NeuronscienceC2.docx

NeuronscienceC2.docx

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NeuronscienceC2

Chapter2NeuronsandGlia

INTRODUCTION

THENEURONDOCTRINE

TheGolgiStain

Cajal'sContribution

Box2.1OfSpecialInterest:

AdvancesinMicroscopy

THEPROTOTYPICALNEURON

TheSoma

TheNucleus

RoughEndoplasmicReticulum

SmoothEndoplasmicReticulumandtheGolgiApparatus

TheMitochondrion

TheNeuronalMembrane

TheCytoskeleton

Microtubules

Box2.2OfSpecialInterestAlzheimer'sDiseaseandtheNeuronalCytoskeleton

Microfilaments

Neurofilaments

TheAxon

TheAxonTerminal

TheSynapse,

AxoplasmicTransport

Box2.3OfSpecialInterestHitchingaRideon"Retrorail"

Dendrites

Box2.4OfSpecialInterestMentalRetardationandDendriticSpines

Box2.5PathofDiscoveryTheStoryofDendriticProteinSynthesis,byOswaldSteward

CLASSIFYINGNEURONS

ClassificationBasedontheNumberofNeurites

ClassificationBasedonDendrites

ClassificationBasedonConnections

ClassificationBasedonAxonLength

ClassificationBasedonNeurotransmitter

GLlA

Astrocytes

MyelinatingGlia

OtherNon-NeuronalCells

CONCLUDINGREMARKS

INTRODUCTION

Alltissuesandorgansinthebodyconsistofcells.Thespecializedfunctionsofcellsandhowtheyinteractdeterminethefunctionsoforgans.Thebrainisanorgan--tobesure,themostsophisticatedandcomplexorganthatnaturehasdevised.Butthebasicstrategyforunravelingitsfunctionisnodifferentfromthatusedtoinvestigatethepancreasorthelung.Wemustbeginbylearninghowbraincellsworkindividuallyandthenseehowtheyareassembledtoworktogether.Inneuroscience,thereisnoneedtoseparatemindfrombrain:

oncewefullyunderstandtheindividualandconcertedactionsofbraincells,wewillunderstandtheoriginsofourmentalabilities.Theorganizationofthisbookreflectsthis"neurophilosophy."Webeginwiththecellsofthenervoussystemtheirstructure,function,andmeansofcommunication.Inlaterchapters,wewillexplorehowthesecellsareassembledintocircuitsthatmediatesensation,perception,movement,speech,andemotion.

Inthischapter,wefocusonthestructureofthedifferenttypesofcellsinthenervoussystem:

neuronsandglia.Thesearebroadcategories,withinwhicharemanytypesofcellsthatdifferbasedontheirstructure,chemistry,andfunction.Nonetheless,thedistinctionbetweenneuronsandgliaisimportant.Althoughtherearemanyneuronsinthehumanbrain（about100billion）,gliaoutnumberneuronsbytenfold.Basedonthesenumbers,itmightappearthatweshouldfocusourattentionongliaforinsightsintothecellularfunctionsofthenervoussystem.However,neuronsarethemostimportantcellsfortheuniquefunctionsofthebrain.Itistheneuronsthatsensechangesintheenvironment,communicatethesechangestootherneurons,andcommandthebody'sresponsestothesesensations.Gliaarethoughttocontributetobrainfunctionmainlybyinsulating,supporting,andnourishingneighboringneurons.Ifthebrainwereachocolate-chipcookieandtheneuronswerechocolatechips,thegliawouldbethecookiedoughthatfillsalltheotherspaceandensuresthatthechipsaresuspendedintheirappropriatelocations.Indeed,thetermgliaisderivedfromtheGreekwordfor‘glue’,I'mgivingtheimpressionthatthemainfunctionofthesecellsistokeepthebrainfromrunningoutofourears!

Asweshallseelaterinthechapter,thesimplicityofthisviewisprobablyagoodindicationofthedepthofourignoranceaboutglialfunction.However,westillareconfidentthatneuronsperformthebulkofinformationprocessinginthebrain.Therefore,wewillfocus90%ofourattentionon10%ofbraincells:

theneurons.

Neuroscience,likeotherfields,hasalanguageallitsown.Tousethislanguage,youmustlearnthevocabulary.Afteryouhavereadthischapter,takeafewminutestoreviewthekeytermslistandmakesureyouunderstandthemeaningofeachterm.Yourneurosciencevocabularywillgrowasyouworkyourwaythroughthebook.

THENEURONDOCTRINE

Tostudythestructureofbraincells,scientistshavehadtoovercomeseveralobstacles.Thefirstwasthesmallsize.Mostcellsareintherangeof0.01-0.05mmindiameter.Thetipofanunsharpenedpencilleadisabout2mmacross;neuronsare40-200timessmaller.（Forareviewofthemetricsystem,seeTable2.1.）Thissizeisatorbeyondthelimitofwhatcanbeseenbythenakedeye.Therefore,progressincellularneurosciencewasnotpossiblebeforethedevelopmentofthecompoundmicroscopeinthelateseventeenthcentury.Eventhen,obstaclesremained.Toobservebraintissueusingamicroscope,itwasnecessarytomakeverythinslices,ideallynotmuchthickerthanthediameterofthecells.However,braintissuehasaconsistencylikeabowlofJello:

notfirmenoughtomakethinslices.Thus,thestudyoftheanatomyofbraincellshadtoawaitthedevelopmentofamethodtohardenthetissuewithoutdisturbingitsstructureandaninstrumentthatcouldproduceverythinslices.Earlyinthenineteenthcentury,scientistsdiscoveredhowtoharden,or"fix,"tissuesbyimmersingtheminformaldehyde,andtheydevelopedaspecialdevicecalledamicrotometomakeverythinslices.

UNITABBREVIATIONMETEREQUIVALENTREAL-WORLDEOUIVALENT

Kilomoterkm103mAbouttwo-thirdsofamile.

Meterm1mAbout3feet.

Centimetercm10-2mThicknessofyourlittlefinger,

Millimetermm10-3mThicknessofyourtoenail.

Micrometerpm10-6mNearthelimitofresolutionforthelightmicroscope.

Nanometernm10-9mNearthelimitofresolutionfor

theelectronmicroscope.

Thesetechnicaladvancesspawnedthefieldofhistology,themicroscopystudyofthestructureoftissues.Butscientistsstudyingbrainstructurefacedyetanotherobstacle.Freshlypreparedbrainhasauniform,cream-coloredappearanceunderthemicroscope:

thetissuehasnodifferencesinpigmentationtoenablehistologiststoresolveindividualcells.Thus,thefinalbreakthroughinneurohistologywastheintroductionofstainsthatcouldselectivelycolorsome,butnotall,partsofthecellsinbraintissue.

Onestain,stillusedtoday,wasintroducedbytheGermanneurologistFranzNisslinthelatenineteenthcentury.Nisslshowedthataclassofbasicdyeswouldstainthenucleiofallcellsandalsostainclumpsofmaterialsurroundingthenucleiofneurons（Figure2.1）.TheseclumpsarecalledNisslbodies,andthestainisknownastheNisslstain.TheNisslstainisextremelyusefulfortworeasons.First,itdistinguishesneuronsandgliafromoneanother.Second,itenableshistologiststostudythearrangement,orcytoarchitecture,ofneuronsindifferentpartsofthebrain.（Theprefixcyto-isfromtheGreekwordfor"cell."）Thestudyofcytoarchitectureledtotherealizationthatthebrainconsistsofmanyspecializedregions.Wenowknowthateachregionperformsadifferentfunction.

TheGolgiStain

TheNisslstain,however,doesnottellthewholestory.ANissl-stainedneuronlookslikelittlemorethanalumpofprotoplasmcontaininganucleus,Neuronsaremuchmorethanthat,buthowmuchmorewasnotrecognizeduntilthepublicationoftheworkofItalianhistologistCamilloGolgi（Figure2.2）.In1873,Golgidiscoveredthatbysoakingbraintissueinasilverchromatesolution,nowcalledtheGolgistain,asmallpercentageofneuronsbecamedarklycoloredintheirentirety（Figure2.3）.Thisrevealedthattheneuronalcellbody,theregionoftheneuronaroundthenucleusthatisshownwiththeNisslstain,isactuallyonlyasmallfractionofthetotalstructureoftheneuron.NoticeinFigures2.1and2.3howdifferenthistologicalstainscanprovidestrikinglydifferentviewsofthesametissue.Today,neurohistologyremainsanactivefieldinneuroscience,alongwithitscredo:

"Thegaininbrainismainlyinthestain."

TheGolgistainshowsthatneuronshaveatleasttwodistinguishableparts:

acentralregionthatcontainsthecellnucleus,andnumerousthintubesthatradiateawayfromthecentralregion.Theswollenregioncontainingthecellnucleushasseveralnamesthatareusedinterchangeably:

cellbody,soma（plural:

somata）,andperikaryon（plural:

perikarya）.Thethintubesthatradiateawayfromthesomaarecalledneuritesandareoftwotypes:

axonsanddendrites（Figure2.4）.

Thecellbodyusuallygivesrisetoasingleaxon.Theaxonisofuniformdiameterthroughoutitslength,andifitbranches,thebranchesgenerallyextendatrightangles.Becauseaxonscantravelovergreatdistancesinthebody（ameterormore）,itwasimmediatelyrecognizedbythehistologistsofthedaythataxonsmustactlike"wires'thatcarrytheoutputoftheneurons.Dendrites,ontheotherhand,rarelyextendmorethan2mminlength.Manydendritesextendfromthecellbodyandgenerallytapertoafinepoint.Earlyhistologistsrecognizedthatbecausedendritescomeincontactwithmanyaxons,theymustactastheantennaeoftheneurontoreceiveincomingsignals,orinput.

Cajal'sContribution

Golgiinventedthestain,butitwasaSpanishcontemporaryofGolgiwhousedittogreatesteffect.SantiagoRamenyCajalwasaskilledhistologistandartistwholearnedaboutGolgi'smethodin1888（Figure2.5）.Inaremarkableseriesofpublicationsoverthenext25years,CajalusedtheGolgistaintoworkoutthecircuitryofmanyregionsofthebrain（Figure2.6）.Ironically,GolgiandCajaldrewcompletelyoppositeconclusionsaboutneurons.Golgichampionedtheviewthattheneuritesofdifferentcellsarefusedtogethertoformacontinuousreticulum,ornetwork,similartothearteriesandveinsofthecirculatorysystem.Accordingtothisreticulartheory,thebrainisanexceptiontothecelltheory,whichst