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Lecture 6

BIOLOGY 2A03 Lecture 6: 6) Lecture- neuro 03A
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Department
Biology
Course
BIOLOGY 2A03
Professor
Graham R.Scott
Semester
Winter

Description
6) Lecture- neuro 03A Monday, January 16, 2017 4:35 PM 6 •Centralnervoussystem is made of brainand spinalcord •Centralnervoussystem receivesinfo from the afferent part of the peripheralnervoussystem, andsendsinfo out through the efferent part of the PNS •Afferent is part of sensory •Efferent innervatestheskeletalmuscles, so we can controlmovement ◦Also controlsautonomicnervoussystem, whichcontrols bodilyprocesses(heartrate, organs,etc.) 7 •Afferent neuronstendto be pseudo-unipolar •Peripheralaxonhasa cell bodycloseto the CNSand a centralaxon, projectingintothe CNS •Visceralreceptorsare on organsinthe internal environment 8 •Cellbody tendsto bein the CNS •Innervate skeletalmusclesor of the autonomous nervoussystem 9 •Cells in the CNS are called interneurons ◦Interneurons are the intermediate between efferent and afferentneurons ◦Most neuronsinour bodyare these, bc most neurons are inthe CNS ◦Sensory processing(integratingsignalsfromthe periphery) 10 •Glia supportthe neuronsin the CNS •OligodendrocytesandSchwanncellsprovideinsulation aroundthe axons •They reduce ion leak and thus shield the axon from electricaldisruption 11 •In the CNS, the Oligodendrocytesinsulatemanyaxons ◦One nucleuscan reachout andwrap aroundmany aroundthe axons •They reduce ion leak and thus shield the axon from electricaldisruption 11 •In the CNS, the Oligodendrocytesinsulatemanyaxons ◦One nucleuscan reachout andwrap aroundmany axons •In the peripheral,asignalSchwanncellinsulatesa singlepart of oneaxon •They both wraparoundcellsto providean insulated layer •Leavelittle areasexposed •Cellin closeproximityto axonandas the cellgrows, it wrapsaround(concentricrings) •Within the ringsare lipids,calledmyelin,in whichions are unabletomove across well(becauseit's lipophilic) •Insulationallowsthe axonsto transmit signalsmore quickly 12 •CNS has whiteandgrey regions(cellbody= grey, axons = white, dueto lipidinmyelin) •Ex, spinalcordhas parts that are whiteand grey, dueto the presenceof lots of axon tracts, or not, etc. •Where there arelots of cell bodies,there arelots of synapses ◦This is whereintegrationoccurs 14 HOW ELECTRICALSIGNALS ARE TRANSMITTED: Electrochemicaldriving force= chemical(conc gradient) and electricalforces (on charged ions) - Electrochemicaldrivingforcecanpush ionsout of cellsby diffusionor pullions intocells. - Overallelectrochemicaldrivingforceis the combinedworkof electricalandchemicaldriving forces on ions One situation- no net diffusionof ions. - Chemicalandelectricalareequal. - Restingpotentialis neg,so k+ willalwaysbe pulledINTO the cell becauseits positive. - Membranepotentialwheretheforces are in equilibrium= equilibrium potential for potassium. - They differ for different ions. - Calculateit withthe Nernst equationdependingon - Restingpotentialis neg,so k+ willalwaysbe pulledINTO the cell becauseits positive. - Membranepotentialwheretheforces are in equilibrium= equilibrium potential for potassium. - They differ for different ions. - Calculateit withthe Nernst equationdependingon their concon eitherside of the membraneandtheir charge. - If membranepotentialisat equliibpotential,there is no netdrivingforce. - If membrane potential is at a diff voltagethan equilibpotential, thereis a driving force. •When the chemicalandelectricaldrivingforcesare not equal,diffusionof ions occursin or out •However,if they are equal,there is nonet electrochemicaldrivingforceandweobtainan equilibrium potential •EP differs for differentions (dependentonconc. And charge) •If the membraneis at a different voltagethan the MembranePotential,then thereis a drivingforce 15 •Voltages aredeterminedbythe thin layerof ions close to the membrane,not all the ions •ChangesinMP areimportant Normalrestingmembranepotential= -70mV (typical). - Voltagesare not causedby massivedifferencesin chargebetweenthe insideandoutsideof the cell. - Its just the very thin layerof ions nearthe cell membrane. - Slight excessof anions JUST in the membrane and a SLIGHT excessof cations JUSToutside the membranecausesthis. - This is a smallproportionof ions. - Diffs occur at the micro level(just nearthe membrane). - Overallin the cell- electricalneutrality(nodiff in negand poscharge) When neurons communicate they make changes in the membranepotential. - Changesinmembraneportentialareimportant. - Why these membranepotentialsexist….. 16 When neurons communicate they make changes in the membranepotential. - Changesinmembraneportentialareimportant. - Why these membranepotentialsexist….. 16 •When we makeconc. Gradients of ions, this contributes to a membrane potential •How ionsmoveacross membranesis importantfor MP Why they exist: 1) When youmake conc gradients of ions , move ionicchargearound,it contributesto a membrane potential 2) Permeability - Howreadilyionsmove across membranes - Changesinpermeabilityareimportantfor underlyingelectricalsignal. - Important are sodium and potassium. - Concentration gradients in these ions are establishedby Na/KATPase - Na/K ATPase create concentrationgradientson the inside/outsideofthe cell. - Createsa voltge diff bc the pumpitself is electrogenic Permeability of ions is important as well. •All cellshave a MP becauseNa/KATPase createsthe conditionsfor a MP to exist •The pump itself iselectrogenic:pumps3 out and 2 in, creating neg. MP insidethe cellbc alwaysmovingmore outthan in 17 Consider- cellwith NO MEMBRANE POTENTIAL. - Cellis onlypermeablyto k+ ions. - Intracellularanions,A- insidethecell - Outside, lots of sodiumand chlorideions - Only thingthat movesacross the membraneis potassium. - Start withno membranepotential,chemical driving force will push k+ OUT (bc its very concetratedinsidethe cell- gradient - chemical drivingforce outward) - Becausepos chargeis leavingthecell, it creates a VOLTAGE. - Start withno membranepotential,chemical driving force will push k+ OUT (bc its very concetratedinsidethe cell- gradient - chemical drivingforce outward) - Becausepos chargeis leavingthecell, it creates a VOLTAGE. - Get a slightneg voltage insideand pos outside as potassiummoves out. - This creates anelectricalforceopposing the chemicaldriving force . - Ie positivevoltageoutsidebut potassiumis moving outside(downits conc gradientbc morepotassium was insideat first). - Eventually,the two forces balanceout - As K+ continuesout, electricaldrivingforcegets greaterand greateras the voltagegets more negativeinside. - At equilibpotential,theycanceleachother out. - But, just becausek+ is moreconc insideand membraneleakspotassium,creates the equilb potentialAS LONGAS DIFF IN ION CONCENTRATIONS ARE MAINTAINED. •Consider:cellwith noMP andcellis only permeableto K+ ◦The chemicaldrivingforce pushesK+ out ◦Then, this creates a voltage(negativeinsideandpositive outside) ◦Eventually,these two thingsbalanceout 18 NernstEquatoin = equilib potential for any ion - Calculateequilibpotentialforanyion. - Differs for different ions(diff concand diff charge). - When membranepotentialis+60 (not typicalof restingcell) - sodiumis equilib - THIS IS WHYNORMALLY,SODIUM IS UNDERa chemicaldrivingforceINTO the cell, because sodiumsmembranepotentialisfar from normal resting. Negand poshas to do with differencesinthe ratios (one higherinsideandonehigheroutside) •EP is differentfor different ionsbc conc. And chargeis resting. Negand poshas to do with differencesinthe ratios (one higherinsideandonehigheroutside) •EP is differentfor different ionsbc conc. And chargeis different •Bottom is simplified 19 First exampleisfor potassium= -94mV. Sodiumequilibpotentialisdifferenteven hthosame chargebc ratio of conc is different For sodium, its +60. •We arecalculatingEPsfor ions •Neg. andpositivehas to do withdifferences inthe ratios 20 Permeability- ionmove acrossmembrane •If permeabilityishigherfor an ion,that meansits conductance is higher(abilityto move across the mmebraneis higher - Normally,k+ permeability is much greaterthan Pna+ - Thus, easierfor potassiumto move acrossa membranethansodium - This is why the membrane potential is negate i. 21 EX (hypothetical) - Lots of potassiumin, sodiumoUT. - Permeableto both, but permeability to potassium is greater. - Potassiummoves more easily. - If we start with a progressingmembranepotential of zero - Chemicaldriving force will push potassium out strongly andsodium IN lessstrongly (bc they have diff conc gradients). (bc sodium is less permeable). - Let ionsleak. - Permeabilityto potassiumis higherthansodium - More bluechannelsforpotassiumthan strongly andsodium IN lessstrongly (bc they have diff conc gradients). (bc sodium is less permeable). - Let ionsleak. - Permeabilityto potassiumis higherthansodium - More bluechannelsforpotassiumthan red(sodium) - If membraneis morepermeableto k+ = potassium can moveout easierand faster than sodiumcan move in. - NETPOSITIVE MOVEMENT OUTOF CELL BECAUSEPOTASSIUM MOVES OUT OF CELL FASTER THAN SODIUM MOVES IN. This establishes a NEGATIVE membrane resting potential. - Buildup of negvoltageinsideas pos net movementis OUT. - Pos voltageoutside. - Once the voltageis established,youget an electricalforce OPPOSING the chemicaldriving force. - So negvoltageis pullingpotassiumbackin cause its pos - Nowelectricalforce is actingin the same direction as the chemicalforce, for sodium. - Sodium has two forces pulling it INTO the cell. - Eventually,potassiummovementslows downbc they willopposeeachother andfor sodium, the forces act together - SO, SODIUM WILL MOVE IN FASTER
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