08 lecture eightcentral nervous systemcellular organization.docx

08 lecture eightcentral nervous systemcellular organization.docx

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08lectureeightcentralnervoussystemcellularorganization

LectureEight

CentralNervousSystem—CellularOrganization

Scope:

Intheprecedinglecture,welearnedthatneuronsandsomenonneuronalcellsarisefromthesameprecursorcellsduringdevelopmentoftheCNS.Oncethesecellsarecommittedtobecomingeitherneuronsorglia,however,theyareverydifferentbothintermsofstructureandfunction.Here,wewillfocusonafewstructuralandfunctionaldifferencesbetweenneuronsandglialcells.

Outline

I.TheadultCNSconsistsprimarily（butnotexclusively）ofneuronsandglialcells;thereareabout100billionneurons,and10-100timesthatmanyglialcellsinbrain.

A.Thereareatleast150differenttypesofneurons,makingthemthemostdiversecelltypeinthebody.Neuronsaredistinguishedprimarilybytheirsize（about4-100microns）andtheirdendriticarbors.

1.Mostneuronsaremultipolarcells,meaningthattheyhaveanumberofprocessesemanatingfromthecellbody;the“generic”neuroninFigure3.1isamultipolarneuron.

2.Inadults,neuronsaregenerallyconsiderednonmitoticcellsandthusdonotgiverisetotumorsintheCNS;however,thereisalowlevelofmitosisofneuronsinsomeareasoftheadultCNS,atopicwewillreturntoinfuturelectures.

B.Glialcellsarefoundinbothgrayandwhitematter.Therearetwomajortypesofglialcellsinthebrain（astrocytesandoligodendrocytes）.

1.Thenameglia（glue）derivesfromthehistoricalviewthatthefunctionofglialcellswastosimplyholdthebraintogether,butmodernneurosciencehasrevealedthatgliaarevitaltothenormalfunctioningofthenervoussystem.

2.Astrocytes（“star”cells）arederivedfromthesameprecursorcellsasneurons.Astrocytessupportandguideneuronsduringdevelopment,actas“sinks”forions（chargedatoms,forexample,potassium[k+]）involvedinneuralactivity,andremoveneuroactiveandpotentiallyneurotoxicsubstancesfromtheextracellularenvironment;astrocytesalsoformscarsintheCNS.

3.OligodendrocytesmyelinateaxonsintheCNS.

C.Anumberofneurologicaldisordersspecificallyinvolveglialcells.

1.Astrocytesandoligodendrocytesremainmitoticthroughoutlife,andthus,giverisetoprimarybraintumors.

2.TheCNSdisordermultiplesclerosis（multiple“scar-like”areas）isbelievedtobeanautoimmunedisorderinwhichantibodiesdestroyoligodendrocytes（whichmyelinateaxons）andthemyelinsheathsurroundingaxons;destructionofthismyelinsheathalterstheabilityoftheaxontotransmitimpulsesnormally.

II.Neuronsaresimilartoothercellsinthebody,exceptthattheyarepolarized.（seeFigure3.1）

A.Polarizationreferstothefactthattheneuroncellbodyanddendritesarespecializedforreceivinginformationfromotherneurons,andtheaxonisaspecializedstructureforconductingimpulsestothedendritesandcellbodiesofotherneurons.

B.Dendritesandtheirtinyprotrusionscalledspinesaretrueextensionsofthecellbodysurface,greatlyincreasingthesurfaceareaofthecell.

C.Asopposedtodendrites,axonsarenotsimplyextensionsofthecellbodysurfacebutareaspecializedstructureforpropagationofanelectricalimpulse.Generally,eachneuronhasoneaxon,althoughtheremaybemanycollateralsorbranches,whichariseatthebreaksornodesbetweenthemyelinsheaths（seeFigure4.1）.

III.Thejunctionbetweentheaxonofoneneuronand（generally）thedendritesorspinesofanotherneuronisreferredtoasasynapse（seeFigure4.1）,atermcoinedbySirCharlesSherrington（1857-1952）whopostulatedthattheremustbea“gap”betweenneuronsingraymatterbecauseconductionwasshowerthatthatseeninnervouscomposedonlyofaxons.

A.Synapsesareextremelysmallandwerefirstseenwithelectronmicroscopyintheearly1950s.

B.AsshowninFigure8.1,asynapseisaspecializedjunctionconsistingofapresynapticcomponent（generallyanaxonalending）,asynapticcleftwhichisthespacebetweenthepre-andpostsynapticcomponents,andapostsynapticcomponent（generallyadendriteorspineofanotherneuron）.

C.Thepresynapticcomponentofthesynapsehasanumberofimportantcellularandmolecularstructures.

1.Synapticvesiclescontainchemicals（neurotransmittersorneuromodulators）whicharereleasedbythepresynapticterminalintothesynapticcleft.

2.Transportermoleculesandotherproteinsmayfunction,forexample,intakinganeurotransmitterbackupintothepresynapticcomponenttoberecycled（referredtoasreuptake）.

D.Thepostsynapticcomponentofasynapsealsohasanumberofimportantcellular/molecularstructuresincludingpostsynapticreceptorsormoleculesthatbindneurotransmittersandleadtochangesinthepostsynapticmembrane.

IV.Eachofthe100billionorsoneuronsinthehumanbraincanmakethousandsofsynapses!

Aconservativeestimateisthatthereareabout100trillionsynapsesintheadulthumanbrain!

FurtherReading:

Anyrecentneurosciencetextwillcovermostofthebasicinformationcoveredinthislecture,althoughinmuchgreaterdetail.TheauthoritativebookinneuroscienceisPrinciplesofNeuralScience,editedbyE.R.Kandel,J.H.Schwartz,andT.M.Jessell.

QuestionstoConsider:

1.Isn’titabsolutelyamazingthatthousandsandthousandsofsynapsesarefiringeverytimeyoureadaword,thinkathought,orplayamove?

Asyoureadthisquestion,howmanydifferentareas/pathwayscanyounamethatarebeingstimulated?

2.Nowthatyouunderstandhowneuronscommunicate,discusswhythefunctionofglialcellslikeastrocytesandoligodendrocytesaresovitallyimportant?

Glue,indeed!

LectureEight—Transcript

CentralNervousSystem—CellularOrganization

Hello,welcomeback.Intheprecedinglectureweweretalkingaboutdevelopmentandhowfromafewcellsthisincrediblestructure,thebrain,develops.Oneofthethingsweweretalkingaboutishowthesameprogenitorcellsgiverisetobothneuronsandtosomenonneuronalcellsinthebrain.Now,thereareactuallyalotofdifferentkindsofcellsinthebrain.Therearecellsthatarerelatedtoliningtheventricularsystem.Therearecellsthatarerelatedtobloodvesselsinthebrain.Thereareothercells,buthereinthiscoursewearegoingtofocusonneurons,whicharenervecells,andtwotypesofcellscalledastrocytesandoligodendrocytes,whicharederivedfromthesameprecursorcells.So,onceacell,however,decidesit’sgoingtobecomeaneuron,oraglialcell,theirstructureandtheirfunctionarevery,verydifferent.Inthislecturewhatwewanttodoisfocusonafewofthosestructuralandfunctionaldifferencesbetweenneuronsandthosetwotypesofglial,orsupporting,cells.

Theadultcentralnervoussystemconsistsprimarily,butnotexclusively,ofneuronsandglialcells.Thereare100billionneuronsafterregressiveeventshavetakenplace.Now,amazingly,ifthisseemsincredible,thereare10to100timesthatmanyglialcellsinthebrain;anincrediblycomplexstructure.Forneurons,thereareatleast150differenttypesofneurons,makingthemthemostdiversecelltypeintheentirebody.Ifyouweretocutthroughtheliver,youdon’tfind150livertypesofcells.So,thebrainhasmorediversity,thebraincellshavemorediversitythananyotherorganinthebody.Theydifferastosize.Forexample,amotorneuronisverylarge,itmightevenbeasmuchas100micronsindiameter,andithasalong-axonthatwillgoallthewaytothespinalcord.Otherneuronsmayonlybefourmicrons,whichisjusttiny,andthesewouldmakeupthoseinterneuronswhichareintercalatedbetweeninputandoutputtoanucleus;sotheyarelittletinycellswhoselittleaxonsneverleavethenucleus,thecellbodyandaxonsareinonestructure,buttheyareincrediblyimportantinthefunctioningofthebrain.

Now,ifweweretolookatdifferentneuronalcelltypes,wecanseelookingwiththeGolgimethodthatneuronsdoindeedcomeindifferenttypes.Wehaveseenthisneuronbefore.Thisisneuronactuallyfromthemotorcortex,andthesurfaceofthebrainisuphere,andthisiswherethecellislocated,inlayerVorVI.Thiscellhasdendritesextendingfromthebaseofthecelloutinalldirections,andithasonehugedendriteextendingallthewayuptothesurfaceofthecortex.Thecellswhichcometobethesemotorneuronswillhavethisparticulartypeofstructure.

Nowlet’slookoverhereatthis;thisisanotherGolgi-impregnatedsection.Theyarealwaysdifficultforthestudenttolookatbecausethereisalotofpartiallyimpregnatedthingsandotherthingsthatarepresent.Butwhatwewanttofocusonisthishugeneuronrighthere.Youseethatithasahugedendritethatcomesuphere,andtheothersideaswell,andmany,manydifferentbranches—manysecondary,tertiary,andmorebranchesthere.Theseparticulartypesofneuronsarefoundinthecerebellumandtheyarecalledcandelabraneurons.So,outofthecellbodyonbothsidescomethesehugedendriticbranchesthatthenbreakintosmallerbranchesandincreasethesurfaceareaofthatneuron.Dendritesarejustextensionsofthecellbodythatincreasethesurfaceareaoftheneuron.It’sgoingtoprocessinformationdifferently.Thisneuronprocessesinformationdifferentlythananeuronthatdifferentiatesintothisparticularneuronaltype.

Andtojustgiveyouanothermethodwhichisused,afluorescentmethodshowsjustadifferentneuronaltype,showingthedendritesextendinginalldirectionsfromthecellbody.

Mostneuronsaremultipolar.Theneuronsyouarelookingathereareallmultipolar.Andthisneuronismultipolar.Thisisanothertypeofneuronthatdifferentiatesinthebrain.Hereisthecellbody.Itisshowin