PhysiologyNevous Review1
Transcript of PhysiologyNevous Review1
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PhysiologyReview:Neurophysiology(BasedonGuytonsandpersonalextras)
WellaslastyearImtryingtomakeafinaldocumentusefulformeandtheentireclasstorehearse
allthosemainconceptswemayfindatthefinalexam,butIwanttobeclear:thisyearIdonothave
topictofocuson,Ijusthavebookschapters.HenceIlltrytounderlinefundamentalsandexplainsomeofthosetrickychaptersyoumayfind,Icantmakeasortofpasse-partoutfile,butreading
thesepageswouldprobablyhelpyoupassingtheexam.
GeneraldesignoftheNervousSystem:CNSvs.PNS
TostartproperlythisworkIthinkisbettertoclarifysomekey
arguments,justpointingsomedefinitions:
CNS:CentralNervousSystem,madeofthenevraxis,e.g.:brainand
spinalcord
PNS:PeripheralNervousSystem,madeofspinal
andcranialnerves
AndhereImentionalsohow
thisnetworkinteract:
1.Sensory(Afferent)Division-
TOtheCNS
Somaticafferents-fromskin,muscle,joints
Visceralafferents-frommembranes&organs
2.Motor(Efferent)Division-
FROMtheCNS
SomaticNervousSystem(Voluntary)-toskeletalmuscles AutonomicNervousSystem(Involuntary)-toorgans&glandsi. SympatheticDivisionii. ParasympatheticDivision
Justtobeclearerforthosenotinconfidenceswiththeseterms:
Somatic:relatedtothebody,mainlytoportionsofthebodyrelated
totheenvironment,arms,legs,skinandsoon
Visceral:relatedtobodysorgansandmembranes,orportionsofthe
bodywedonothaveaproperconsciousness.
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Nowthatourbasementisbuiltup,itwouldbebettertorevisitthoseconceptsof
firstyearsgeneralphysiologystudiesaboutneurons,oratleastthisiswhatIsuggestyoutogetabetterreadingofwhatfollows.
Sotofinishthischapter:aspecialfeatureofmostsynapsesisthe
forwarddirection(Axon->dendrites),andactivitiesareinitiatedby
experiencesthatexcitesensoryreceptors.Informationarrivingtothe
CNSpassthroughallnevraxis;spinalcord,medulla,pons,cerebellum
untilthalamusandcortex.LaterifrequiredeffectorsfromtheCNS
taketheoppositeroutetriggeringbodilyactivities.
Theresponsetoenvironmentalexcitementshastobeappropriateto
thesituationinwhichwearemovingortalkingandsoon,sothe
humannervoussystemhasacquiredanintegrativefunction.For
exampleusuallywedonotcareaboutfeelingsgivenbyashirtonour
skinorhowarearrangedthingsinourdomesticpanorama,butif
somethingoccursinthedailyactivities(anewstepinthestairs,a
newpositionofobjects)itwillbefocusedandelaborated.
However,itisimportanttopointoutherethatsomesynapsesare
easierthanothers,andalsosomeothermechanismsparticipateto
facilitateorinhibitsignalsformodelingthepreciseresponse.
Alsothestorageofinformation(memory)isperformedbysynapses,
andthemoreasignalfollowthesameroutethemorethisroutewill
befacilitated,inordertokeepinmindthatfeeling/action.Some
timesthesepatternsaresomuchfacilitatedthatwecangetthe
perceptionoftherealeventwhenitisonlyitsmemories.
Oncememoriesarestoredinthebraintheywillbecomepartofthe
morecomplicatedprocessofthinking,comparinganexperienceto
oneother.
MajorLevelsofNSfunctions:
Neuronalcircuitsinthecordcancause:
Walkingmovements Reflexesforpainreacting Reflexesofcontractioninresponsetogravityorweightlifting Reflexesactingonvariousvisceralactivities
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Lowerbrainiscapabletocontrolsubconsciousactivitieslikearterial
pressure,equilibrium,salivation,andmanyemotionalandsex
connectedstimuli.
Higherbrainisfirstofallastorageplaceforinformation,butalso
herefunctionsareconvertedtopreciseandcontrolledstimuli.Finallythecorticalportionisessentialforthinkingprocesses.Thisis
whyinourbookIfoundtotaluselesssub-chapterthatcomparesour
nervoussystemtoacomputerWhatawaistofInk!
CentralNervousSystemSynapses:
Typesofsynapses:ChemicalandElectrical,nothingnew
Theonlythingtoremarkofthissub-chapteristhatchemicaltypetransmitsonlyfromthepresynapticneurontothepostsynapticone,
inonewayonlycomparedtothepossibilitiesofelectrictype.
AtthispointIgottojumpsomeofthefollowingsubchapters,Imean
themechanismofpretopostsynapticneuronsiswrittenmuchmore
complicatedthanitreallyis:anactionpotentialcannotcrossthe
synapticcleftbetweenneurons.Insteadchemicalscalled
neurotransmitterscarrythenerveimpulse.Inthefollowingway:
1. Attheendofthepre-synapticneuronetherearevoltage-gated
calciumchannels.Whenanactionpotentialreachesthesynapse
thesechannelsopen,causingcalciumionstoflowintothecell.
2. Thesecalciumionscausethesynapticvesiclestofusewiththecellmembrane,releasingtheircontents(theneurotransmitter
chemicals)byexocytosis.
3. Theneurotransmittersdiffuseacrossthesynapticcleft.4. Theneurotransmitterbindstotheneuro-receptorsinthepost-
synapticmembrane,causingthechannelstoopen.Intheexampleshownthesearesodiumchannels,sosodiumionsflowin.
5. Thiscausesadepolarizationofthepost-synapticcellmembrane,whichmayinitiateanactionpotential,ifthethresholdisreached.
6. Aspecificenzymeinthesynapticcleftbreaksdowntheneurotransmitter;forexampletheenzymeacetylcholinesterase
breaksdowntheneurotransmitteracetylcholine.The
breakdownproductsareabsorbedbythepre-synapticneurone
byendocytosisandusedtore-synthesizemoreneurotransmitter,
usingenergyfromthemitochondria.Thisstopsthesynapsebeingpermanentlyon.
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Thehuman
nervous
systemusesanumber
ofdifferent
neurotrans
mitterand
neuro-
receptors,
andthey
dontall
workinthesameway.
Wecan
groupsynapsesinto5types:
1. ExcitatoryIonChannelSynapses.Thesesynapseshaveneuroreceptorsthataresodiumchannels.
Whenthechannelsopen,positiveionsflowin,causingalocal
depolarizationandmakinganactionpotentialmorelikely.This
wasthekindofsynapsedescribedabove.Typicalneurotransmittersareacetylcholine,glutamateoraspartate.
2. InhibitoryIonChannelSynapses.Thesesynapseshaveneuroreceptorsthatarechloride
channels.Whenthechannelsopen,negativeionsflowin
causingalocalhyperpolarizationandmakinganaction
potentiallesslikely.Sowiththesesynapsesanimpulseinone
neuroncaninhibitanimpulseinthenext.Typical
neurotransmittersareglycineorGABA.
3.NonChannelSynapses.Thesesynapseshaveneuroreceptorsthatarenotchannelsat
all,butinsteadaremembrane-boundenzymes.Whenactivated
bytheneurotransmitter,theycatalyzetheproductionofa
messengerchemicalinsidethecell,whichinturncanaffect
manyaspectsofthecellsmetabolism.Inparticulartheycan
alterthenumberandsensitivityoftheionchannelreceptorsin
thesamecell.Thesesynapsesareinvolvedinslowandlong-lastingresponseslikelearningandmemory.Typical
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neurotransmittersareadrenaline,noradrenaline(NB
adrenalineiscalledepinephrineinAmerica),dopamine,
serotonin,endorphin,angiotensin,andacetylcholine.
4.NeuromuscularJunctions.Thesearethesynapsesformedbetweenmotorneuronesandmusclecells.Theyalwaysusetheneurotransmitter
acetylcholine,andarealwaysexcitatory.Motorneuronsalso
formspecializedsynapseswithsecretorycells.
5. ElectricalSynapsesInthesesynapsesthemembranesofthetwocellsactually
touch,andtheyshareproteins.Thisallowstheactionpotential
topassdirectlyfromonemembranetothenext.Theyareveryfast,butarequiterare,foundonlyintheheartandtheeye.
TheSpaceSummationinneurons: itonlystatesthat,asinthe
picturebelow,simultaneousfiringofonlyfewsynapseswillnot
causeasufficientsummatedpotentialtoreachtheactionpotential.
Thisisjustbecauseonlyonepresynapticterminalcarriesonlyafew
neurotransmittersubstances.
Hencewecanfindalsoakindofsummationcalled
Temporal,itstatesonlythat
moreopeningofthepre-S-
channelwefindinashorter
periodoftimecangivea
higherstimulationsofthe
postregion,thatsit!Thenif
thistemporalsummationis
repeateditwillsworksfirstbutitwillleadtoastressto
bothpreandpostneurons,leadingtheirfatigue.Thisprocessgivesprotectiontothesystemtooverexcitationsandexaggeratedstress.AlsoHighbloodpH
raiseexcitabilitywhilelowpHgreatlydecreasesit.Hypoxiainfewsecondscancause
inexcitabilityandsomedrugscangiveusboththeeffectsdescribedbefore.
Thatsitforthe1reviewhopeitwouldbeusefulforyou,buddies
Thisargumentisthenervoussystem,butthisnamedoesnotbelong
tothebrainbuttoitsmainfeature:itmakesusnervous!
Yourbeloved,nervously,colleague,AlessandroMotta.