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ESSENTIAL Electromyography
John A. Jarratt
Essential Electromyography
JohnA.Jarratt
EmeritusofShe
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Informationonthistitle: www.cambridge.org/9781009381062
DOI: 10.1017/9781009381055
©JohnA.Jarratt2023
Thispublicationisincopyright.Subjecttostatutoryexceptionandtotheprovisions ofrelevantcollectivelicensingagreements,noreproductionofanypartmaytake placewithoutthewrittenpermissionofCambridgeUniversityPress&Assessment.
Firstpublished2023
PrintedintheUnitedKingdombyTJBooksLimited,PadstowCornwall
AcataloguerecordforthispublicationisavailablefromtheBritishLibrary.
LibraryofCongressCataloging-in-PublicationData
Names:Jarratt,JohnA.,author.
Title:Essentialelectromyography/JohnA.Jarratt.
Description:Cambridge,UnitedKingdom;NewYork,NY:CambridgeUniversityPress,2023.|Includes bibliographicalreferencesandindex.
Identifiers:LCCN2023029575|ISBN9781009381062(paperback)|ISBN9781009381055(ebook)
Subjects:MESH:Electromyography|NerveConductionStudies
Classification:LCCRC77.5|NLMWE560|DDC616.7/407547–dc23/eng/20230726
LCrecordavailableat https://lccn.loc.gov/2023029575
ISBN978-1-009-38106-2Paperback
CambridgeUniversityPress&Assessmenthasnoresponsibilityforthepersistence oraccuracyofURLsforexternalorthird-partyinternetwebsitesreferredtointhis publicationanddoesnotguaranteethatanycontentonsuchwebsitesis,orwill remain,accurateorappropriate.
Everyefforthasbeenmadeinpreparingthisbooktoprovideaccurateandup-to-dateinformationthat isinaccordwithacceptedstandardsandpracticeatthetimeofpublication.Althoughcasehistoriesare drawnfromactualcases,everyefforthasbeenmadetodisguisetheidentitiesoftheindividualsinvolved. Nevertheless,theauthors,editors,andpublisherscanmakenowarrantiesthattheinformation containedhereinistotallyfreefromerror,notleastbecauseclinicalstandardsareconstantlychanging throughresearchandregulation.Theauthors,editors,andpublishersthereforedisclaimallliabilityfor directorconsequentialdamagesresultingfromtheuseofmaterialcontainedinthisbook.Readersare stronglyadvisedtopaycarefulattentiontoinformationprovidedbythemanufacturerofanydrugsor equipmentthattheyplantouse.
To
Indy,Scarlett,TheoandZachary Intheforlornhopethattheymightbeimpressed.
Figures
7.1Theuseoftriggeranddelaytodisplayamotorunitpotential. page 21
7.2Twomotorunitpotentialsmasqueradingasfour. 22
10.1Amotorunitpotentialofincreaseddurationincorporating asatellitepotentialdenotedbytheasterisk. 32
10.2Examplesofmotorunitpotentialsrecordedfromanormal muscle(A),amyopathy(B)andaneuropathy(C). 33
10.3Anormalrecruitmentpattern,sometimescalledafull interferencepattern. 36
10.4Amarkedlyreducedrecruitmentpattern,sometimes calledapatternofdiscreteactivity. 37
10.5End-platenoise. 38
10.6End-platepotentials. 38
10.7Fibrillationpotentials. 39
10.8Positivesharpwaves. 40
10.9Fasciculationpotentials. 41
10.10Incompleterelaxation. 41
10.11Myotonia. 42
10.12Myokymiarecordedataslowsweepspeedshowingthe ‘marching’ patternofthedischarge. 43
10.13Myokymiarecordedatafastersweepspeedshowingthe ‘triplets’ . 43
10.14Complexrepetitivedischarges. 43
12.1Anormalulnarnervesensoryconductionstudy. 52
12.2Anormalulnarnervemotorconductionstudy. 53
12.3Submaximalstimulationcausingfalsemeasurementofmotor conductionvelocity. 54
13.1Anormalmediansensorynerveactionpotential. 58
13.2Amediansensorynerveactionpotentialofreducedamplitude. 59
14.1Anormalmediannervesensorystudy. 62
14.2Slowedsensoryconductioninamediannerve. 62
14.3Anormalmediannervemotorstudy. 62
14.4Amediannervemotorstudyshowingaprolongeddistallatency toabductorpollicisbrevis. 63
14.5Anulnarnervesensorystudyshowingdesynchronisationofthe sensorynerveactionpotentialrecordedabovetheelbow. 63
14.6Anulnarnervemotorstudyshowingslowedconductionacross theelbowanddesynchronisationofthecompoundmuscleaction potentialwhenthenerveisstimulatedproximaltotheelbow. 64
14.7Anulnarnervemotorstudyshowingmarkedslowingand conductionblockacrosstheelbow. 65
14.8Anormalmediannervesensorystudy,recordingatthepalm andthewrist. 66
14.9Amediannervesensorystudyshowingslowedconduction betweenpalmandwrist. 66
14.10Anulnarnervemotorstudyfromapatientwithentrapment ofthenerveattheelbow. 67
15.1Nascentmotorunitpotentialsindicatingmotornerveregeneration. 70
15.2Anascentmotorunitpotentialshowinginstabilityofitscomponents. 70
16.1Repetitivenervestimulationinanormalsubject. 73
16.2Repetitivenervestimulationinapatientwithmyastheniagravis showinginitialdecrementintheamplitudeofthecompound muscleactionpotential. 74
16.3Repetitivenervestimulationinapatientwithmyasthenicsyndrome showinginitialdecrementintheamplitudeofthecompound muscleactionpotential.(Study1). 75
16.4Repetitivenervestimulationinapatientwithmyasthenic syndromeshowingover100%incrementintheamplitudeofthe compoundmuscleactionpotentialafterashortperiodofvoluntary musclecontraction.(Study7).Thatthiseffectisreproducible isshowninStudy14. 76
16.5Repetitivenervestimulationshowingpseudofacilitation. 77
16.6Single-fibreelectromyographyshowinganormaljittervalue. 80
16.7Single-fibreelectromyographyinapatientwithmyasthenia gravisshowingmarkedlyincreasedjitterandalsoblocking. 81
17.1AnormalF-wavestudy. 83
17.2AnormalH-reflexstudy. 85
18.1Generaliseddegeneratingperipheralneuropathyshowing distallypredominantmotorandsensorynerveabnormalities. 87
18.2Generaliseddemyelinatingperipheralneuropathyshowingslowing andconductionblockinthemedianmotorstudy. 88
18.3Generaliseddemyelinatingperipheralneuropathyshowingslowing inall fibresespeciallythesmallerdiameteronesinthemedianmotorstudy. 89
18.4AnormalF-wavestudytoextensordigitorumbrevis. 90
18.5AprolongedF-wavelatencytoextensordigitorumbrevisinageneralised demyelinatingperipheralneuropathy. 90
18.6Peronealmotornervestudyshowingslowedconductionandsome conductionblockacrosstheheadofthe fibula. 91
18.7Rightperonealnervelesion.ThetypicalEMGandNCS findings. 93
18.8Normallatencyfrommedialmalleolustoabductorhallucis. 94
18.9Lefttarsaltunnelsyndromeshowingaprolongeddistallatencyfrom medialmalleolustoabductorhallucisandamarkedlydesynchronised compoundmuscleactionpotential. 94
18.10Possiblerighttarsaltunnelsyndrome.Motorunitpotentialsofincreased durationinabductorhalluciswasthesolitaryabnormal finding. 95
18.11Cervicalradiculopathy.Motorunitsofincreaseddurationinright extensordigitorumcommunisandright flexordigitorumsuperficialis associatedwithnormalsensorystudies. 96
19.1Martin–Gruberanastomosisinapatientwithcarpaltunnelsyndrome. 99 19.2Anaccessoryperonealnerve. 99
19.3Anaxonreflex.TheA-wave(denotedbyanasterisk)isconstant inlatency,amplitudeandformdistinguishingitfromthelaterF-wave. 100
Diagrams
1.1AnoverviewoftheanatomicalstructuresinvestigatedbyEMGandNCS. page 2
1.2AnoverviewoftheapplicationofEMGandNCStodiagnosis. 2
4.1Low-powerandhigh-powerrepresentationsofamyelinatednerve fibre. 11
4.2The flowofsodiumionsalongamyelinatednerveduringimpulse transmission. 12
5.1Basicstructureandfunctionoftheneuromuscularjunction. 14
7.1Adifferentialamplifier. 20
8.1Volume-conductedtriphasicpotentialscanaugmenteachother. 25
9.1Thedifferenttypesofpathologyaffectingmyelinatedperipheralnerves. 28
10.1Theeffectofelectrodedistancefromthegeneratorsiteontheamplitudeand durationoftherecordedpotential. 30
10.2aMuscleactionpotentialstravelinbothdirectionsfromtheend-plate. 31 10.2bThedurationofthemotorunitpotentialisproportionaltothespatialscatterofthe end-plates. 31
10.3Theincreaseinmotorunitpotentialdurationduetocollateralre-innervation. 31
10.4Thereductioninmotorunitpotentialdurationduetolossofmuscle fibres. 32
10.5Amethodtoavoidrepeatedsamplingofthesamemotorunit. 35
10.6Therecruitmentpatternisproportionaltothenumberofmotorunits firingand their firingfrequency. 36
11.1Principlesunderlyingtheindependentexaminationofmotorandsensory nerves. 45
12.1Measurementofsensoryconductionvelocity. 49
12.2Measurementofdistallatencyandmotorconductionvelocity. 50
12.3Theeffectofdifferentconductionvelocitiesontheamplitudeoftherecordednerve actionpotential. 51
13.1Theeffectofdegenerationindifferentdiameter fibresonnerveconduction. 55
13.2Slowedconductionvelocityduetolossof fibres. 56
13.3Pathologyproximaltothedorsalrootdoesnotaffecttheamplitudeofthesensory nerveactionpotential. 57
13.4Thelimitedcapacityofnerveconductionstudiestolocalisedegeneration. 58
14.1Theeffectofdemyelinationindifferentdiameter fibresonnerveconduction. 61
14.2Hownerveconductionstudieslocalisedemyelination. 65
15.1Adistalulnarsensorystudyshowingtheeffectsofproximallesionsofdifferent degrees. 69
16.1Theinterpeakintervalbetweentheactionpotentialsfromtwosinglemuscle fibresis mainlyduetothedifferenceindistanceofeachend-platefromtherecording electrode. 78
16.2Neuromusculartransmissionattwonormalend-platesgivesanormaljittervalue. HereNMTdenotesneuromusculartransmission. 78
16.3Delayedneuromusculartransmissionattwoend-platesgivesanincreasedjitter value.Transmissionacrossonlyoneend-platecausesblocking.HereNMTdenotes neuromusculartransmission. 79
17.1Thelatencytothesmall,inconstantF-wavesshortensasthestimulussiteismoved proximally. 83
17.2ThesmallH-reflexisobliteratedasthestimulusstrengthisincreased. 84
19.1Atwigfromaperipheralnerveendinghasdegeneratedresultinginasmallerendplatezone,leadingtoamotorunitpotentialofreducedduration. 101
19.2Conductioninregeneratingmuscle fibresisabnormallyslowleadingtoamotorunit potentialofincreasedduration. 101
1.1Asummarytableoutlinestheplanofthebook.Thebookconcludes withitscompletion. page 3
3.1Peripheralnerve fibretypes. 8
6.1Muscle fibretypes. 17
10.1Motorunitpotentialsinneuropathyandmyopathy. 34
21.1Summarytable(Table1.1)completed. 106
Preface
TheclinicwhereI firsttrainedwascalledtheDepartmentofAppliedElectrophysiology.No doubtthemenacethisimpliedofsomejunta-likeoperativeextractingadiagnosisby whatevermeansnecessarypromptedare-brandingexercise.DepartmentsofClinical Neurophysiologysprangup,whichseemedtoplacethemwithintheirnaturalneurological habitatandatthesametimedistinguishingthemfromthescientifichothousesofacademic neurophysiology.Therenowseemstobeabackwardtrendtolabelthespecialty Electrodiagnosisor,consonantwiththezeitgeistofsocialmedia,EDX.
Thisleadsustothedifficultyincreatingatitleforthisbook.Thenameofthespecialty wouldbeanobviouschoicebutthiscultofincreasingconcisionisoffsetbyitsdiminished allure.Ihopethat EssentialElectromyography capturestheaimofprovidingabriefaccount oftheprinciplesunderlyingthetechniquesinvolvedinelectromyographyandnerve conductionstudiesratherthandetaileddescriptionsofthetechniquesthemselves. Changingfashionsinnomenclatureandeventechniqueshouldnotinvalidatethese principles.
Anadditionalaimofthebookistointroducetoavarietyofreaderswhataprofessorof medicineoncepejorativelyifnotcondescendinglydescribedtotheauthorasthearcane worldofclinicalneurophysiology.Thisunderlinedwhatmostpractitionersofthespecialty alreadyknow;namely,thatmanyoftheircolleagues findthejargonasimpenetrableasthe basicprinciplesunderlyingitsexercise.Withthisinmind,anattempthasbeenmadeto describeordefinetechnicaltermswhentheyare firstencountered.Aglossaryisalso provided.
The findingsincommonlyoccurringconditionsandhowtheyarerelatedtothe underlyingpathologyareexplained.Thetechniquesinvolvedarementionedonlywhere necessaryandthenbriefly.InthiswayIhopethebookwillappealnotonlytojuniortrainees inthesubjectbutalsotoawiderangeofclinicianssuchasneurologists,orthopaedic surgeons,generalphysicians,rheumatologistsandendocrinologistswhoreferpatientsfor investigation.Thisshortsummaryshouldaidtheirselectionofpatientsforreferralandtheir appreciationoftheimplicationsoftheresults.Experiencesuggeststhatlawyersinvolvedin medico-legalcasesmightalsobeinterested.
Acknowledgements
ItisapleasuretothankmyesteemedformercolleaguesProfTonyBarker,DrArup ChattopadhyayandDrRosKandlerforalltheirsupportandwisecounsel.Iamgrateful totheskilledartistsintheDepartmentofMedicalIllustrationattheSheffieldTeaching HospitalsfordrawingmanyofthediagramsandforthekindpermissionfromtheSheffield TeachingHospitalsNHSFoundationTrusttoreproducetheseimages.Ithanktheteamat CambridgeUniversityPress,AnnaWhiting,CamilleLee-Own,BethSexton,ReshmaXavier andUrsulaActon,wholedthisnoviceauthorthroughthedauntingprocessofpublication withunderstandingandprofessionalism.
Abbreviations
m/sMetrespersecond. msMillisecondsAlsosometimescalledmsec.Thousandthsofasecond. mVMillivoltsThousandthsofavolt.
μVMicrovoltsMillionthsofavolt.
AChAcetylcholineAchemicalinvolvedintransmittingimpulsesbetween nerves,andbetweenanerveandthemuscleitsupplies.
CMAPCompoundmuscleactionpotentialThepotentialrecordedfromamuscleafterstimulatingits nervesupply;representingthesumofalltheindividual muscleactionpotentialsgenerated.
CNEConcentricneedleelectrodeArecordingelectrodeproducedbypassinganinsulated wiredownthecannulaofahollowneedle.
CVConductionvelocity.
EPZEnd-platezoneThepointatwhichamotornerveconnectstoitsmuscle. SeealsoNMJ,neuromuscularjunction.
F-waveAlateandsmallcompoundmusclepotentialgenerated byantidromicstimulationofamotornerveandsubsequent firingoftheanteriorhorncell.
H-reflexAlateandsmallcompoundmuscleactionpotentialgeneratedbyorthodromicstimulationofmusclespindle afferentswhichconnectmonosynapticallytotheanterior horncell.Similartoatendonreflex.
MAPMuscleactionpotentialThepropagatedpotentialgeneratedbyanactivesingle muscle fibre.
MCVMotorconductionvelocity.
MNAPMixednerveactionpotentialThepotentialrecordedfromamixednerverepresenting thesumoftheactionpotentialsgeneratedbyindividual activesensoryandmotor fibres.
MUAPMotorunitactionpotentialThepotentialgeneratedbyanactivemotorunit,representingthesumofalltheindividualmuscleactionpotentialswithinthatunit.
MUPSameasMUAP.
M-waveSameasCMAP.
NMJNeuromuscularjunctionThepointatwhichamotornerveconnectstoitsmuscle. SeealsoEPZ,end-platezone.
NMTNeuromusculartransmissiontimeThetimetakenforanerveimpulsearrivingattheendplatezonetogenerateamuscleactionpotential.
SAPSensoryactionpotentialSameasSNAP.
SCVSensoryconductionvelocity.
SNAPSensorynerveactionpotentialThepotentialrecordedfromasensorynerverepresenting thesumoftheactionpotentialsgeneratedbyindividual active fibres.
1 Introduction
Therearefewdifficultconceptsinclinicalmedicine.Rocketscienceitdefinitelyisnot.But thebewilderingprofusionofnomenclatureisundoubtedlyabarriertotheunderstandingof manydisciplines,clinicalneurophysiologyincluded.
Clinicalneurophysiologyistheapplicationofelectronictechniquestothenervous systemanditsconnectionsforthepurpose sofdiagnosis,monitoringand,occasionally,treatment.Thisbookdealsonlywithelectromyography(EMG)andnerve conductionstudies(NCS)asused indiagnosisandmonitoring.
Aswithalldiagnosticmethods,betheypurelyclinical,orinvestigativeoracombination, anumberofgeneralquestionsneedtobeaddressed.Thesearelistedheretogetherwiththe issuesspecifictoclinicalneurophysiology:
• Whatisthelocationofthedisorder?(Isitinmuscle,nerveortheneuromuscular junction,andifinthenerve,istheconditionlocalorwidespread?)
• Whatisthepathology?(Ifmuscleisa ff ected,canitbede fi ned?Ifnerveis implicated,isitdegenerating,inwhichthenerve fi breitselfisinvolved,orisit demyelinating,inwhichtheinsulatingsheatharoundthenerveisdamaged?)
• Whatistheseverityandthustheprognosis?(Whatisthedegreeofchange?Andwhatis thelikelyclinicaldiagnosisandthusprognosis?)
• Havingidentifiedanabnormality,canitbemonitored?
Westartbyde fi ningthescopeofthebook.The fi rstpartdealswithbasicelements ofanatomy,physiologyandtechnicalmattersinaneff orttoprovidesomesimplebut su ffi cientbackgroundmaterial.Thesecondpartthendescribestheprinciplesofthe examinationmethodsandhowtheyareusedinclinicalpractice.
Peripheralnervescarrynerveimpulses fromtheskinviathedorsalrootganglion tothespinalcordandthencetootherpartsofthecentralnervoussystem.Theseare sensorynerves.Nerveimpulsestoamusclearesentfromananteriorhorncellwithin thegreymatterofthespinalcordtotheneuromuscularjunctionfromwheretheyare transmittedtothemuscle fi bres.Thesearemotornerves.Bothtypesareshownin Diagram1.1.
Electromyographyinvestigatesdisordersofneuromusculartransmissionandalso abnormalitieswithinmusclearisingfromprimarymusclediseaseorasaconsequenceof pathologywithinitsnervesupply.
Dorsal root ganglion
PERIPHERAL NERVE (Sensory and motor)
Anterior horn cell
Skin
NEUROMUSCULAR JUNCTIONMUSCLE
Diagram1.1 AnoverviewoftheanatomicalstructuresinvestigatedbyEMGandNCS.(Imageincludedwith permissionfromtheSheffieldTeachingHospitalsNHSFoundationTrust.)
NERVE
CONDUCTION STUDIES
ELECTROMYOGRAPHY
PERIPHERAL NERVE (Sensory and motor)
NEUROMUSCULAR JUNCTION
MUSCLE
Diagram1.2 AnoverviewoftheapplicationofEMGandNCStodiagnosis.(Imageincludedwithpermissionfrom theSheffieldTeachingHospitalsNHSFoundationTrust.)
Nerveconductionstudiesarealsousedtoinvestigateneuromusculartransmission. Theirmainfunctionistodetectchangesinperipheralnervesarisingasaresultofcompression,orotherformsoftrauma,orsystemicdisease(Diagram1.2).
Thesummarytable, Table1.1,outlinestheplanofthetextshowinghowthesetechniquesareusedintheclinictotryandanswerthediagnosticquestionsposedearlier.
Thecompletedtable, Table21.1,isgivenattheendofthebook.
Table1.1 Asummarytableoutlinestheplanofthebook.Thebookconcludeswithitscompletion.
AnatomyPathology
MuscleMyopathy
NeuromuscularjunctionMG
LEMS
Peripheralnervoussystem
Neurophysiology
EMGNCS
D/MNeuropathy
Compressionlesions* GPN***
Peripheralnerve lesions*
Plexuslesions** Radiculopathy** GPN***
AHCdisease***
Keytoabbreviations: *Localchanges **Regionalchanges ***Widespreadchanges
D/GNeuropathy
MGMyastheniagravis LEMSLambert–Eatonmyasthenicsyndrome GPNGeneralisedperipheralneuropathy AHCdisease
D/MDemyelinating D/GDegenerating
2
BasicAnatomyandaLittle Physiology
Thenervoussystemcanbeconsideredtoconsistoftwoparts:thecentralnervoussystem andtheperipheralnervoussystem.Athirdcomponent,theautonomicnervoussystem, featureslittleifatallintheclinicalapplicationofelectrophysiologicaltesting.Thisandits complexitymightpermitusthenotionthatitshouldbeenjoyedratherthanunderstood.
Thefunctionoftheindividualelements,theneurons,ofbothpartsofthenervoussystem istotransmitinformationfromonesitetoanother.Eachneuroncomprisesacellbody,the soma,bearinganextrusion,theaxon,whichisusuallyofsuchimpressivelengththatwhen referredtoasa ‘ nerve ’ itiseasytooverlookthefactthatitismerelyaconduitbetweenthe somaanditsdestination.Sensoryneuronsintheperipheralnervoussystemhavetwosuch axonsandarethereforecalledbipolarcells.
Thetransmissionofinformationmaytakeplacebetweensensoryreceptorsand aneuron,betweenneuronsorbetweenaneuronandamuscle.Theaxonterminalsfrom oneneuronconnecttothesomaofanotherneuronatjunctionscalledsynapses.Theseare mainlylocatedondendrites,whicharealsoextrusionsofthesomabutmuchshorterthan theaxon.Amotornerveconnectstomuscle fibresattheneuromuscularjunction.
Thebrainandspinalcordcomprisethecentralnervoussystem.Withinthissystem, areascontainingthecellbodiesofnervesappeardarkerandarereferredtoasgreymatter. Atthebaseofthebrainisastalk-likestructure,thebrainstem,whichformsacontinuation ofthespinalcord.
Greymatterinthebrainislocatedoverthesurface,formingthecerebralcortex,orin clusterssuchasthethalamusandbasalgangliaburiedwithinthesubstanceofthehemispheres.Thegreymatterinthespinalcordisdeeplysituated.ItisroughlyH-shapedhaving twoventral,oranterior,hornsandtwodorsal,orposterior,horns.
Theinterconnectingnervesbetweenthecellbodiesinthecentralnervoussystemare bundledintotractsknownaswhitematter.Theyaresheathedinmyelinwhich,containing lipids,impartstheirlighterappearance.Thisinsulatesthemfromoneanotherthuspreventing unwanted ‘cross-talk’ betweenadjacentnerves.Asweshallseein Chapter4, ‘PeripheralNerve Function’,thepresenceofmyelinalsoincreasestheconductionvelocityalongthenerve.
Nervessupplyingthelimbsandtrunkformtheperipheralnervoussystem.Thenervesto theheadandneckhavecomplexandindividualanatomiesandsoratherthanbeingthought ofasasystem,theyarereferredtobytheirindividualcranialnervenames.
Themajorityofperipheralnervesareunmyelinatedbutinthosethatare,themyelinis appliedinmultiple,shortsegments.
Nervescarryingimpulsesintothecentralnervoussystemarecalledafferentorsensory nerveswhilstthosecarryingimpulsesfromthecentralnervoussystemtomusclesarecalled
efferentormotornerves.Mostbutnotallperipheralnervescontainsomeofbothtypesof nerveandarethereforecalledmixednerves.
MotorSystem
Themotornerveswhichsupplythelimbsandtrunkariseinthecerebralcortexandthenrun throughthepartofthebrainstemknownasthemedullaandthencedownintothespinal cordwheretheyformasynapticlinkwiththeanteriorhorncellsintheventralgreymatter. Mostofthese fibrescrosstotheothersideastheypassthroughtheregionofthemedulla knownasthepyramidstoformthepyramidalorlateralcorticospinaltract.Theremainder formtheanteriorcorticospinaltract.
Thespinalcord,althoughacontinuousstructure,canbethoughtofasasequentialseries ofsegments.Themotoroutflowfromagivensegmentsuppliesaseriesofmuscles,the myotome.Theanteriorhorncellpoolofmotorneuronessupplyingthemyotomereceives connectionsfromthepyramidaltractandfromtheanteriorcorticospinaltractafterithas decussated(crossedsides)atthatlevel.
Thenervesissuingfromtheanteriorhorncellsdestinedforthelimbsandtrunkexit fromthespinalcordviatheventralnerverootsandthennegotiateplexuseswheresensory andmotornervesarisingfromdifferentsegmentallevelsinthespinalcordcombine.Each paraspinalmusclereceivesitsnervesupplyfromthedorsalramuswhicharisesjustdistalto thepointwherethedorsalandventralrootsatthatsegmentallevelmerge.
Theanteriorhorncell,itsperipheralnerveandallthemuscles fibresitinnervatesis calledamotorunit.Thesizeofthemotorunitisproportionaltothenumberofmuscle fibres itcontains.
SensorySystem
Sensoryneuronswithintheperipheralnervoussystemarelocatedinthedorsalrootganglia justoutsidethespinalcord.Theydifferfrommotorneuronsinhavingnotonebuttwo extrudednerve fibres,hencethenamebipolarcells.Theperipheral,distal fibrebringsin impulsesfromthelimbsortrunk.Italsoparticipateswiththemotornervesintheformation ofplexuses.Thecentrallyprojecting fibrefromthedorsalrootganglionrunsintothespinal cordviathedorsalrootandthenfollowsoneoftwomainpathways.
Nervescarryingpain,temperatureanddeeptouchsensationscrossthemidlineand formsynapsesintheposteriorhornsofthespinalgreymatter.Fromherethelateral spinothalamictractsrelaysignalstothecerebralcortexaftermakingfurthersynaptic connectionsinthethalamus.
Nervescarryinglighttouchandproprioceptivesensationsdonotcrossthemidlineat thisstage.Theyentertractscalledthedorsalcolumnswhichsynapseinthecuneateand gracilenucleilocatedinthemedulla.Theythencrossthemidlineinthemediallemniscus tracttothethalamus.Heretheyalsoengageinfurthersynapticactivitybeforetheironward journeytothecerebralcortex.
Thereisanexceptiontothisgeneraltrendofrelaysmediatedviamultiplesynapses. Nervesfromtheintrafusalmusclespindles,whichsignalinformationaboutitslength,form amonosynapticlinkwiththeanteriorhorncellssupplyingtheforce-producingextrafusal fibresofthesamemuscle.ThiswillbediscussedfurtherwhenweconsidertheH-reflexin Chapter17, ‘OtherTechniques:F-wavesandH-reflexes ’.Theintrafusalandextrafusal muscle fibresarediscussedmorefullyin Chapter6, ‘Muscle’ .
Soma,AxonHillockandInitialSegment
Wearenowinapositiontoconsiderhowanimpulsefromthespinalcordbeginsitsjourney toamuscle.
Aswehaveseen,thesoma – inthiscase,theanteriorhorncell – hasnumeroussmall projectionscalleddendritesandalong,extrudedportion,theaxon,whichformsthe peripheralmotornerve fibre.Theactivityintheconnectionsbetweentheaxonterminals fromothernervesandtheseanteriorhorncelldendritesdeterminestheactivityofthesoma andhenceitsnerve.
Neurotransmitterscrossthejunctionsbetweentheseconnection,thesynapses,and induceeitheranexcitatoryorinhibitorypotentialinthesoma.Theseareknownas excitatorypost-synapticpotentials(EPSPs)orinhibitorypost-synapticpotentials(IPSPs), respectively.AsingleEPSPisinsufficienttogenerateaso-calledactionpotentialintheaxon, thatistosay,apotentialthatwillbepropagateddownthenerve.BothEPSPsandIPSPsmay beaugmentedbyspatialand/ortemporalsummation.Inspatialsummation,theeffectsof activityinmultipledendritesaresummed.Intemporalsummation,theeffectsofrepeated activityatasingledendritearesummed.ThealgebraicsummationsoftheEPSPsandIPSPs thendetermineifthesomahasbeensufficientlydepolarisedtogenerateanactionpotential. Ifso,thesomaissaidto fire.Howisthisachieved?Thecurrentsfromthesepotentialsare routedtoabulgeinthesomacalledtheaxonhillockfromwhichtheaxonitselfarises.The axonhillockandtheso-calledinitialsegmentoftheaxonleadingfromitarebothespecially sensitivetodepolarisationastheycontainveryhighconcentrationsofsodiumchannels whichfacilitatetheentryofsodiumions.
Inthisway,thesomaweighstheevidenceofincomingsignalsindeterminingwhetheror notto fire.Whenitdoesdecidetodoso,thephysiologyoftheperipheralnervemeansthat thereisnogoingbackineithersenseoftheterm.Thisrelatestosomethingcalledthe absoluterefractoryperiodwhich,togetherwithfurtherdetailsofthedepolarisationprocess, willbedescribedin Chapter4, ‘PeripheralNerveFunction’
PeripheralNerveTypes
PeripheralNerveClassification
Peripheralnerveswereoriginallyclassi fi edasA,BorC,indescendingorderof diameter.NervetypesAandBaremyelinated;Cisnot.Amorerecentclassifi cation de fi nesfoursubclassesoftheA fi bres,namely α , β, γ and δ ,againindescendingorder ofdiameter.TheA α fi bresarethee ff erentstotheextrafusalmuscle fi bres;thatis, fi bresnotwithinthemusclespindle.TheA γ fi bresarethee ff erentstothemuscle spindles.Aff erent fi breswithinperipheralnervesnowhaveaRomannumeralclassi fication.TheIa fi bressupplytheannulospiralreceptorsofthemusclespindle;theIb supplytheGolgitendonorgan.Smallerdiameter fi bres,typeII,supplythe fl owersprayendingsinthemusclespindlesandalsothecutaneousmechanoreceptors.The smallestmyelinated fi breswithinthegroup,typeIII,supplyfastpainandcold receptorsintheskinandalsothefreenerveendingssubservingtouchandpressure. TypeIV fi bres,thetypeCoftheearlierclassi fi cation,areunmyelinated fi bresrelaying sensationsofpainandheat.Thesesubtypesbasedondiameter/conductionvelocity andfunctionarehelpfuleventhoughthereisconsiderableoverlapbetweenthe categories. Table3.1 summarisesthedi ff erencesbetweenthem.
Nerveswiththelargestdiameters,upto20microns(i.e.micrometresor μm),arefound intheIa,IbandAα categories.Thesmallestdiameter fibres,ofabout1 μm,belongtothe unmyelinatedtypeIVnerves.
Whilstitisimportanttohaveanappreciationofthisclassi fi cation,themessage thatneedstobekeptinmindwhenperformingnerveconductionstudiesisthat peripheralnervescontain fi bresofdi ff erentdiametersandtheseconductatdiff erent speeds.
Thedetailedanalysisofthebehaviourofthesedifferentcomponentsofperipheralnerves fallswithintheremitofacademicneurophysiologybutabriefsummaryofthemore pertinentaspectsfollows.
SensoryNerves
Sensorynervesfromthemusclespindlesandtendonsaredesignedtomonitormuscle lengthandtension,respectively,andoperateatthesubconsciouslevel.Sensory fibres supplyingthemusclespindles,theIaafferents,arethelargestandfastest-conducting fibres intheperipheralnervoussystem.Theyprovideinformationaboutmusclelengthandthe rateofanychange.Theyarerelayedinthecentralnervoussystemtothecerebellumwhich co-ordinatesmovement.Inthespinalcord,theyalsoformaconnectionwiththealpha motorneuronssupplyingthesamemuscle.Ifthemuscleisstretched,theyexcitethisalpha
Table3.1 Peripheralnerve fibretypes.
Fibre type
Aα Myelinated Muscle extrafusal fibres
Aγ Myelinated Musclespindle
Unmyelinated SeeIV
IaMyelinated Musclespindle annulospiralendings
IbMyelinated Golgitendonorgans
IIMyelinated Musclespindle flowersprayendingsand specialisedreceptorsfor touch,pressureand vibration
IIIMyelinated Mainlyfreenerve endingsforfastpainand cold,andtouch
IV Unmyelinated Mainlyfreenerve endingsforslowpain, heatandcold
motorneurontoelicitacontractiontherebyrestoringmusclelength.Becausethisreflexarc isbasedononesensoryneuronandonemotorneuronitiscalledamonosynapticreflex. Andsincethiscompensatorycontractionwouldstretchantagonistmuscles,theIaafferents alsoformaninhibitoryconnection,viaaninterneuron,withthealphamotorneurons supplyingthem.Thephenomenonisfamiliarasthetendonreflexoftheknee-jerk.The tendonsalsocontainreceptorswhichsignalmuscletensionviatheIbafferents.
Otherspecialisedcutaneoussensoryreceptorsrespondtospecificstimulisuchas pressure,vibrationorlighttouch.Attemptstorefinesensorynerveconductionstudiesby usingmodality-specificstimulihavenotsofarbeenclinicallyuseful.Fortunately,thereare abundantperipheralsensorynerveswhichcanbeeasilystimulatedtoprovidevaluable diagnosticinformation.
MotorNerves
Thealphamotorneuronswhichariseintheventralgreymatter(alsoknownastheanterior horn)ofthespinalcordareresponsibleformusclecontraction.Theyarealsofastconductingnerves,onlyslightlylesssothantheIaafferents.
Moreslowlyconductinggammamotorneurons,whichalsoariseintheventralgrey matter,supplythemusclespindles.Theymaintaintensiononthespindletomatchthe desiredlengthoftheextrafusalmuscle fibres.Thisiscalledalpha–gammaco-activation. UnintendeddeparturefromthisstateissignalledbytheIaafferentstothealphamotor neuronswhose firingrates,whichdeterminemuscletension,arecorrespondinglyadjusted.
PeripheralNerveFunction
Wenowneedtoconsiderinmoredetailthestructureofaperipheralnerveandhowthis relatestoitsfunctioning.Theperipheralnervehasasemipermeablemembrane.Outsidethe nerve,thereisapredominanceofsodiumions.Withinthenerve,potassiumionspredominate.Anactiveenergy-dependentprocess,thesodium–potassiumpump,pushesoutthree potassiumionsforeverytwosodiumionsthatenter.Thisleadstoarestingmembrane potentialinwhichtheinteriorofthenerveisapproximately –70millivolts(mV)relativeto theexterior.Giventhatthereisanexcessconcentrationofsodiumionsoutsidethenerve withinapositivelychargedenvironment,onehastoaskwhytheconcentrationand/or electricalgradientsfailtopropelthemintothecell.Onereason,butnotthemostimportant, isthatsodiumionsarehydrated,makingthemlargerandsolessdiffusible.Theotherand criticalfactoristhatentryofsodiumionstakesplaceatspecialisedsitesincorporating voltage-gatedchannels.Theseareionchannelswhichonlyopeninresponsetospecific changesinmembranepotential.Inthecaseofsodiumions,thisiswhenthemembraneis depolarised,thatistosay,whentheinteriorbecomesmorepositiveandtheexterior becomesmorenegative.
Ifaperipheralnerveisstimulatedas,forexample,inanerveconductionstudy,andifthe stimulusstrengthisverylow,themembranewillbedepolarisedbutnotsufficientlyto produceapotentialthatwillbepropagatedalongthenerve.Bydefinition,thisis asubthresholdstimulus.Butifthestimulusstrengthissufficientlygreattoexceedthe threshold,about15to20mV,thepotentialwillbetransmittedalongthenerve.Thisiscalled anactionpotentialandthenerveissaidto fire.Oncethethresholdhasbeenbreached,many morelocalsodiumchannelsareopenedandtheionspourintothenerve,reducingthe membranepotentialevenfurther.
Aswehaveseenin Chapter2, ‘BasicAnatomyandaLittlePhysiology;Soma,Axon HillockandInitialSegment’,thedepolarisationoftheanteriorhorncellandinitialsegment oftheaxonasaresultoftheopeningofthesesodiumchannelsiswellabovethresholdto produceanactionpotential.
Beforeweaddresstheissueofhowtheactionpotentialisthenpropagatedalongthe nerve,weneedtoreflectamomentonperipheralnervestructure.Some,butnotmost,ofthe peripheralnervesaremyelinatedandsincethesearetheoneswestudyintheclinic,theyare theonesweconsider first.
Diagram4.1 showsamyelinatednerve fibreatlow-powermagnificationand,belowit, cross-sectionalandlongitudinalsectionaldiagramsathighermagnification.Inreality, myelinappearsasconcentricringsinacross-sectionalview,whichtheauthorhopeswill justifytheartisticlicenceindepictingitassuch.
MyelinInternode
Schwanncellscontainingmyelinandlyingadjacenttotheaxonwraparounditratherin themannerofaSwissroll.Betweeneachcell,theaxonisexposedatthenodeofRanvier.If anadditionalfoodstuff metaphorisallowed,amyelinatednerveresemblesastringof sausages.Thedensityofthevoltage-gatedsodiumchannelsisespeciallyhighatthenodes.
Themyelinsegmentsbetweeneachnodegreatlyfacilitatethe flowofsodiumionsalong theinteriorofthenerveratherthanacrossitsmembrane.Twofactorsaccountforthis.First, themyelinincreasestheresistanceofthenervemembrane,thusreducingtheoutward flowof sodiumions.Second,thestructureandfattycompositionofthemyelinsheathreducethe electricalcapacitanceofthemembrane.Ifwehavetwoconductingmedia,inthiscasethe internalaxoplasmwithinthenerveandtheexternalextracellular fluid,whichareseparatedby aninsulatingmaterial,themyelin,thenfromq=CVwhereq=charge(i.e.current×time), thevoltage(V)acrossthenervemembraneisdeterminedbythechargeonitdividedbyits capacitance(C):
Ifthecapacitanceisreduced,thenthetimetoreachthedepolarisingvoltagefromagiven currentwillbereduced.Thismeansthattheconductionvelocitywillbeincreased.Wecan nowreviewwhathappensatthedepolarisednode.
Themassiveandself-perpetuatingincreaseinsodiumconductanceisshort-lived.Asitis switchedoff,aslowerbutmoresustainedoutwardpotassiumconductancetakesover.This notonlyrestoresthemembranepotentialbut,becauseofitsrelativelylongduration,thereis abriefovershootperiodduringwhichthemembranebecomesslightlyhyperpolarised.
Theinwardrushofsodiumionslastsonlyoneortwomilliseconds.Itishaltedbythe sodiumchannelsbecominginactivated.Duringthisperiodofinactivationnostimulus, howeverstrong,willdepolarisethenerve.Thisiscalledtheabsoluterefractoryperiodand normallylaststhreetofourmilliseconds.Afterthat,twoopposingfactorscomeintoforce. Ontheonehand,sodiumchannelsprogressivelyregaintheiractivity,favouringareturnto
Diagram4.1 Low-powerandhigh-powerrepresentationsofamyelinatednerve fibre.(Illustrationbyauthor.)
