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AndrewAllen

NEUR30002Neurophysiology:NeuronsandCircuits

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CellularComponentsoftheBrain

Thenervoussystemconsistsofseveraltimesmoregliathanneurons.

AstrocytesThe number of astrocytes per neuron of the CNS of an organism is proportional to the

complexityoftheCNSoftheorganism.

Astrocytesareinvolvedininformationprocessing.

SpatialBufferingGapjunctionsconsistoftwoconnexons,whicheachconsistofsixconnexins.

Astrocytes take up potassium, which is released by active neurons, and dissipates it

throughoutneighbouringastrocytesviagapjunctions.

Astrocytes can transmit electrical and chemical signals toneighbouringastrocytes via gap

junctions.

ResponsiveAstrocytesAstrocytescanrespondtostimulationbyactivatingintracellularcalciumviaIP3.

Inresponsetostimulation,astrocytescanreleasegliotransmitters,whichcanstimulateother

astrocytes,bycalcium-dependentexocytosis.

Glialcellsofthenervoussystem.

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Astrocytic responses to stimulation by glutamate

(http://www.sciencedirect.com.ezp.lib.unimelb.edu.au/science/article/pii/S0166223603002376).

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RestingMembranePotentialTherestingmembranepotentialofatypicalneuronis−65%&.

Extracellular and Intracellular Ion Concentrations and EquilibriumPotentialsPotassiumtypicallyhasanECFconcentrationof5%',anICFconcentrationof150%',and

anequilibriumpotentialof−90%&.

SodiumtypicallyhasanECFconcentrationof150%',anICFconcentrationof10%',and

anequilibriumpotentialof+60%&.

ChloridetypicallyhasanECFconcentrationof100%',anICFconcentrationof10%',and

anequilibriumpotentialof−60%&.

CalciumtypicallyhasanECFconcentrationof1%',andanICFconcentrationof0.0001%',

andanequilibriumpotentialof+120%&.

IonTransportersandIonChannelsIon transportersmove ionsagainst theirconcentrationgradient,andcreateconcentration

gradients.

IonPumpsIonpumps,suchassodium-potassiumpumpsandcalciumpumps,areiontransporters.

Sodium-potassiumpumpsmovethreesodiumionsoutofacellandtwopotassiumionsinto

a cell per molecule of ATP, and are primarily responsible for creating the sodium and

potassiumconcentrationgradientsofcells.

Sodium-potassium pumps consist of an alpha subunit that has an Ouabain binding site, a

phosphorylationsite,asodiumbindingsite,apotassiumbindingsite,andanATPbindingsite,anda

betasubunit.

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IonExchangersIonexchangersareiontransporters,andmoveionsagainsttheirconcentrationgradientby

allowinganotheriontodiffusedownitsconcentrationgradient.

IonChannelsIonchannelsallowionstodiffusedowntheirconcentrationgradient,canbevoltage-gated,

canbeligand-gated,andcanbemechanically-gated.

Therestingmembranepotentialofaneuron isprimarilyaproductof theelectrochemical

gradientestablishedbythediffusionofpotassiumionsthroughpotassiumchannels.

Ionexchangers.

Theselectivityporeofanionchannelreplicatestherestinghydrationstateof

itsselectedion.

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PotassiumChannels

Kv, HERG, inward rectifier, calcium-activated, and two-pore potassium channels form

tetramers.

Kv,HERG,andinwardrectifierpotassiumchannelsarevoltage-gated.

Two-porepotassiumchannelscanbesensitivetoanaesthetics,lipids,heat,voltage,stretch,

andpH.

Inward rectifier and two-pore potassium channels contribute to the resting membrane

potentialofacell.

ThestructuresofKv,HERG,inwardrectifier,calcium-activated,andtwo-porepotassiumchannels.

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IonotropicReceptorsFastsynaptictransmissioninvolvesionotropicneurotransmitterreceptors.

FastExcitatorySynapticTransmission

IonotropicGlutamateReceptorsIonotropicglutamatereceptorsaretheprincipalfastexcitatoryneurotransmitterreceptors

of the central nervous system, are non-selective cation channels, and are tetramers of

subunitsthateachhavethreetransmembranehelicesandare-entrantloop.

AMPAglutamatereceptorsarepermeabletosodium,arepermeabletopotassium,andare

minimallypermeabletocalcium.

KainateglutamatereceptorsaresimilartoAMPAglutamatereceptors.

NMDA glutamate receptors are permeable to sodium, are permeable to potassium, are

permeable to calcium, have a glycine binding site, and are voltage-dependent due to a

magnesiumbindingsiteinthechannelporethatblockstheporewhenboundtomagnesium.

Therecyclingofglutamateatasynapse.

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NicotinicAcetylcholineReceptorsNicotinicacetylcholinereceptorsarefoundonskeletalmusclefibres,arenon-selectivecation

channels,andarepentamersofsubunitsthateachhavefourtransmembranehelices.

TheEPSCsproducedbytheopeningofAMPAandNMDAglutamatereceptors.

Therecyclingofacetylcholineatasynapse.

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FastInhibitorySynapticTransmissionFastinhibitorysynaptictransmissioninvolveschlorideionchannels,whichmaintainresting

membranepotentialbyshuntingEPSPs.

GABAAReceptorsGABAAreceptorsaretheprincipalfastinhibitoryneurotransmitterreceptorsofthebrain,are

pentamers,andarepermeabletochlorideions.

ThefunctionofGABAAreceptorsisaffectedbybenzodiazepines,barbiturates,steroids,and

picrotoxin.

TherecyclingofGABAatasynapse.

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GlycineReceptorsGlycinereceptorsaretheprincipal fast inhibitoryneurotransmitter receptorsof thespinal

cord,arepentamers,andarepermeabletochlorideions.

Therecyclingofglycineatasynapse.

GABAAreceptorsproduceEPSPsinimmatureneurons.