05 lecture fivecortexlobes and areas.docx

05 lecture fivecortexlobes and areas.docx

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05lecturefivecortexlobesandareas

LectureFive

Cortex—LobesandAreas

Scope:

Thecerebralcortexisconsistedtheseatofthemind.Thus,cognitionandotherhigher-orderfunctionssuchaslanguage,intellect,andmemoryarelargelycorticalprocesses.Elaborationanddifferentiationofthecortexis,infact,oneofthemostdistinguishingcharacteristicsofthehumanbrain,andisbelievedtounderliemanyoftheabilitiesandbehaviorsassociateduniquelywithoutspecies.Forthisreason,thecortexhaslongbeentheprojectofintensescientificinvestigation.Inthisandthefollowinglecture,wewillexplorehowthiscomplexsheetofcellsisorganized,andhowdifferentclassificationschemeshavebeenusedtopartitionit.

Outline

I.Thecerebralcortexisbelievedtobetheseatofthemind.Itisthe“bark”oroutermantleofgraymatter（neurons）coveringthecerebralhemispheresofthetelencephalon.

A.Thecortexappears“wrinkled”andfoldedbecauseitiscompressedinordertofitwithintheskull;itisestimatedthatiffattenedout,thehumancortexwouldbeabout21/2squarefeetinarea.

B.Thewrinklesorfoldsvisibleontheexternalsurfacearereferredtoasgyri（singular=gyrus）.Thevalleysorinfoldingsofcorticaltissuearereferredtoassulci（singular=sulcus）;whenasulcusisdeep,itisreferredtoasafissure.

II.Thecerebralcortex,however,isnotauniformstructure.

A.Itvariesfromabout1-4millimetersinthickness.

B.Differentcorticalareasvaryintermsofphylogenetic（evolutionary）agewhichisreflectedinthenumberoflayers.

1.Cortexthatisoldestintermsofphylogeny,referredtoasarchi-（ancient）andpaleo-（old）cortex,ismadeupoffewercelllayersthanneo-（new）cortexofmorerecentevolutionaryage.

2.Forexample,thehippocampus,whichisanoldcorticalareainvolvedinlearningandmemory,hasthreecelllayers.Neocortexiscomposedofsixcelllayersandisthemostrecentlyevolvedcortex.Whilewealsohavearchi-andpaleocortex,mostofthehumancerebralcortexismadeupofneocortex.

C.Thecortexdiffersinhowthevariouscelllayersarearranged.

1.Anumberofneuroanatomistsaroundtheturnofthecenturydividedthecortexintoabout50-200differentareasonthebasisofhowtheneuronswereorganizedintolayers（referredtoascytoarchitecture）.

2.Figure5.1showsthenumberingsystemgiventothelateralandmedialcortexbyKorbinianBrodmann（1868-1918）,whodividedthecortexintoabout50differentareasbasedoncellarrangement.

3.Whilewenowknowthatmanyoftheseareascanbefurthersubdivided,Brodmann’snumberingsystemisstillusedinmodernneuroscience.

4.DifferentcorticalareascanbereferredtobyaBrodmannnumber,aname,oraRomannumeral;forexample,becauseArea17isthefirstcorticalareatoreceiveinformationaboutvision,itisalsoreferredtoasprimaryvisualcortexorVI;othernamesgivenacorticalareamaybedescriptive（forexample,Area17isalsocalled“striate”cortexforitsstripedappearanceinthefreshbrain）.

D.InadditiontoahorizontalorganizationtheBrodmannareasgivetothebrain,thecortexalsohasaverticalorganization.

1.Each1-2mm2ofcortex,extendingthroughthelayers,representsafunctionalunitormodule.

2.Thus,thelargersurfaceareaofthecortexinhumans,whichallowsforanenormousincreaseinthenumberofsuchfunctionalmodules,isbelievedtounderliethetremendousadvantageofourspeciesinprocessinginformationascomparedtootherspecies.

III.Thecortexcanbegrosslydividedintofourlobesbymajorsulciorfissures（Figure5.2）.Eachofthelobescontainsanumberofimportantareas;thefunctionofmanyoftheseareaswillbediscussedatgreaterlengthinfuturelecturesandwillbecommentedononlybrieflyhere.

A.ThefrontallobeisdemarcatedbythecentralsulcusofRolandoandtheSylvian（lateral）fissure.

1.Importantareaswithinthefrontallobeincludetheprecentralgyrus（Area4）,whichisimmediatelyrostraltothecentralsulcusofRolando;Area4istheprimarymotorcortex,thecorticalarearesponsiblefortheinitiationofamotormovement;neuronsinArea4giverisetothecorticospinalpathway.

2.AreasrostraloranteriortoArea4includethoseinvolvedintheplanningofamotormovement,andlanguage（leftordominanthemisphere）.

3.Themostrostralpartofthefrontallobeisreferredtoastheprefrontalcortex,whichBrodmanndividedintosubareas;differentprefrontalareasplayaroleinavarietyofhighercognitivefunctionsincludingtheabilitytoinhibitbehaviorandtounderstandtheconsequencesofone’sbehavior.

B.TheparietallobeisborderedanteriorlybythecentralsulcusofRolando,posteriorlybytheparieto-occipitalsulcus.

1.Thepostcentralgyrus（Areas3,1and2）liesimmediatelycaudaltothecentralsulcusofRolandoand