Oligodendrocytes and Schwann cells are critical for the function of axons. Many axons are surrounded by a myelin sheath, which is a spiral multilayered wrapping of glial cell membrane (
Fig. 4.5A–B ). In the CNS, myelin is formed by the oligodendrocytes, whereas in the PNS Schwann cells form myelin. Myelin increases the speed of action potential conduction, in part by restricting the
CHAPTER 4 The Nervous System: Introduction to Cells and Systems •Fig. 4.5 Axonal/glialassociations.A,MyelinatedaxonsintheCNS.Asingleoligodendrocyte(G)emitsseveralprocesses,eachofwhichwindsinaspiralfashionaroundanaxontoformamyelinsegment.Theaxonisshownincutaway.Themyelinfromasingleoligodendrocyteendsbeforethenextwrappingfromanotheroligodendrocyte.ThebareaxonbetweenthemyelinatedsegmentsisthenodeofRanvier(N).B,ElectronmicrographofmyelinatedaxoninthePNSshownincrosssection.Theaxon(Ax)isseenatthecenterwithinasheathconsistingofmultiplewrappingsoftheSchwanncell’scytoplasmicmembrane.TheSchwanncellsoma(SC)isattheupperright.mes,mesaxon,internalandexternal.C,Electronmicro-graphofunmyelinatedaxonsinPNS.Nineaxons(asterisks) cutincrosssectionareseenembeddedinaSchwanncellwhosenucleusisatthecenter(N).Atlowerrightaportionofamyelinatedaxonisvisible.(B,FromPetersAetal.The Fine Structure of the Nervous System. NewYork:OxfordUniversityPress;1991,Fig.6.5.C,FromPanneseE.Neurocytology. 2nded.Basel,Switzerland:SpringerInternational;2015.)
Astrocytesarecoupledtoeachotherbygapjunctionssuchthattheyformasyncytiumthroughwhichsmallmoleculesandionscanredistributealongtheirconcentrationgradientsorbycurrentflow.Whennormalneuralactivitygivesrisetoalocalincreaseinextracellular[K+],thiscouplednetworkcanenablespatialredistributionofK+ overawideareaviacurrentflowinmanyastrocytes.
Underconditionsofhypoxia,suchasmightbeassociatedwithischemiasecondarytoblockageofanartery(i.e.,astroke),[K+]intheextracellularspaceofabrainregioncanincreasebyafactorofasmuchas20.Thiswilldepolarizeneuronsandsynapticterminalsandresultinreleaseoftransmitterssuchasglutamate,whichwillcausefurtherreleaseofK+ fromneurons.Theadditionalreleaseonlyexacerbatestheproblemandcanleadtoneuronaldeath.Undersuchconditions,localastrogliawillprobablytakeuptheexcessK+ byK+-Cl− symportratherthanbyspatialbuffering,becausetheelevationinextracellular[K+]tendstobewidespreadratherthanlocal.
flow of ionic current to small unmyelinated portions of the axon between adjacent glial cells, the nodes of Ranvier (see ). Although both act to increase the speed of conduction, there are several important differences in the relationship between axons and either oligodendrocytes or Schwann cells. One major difference is that a single oligodendrocyte typically helps myelinate multiple axons in the CNS, whereas each Schwann cell helps myelinate only a single axon in the PNS. A second difference is that in the CNS, unmyelinated axons are bare, whereas in the PNS, unmyelinated axons are not. Rather, they are surrounded by Schwann cell processes; the Schwann cell, however, does not form a multilayered covering (i.e., myelin), but instead extends processes that surround parts of several axons (the Schwann cell with its set of unymyelinated axons is called a Remak bundle) (see