05 lecture fivecortexlobes and areas.docx
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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
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