Oligodendrocytes and Schwann cells play a crucial role in the functioning of axons. The myelin sheath, which consists of multiple layers of glial cell membrane, wraps around many axons (Fig. 4.5A-B). In the central nervous system (CNS), oligodendrocytes form myelin, while in the peripheral nervous system (PNS), Schwann cells are responsible for myelin formation. Myelin enhances the speed of action potential conduction by limiting the flow of ionic current to the nodes of Ranvier, which are small unmyelinated segments of the axon situated between adjacent glial cells. It is important to note that there are several distinctions between axons and oligodendrocytes or Schwann cells. One key difference is that a single oligodendrocyte typically myelinates multiple axons in the CNS, whereas each Schwann cell myelinates only one axon in the PNS. Additionally, unmyelinated axons in the CNS are bare, while in the PNS, they are enveloped by Schwann cell processes. However, unlike myelinated axons, these processes do not form a multilayered myelin covering but instead surround parts of several axons, forming a structure known as a Remak bundle (see Fig. 4.5C).

Astrocytes are interconnected through gap junctions, creating a syncytium that allows for the redistribution of small molecules and ions along their concentration gradients or through current flow. During normal neural activity, an increase in extracellular potassium (K+) concentration can be spatially redistributed over a wide area via current flow in many astrocytes. However, under hypoxic conditions, such as those associated with ischemia resulting from artery blockage (e.g., stroke), extracellular potassium concentration in a specific brain region can increase up to 20 times. This depolarizes neurons and synaptic terminals, leading to the release of transmitters like glutamate, which further promotes the release of potassium from neurons. This additional release exacerbates the problem and can cause neuronal death. Under such conditions, local astroglia are likely to take up the excess potassium through K+-Cl− symport rather than spatial buffering, as the elevation in extracellular potassium tends to be widespread rather than localized.
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