Introduction to Anatomy (Lec 7) muscular+Nervous tissue.pdf

Introduction to Anatomy (Lec 7) muscular+Nervous tissue.pdf

1. TISSUES OF BODY
Pharm D 1
st
Prof
Bahauddin Zakariya University Multan
Waseem Ashraf

2. Recommended Books:
•Fundamentals of Anatomy & Physiology. Martini F, Nath JL, Bartholomew EF. 10th Edition
•Principles of Anatomy and Physiology. Tortora GJ, Derrickson B. 13th Edition

3. Tissue Membranes
Tissuemembranesarephysicalbarriersoffourtypes:mucous,serous,cutaneous,andsynovial.
Atissuemembraneformsaphysicalbarrier.Therearemanydifferenttypesofanatomicalmembranes.
Eachconsistsofanepitheliumsupportedbyconnectivetissue.Foursuchmembranetypesarefoundinthebody:(1)
mucousmembranes,(2)serousmembranes,(3)thecutaneousmembrane,and(4)synovialmembranes
MucousMembranes
Mucousmembranes,ormucosae,line,passagewaysandchambersthatcommunicatewiththeexterior,includingthosein
thedigestive,respiratory,reproductive,andurinarytracts.Theepithelialsurfacesofthesepassagewaysmustbekeptmoist
toreducefrictionand,inmanycases,tofacilitateabsorptionorsecretion.Theepithelialsurfacesarelubricatedeitherby
mucusproducedbymucouscellsormulticellularglands,orbyfluids,suchasurineorsemen.Theareolartissue
componentofamucousmembraneiscalledthelaminapropria.Manymucousmembranescontainsimpleepitheliathat
performabsorptiveorsecretoryfunctions,suchasthesimplecolumnarepitheliumofthedigestivetract.However,other
typesofepitheliamaybeinvolved.Forexample,astratifiedsquamousepitheliumispartofthemucousmembraneofthe
mouth,andthemucousmembranealongmostoftheurinarytractcontainsatransitionalepithelium.

5. Tissue Membranes
SerousMembranes
Serousmembraneslinethesealed,internalcavitiesofthetrunk—cavitiesthatarenotopentotheexterior.These
membranesconsistofamesotheliumsupportedbyareolartissue.Thethreetypesofserousmembranesare(1)thepleura,
whichlinesthepleuralcavitiesandcoversthelungs;(2)thepericardium,whichlinesthepericardialcavityandcovers
theheart;and(3)theperitoneum,whichlinestheperitonealcavityandcoversthesurfacesoftheenclosedorgans.Serous
membranesareverythin,buttheyarefirmlyattachedtothebodywallandtotheorganstheycover.Whenlookingatan
organsuchastheheartorstomach,youarereallyseeingthetissuesoftheorganthroughatransparentserousmembrane.
Theprimaryfunctionofanyserousmembraneistominimizefrictionbetweenthesurfacesthatitcovers.Eachserous
membranecanbedividedintoaparietalportion,whichlinestheinnersurfaceofthecavity,andanopposingvisceral
portion,orserosa,whichcoverstheoutersurfacesofvisceralorgans.Theseorgansoftenmoveorchangeshapeasthey
carryouttheirvariousfunctions,andtheparietalandvisceralsurfacesofaserousmembraneareinclosecontactatall
times.Frictionbetweenthesesurfacesiskepttoaminimumbecausemesotheliaareverythinandpermeable,andtissue
fluidscontinuouslydiffuseontotheexposedsurfaces,keepingthemmoistandslippery.

6. Tissue Membranes
Thefluidsecretedbythemesotheliumofaserousmembraneiscalledserousfluid.Serousfluidpreventsfrictionbetween
thecavitywallsandthesurfacesofinternalorgans.However,afteraninjuryorincertaindiseasestateswherethereisan
imbalanceofhydrostaticandosmoticforces,thevolumeofserousfluid,nowcalledtransudatemayincreasedramatically.
Thischangemaycomplicateexistingmedicalproblemsorproducenewones.
TheCutaneousMembrane
Thecutaneousmembraneistheskinthatcoversthesurfaceofyourbody.Itconsistsofastratifiedsquamousepithelium
andalayerofareolartissuereinforcedbyunderlyingdenseirregularconnectivetissue.Incontrasttoserousandmucous
membranes,thecutaneousmembraneisthick,relativelywaterproof,andusuallydry.
SynovialMembranes
Adjacentbonesofteninteractatjoints,orarticulations.Therethetwoarticulatingbonesareveryclosetogetherorin
contact.Jointsthatpermitsignificantamountsofmovementarecomplexstructures.Suchajointissurroundedbya
fibrouscapsule,andtheendsofthearticulatingbonesliewithinajointcavityfilledwithsynovialfluid.Thesynovialfluid
isproducedbyasynovialmembrane,whichlinesthejointcavity.Asynovialmembraneconsistsofanextensiveareaof
areolartissuecontainingamatrixofinterwovencollagenfibers,proteoglycans,andglycoproteins.

7. Tissue Membranes
Anincompletelayer,orsheet,ofmacrophagesandspecializedfibroblastsoverliesandseparatestheareolartissuefromthe
jointcavity.Thesecellsregulatethecompositionofthesynovialfluid.Thisincompletelayerofcellsisoftencalledan
epithelium,butitdiffersfromtrueepitheliainfourrespects:(1)Itdevelopswithinaconnectivetissue;(2)nobasement
membraneispresent;(3)gapsofupto1mmmayseparateadjacentcells;and(4)fluidandsolutesarecontinuously
exchangedbetweenthesynovialfluidandcapillariesintheunderlyingconnectivetissue.
Eventhoughasmoothlayerofarticularcartilagecoverstheendsofthebones,thesurfacesmustbelubricatedtokeep
frictionfromdamagingtheopposingsurfaces.Synovialfluidprovidesthemuch-neededlubrication.Thisfluidissimilar
incompositiontothegroundsubstanceinlooseconnectivetissues.Synovialfluidcirculatesfromtheareolartissueinto
thejointcavityandpercolatesthroughthearticularcartilages,supplyingoxygenandnutrientstothechondrocytes.Joint
movementisimportantinstimulatingtheformationandcirculationofsynovialfluid:Ifasynovialjointisimmobilizedfor
longperiods,thearticularcartilagesandthesynovialmembraneundergodegenerativechanges.

8. Muscle Tissue
Muscletissuesarespecializedforcontractionandproducingmovements.
Musclecellshaveorganellesandpropertiesdistinctfromothercells.
Therearethreetypesofmuscletissue:(1)skeletalmuscle,whichformsthelargemusclesthatproducegrossbody
movements;(2)cardiacmuscle,foundintheheart,isresponsibleforcirculatingtheblood;and(3)smoothmuscle,foundin
thewallsofvisceralorgansandavarietyofotherlocations,whereitprovideselasticity,contractility,andsupport.The
contractionmechanismissimilarinallthreetypesofmuscletissue,butthemusclecellsdifferininternalorganization.
SkeletalMuscletissue
Skeletalmuscletissuecontainsverylargemusclecells—upto0.3m(1ft)ormoreinlength.Theindividualmusclecellsare
usuallycalledmusclefibersbecausetheyarerelativelylongandslender.Eachmusclefiberhasseveralhundrednucleijust
insidetheplasmamembrane.Forthisreason,thefibersaredescribedasmultinucleate.Skeletalmusclefibersareincapable
ofdividing.Newmusclefibersareproducedinsteadthroughthedivisionsofmyosatellitecells(satellitecells),stemcells
thatpersistinadultskeletalmuscletissue.Asaresult,skeletalmuscletissuecanatleastpartiallyrepairitselfafteraninjury.
Skeletalmusclefibersalsocontainsactin&myosinfilamentshowever,thesefilamentsareorganizedintorepeatingpatterns
thatgivethecellsastriated,orbanded,appearance.Thestriations,orbands,areeasytoseeinlightmicrographs.

9. Skeletalmusclefibersdonotusuallycontractunlessstimulatedbynerves,andthenervoussystemprovidesvoluntary
controlovertheiractivities.Thus,skeletalmuscleiscalledstriatedvoluntarymuscle.
CardiacMuscletissue
Cardiacmuscletissueisfoundintheheart.Atypicalcardiacmusclecell,alsoknownasacardiocyte,issmallerthana
skeletalmusclecell.Atypicalcardiacmusclecellhasonecentrallypositionednucleus,butsomecardiocyteshaveasmany
asfive.Prominentstriationsresemblethoseofskeletalmusclebecausetheactinandmyosinfilamentsarearrangedthesame
wayinthecellsofbothtissuetypes.Cardiacmuscletissueconsistsofabranchingnetworkofinterconnectedcardiacmuscle
cells.Thecellsareconnectedatspecializedregionsknownasintercalateddiscs.Therethemembranesarelockedtogether
bydesmosomes,proteoglycans,andgapjunctions.Ionmovementthroughgapjunctionshelpssynchronizethecontractions
ofthecardiacmusclecells,andthedesmosomesandproteoglycanslockthecellstogetherduringacontraction.
Cardiacmuscletissuehasaverylimitedabilitytorepairitself.Somecardiacmusclecellsdodivideafteraninjurytothe
heart,buttherepairsareincomplete.Someheartfunctionisusuallylost.
Cardiacmusclecellsdonotrelyonnerveactivitytostartacontraction.Instead,specializedcardiacmusclecellscalled
pacemakercellssetaregularrateofcontraction.Thenervoussystemcanaltertherateofpacemakercellactivity,butitdoes
notprovidevoluntarycontroloverindividualcardiacmusclecells.Forthisreason,cardiacmuscleiscalledstriated
involuntarymuscle.

10. SmoothMuscletissue
Smoothmuscletissueisfoundinthewallsofbloodvessels,aroundholloworganssuchastheurinarybladder,andinlayers
aroundtherespiratory,cardiovascular,digestive,andreproductivetracts.Asmoothmusclecellissmallandspindle-shaped,
withtaperingendsandasingle,ovalnucleus.Thesecellscandivide,sosmoothmuscletissuecanregenerateafteraninjury.
Theactinandmyosinfilamentsinsmoothmusclecellsareorganizeddifferentlyfromthoseofskeletalandcardiacmuscles.
Oneresultisthatsmoothmuscletissuehasnostriations.
Smoothmusclecellsmaycontractontheirown,withgapjunctionsbetweenadjacentcellscoordinatingthecontractionsof
individualcells.Thecontractionofsomesmoothmuscletissuecanbecontrolledbythenervoussystem,butcontractile
activityisnotundervoluntarycontrol.
(Imaginetheeffortyouwouldneedtoexertconsciouscontroloverthesmoothmusclesalongthe8m[26ft.]ofdigestive
tract,nottomentionthemilesofbloodvessels!)Smoothmuscleisknownasnonstriatedinvoluntarymusclebecauseofits
appearanceandthefactthatthenervoussystemusuallydoesnotvoluntarilycontrolsmoothmusclecontractions.

14. Neural or Nervous Tissue
Neuraltissue,alsoknownasnervoustissueornervetissue,isspecializedforthepropagation(movement)ofelectrical
impulsesfromoneregionofthebodytoanother.Ninetyeightpercentoftheneuraltissueinthebodyisinthebrainand
spinalcord,whicharethecontrolcentersofthenervoussystem.
Neuraltissuecontainstwobasictypesofcells:(1)neurons(neuro,nerve)and(2)severalkindsofsupportingcells,
collectivelycalledneuroglia,orglialcells(glia,glue).
Ourconsciousandunconsciousthoughtprocessesreflectthecommunicationamongneuronsinthebrain.Such
communicationinvolvesthepropagationofelectricalimpulses,intheformofreversiblechangesinthemembranepotential.
Thefrequencyandthepatternoftheimpulsesconveyinformation.
Neurogliasupportandrepairneuraltissueandsupplynutrientstoneurons.
Neuronsarethelongestcellsinyourbody.Manyareaslongasameter(39in.)!Mostneuronscannotdivideundernormal
circumstances,sotheyhaveaverylimitedabilitytorepairthemselvesafterinjury.Atypicalneuronhasalargecellbody
withalargenucleusandaprominentnucleolus.Extendingfromthecellbodyaremanybranchingprocesses(projectionsor
outgrowths)termeddendrites(DEN-drı.ts;dendron,atree),andoneaxon.Thedendritesreceiveinformation,typicallyfrom
otherneurons.Theaxonconductsthatinformationtoothercells.Becauseaxonstendtobeverylongandslender,theyare
alsocallednervefibers.

15. Neuron
TheCellBody
Thecellbody,orsoma,containsalarge,roundnucleuswithaprominentnucleolus.Thecytoplasmsurroundingthenucleus
istheperikaryon(peri,around+karyon,nucleus).Thecytoskeletonoftheperikaryoncontainsneurofilamentsand
neurotubules,whicharesimilartotheintermediatefilamentsandmicrotubulesofothertypesofcells.Bundlesof
neurofilaments,calledneurofibrils,extendintothedendritesandaxon,providinginternalsupportforthem.Theperikaryon
containsorganellesthatprovideenergyandsynthesizeorganicmaterials,especiallythechemicalneurotransmittersthatare
importantincell-to-cellcommunication.Thenumerousmitochondria,freeandfixedribosomes,andmembranesofrough
endoplasmicreticulum(RER)givetheperikaryonacoarse,grainyappearance.MitochondriagenerateATPtomeetthehigh
energydemandsofanactiveneuron.TheribosomesandRERsynthesizeproteins.Someareasoftheperikaryoncontain
clustersofRERandfreeribosomes.Theseregions,whichstaindarkly,arecalledNisslbodies,aftertheGerman
microscopistFranzNissl,whofirstdescribedthem.Nisslbodiesgiveagraycolortoareascontainingneuroncellbodies—
thegraymatterseeningrossdissectionofthebrainandspinalcord.
Mostneuronslackcentrioles,importantorganellesthathelptoorganizethecytoskeletonandthemicrotubulesthatmove
chromosomesduringmitosis.Asaresult,typicalCNSneuronscannotdivide.Forthisreason,theycannotbereplacediflost
toinjuryordisease.

17. Neuron
Neuralstemcellspersistintheadultnervoussystem,buttheyaretypicallyinactiveexceptinthenose,wherethe
regenerationofolfactory(smell)receptorsmaintainsoursenseofsmell,andinthehippocampus,apartofthebraininvolved
instoringmemories.Researchersareinvestigatingthecontrolmechanismsthattriggerneuralstemcellactivity,withthe
goalofpreventingorreversingneuronlossduetotrauma,disease,oraging.
Dendritesandaxons
Avariablenumberofslender,sensitiveprocesses(extensions)knownasdendritesextendoutfromthecellbody.Dendrites
playkeyrolesinintercellularcommunication.Typicaldendritesarehighlybranched,andeachbranchhasfine0.5-to1-μm-
longstuddedprocessescalleddendriticspines.
IntheCNS,aneuronreceivesinformationfromotherneuronsprimarilyatthedendriticspines,whichrepresent80–90
percentoftheneuron’stotalsurfacearea.
Anaxonisalongcytoplasmicprocesscapableofpropagatinganelectricalimpulseknownasanactionpotential.The
axoplasmorcytoplasmoftheaxon,containsneurofibrils,neurotubules,smallvesicles,lysosomes,mitochondria,and
variousenzymes.Theaxolemma(lemma,husk),aspecializedportionoftheplasmamembrane,surroundstheaxoplasm.In
theCNS,theaxolemmamaybeexposedtotheinterstitialfluidor,itmaybecoveredbythecellularprocessesofneuroglia.

18. Neuron
Thebase,orinitialsegment,oftheaxoninamultipolarneuronjoinsthecellbodyatathickenedregionknownastheaxon
hillock.
Anaxonmaybranchalongitslength,producingsidebranchesknownascollaterals.Collateralsenableasingleneuronto
communicatewithseveralothercells.Themainaxontrunkandanycollateralsendinaseriesoffineextensionscalled
telodendria(telo-,end+dendron,tree)orterminalbranches.Thetelodendria,inturn,endataxonterminals(alsocalled
synapticterminals,synapticknobs,andsynapticboutons),whichplayaroleincommunicationwithanothercell.
Thestructureoftheaxonterminalvarieswiththetypeofpostsynapticcell.Arelativelysimple,roundaxonterminaloccurs
wherethepostsynapticcellisanotherneuron.Atasynapse,thenarrowsynapticcleftseparatesthepresynapticmembrane
whereneurotransmittersarereleased,fromthepostsynapticmembrane,whichbearsreceptorsforneurotransmitters.The
axonterminalataneuromuscularjunctionismuchmorestructurallycomplex.
Eachaxonterminalcontainsmitochondriaandthousandsofvesiclesfilledwithneurotransmittermolecules.Theaxon
terminalreabsorbsbreakdownproductsofneurotransmittersformedatthesynapseandreassemblesthem.Italsoreceivesa
continuoussupplyofneurotransmitterssynthesizedinthecellbody,alongwithenzymesandlysosomes.Thesematerials
travelthelengthoftheaxononneurotubules,pulledalongbyprotein“molecularmotors,”calledkinesinanddynein,that
runonATP.

19. Neuron
Themovementofmaterialsbetweenthecellbodyandaxonterminalsiscalledaxoplasmictransport.Somematerialstravel
slowly,atratesofafewmillimetersperday.Thistransportmechanismisknownasthe“slowstream.”Vesiclescontaining
neurotransmittersmovemuchmorerapidly,travelinginthe“faststream”at5–10mmperhour.
Axoplasmictransportoccursinbothdirections.
Theflowofmaterialsfromthecellbodytotheaxonterminalisanterograde(antero-,forward)flow,carriedbykinesin.
Atthesametime,othersubstancesaretransportedfromtheaxonterminaltowardthecellbodyinretrogradeflow(retro,
backward),carriedbydynein.Ifdebrisorunusualchemicalsappearintheaxonterminal,retrogradeflowsoondelivers
themtothecellbody.Theretheymaythenaltertheactivityofthecellbyturningcertaingenesonoroff.

21. Structural Classification of Neurons
Anaxonicneuronsaresmallandhavenumerousdendrites,butnoaxon.Anaxonicneuronsarelocatedinthebrainandin
specialsenseorgans.Theirfunctionsarepoorlyunderstood.
Bipolarneuronshavetwodistinctprocesses—onedendritethatbranchesextensivelyintodendriticbranchesatitsdistaltip,
andoneaxon—withthecellbodybetweenthetwo.Bipolarneuronsarerare.Theyoccurinspecialsenseorgans,wherethey
relayinformationaboutsight,smell,orhearingfromreceptorcellstootherneurons.Bipolarneuronsaresmall.Thelargest
measurelessthan30µmfromendtoend.
Inaunipolarneuron,orpseudounipolarneuron,thedendritesandaxonarecontinuous—basically,fused—andthecell
bodyliesofftooneside.Insuchaneuron,theinitialsegmentlieswherethedendritesconverge.Therestoftheprocess,
whichcarriesactionpotentials,isusuallyconsideredanaxon.Mostsensoryneuronsoftheperipheralnervoussystemare
unipolar.Theiraxonsmayextendameterormore,endingatsynapsesinthecentralnervoussystem.Thelongestcarry
sensationsfromthetipsofthetoestothespinalcord.
Multipolarneuronshavetwoormoredendritesandasingleaxon.TheyarethemostcommonneuronsintheCNS.Allthe
motorneuronsthatcontrolskeletalmuscles,forexample,aremultipolarneurons.Theaxonsofmultipolarneuronscanbeas
longasthoseofunipolarneurons,andthelongestcarrymotorcommandsfromthespinalcordtosmallmusclesthatmove
thetoes.

24. Neuroglia of the Central Nervous System
Thecentralnervoussystemhasfourtypesofneuroglia:(1)ependymalcells,(2)astrocytes,(3)oligodendrocytes,and(4)
microglia.
EpendymalCells
Afluid-filledcentralpassagewayextendsalongthelongitudinalaxisofthespinalcordandbrain.Cerebrospinalfluid(CSF)
fillsthispassagewayandalsosurroundsthebrainandspinalcord.Thisfluid,whichcirculatescontinuously,providesa
protectivecushionandtransportsdissolvedgases,nutrients,wastes,andothermaterials.Thediameteroftheinternal
passagewayvariesfromoneregiontoanother.Inthespinalcord,thisnarrowpassagewayiscalledthecentralcanal.In
severalregionsofthebrain,thepassagewayformsenlargedchamberscalledventricles.Ependymalcellslinethecentral
canalandventricles,wheretheyformasimplecuboidaltocolumnarepitheliumknownastheependyma.Unlikeother
epithelia,itlacksabasementmembrane.Ependymalcellsassistinproducing,monitoring,andcirculatingCSF.Thereare
threetypesofependymalcells:ependymocytes,tanycytes,andspecializedCSF-producingependymalcells.
EpendymocyteshavemotileciliathataidinthecirculationofCSFandalsomicrovilli.Inadults,theciliadonotpersistin
allareasoftheventriclesandcentralcanal.Ependymocyteshavelong,slenderbasalprocessesthatbranchandmakecontact
withneuroglia.Tanycytesarespecialized,non-ciliatedependymalcellswithmicrovilliontheirapicalsurfaces.Theyare
foundinonlyonebrainventricle.ItisthoughtthattheytransportsubstancesbetweentheCSFandthebrain.

27. Neuroglia of the Central Nervous System
Astrocytes
Astrocytes(AS-tro.-sı.ts;astro-,star+-cyte,cell)arethelargestandmostnumerousneurogliaintheCNS.
Thesecellshaveavarietyoffunctions,manyofthempoorlyunderstood:
•Maintaining the Blood–Brain Barrier.
•Creating a Three-Dimensional Framework for the CNS.
•Repairing Damaged Neural Tissue.
•Guiding Neuron Development.
•Controlling the Interstitial Environment.
Oligodendrocytes
Likeastrocytes,oligodendrocyteshaveslendercytoplasmicextensions,butthecellbodiesofoligodendrocytesaresmaller,
withfewerprocesses,thanastrocytes.Theprocessesofoligodendrocytesgenerallyareincontactwiththeexposedsurfaces
ofneurons.Thefunctionsofprocessesendingattheneuroncellbodyhaveyettobedetermined.Muchmoreisknownabout
theprocessesthatendonthesurfacesofaxons.ManyaxonsintheCNSarecompletelysheathedintheseprocesses,which
insulatethemfromcontactwiththeextracellularfluid.

28. Neuroglia of the Central Nervous System
Duringitsmaturation,theplasmamembranenearthetipofeachprocessofanoligodendrocyteexpandstoforman
enormouspad,andthecytoplasmtherebecomesverythin.Thisflattened“pancake”somehowgetswrappedaroundthe
axolemma,formingconcentriclayersofplasmamembrane.Thismembranouswrapping,calledmyelin,servesaselectrical
insulationandincreasesthespeedatwhichanactionpotentialtravelsalongtheaxon.Manyoligodendrocytescooperatein
formingamyelinsheathalongthelengthofanaxon.Suchanaxonissaidtobemyelinated.Eacholigodendrocyte
myelinatessegmentsofseveralaxons.Thefairlylargeareasoftheaxonthatarewrappedinmyelinarecalledinternodes.
Internodesaretypically1–2mminlength.Thesmallgapsofafewmicrometersthatseparateadjacentinternodesarecalled
nodes,ornodesofRanvier.Anaxon’scollateralbranchesoriginateatnodes.
Indissection,myelinatedaxonsappearglossywhite,primarilybecauseofthelipidsinthemyelin.Asaresult,regions
dominatedbymyelinatedaxonsareknownasthewhitematteroftheCNS.
Unmyelinatedaxonsarenotcompletelycoveredbyoligodendrocytes.Suchaxonsarecommonwhereshortaxonsand
collateralsformsynapseswithdenselypackedneuroncellbodies.Areascontainingneuroncellbodies,dendrites,and
unmyelinatedaxonshaveaduskygraycolor,andmakeupthegraymatteroftheCNS.
Insummary,oligodendrocytesplayanimportantroleinstructuralorganizationbytyingclustersofaxonstogether.Theyalso
improvethefunctioningofneuronsbywrappingaxonswithinamyelinsheath.

29. Neuroglia of the Central Nervous System
Microglia
TheleastnumerousandsmallestneurogliaintheCNSarephagocyticcellscalledmicroglia.Theirslenderprocesseshave
manyfinebranches.Thesecellscanmigratethroughneuraltissue.Microgliaappearearlyinembryonicdevelopment,
originatingfrommesodermalstemcellsrelatedtostemcellsthatproducemonocytesandmacrophages.Microgliamigrate
intotheCNSasthenervoussystemforms.Theretheyremain,actingasawanderingjanitorialserviceandpoliceforceby
engulfingcellulardebris,wasteproducts,andpathogens.

30. Neuroglia of the Peripheral Nervous System
ThecellbodiesofneuronsinthePNSareclusteredinmassescalledganglia(singular,ganglion).Theprocessesofneuroglia
completelyinsulateneuronalcellbodiesandmostaxonsinthePNSfromtheirsurroundings.
ThetwotypesofneurogliainthePNSaresatellitecellsandSchwanncells.
Satellitecellssurroundneuroncellbodiesinganglia.Theyregulatetheenvironmentaroundtheneurons,muchasastrocytes
dointheCNS.
Schwanncells,orneurilemmacells,eitherformathick,myelinsheathorindentedfoldsofplasmamembranearound
peripheralaxons.WhereveraSchwanncellcoversanaxon,theoutersurfaceoftheSchwanncelliscalledtheneurilemma.
MostaxonsinthePNS,whethermyelinatedorunmyelinated,areshieldedfromcontactwithinterstitialfluidsbySchwann
cells.AmyelinatingSchwanncellcanmyelinateonlyonesegmentofasingleaxon,whereasanoligodendrocyteintheCNS
maymyelinateportionsofseveraladjacentaxons.Non-myelinatingSchwanncellscanenclosesegmentsofseveral
unmyelinatedaxons.AseriesofSchwanncellsisrequiredtoencloseanaxonalongitsentirelength.