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Some nerve cells, for example, are electricallycoupled, allowing action potentials to spread rapidly from cell to cell, withoutthe delay that occurs at chemical synapses.This is advantageous when speedand reliability are crucial, as in certain escaperesponsesin fish and insects, orwhere a set of neurons need to act in synchrony. Similarly, in vertebrates, electrical coupling through gap junctions synchronizes the contractions of heartmuscle cells as well as those of the smooth muscle cells responsible for the peristaltic movements of the intestine.Gap junctions also occur in many tissues whose cells are not electricallyexcitable.
In principle, the sharing of small metabolites and ions provides amechanism for coordinating the activities of individual cells in such tissues andfor smoothing out random fluctuations in small-molecule concentrations in different cells. Gap junctions are required in the liver, for example, to coordinatethe response of the liver cells to signals from nerve terminals that contact only apart of the cell population (see Figure f 5-7).
The normal development of ovarian follicles also depends on gap-junction-mediated communication-in thiscase,between the oocyte and the surrounding granulosa cells.A mutation in thegene that encodes the connexin that normally couples these two cell typescauses infertility.Mutations in connexins, especially connexin-26, are the commonest of allgenetic causes of congenital deafness: they result in the death of cells in theorgan of Corti, probably because they disrupt functionally important pathwaysfor the flow of ions from cell to cell in this electrically active sensory epithelium.Connexin mutations are responsible for many other disorders besides deafness,ranging from cataracts in the lens of the eye to a form of demyelinating diseasein peripheral nerves.Cell coupling via gap junctions also seems to play a part in embryogenesis.In early vertebrate embryos (beginning with the late eight-cell stage in mouseembryos), most cells are electrically coupled to one another.
As specific groupsof cells in the embryo develop their distinct identities and begin to differentiate, they commonly uncouple from surrounding tissue. As the neural platefolds up and pinches off to form the neural tube, for instance (see Figure19-16), its cells uncouple from the overlying ectoderm. Meanwhile, the cellswithin each group remain coupled with one another and therefore tend tobehave as a cooperative assembly, all following a similar developmental pathway in a coordinated fashion.CellsCanRegulatethe Permeabilityof TheirGapJunctionsLike conventional ion channels (discussedin Chapter 11), individual gap-junction channels do not remain continuously open; instead, they flip between openand closed states.Moreover, the permeability of gap junctions is rapidly (withinseconds) and reversibly reduced by experimental manipulations that decreasethe cytosolic pH or increase the cytosolic concentration of free Caz*to very highlevels.The purpose of the pH regulation of gap-junction permeability is unknor.vn.In one case, however, the purpose of Caz* control seems clear.
lVhen a cell isdamaged, its plasma membrane can become leaky.Ions present at high concentration in the extracellular fluid, such as Ca2* and Na*, then move into the cell,and valuable metabolites leak out. If the cell were to remain coupled to its't16"11162Chapter19:CellJunctions,MatrixCellAdhesion,and the ExtracellularFigure19-37The regulationof gapjunction coupling by a neurotransmitter.(A)A neuron in a rabbit retinawas injectedwith the dye Luciferyellow,which passesreadilythroughgapjunctionsand labelsother neuronsof the sametype that areconnectedto the injectedcellby gapjunctions.(B)The retinawasfirsttreatedwith the neurotransmitterdopamine,beforethe neuronwas injectedwith dye.As can be seen,the dopaminetreatmentgreatlydecreasedthe permeabilityof thegapjunctions.DopamineactsbyincreasingintracellularcyclicAMPlevels.(Courtesyof DavidVaney.)healthy neighbors, these too would suffer a dangerous disturbance of theirinternal chemistry.
But the large influx of Ca2*into the damaged cell causesitsgap-junction channels to close immediately, effectively isolating the cell andpreventing the damage from spreading to other cells.Gap-junction communication can also be regulated by extracellular signals.The neurotransmitter dopamine, for example, reduces gap-junction communication between a classof neurons in the retina in responseto an increasein lightintensity (Figure f 9-37).
This reduction in gap-junction permeability helps theretina switch from using rod photoreceptors, which are good detectors of lowlight, to cone photoreceptors, which detect color and fine detail in bright light.In Plants,PlasmodesmataPerformManyof the SameFunctionsa sG a pJ u n c t io n sThe tissues of a plant are organized on different principles from those of an animal. This is becauseplant cells are imprisoned within tough cell walls composedof an extracellular matrix rich in cellulose and other polysacharides, as we discuss later. The cell walls of adjacent cells are firmly cemented to those of theirneighbors, which eliminates the need for anchoring junctions to hold the cellsin place.
But a need for direct cell-cell communication remains. Thus, plant cellshave only one class of intercellular junctions, plasmodesmata. Like gap junctions, they directly connect the cytoplasms of adjacent cells.In plants, the cell wall between a tlpical pair of adjacent cells is at least 0.1pm thick, and so a structure very different from a gap junction is required tomediate communication across it. Plasmodesmata solve the problem. With afew specialized exceptions, every living cell in a higher plant is connected to itsliving neighbors by these structures, which form fine cytoplasmic channelsthrough the intervening cell walls. As shown in Figure l9-38A, the plasma membrane of one cell is continuous with that of its neighbor at each plasmodesma,which connects the cltoplasms of the two cells by a roughly cylindrical channelwith a diameter of 20-40 nm.Running through the center of the channel in most plasmodesmata is a narrower cylindrical structure, the desmotubule,which is continuous with elementsof the smooth endoplasmic reticulum in each of the connected cells (Figurel9-38B-D).
Between the outside of the desmotubule and the inner face of thecylindrical channel formed by plasma membrane is an annulus of cytosolthrough which small molecules can pass from cell to cell. As each new cell wallis assembled during the cytokinesis phase of cell division, plasmodesmata arecreated within it. They form around elements of smooth ER that becometrapped acrossthe developing cell plate (discussedin Chapter 17).They can alsobe inserted de nouo through preexisting cell walls, where they are commonlyfound in dense clusters called pit ftelds. \Alhen no longer required, plasmodesmata can be readilv removed.a u e r q u a u Je u s e l orxuszarDsuoxauuoJmau :saJnJJn4srllaqutp alo suolpunf doD 'suolpun[ asaq]to lgntqoala-rad aqi ataruSalual sIpJ aqJ 'txauatilto ap$w at4ioi IpJ auoto apcm aqt uto{ fpcanpssadot suotf)p 000t noqq uqqj JaIIawssalnralow mo71a[\\onsn ]Dql suoxauuoJpalal-sn1ctosanba1dtorutol ar4layat asar.lJ'suortcunfdaBpallar suoltJuntBurtutol-1auuar4c[q pa1dnot an 'p1aqildauou puD pqaquda q]oq'sanssu louarua [uaw lo sIIaJaLIJ[reuun5'll q8norql ssedol sB os leuueqf, aq] Jo saruadord uodsueJ] aql aleln8ar salnJaloruoJJeruIUJIAro snouaSopua ^aoq,rfuatsfure ill]s sl ll pue 'u^ oulun eJe lJeJJeslq] a]elpauleq] Bleusepourseld;o sluauoduroJ leuollcunJ aq1 ,tlpctlelueJp Iauueqo eqlur Elerusepouseld aql;o sluauodJo azrs arod anqca;;e aql aseaJJurleq] s.,(e,nn-ruoc ot purq ]eqt suratord acnpord sesnJIAasaql Lert sql ul IIaJ ol IIaJ ruoqssed uec 'salcrlred snJrl ]Jelur uala Jo 'ygg pryr snollJaJul :elnoJ slq] lloldxaosle sesnrrl 1ue1dauog 'q3norqr ssed osp uec uralord aq] sepoJua ]pqt VNutueq] 'saspo aruos uI 'a8essed Jlaql lue^eJd asu,traqlo plnolv\ ]Bq] rusluPqceuuorsnloxa ezrs aq] eprrralo pue eletusaporuseydeq] Jo sluauodruoc ol pqqdaql 'elerusaporuseld q8norqt ilac ot IIaJ ruo4 ssed uorleuluJetap alEJIIat Jossacord slq] uI pe^lo^ur suralord rfuolep8ar aua8 auog '(77 rc1deq3 uI pessnJ-srp) seleJ ernlnJ rraql Surugap yo ssacord aq] uI Jeqloue euo o1 pu8ts sruals-rJaru]ooJ pue looqs aql ulqtl,|/I sllal Jo sdnorS 'luaurdolanap tueld Sulrnq'poolsJepunlou aJesesuJasaq] uI uollEslunlutuoJ lclJlseJ lEql sl'uslu-eqJeu eqt letpusapourseld.'tpuaredde ,(q palcauuoJ are leq] 'slleoIEuJou'slleJuala Josalnceloruureualueemlaq]q31arr,r-relnceloru-Mol;o sdnor8 ro'eldurexe roJ 'sluarulradxa uorl]uaruenoru eql ol sJerJJBqaq ueo ereq] ]eq] lrtoqs-cafur-ar(q 'paleln8ar sr elurusaporuseld q8norqt ]rodsue;l 'suotlcunf de8 qlpnsV 'suorlJunt de8 rol gornc lq8ram-repcalou aql ol JelIuIS sI qolq ^ '008 ]noqeueq] ssal ;o lq8raa,rrelnJaloru e qllm salnJeloru ;o aBessedeql ,vrolleelerusap-oruseldsazts ]ueJeJJIpJo selnoaloru racer] Sutlcafut .,(qpaurelqoslsaSSnsleqlaf,uapng 's,{e,,r,rrepurs .{lqeryeural uI uoI}JunJ 01 ulaes daq} 'suollcunf de8pue eleusaporuseld uaa,t{leq arnlcnJts uI eJueraJJlpIPoIpBJaq};o alrds u1'Ot pueGl 'dd'6('Z86:6raq;apragIL'6elra-ra6uud5'lleie^O'ULuollC pueJ) 'uorl)aseuseldolor6ur'6uruun9'Sl'B pupa;loM'f'uaasaq osleue)alnqnloulsaple;rurrsssoJ)uruaaseursopor.lrseldV (O)'lxau aql ols]l pue Lunln)llal)lLuseloopufllallerlua)aql qllM uolleDosseauo uror]snonurluofsrpue arod aql seu!lauerqulauleulseloaql'ulal raleMuot1ral3(1)e 1oqde.r6ortttue urorJeulsapor.use;de;o uorl)asleurpnlrbuol'Lunln)rlaraqt'aln])nllsqlooLUsLrjor;panuap'alnqnlousaprrurse;dopuaJelnqn]auUe sureluo)osle^llensnll 'slla)pal)auuofoMl ol uol'.tlL.uolq)el (8) 'raq1a6o1qrlm paull st eulsapoLrrsPldsrleql auerquraureu.rseldrueldeql a>latdeleu.rsapouse;d1ue;de ur sllal llel)auuo) pue lleM1;a>jo slauueqlrrr.use;doy{raqf (V)'eteusapouseld95-61 atn6t1(v)(8)sllar lua)efpe o^^]6ullrauuo:'eutsapo|.Useld6 u r u r ;a u e t q u r e uer u r s e ; deleursapourselos l l e )l u e l oluerelpe 1os l l e Ml l a )losoll)u n l n )r l a l>ruseldopua€9rrdV9:'II]f OIII]fVIVWS]COhISVIdCNVSNOIIfNNTIAIOUJSAVM]9VSSVd'taaqsa u e r q u J a u r e u J s e l ol l a ) a l ) s n ue u r u r e ll e s e q a n s s ral A r l ) a u u o )llel lerlaqlroeletlaqltoa e u r u r Plle s e qaql ur sde6relnllafralul| .,.\' ."'saql uedsvgl'sLu6uLldolpllls*!- "palle)'sarnl)nrls paseq-utaloldraqlo uo spuaoaposleuorlPrlluaql'poolq aqt uorj auunaql olu! 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