4.1Drawandlabeladiagramofamotorunit.
● Nervoussystemmadeupofmillionsofnervefibers,transferringelectricalsignalsfromthebrain
○ Thecentralnervoussystem(CNS)consistsofthebrainandspinalcord
○ Theperipheralnervoussystemisthearrangementofnervesextendingfromhtespinalcordto
otherpartsofthebody
○ Motorneurons(motoneurons)arenervesthatcarryinfofromtheCNStothemusclesand
signalforcontraction.
● Structureofneurons
○ Cellbody-containedinthespinalcordorinclustersjustoutsideitcalledganglia
○ Dendrites-linktheneurontootherneuronsandinformationtoflow
○ Axon-maincomponenttonervesignaltransmission,similartoanelectricalwire.Encasein
myelinforinsulation.
■ GapsinmyelincallednodesofRanvier
○ Neuromuscularjunction(NMJ)(motorendplate)-wheretheneuronmeetsthemuscle
■ Smallgapbetweenthetwocalledthesynapse
● Motorunit-asinglemotorneuronandthemuscleitinnervates
○ Typicallythelargerthemusclethemoremusclefibersareinnervatedbyeachmotorneuron.
Allowsasinglemotorneurontogeneratelargemuscularforces
○ Asmallnumberofmusclefiberspermotorneurongivesasmallforcebutgreatprecision(ex
eye).
○ Whenthemotorunitisinnervatedallthemusclefibersattachedtoitarecontracted.
● Typesofmotorunits(fast/slowtwitch)
○ TypeI-slowtwitchmotorunitsconsistofmainlyslowtwitchmusclefibersandhaveslower
nervetransmissionspeedsandsmallmuscleforces.
■ Canmaintaincontractionsforalongperiodoftime
■ Fatigueresistant
■ aerobic
○ TypeIIa-fasttwitchoxidative(usesoxygen)motorunitsconsistmainlyoftypeIIamusclefibers
andhavefastnervetransmissions
■ Strongercontractionforcesandaremoreresistanttofatigue
■ Anaerobicandaerobic
○ TypeIIb-fasttwitchmotorunitswithmostlyfasttwitchmusclefibers.
■ Fastestcontractiontimesandlargestforces
■ Highfatiguerateandcan’tmaintaincontractionsforalongperiodoftime
■ Anaerobic
● Mechanicsofmusclecontraction
○ Striations-musclefibersthatappearstripedduetotheoverlapofactinandmyosinproteins
withinthemusclefiber
○ Musclecontractionstartswithelectricalimpulsefrombrain(eithervoluntarilyorbyreflex).
SignaltravelsalongmotorneurontothemuscleviatheNMJacrossthesynapse.
○ Whensignalreacheshere,theneurotransmitter,Acetylcholine,isreleasedandchangesthe
electricalstateofthemuscle.
■ Thesignaltravelsthroughthemusclefibersstimulatingthesarcoplasmicreticulum
whereitreleasescalcium(Ca2+)
■ Myosinbindingsitesonactinarecoveredbytropomyosin.
■ Calciumbindstotroponinonthetropomyosinwhichcausesittomoveandrevealthe
mysosinbindingsitesontheactin.ATPontheMyosinheadishydrolysedtoformADP+
Phosphate
■ Crossbridgeformed-myosinheadsareshapedlikelittlegolfclubsanditistheendsof
theheadsthatattachtotheactin.MyosinheadremainsbounduntilATPmolecule
releasesit.Aslongasthereiscalciumavailablecrossbridgeformationwillcontinue
untilmaximumcontractionofthemusclefiberisreached.
■ Themotorneuroninitiatesarestingpotentialthroughrepoliraziation.Cholinesterase,
anenzymethatbreaksdownacetylcoline,isreleasedandcausethemusclecellto
replorizeandrelax.
■ Calciumionsareremovedfromthecellandreturnedtothesarcosplasmicreticulumvia
thecalciumpump
■ Crossbridgeformationisterminatedasthereisnocalciumwhichmeansthemyosin
bindingsitesontheactinfilamentarecoveredbytropomyosin.Myosinheadstoa
restingstate.
● Controlofmuscleforce
○ Whenamuscleissignalledtocontract,theforceofthecontractionisappropriatesothebody
segmentmovesappropriately
■ Quadsrequirealargefor(bigmusclegroup),orasmallforcelikefingersforwriting.