactile neurons with RFs in the mouth.
50
Among these mechanoreceptive neurons, taste neurons are
found.
36,44,50
Areas G and3 contain neurons withdifferent taste response characteristics
51
: neurons in
AreaGtendtoresponddifferentiallytohedonicallypositive(sweetorsalty)ornegative(sourorbitter)
tastants,butthoseinArea3donot.Ineachbestcategory,sometasteneuronsdisplayresponseprofiles
with two modes at the best stimulus and another one, by producing the largest responses to the two
tastants and a difference in response between the two (Fig. 13.8). For example, some sucrose-best
neuronsshowlargerresponsestoquininebutsmalleronestoNaClorHCl.Thesefindingssuggestthatthe
primary cortex is concernedwith taste contrast,as inrats.
52
Neurons mostresponsive tomonosodium
glutamatearealsofoundinAreas3andG(Fig.13.8),
36
probablyrepresentingumami.
Studyingthegustatorycorticesinbehavingmonkeysrevealedthatalmostalloftheneuronsinboth
Area3andAreaGrespondtothesamechemicalcueswithconsistentresponsepatternsirrespective
ofchangesinbehavioralresponses,suchasleverpressingtogainareward
53
;thisindicatesthatthe
neuronsintheprimarycorticesarerealtasteneurons.
HigherOrderCortexandOtherAreas
Anatomy
The PrCO,Area 12, andprobably the OFO,receivingafferents from theprimaryareas,
46,47
arehigher
order gustatoryareas. They are dysgranularneocortex. Tasteneuronsare also foundinArea 13 inthe
medialpartoftheorbitofrontalcortex,thelimbiccortex.
25
Area12anddysgranularinsulaprojecttothe
amygdala.
36,44,54
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Fig.13.7GustatorypathwayfromVPMpctotheprimarygustatorycortexandtohigherordergustatorycortex.G,areaG;IA,agranularpart
of the insula; OFO,orbitofrontal operculum; PrCO,precentral operculum; ai, arcuate sulcus, inferior branch; as, arcuate sulcus, superior
branch;ipc,inferiorprecentralsulcus;p,principalsulcus;ss,Sylviansulcus;1–2,area1–2;3,area3.Otherabbreviations,seethelegendfor
Fig.13.6.(FromOgawaetal.
48
)
Fig.13.8ResponseprofilesoftasteneuronsinAreas3andG.Responses(numberofimpulsespersecond)areplottedagainsttastestimuli.
AArea3;BAreaG;asucrosebest;bNaClbest;cHClbest;dquininebest;eglutamatebest;0,spontaneousdischargerate;G,glutamate;
H,hydrochloricacid;N,sodiumchloride;Q,quinine;S,sucrose.
ResponseFeatures
NeuronsinArea12respondtotastestimuliandothermodalitiessuchassomatosensory,visual,and
olfactory stimuli.
36,44
Pleasant stimuli are effective. Neurons in Area 12 show sensory-specific
satiety,withtasteresponsesfacilitatedbyhungertoacertainstimulusbutsuppressedbysatietytoit.
HypothalamicsignalsmaygatetasteinputstoArea12.
36
Inbehavingmonkeys,almostalloftheneuronsinArea12andhalftheneuronsinthePrCOrespondtothe
samechemicalcues,withtheresponsepatternreversedafterchangesinbehavioralresponse(Fig.13.9).
These areas do notrepresent taste, but rather behavioral context,or whatthe monkey wants to do in
responsetocues.
53
Neuronsinthe amygdala tendto respondto aversivestimuli. Neuronsinthe amygdala ortemporal
poledifferentiallyrespondtofoodornon-food.
55
Theamygdalaisalsoinvolvedinlearnedtasteaversion
inrats.
56
Ablation of the lower part of the parietal cortex and the temporal pole in monkeys yields
hyperorality,orKluever–Bucysyndrome.
54
Fig.13.9a,bResponsesofcorticalneuronstocues inmonkeyengagedina delayedversionoftheNaCl–water discriminationGO/NOGO
task.Thetaskwasreversedbeforeandafterthearrow.(FromIfukuetal.
53
)
aNeuronconsistentlyrespondingtoNaClorwaterirrespectiveoftaskreversal,representingchemicalnatureofcues(AreaG).
bNeuronchangingresponsetocuesaccordingtothetask,representingthebehavioralcontext(Area12).
Taste neurons in the two primary areas show invariant taste responsiveness irrespective of
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behavioralcontext,whilethoseinhigher-orderArea12,showchangesinresponsivenessdepending
onbehavioralcontext.Thelatteralsoshowssensory-specificsatiety.
GustatoryCorticesinHumans
PrimaryCortex
Thelocationoftheprimarygustatorycortex(PGA)inthehumancerebralcortexhasbeenarguedformany
decades. Börnsteinstudied patients withageusia after bullet wounds involving the parietal lobe, and
locatedagustatorycorticalareainArea43ortheRolandoperculum.
57
PenfieldandJasper,
58
ontheother
hand,suggestedtasterepresentationinthecircuminsular cortex superior tothe circular sulcusnear the
footofthecentralsulcusandextendingtotheinsularcortex,eventhoughtherelationtosecondarysensory
areasisnotcertain.Bothreportssuggestthatthegustatorycortexliesatthefootofthecentralsulcus,but
differastowhetheritisatthelateralsurfaceorburiedinthecerebralcortex.Asdescribedonp.158,
twoprimarygustatorycorticesarerecognizedinsquirrelandmacaquemonkeys.
Recentdevelopmentsinimagingtechniqueshaveyieldedvariousnoninvasivemethods(e.g.,functional
magneticresonanceimaging[fMRI],positronemissiontomography[PET],andmagnetoencephalography
[MEG]) that allow us to measure the brain activity in healthy living human subjects. fMRI measures
changesinblood flow bydetecting thecontrastinmagnetic susceptibility betweenoxyhemoglobinand
deoxyhemoglobin. PET measures the density of oxygen-15-water (H
2
15
O) in blood, by detection of
gamma rays caused by electron–positron annihilation. Both methods mainly measure changes in
metabolism associated with neural activity, which provides better spatial resolution than detecting
activatedregions.MEGorelectroencephalography(EEG),however,measurechangesinmagneticfields
orelectricpotential,respectively,onthesurfaceoftheskull,causedbyelectricalactivationinclustersof
neurons.MEGandEEG,therefore,providebettertemporalresolutionofcorticalactivation.Additionally,
MEG is superior to EEG for localizing cortical activity because of the uniformity of magnetic
permeabilityinaskull,comparedwiththelargedistortionofelectricsignalsforEEG.
AsthePGAisthefirstcorticalareareceivingprojectionsfromthethalamus,MEGisthemostsuitable
method of identifying thePGA.MEG experiments revealed that taste stimuli evoked the first cortical
activationattworegions:First,thelowerendofthecentralsulcus,probablycorrespondingtothefrontal
extension of Area 3 in primates; and, second, the transition area between the parietal or Rolandic
operculumandtheposteriorpartoftheinsula,probablycorrespondingtoAreaGneartheanteriorendof
thelateralsulcusinprimates(humanAreaG),asshowninFig.13.10.Activationoftheformerwasfound
tocontinuetoArea43inanfMRIstudy.Theformer,includingArea43andthefootofthecentralsulcus,
probablycorrespondstoBörnstein'sgustatoryarea,andthehumanAreaG(insidethelateralsulcus)to
that of Penfield and Jasper. The onset latency was ~150 milliseconds for sodium chloride and 250
millisecondsforsaccharin(artificialsweetsubstance)inthelatterarea;theselatenciesintheformerarea
are much shorter(90 milliseconds for sodium chloride).
59
Compared with the location of thePGA in
primates,
43,44
thatofhumanbeingsmovesposteriorly,traversingthecentralsulcustotheparietalcortex.
Suchmovementofsensoryareasonthecerebralcortexwithphylogeneticevolutionisalsoknowninother
areas, such as the visual cortex. Factor analysis of correspondences suggested that gustatory stimuli
activatedtheinsularregion,andsomatogustatorystimuliactivatedtheRolandicopercula,includingapart
ofthesensoryhomunculusdedicatedtothetactilerepresentationoforalstructures.
60
Fig.13.10a,bLocationoftheprimarygustatorycortex(PGA)inhumans.
aCoronalsectionshowsthatoneofthePGAs(humanAreaG)locatesatatransitionareabetweentheinsularcortexandparietalorRolandic
operculum.
bIllustratesthePGAwhentheinsularcortexisexposedbyclamps.
Some reports
61,62
insist that the frontal operculum and anterior insula are PGA in humans on the
groundsthatthePGAofprimatesislocatedinthesecorticalareas.AsfMRIorPETmeasuremetabolic
changesinthebrainandhavepoor temporal resolution,itisdifficult toestablish whether the PGA is
locatedinparietalareaswithfMRIorPETstudyalone.MEGstudy
59
revealedthatthefrontaloperculum
andanteriorinsulawereactivatedafterAreaGwithlongerlatency(286milliseconds).Itisreasonable,
therefore,toassumethatthesefrontalareasarehigherareasrelatedtotasteperceptionratherthanPGA.
The PGAisinvolved intasteconcentration coding. Theamplitude ofactivity inthe PGAhadgood
linearityaccording tothe logarithm offoursodiumchloride concentrations, asshowninFig. 13.11a.
63
The latency,however,did notchangewithchangesinconcentrations.Fig. 13.11bshows,however,that
psychological perceived intensity has a partial discrepancy from cortical activation in PGA at high
concentration.Theinterpretationforthisphenomenonwillbedescribedinthenextsection.
Inaddition,thePGA ischaracterizedbysensitivitytothe repetition oftastestimulation,asfoundin
otherprimarysensorycortices,suchastheprimaryvisualcortexinwhichchangesinregionalcerebral
bloodflow(rCBF)areobservedbyrepetitivestimuliorchecker-boardreversal.
64
Althoughmanystudies
have reported weakor rare activation of the PGA by long-lasting taste stimuli, significantchangesof
rCBFatthePGAwerealsoobservedbyfrequenttastestimuli,
65
asshowninFig.13.12a,andthelocation
revealedbyfMRIisconsistentwithMEGresults(Fig.13.12b).
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