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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.