Nerves, Muscles & Movement Notes 09a - PBworks

HLBiology NotesforNerves,Muscles&Movement

A.DeJong/TFSS2007 1of21

ThenotesinthisdocumentcoverIBtopics11.2andoptionE.1,E.2,E.4andE.5.

OrganizationoftheNervousSystem

The nervous system is divided into the peripheral nervous system (PNS) and central nervous system(CNS).

ThePNSconsistsof

sensoryneuronsrunningfromstimulusreceptorsthatinformtheCNSofthestimuli motorneurons running from theCNS to the effectors (muscles and glands) that respond to the

stimuli

TheCNSconsistsof

thespinalcord thebrain

ThePeripheralNervousSystem

ThePNSissubdividedinto

thesensory‐somaticnervoussystemwhichconnectstheexternalenvironmentandtheCNS

thereare12pairsofcranialnerves,whichconnectdirectlytothebrain(e.g.theopticnerve),andtheymaybesensory,motor,ormixednerves

thereare31pairsofspinalnerves,allofwhicharemixed all our conscious awareness of the external environment and all ourmotor activity to cope

withitoperatethroughthesensory‐somaticdivisionofthePNS actions of the sensory‐somatic nervous system are largely voluntary – skeletal muscle is

controlledbythissystem

theautonomicnervoussystem(ANS)whichconnectstheinternalenvironmentandtheCNS

consistsofsensoryneuronsandmotorneuronsthatrunbetweentheCNSandvariousinternalorgans

it is responsible for monitoring conditions in the internal environment and bringing aboutappropriatechangesinthem

actions of the autonomic nervous system are largely involuntary ‐cardiac muscle (heart),bloodvessels,digestivesystem,smoothmuscle,andglandsarecontrolledbythissystem

usestwogroupsofmotorneuronstostimulatetheeffectors

CNS External Environment

Internal Environment

Sensory- Somatic NS

Autonomic NS

Sensory Neurons Sensory Neurons

Motor Neurons Motor Neurons



■ preganglionicneuronsariseintheCNSandruntoaganglioninthebody■ postganglionicneuronsruntotheeffectororgan(synapseoccursintheganglion

further subdivided into the sympathetic and parasympathetic nervous systems, which arelargelyantagonistictoeachother:

Imagefromhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/autonomic.gif

TheSympatheticNervousSystem

Theneurotransmitterofthepreganglionicneuronsisacetylcholine(Ach).Itstimulatesactionpotentialsinthepostganglionicneurons.Theneurotransmitterofthepostganglionicneuronsisnoradrenaline.Theactionofnoradrenalineonaparticularglandormusclemaybeexcitatoryorinhibitory.

StimulationofthesympatheticbranchoftheANSpreparesthebodyforemergencies:“fightorflight”.

TheParasympatheticNervousSystem

Themain nerves of the parasympathetic nervous system are the vagus nerves, which originate in themedullaoblongata. Acetylcholine is theneuro‐transmitteratallpre‐andmanypostganglionicneurons.Somepostganglionicneuronsreleasenitricoxideastheirneurotransmitter.

The parasympathetic nervous system returns the body to normal after they have been altered bysympatheticstimulation:“restanddigest”.



AlthoughtheANSisconsideredtobeinvoluntary,thisisnotentirelytrue.AcertainamountofconsciouscontrolcanbeexertedoveritashaslongbeendemonstratedbypractitionersofyogaandZenBuddhism.During their periods of meditation, these people are able to alter a number of autonomic functionsincludingheart rate and the rateof oxygen consumption. These changes arenot simply a reflectionofdecreasedphysicalactivitybecausetheyare lowerthan levels foundduringsleeporhypnosis. AnotherexampleofconsciouscontroloftheANSisthecontrolofemptyingofthebladderandbowels.

TheCentralNervousSystem

ThespinalcordconductssensoryinformationfromthePNStothebrain,andconductsmotorinformationfromthebraintotheeffectors,includingskeletal,smoothandcardiacmuscle,andglands.Italsoservesasaminorreflexcentre.

The brain receives sensory input from the spinal cord and its own nerves. It devotes most of itscomputationalpowertoprocessingitsvarioussensoryinputsandinitiatingappropriateandcoordinatedmotoroutputs.

Boththespinalcordandbrainconsistofwhitematter(bundlesofaxonscoatedwithmyelinsheaths)andgreymatter(cellbodies&dendrites,coveredinsynapses).Theyarealsocoveredwithconnectivetissuecalledthemeninges.

Anextracellular fluid thatdiffers in its composition from theECF in the restof thebodysurrounds thecells of the CNS. Cerebrospinal fluid (CSF) contains less protein than ECF, and is found within thecerebrospinalcanalofthespinalcordandwithinthefourventriclesofthebrain.

TheSpinalCord

Imagefromhttp://neuro.wehealny.org/images/14_01.jpg

There are 31 pairs of spinal nerves. These are all classed asmixed nerves because they contain bothsensoryandmotoraxons.

sensoryaxonspassintothedorsalrootganglionwheretheircellbodiesarelocatedandthenonintothespinalcorditself

motoraxonspass intopass into theventralrootsbeforeunitingwith thesensoryaxons to formthemixednerves



Theprimaryfunctionsofthespinalcord:

itconnectsalargepartofthePNStothebrain

it is a minor coordinating centre responsible for some simple reflexes such as the withdrawalreflex

TheBrain

MedullaOblongatacontrolsinvoluntaryandvisceralactivities

Cerebellumcontrolsbodybalance,muscularcoordinationandequilibrium.

Hypothalamusmaintainstheinternalenvironment

regulatesbodytemperature,thirst,hunger,metabolism,pleasure,pain,etc.

Thalamussortsincomingandoutgoingimpulsesandsendstotheappropriatecentre

CerebralCortexcentreofallvoluntarymusclecontrolandmentalactivity

analysis coding info.storage recognition

memory understanding intelligence senseintegration

Imagefromhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/brain_3.gif

Thepituitarygland,locatedatthebaseofthebrain,isapproximatelythesizeofapea,andiscomposedoftwolobes.

anteriorlobe:stimulatedbythehypothalamusofthebraintosecreteseveralhormones

thyroidstimulatinghormone(TSH) follicle‐stimulatinghormone(FSH) luteinizinghormone(LH)

prolactin growthhormone adrenocorticotropichormone(ACTH)

posteriorlobe:releasestwohormones,synthesizedbythehypothalamus,intothebloodstream

antidiuretichormone(ADH) oxytocin



Howdoweknowwhatthebraindoes?

Investigatingbrain‐damagedpatients

o Forexample,theexperiencesofsoldierssurvivingbullet‐woundstotherearoftheskullledtothediscoveryoftheroleofthevisualcortexontherearofthecerebralhemispheres.

o Patientswhoarenotimmediatelykilledbyastrokeoftenexperienceparalysisorlossofaspecificbodyfunction–post‐mortemanalysisidentifiestheparticularpartofthebrainaffectedbythestroke.

Animalexperiments

o Wehavelearnedalotaboutbrainfunctionbystudyingmammalsandothervertebrates,removingpartsofahealthybrainorseveringconnectionsbetweenneurons.

o Inoneinvestigationusingcats,severingthefibresthatcrossoverinthecentreofthebrainbelowthetwohalvesofthecerebralhemispheresgavecluestotheinteractionofleftandrighthalvesofthebrain.

fMRI

o FunctionalmagneticresonanceimagingisanadvancedformofMRIthatdetectsthepartsofthebrainthatareactivewhenthebodyperformsspecifictasks.Thereisalwaysademandforoxygenandglucose(foodenergy)inthebrain,buttherearelocalincreasesindemandwhenaparticularareaofthebrainisinuse.fMRIdetectsincreasesinredbloodcelloxygenationatthesiteofneuralactivity.



Neurons

All neurons are specialized cells that carry anelectrochemicalimpulsecalledanactionpotential.

Sensory neurons run from the stimulus receptors (e.g. fortouch,vision,sound,odourandtaste)totheCNS.

Interneurons, foundonly in theCNS,andarestimulatedbysensoryneurons,other interneurons,orboth. Thebrain isestimated to contain 100 billion interneurons averaging1000synapseseach.

Motor neurons such as the one pictured can have an axonthat is up to onemetre in length. They transmit impulsesfromthebraintotheeffectors–musclesandglands.

Imagefromhttp://www.gonzaga.k12.nf.ca/academics/science/sci_page/biology/neuron1.gif

NerveImpulseTransmissionNeuronssendmessageselectrochemically,whichmeansthatchemicalscauseanelectricsignal.Ionshaveeitherapositive(+)ornegative(‐)charge.Importantionsfornerveimpulsetransmissionare:

sodium(Na+) potassium(K+)

calcium(Ca++) chloride(Cl‐)

Somedefinitions:

Membranepotential:theelectricalpotentialdifference(voltage)acrossacell'smembrane.

Actionpotential:awaveofelectricaldischargethattravelsalongthemembraneofacell.Actionpotentialsareusedbythenervoussystemtotransmitinformationbetweenneurons,andbetweenneuronsandeffectors.

Resting potential: the membrane potential that would be maintained if there were no actionpotentials,synapticpotentialsorotheractivechangesinthemembranepotential.Formostcells,this is anegativenumber. The restingpotential of aneuron isusually ‐70mV.

Atrest,K+caneasilycrossthroughthemembrane,whileCl‐andNa+havemoretroublecrossing.Thenegatively‐chargedproteinmolecules(A‐)cannotcrossthemembrane.Inaddition,thesodium‐potassiumionpumpisactivelypumpingthreeNa+outforeverytwoK+itputsin.

Imagefromhttp://faculty.washington.edu/chudler/gif/ioncon.gif



Anactionpotentialoccurswhenaneuronsendsanimpulsedownanaxon,awayfromthecellbody.Itisanexplosionofelectricalactivitythatiscreatedbyadepolarizingcurrent.(Thismeansthatastimulushascausedtherestingpotentialtomovetoward0mV.)Whenthedepolarizationreachesabout‐55mV,aneuronwillfireanactionpotential.Thisvalueiscalledthethreshold.Ifthisvalueisnotreached,theactionpotentialwillnotfire.

* The action potential for any given neuron is always the same. There is no “big” or “small” actionpotentialforaneuron.

Actionpotentialsarecausedbyanexchangeofionsacrossthemembraneofaneuron:

Astimuluscausessodiumchannelstoopen,allowingsodiumionstoenterthecell.

Thiscausesdepolarization,becausethesodiumionsarepositivelycharged.

The potassium channels open after depolarization begins, which causes potassium to leave thecell,reversingthedepolarization.

Around this time, sodium channels begin to close, which causes a repolarisation, as the actionpotentialgoesbacktoward‐70mV.

The action potential actually goes past ‐70 mV (a hyperpolarisation) because the potassiumchannelsstayopenabittoolong.

Gradually,theionconcentrationsgobacktorestinglevelsandthecellreturnsto‐70mV.

Imagefromhttp://faculty.washington.edu/chudler/ap3.gif

Synapses

Nerveimpulsesaretransmittedalonganindividualneuronbymeansofanactionpotential.Sincethesesignalsmustbetransmittednotonlyalongasingleneuron,butfromoneneurontoanother,orfromaneurontoaneffector,theremustbeameansofpassingthesignalfromoneneurontoanother.

Ajunctionbetweentwoneuronsiscalledasynapse.Forinformationtopassbetweenneurons,itmustcrossthesynapse.Invertebrates,andsomefishhaveelectricalsynapses,inwhichtheactionpotentialinthepre‐synapticneuroncantriggeranactionpotentialinthepost‐synapticneuronbecausethereisaphysicalconnectionbetweenthetwoneurons.Electricalsynapsesarefasterthanchemicalsynapses.

Mostnervesareconnectedbychemicalsynapses,whichconsistof: apre‐synapticendingthatcontainsneurotransmitters,mitochondriaandothercellorganelles

aneurotransmitterisasubstance(suchasnorepinephrineoracetylcholine)thattransmitsnerveimpulsesacrossasynapse

apost‐synapticendingthatcontainsreceptorsitesforneurotransmitters



asynapticcleftorspacebetweenthepre‐synapticandpost‐synapticendings

Forcommunicationbetweenneuronstooccur,anelectricalimpulsemusttraveldownanaxontothesynapticterminal.

ActionofNeurotransmitters:

1. Atthepre‐synapticterminal,anelectricalimpulse(actionpotential)causesachangeinmembranepermeabilitytoCa++,whichallowsCa++toflowintothesynapticknob.

Imagefromhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Synapse.gif

2. PresenceofCa++willtriggerthemigrationofvesiclescontainingneurotransmitterstowardthepre‐synapticmembrane.

3. Thevesiclemembranewillfusewiththepre‐synapticmembrane,releasingneurotransmittersintothesynapticcleft.(anexampleofexocytosis)

4. Neurotransmittermoleculesdiffuseacrossthesynapticcleftwheretheycanbindwithreceptorsitesonthepost‐synapticendingtoinfluencetheelectricalresponseinthepost‐synapticneuron.

Whenaneurotransmitterbindstoapost‐synapticreceptor,itchangesthepost‐synapticcell'sexcitability,makingiteithermoreorlesslikelytofireanactionpotential.

Ifthenumberofexcitatorypost‐synapticeventsislargeenough,theywilladdtocauseanactionpotentialinthepost‐synapticcellandacontinuationofthe“message”.

Manypsychoactivedrugsandneurotoxinscanchangethepropertiesofneurotransmitterrelease,neurotransmitterreuptakeandtheavailabilityofreceptorbindingsites.

NeurotransmittersandSynapses

SynapsesofthePNSareclassifiedaccordingtotheneurotransmitterused.Eachsynapseusesonlyoneneuro‐transmitter.

Mostsynapsesintheparasympatheticnervoussystemarecholinergicsynapses,anduseacetylcholine.Neuromuscularjunctionsarealsocholinergic.

Mostsynapsesinthesympatheticnervoussystemareadrenergicsynapses,andusenoradrenaline.

Synapsesofthebrainuseamuchwiderrangeofneurotransmitters,includingdopamineandenkephalins.



Neurotransmittersbindtoreceptorsonthepostsynapticmembrane,causingtemporarychangesinitspermeability.

SomeneurotransmitterscauseNa+orotherpositiveionstoenterthepost‐synapticneuron,helpingtodepolarizeitandcauseanactionpotential.

Thesearecalledexcitatorysynapses.

OtherneurotransmitterscauseCl‐tomoveintothepost‐synapticneuron–thiscauseshyperpolarisation.Hyperpolarisationmakesitmoredifficulttocreateanactionpotential(furtherfromthreshold).

Thesearecalledinhibitorysynapses.

Mostpost‐synapticneuronshavesynapseswithmorethanonepre‐synapticneuron–thesemaybeamixofexcitatoryandinhibitorysynapses,andwhetheranactionpotentialisinitiatedinthepost‐synapticneuronisdeterminedbythesumofallneurotransmittermessages.

Parkinson'sDiseaseiscausedbythedeathofneuronsinapartofthebraincalledthesubstantianigra.Theseneuronsreleasetheneurotransmitterdopamineatinhibitorysynapseswithneuronsthathelptocontrolmusclecontractions.Withoutdopamine,musclecontractionscannotbeproperlycontrolled–thiscausesthesymptomsofParkinson's:

earlysymptomsincludefeelingtiredandshaky,andalossofconcentration eventually,thebodybecomesstiffbecauseantagonisticmusclescannotrelax uncontrollableshakingaffectsthehandsandotherbodypartsandmovementsbecomeveryslow

Painreceptorsarefoundintheskinandotherorgans.Theyconsistoffreenerveendings,whichperceivemechanical,chemicalorthermalstimuli.Painsignalsaresentfromthesenerveendingstothespinalcordvianervefibres,whichcarrythemuptothethalamusorbrainstem.Fromhere,painsignalsmaybepassedontosensoryareasofthecerebralcortex,givingconsciousrecognitionofpain.Sincetherearebothfastandslownerveendings,apainfulstimuluscausesaninitialsharppainsensation,followedbyaslow,burningpain.

Thesensationofpainisnecessarytotellthebodywhenitisbeingdamaged–thisallowsthepainwithdrawalreflexorotherreactionstooccur.Sometimespaininterfereswiththeabilitytoconcentrate.Inthesesituations,paincontrolsystemsinthebrainandspinalcordcanbeusedtoreduceorpreventfeelingsofpain.Thisinvolvestwonaturalpainkillers:

enkephalinsreleasedbythebrainblockcalciumchannelsinthemembraneofthepre‐synapticneurons,blockingsynaptictransmissionsothatpainsignalsdonotreachthebrain

endorphinsproducedbythepituitaryglandarecarriedtothebrainandotherorgansbytheblood,andbindtoreceptorsinthemembranesofneuronsthatsendpainsignalstothebrain–endorphinsaresecretedduringstressfultimes,afterinjuries,andsometimesduringphysicalexercisesuchasrunning

PsychoactiveDrugs

Psychoactivedrugsaffectthebrainandpersonality.Theyeitherincreaseordecreasesynaptictransmission:

theycanbindtothereceptorsiteonpost‐synapticmembranes,mimickingtheneurotransmitterorblockingthebindingoftheneurotransmitter



theycanalsoreducetheeffectoftheenzymewhichnormallybreaksdowntheneurotransmitter,whichcausesanincreaseintheeffectoftheneurotransmitter

nicotinemimicsacetylcholine,whilecurareblocksacetylcholine

ExcitatoryPsychoactiveDrugsincreasetheactivityofthenervoussystem,andmayhavedifferenteffectsonbehaviour:

Nicotinestimulatessynaptictransmissionatcholinergicsynapsesinmanypartsofthebrain,andcausesreleaseofadrenalinefromtheadrenalgland.Thisresultsinincreasedbloodpressureandcardiacfrequency.Itaffectsmood,actinglikeastimulantandcausingeuphoria.

Cocaineblocksthereabsorptionofdopamineandnoradrenalineatsynapsesinthebrain,causingincreasedenergy,alertnessandtalkativeness.Itgivesanintensefeelingofeuphoria.Physicaleffectsincludeincreasedcardiacfrequencyandbodytemperature,anddilationofthepupils.

Amphetaminesstimulatetransmissionatadrenergicsynapsesandhavesimilareffectstococaine.Usersexperienceincreasedalertnessandreducedappetite.“Ecstasy”isaderivativeofamphetamines.Itcausesfeelingsofempathy,opennessandcaring,loweringaggressionandincreasingsexualbehaviour.

Caffeineincreasesheartrateandurineproduction.Itcausessomemoodelevationandincreasesalertness.

InhibitoryPsychoactiveDrugsdecreasetheactivityofthenervoussystem.

BenzodiazepinessuchasValium®relaxmuscles,decreasecirculation,respirationandbloodpressure.Theyreduceanxietyandelevatemood.Inhighdosestheycausedrowsiness,slurredspeechandlossofmusclecontrol.Doctorsprescribethemforuseastranquillizers.

Cannabiscontainsmanychemicals,includingTHC,whichbindstocannabinoidreceptorsintherain,blockingsynaptictransmission.Itsusersclaimitincreasestheintensityofsensoryperception,givesafeelingofemotionalwell‐beingandallowsclearthinkingaboutcomplexideas.Thereisstrongevidence,though,thattheabilitytoconcentrate,controlmusclecontractionsandjudgetimesanddistancesisdiminished.

Alcoholactsasaninhibitorinatleasttwoways(enhancesGABA,aninhibitoryneurotransmitter,andbydecreasingtheactivityofglutamate,anexcitatoryneurotransmitter.)Insmallquantities,alcoholreducesinhibitions,makingpeoplemoreconfidentandtalkative.Italsoreducesreactiontimesandfinemusclecoordination.Inlargerquantitiesitcausesmemoryloss,slurredspeech,lossofbalanceandpoormusclecoordination,andmaycauseviolentbehaviour.

Addictionisastateoftakingamood‐alteringdrughabituallyandbeingunabletogiveitupwithoutexperiencingunpleasantsideeffects.Ithasmanycauses:

THCinterfereswithdopaminemetabolism–thisproducesastateofdependence,withmore&moreofthedrugbeingrequiredtoproduceitseffect.

Geneticpredispositionmaybeafactorwithsomepeople–insufficientlevelsoftheenzymesrequiredtobreakdownthedrug,forexample,orapersonalitytypethatisinclinedtowardsunnecessaryrisk‐taking.

Socialfactors:poordiet,highunemployment&limitedaccesstoeducation&trainingthatcouldleadtorewardingemployment,combinedwithlittleopportunityforself‐fulfilmentcangenerateasenseofhopelessnessthatcouldleadtoseeingdrugsasanescapemechanism.



PerceptionofStimuli

Sensoryreceptorsactasenergytransducers.Thismeansthattheyconvertanon‐electricalsignal(e.g.lightorsound)toanelectricalone.Thisresultsinanactionpotentialinasensoryneuronbecausegatedionchannels(forNa+)areopened.

TypesofSensoryReceptors

chemoreceptorshavemembraneproteinswhichbindaparticularsubstance bindingresultsindepolarizationofthemembrane actionpotentialbringsmessagetothebrain e.g.scent,taste,pHofblood

mechanoreceptorsaresensitivetomovement inhumans,semi‐circularcanalsintheinnerearassociatedwithasystemofhaircells achangeinspeedordirection(ofthebody)movesfluidinthecanals,whichbendsthehairs thiscausesactionpotentialstothebrain

thermoreceptorsaresensitivetotemperature coldreceptorsintheskinsendanactionpotentialwhenthetemperaturedrops warm receptors (deeper in the skin than cold receptors) send an action potential when

temperatureincreases the temperature centre in the hypothalamus also contains thermoreceptors, whichmonitor

thetemperatureoftheblood(body)

photoreceptorsaresensitivetolight rodsandconesintheeyecontainphotopigmentswhichbreakdownwhenexposedtolight thiscausesanactionpotentialtothebrain

■ rodscontainrhodopsinandaresensitivetolightintensity■ conescontainiodopsins(red,greenorblue)andareresponsibleforcolourvision

Reflexesareafastresponsetoastimulus.

SpinalReflexesinvolvethespinalcordandnotthebrain.Theyarepartofinnatebehaviour,andinvolveonlytwoorthreenervecells.

KneeJerkReflex thekneeistapped;thisstretchesthetendon stretchreceptorinthemusclesendsanactionpotentialtothespinalcord theactionpotentialispassedtoamotorneuron,whichmakesthemusclecontract thelowerlegmoves

PainWithdrawalReflex youprickyourfinger(orstubyourtoe) apainreceptorneuronsendsandactionpotentialtothespinalcord anassociationneuronpassestheactionpotentialtoamotorneuron thiscausesthebicepstocontract,movingyourfingerawayfromthesourceofthepain

CranialReflexesinvolvethenervesofthebrain: PupilReflex

whenbrightlightisperceived,theiriswillimmediatelycontract thiswillreducetheamountoflightupontheretinasothatitisnotdamaged thebrainstemisresponsibleforthisreflex‐absenceofthepupilreflexcanindicatedamageto

thebrainstem(braindeath)



BlinkReflex whenanobjectcomesclosetotheeye,youwillblinkorcloseyoureye thishelpspreventdamagetotheeye youcanlearntocontrolyourblinkreflex,forexample,learningtoputincontactlenses

ReflexArc

Imagefromhttp://www.biotopics.co.uk/humans/refarc.gif

StructureoftheEye

Imagefromhttp://www.ai.rug.nl/~lambert/projects/BCI/literature/misc/oog‐retina.gif

fovea



StructureoftheRetina

Imagefromhttp://www.rhsmpsychology.com/images/retina.jpg

ProcessingVisualStimuli

Theretinacontainsrods,cones,andnervecellsresponsibleforvision.

Rodsareresponsiblefordetectinglightintensity,whileconesareresponsibleforcolourvision.

Thefoveaisa“yellowspot”ontheretinawhichisentirelycomposedofcones–thisisthesiteofmostaccuratevision.Thefoveaisfoundjustabovetheblindspot,wheretheopticnerveconnectsatthebackoftheeye.

Whitelighthittingthefoveatriggersactionpotentialsinallconesandisperceivedaswhitebythebrain.

Bluelighthittingthefoveatriggersactionpotentialsinbluecones,andisperceivedasbluebythebrain.

Thereisacertainamountofoverlapintheabsorptionofcolour,particularlybetweengreenandred–thismeansthatredorgreenlightcouldtriggeractionpotentialsinbothredandgreencones.

Lightenteringtheeyeisrefractedbythecorneaandlens.Itpassesthroughthevitreoushumour(clear)toreachtheretina.

lightmustalsopassthroughgangliaandbipolarneuronstoreachtherodsandcones

conesaremostlylocatedinthefovea rodsarefoundthroughouttheretina(exceptthefovea)

Conesarelinkedindividuallytobipolarneurons.Thismakesthemlesssensitivetolightbutincreasestheiraccuracy.



Severalrodsareconnectedtoasinglebipolarneuron.Thismakesthemmoresensitivetolightbutreducestheiraccuracy.

Whenlighthascausedanactionpotentialintherodsorcones,itispassedontothebipolarneurons.

actionpotentialsfromothercellsmayinhibitorfurtherexciteabipolarcell actionpotentialsarepassedfrombipolarcellstogangliaandontotheopticnerve

Theopticnerveiscomposedofmanynervefibres,whichareconnectedtodifferentpartsoftheretina.

somefibresconnectintheopticchiasma,whileothersdonot asaresultacompletepictureistransmittedtothebrain

Imagefromhttp://media‐2.web.britannica.com/eb‐media/48/63348‐004‐3D434AC1.gif

Contralateralprocessingisduetotheopticchiasma,wheretherightbrainprocessesinformationfromtheleftvisualfield,andviceversa,asillustratedabove.

Edge enhancement occurs within the retina, and is bestdemonstratedbytheHermanngridillusion(atright):

• dark,greyblobsappearatthe‘crossroads’wherethewhitelinesintersect–unlessyouaredirectlylookingatthatspot

• thishastodowiththereceptivefieldsoftheretina,whicharesmallerwhenlookingdirectlyattheintersectionpoints(seeleft)

Imagesfromhttp://www.michaelbach.de/ot/lum_herGrid/index.html



ColourBlindnessisaresultofadeficiencyinoneormoretypesofcones.Themostcommontypeisared‐greendeficiency,inwhichitisdifficulttodistinguishbetweencertainshadesofredandgreen.Thosewithred‐greencolour‐blindnesswouldbeunabletoseethenumber“15”intheimagebelow:

Imagefromhttp://www.biologie.uni‐hamburg.de/b‐online/library/falk/vision/colorblind.jpg

ControllingHowLightEnterstheEye

Lightenterstheeyethroughthepupil,andopeninginthecentreintheiris(thecolouredpartoftheeye).

PupilSizechangesinresponsetobrightnessoflight.

Inbrightlight,thecircularmusclesoftheiriscontract,andthepupilbecomessmaller.Thisreducestheamountoflightenteringtheeyetopreventretinadamage.

Indimlight,thesemusclesrelax,openingthepupil.Thisincreasestheamountoflightthatenterstheeye.

Imagefromhttp://www.schools.net.au/edu/lesson_ideas/optics/images/eye_contract.gif

LensThicknesschangesinordertofocuslightontheretina.

Lightreflectedoffadistantobjecthasparallelrays.Refractionthroughthelensfocusesitontheretina.

Lightreflectedoffanearobjecthasdivergentrays.Lighthastoberefractedmoreinordertofocusproperlyontheretina,sothelensthickens.



BrainDeathandthePupilReflex

Braindeathisdefinedastheirreversiblecessationofallbrainfunctions.Modernmedicaltechnologycankeepapatientalive(heartbeating,lungs‘breathing’)longafterthebrainstopsdirectingthesefunctions–becauseofthis,andthepossibilityofusingbrain‐deadpatients’organsfortransplantsurgery,itisnecessarytohaveindicatorsofbraindeath.

Theagreedcriteriaforbraindeath(absenceofallbrainfunction)are:

absenceofpupilreflex absenceofblinkreflex eyesdonotrotateintheirsocketswhentheheadismoved eyesdonotmovewhenicedwaterisplacedintheouterearcanal nocough(orgagging)whenasuctiontubeisplaceddeepintothetrachea breathingdoesnotcommencewhenthepatientistakenofftheventilator

StructureoftheHumanEar

Imagefromhttp://www.perceptualentropy.com/wiki/images/7/7c/HumanEar.jpg

Themalleus(hammer),incus(anvil)andstapes(stirrups)aretheossicles(bones)oftheinnerear.



PerceptionofSound

Theearconsistsofthreebasicparts‐theouterear,themiddleear,andtheinnerear.Eachpartoftheearservesaspecificpurposeinthetaskofdetectingandinterpretingsound:

Theouterear(pinnaandearcanal)servestocollectandchannelsoundtothemiddleear.o Soundenteringtheearcanalisapressurewave,withalternatinghighandlowpressure

regions:

o Whenthesoundreachestheeardrum(tympanicmembrane),theenergycausesitto

vibrate.

Themiddleearservestotransformtheenergyofasoundwaveintotheinternalvibrationsofthebonestructureofthemiddleearandultimatelytransformthesevibrationsintoacompressionalwaveintheinnerear.

o Themiddleearisanair‐filledcavity.

o Vibrationofthetympanicmembranecausestheinterconnectedossiclestovibrate,transmittingthesoundwavetothefluidoftheinnerear.

o ThemiddleearisconnectedtothemouthbytheEustachiantube,whichallowsforequalizationofpressurewithinthemiddleear.

Theinnerearservestotransformtheenergyofacompressionalwavewithintheinnerearfluidintonerveimpulsesthatcanbetransmittedtothebrain.

o Theinnerearconsistsofthecochlea,semicircularcanalsandtheauditorynerve.

o Thesemicircularcanalshavenoroleinhearing–theyactasaccelerometersthatassistwithbalance.

o Thecochleaisfluid‐filledandlinedwithhair‐likecells.Whentheossiclesvibrate,theytransmittheenergyofthevibrationtothecochleaviatheovalwindow.

o Becauseeachofthehair‐likenervecellsdiffersinlengthandsensitivitytothefluid’smotion,eachrespondstoadifferentfrequency.Whenstimulatedbyitsnaturalfrequency,thenervecellwillvibrate,triggeringanactionpotentialintheauditorynerve.



MusclesHumanshavethreetypesofmuscletissue:

skeletalmuscleisattachedtobonesviatendons

called striated muscle because of its stripedappearanceunderthemicroscope

cancontractquicklyandpowerfullybuttireseasily undervoluntarycontrol

smoothmuscleisnotstriated

itiscontrolledautomaticallybythenervoussystem■ thereforeitisinvoluntarymuscle

foundinthedigestivetractandbloodvessels takes longer tocontract,butdoesnot tireaseasily

asskeletalmuscle

cardiacmuscleisfoundintheheart

itismyogenic(beatsofitsownaccord)andisunderinfluenceofthenervoussystem

Imagesfromhttp://biodidac.bio.uottawa.ca

TheMuscleFibre

Skeletalmuscleismadeupofthousandsofcylindricalmusclefibresoftenrunningallthewayfromoriginto insertion. The fibres are bound together by connective tissue throughwhich run blood vessels andnerves.Eachmusclefibrecontains:

anarrayofmyofibrilsthatarestackedlengthwiseandruntheentirelengthofthefibre. mitochondria anextensivesmoothendoplasmicreticulum(SER) manynuclei.

Themultiple nuclei arise from the fact that eachmuscle fibre develops from the fusion of many cells(calledmyoblasts).

Becauseamuscle fibre isnota single cell, itspartsareoftengivenspecialnames suchassarcolemma(plasmamembrane),sarcoplasmicreticulum(endoplasmicreticulum),sarcosome(mitochondrion)andsarcoplasm(cytoplasm);althoughthistendstoobscuretheessentialsimilarityinstructureandfunctionofthesestructuresandthosefoundinothercells.

nucleiandmitochondriaarelocatedjustbeneaththeplasmamembrane theendoplasmicreticulumextendsbetweenthemyofibrils.



Seenfromthesideunderthemicroscope,skeletalmusclefibresshowapatternofcrossbanding,whichgivesrisetotheothername:striatedmuscle.

Thestriatedappearanceofthemusclefibreiscreatedbyapatternofalternating

darkAbandsandlightIbands.o TheAbandsarebisectedbytheHzoneo TheIbandsarebisectedbytheZline.

Eachmyofibrilismadeupofarraysofparallelfilaments.

Thethickfilamentshaveadiameterofabout15nm.Theyarecomposedoftheproteinmyosin. Thethinfilamentshaveadiameterofabout5nm.Theyarecomposedchieflyoftheproteinactinalongwithsmalleramountsoftwootherproteins:troponinandtropomyosin.

Imagefromhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/sarcomere.gif

Theanatomyofasarcomere ThethickfilamentsproducethedarkAband. ThethinfilamentsextendineachdirectionfromtheZline.Wheretheydonotoverlapthethickfilaments,theycreatethelightIband.

TheHzoneisthatportionoftheAbandwherethethickandthinfilamentsdonotoverlap.

The entire array of thick and thin filaments between the Z lines is called a sarcomere. Shortening of thesarcomeresinamyofibrilproducestheshorteningofthemyofibriland,inturn,ofthemusclefibreofwhichitisapart.

Imagefromhttp://www.mrothery.co.uk/images/Imag109.gif



NeuromuscularJunctions

Asynapsebetweenamotorneuronandamuscleiscalledaneuromuscularjunction:

Imagefromhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/synapse.gif

MuscleContraction&SlidingFilamentTheory

Duringmusclecontractionthemyofilamentsmyosinandactinslidetowardeachotherandoverlap.Thisshortensthesarcomeresandtheentiremuscle.Musclecellsare“shocked”bynerveimpulsesfrommotorneurons. The point of attachment of the nerve to themuscle is called the neuromuscular junction. Amotorneuronanditsmusclecellsarereferredtoasamotorunit.

Thenerveimpulseiscarriedfromthemotorneuronacrossthegaptothesarcolemma(membrane)ofthemuscle cell by aneurotransmitter called acetylcholine (ACh).After the impulsehaspassed, an enzymecalledcholinesterasedeactivatesacetylcholine,readyingthemuscleforthenextnerveimpulse.

Stimulation of the muscle cells causes Ca++ to be released into the cell. The Ca++ binds to the actinfilamentscausingthemtoexposeactivesitestothemyosincross‐bridges.Thecrossbridgesbindtotheactivesites,forminganewmolecularstructure,whichcausesthecross‐bridgetobendtowardthecentre,pullingtheactinfilamentwithit.EnergyfromATPisusedtobreakthebond,straightenthecrossbridge,andallowthecrossbridgetoformanewbondwithanotheractivesitefurtherdowntheactinfilament.

Thiscyclecontinuesuntilthemusclecontractioniscomplete.Then,ATPisusedtocauseactivetransport,movingthecalciumionsoutofthemusclefibre,resultinginrelaxationofthemuscle.

TheNervousSystem&Movement

Nervesstimulatemusclecontraction.Eachdifferentmuscleusedinlocomotionmustcontractatthecorrecttime,sothemovementiscoordinated.Sincemusclesareconnectedtobones(bytendons),contractioncausesthebonestomove.Themovementisusuallyreversedbyanothermuscleontheoppositesideofthebone–anantagonisticpair.Jointsareplaceswherebonesmeet,andareclassifiedbytherangeofmotionatthejointandtypeofconnection:

o fibrous:nomovement(e.g.suturesbetweenbonesofthecranium)o cartilaginous:bonesconnectedbycartilage;limitedrangeofmotion(e.g.betweenvertebrae)o synovial:fluid‐filledcavitiesbetweenthebonesallowgreaterrangeofmotion

elbow:ahingejointallowsforextension&retraction hip:aballandsocketjointwithawiderangeofmotion



AntagonisticPair:

Imagesfromwww.saburchill.com/chapters/chap0009.htmlTheelbowisatypicalhingejointinvolvingbones,cartilage,ligaments,tendonsandmuscles:

Imagefromhttp://www.botany.uwc.ac.za/sci_ed/grade10/manphys/images/man/hinge.gif

bonessupportthebodyandallowforlocomotion musclesmovethebones nervesstimulatemusclecontraction synovialfluidprotectsthebonesandlubricatesthejointandiscontainedwithinbursa ligamentsconnectbonestobones tendonsconnectmusclestobones

Nerves, Muscles & Movement Notes 09a - PBworks

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