Graded and Action Potentials PDF

BIRUNI UNIVERSITY FACULTY OF PHARMACY BIOPHYSICS 7. GradedPotentialsAnd Action Potentials Lect. Demet ERDAG  Nervous System Nervoussystemcellsarecomprisedof gliaand neurons. Neuronsareresponsibleforreceive,process,andtransmit informationin nervous system. Gl...

BIRUNI UNIVERSITY FACULTY OF PHARMACY BIOPHYSICS 7. GradedPotentialsAnd Action Potentials Lect. Demet ERDAG  Nervous System Nervoussystemcellsarecomprisedof gliaand neurons. Neuronsareresponsibleforreceive,process,andtransmit informationin nervous system. Glia – Not specialized for information transfer – Supportneurons Neurons(NerveCells) – Receive,process,and transmit information Information travels in one direction Dendrite→soma →axon  Voltmetre (mV) 0 30 -30 50 -50 -70 outside ATP inside All cells have electrical potential difference betweeninsideandoutside ofthecell.  Transient changesinthemembrane potentialof itsrestinglevel produce electricalsignals. – Suchchangesarethemostimportantwaythat nervecells processandtransmitinformation. These signals occur in two forms: 1. graded potentials 2. action potentials Gradedpotentialsareimportantinshortdistances. Actionpotentialsarethelongdistancesignalsof nerve and muscle membranes.  Nerve and muscle cells as well as some endocrine, immune, and reproductive cells have plasma membranes capableof producingactionpotentials. These membranes – are called excitable membranes. – Their ability to generate action potentials is known as excitability.  Allcellsarecapableof conductinggradedpotentials, but excitable membranes can conduct action potentials.  Changes in Membrane Potential  depolarize Theterms  repolarize areusedto describe  hyperpolarize the directionofchangesinthemembranepotentialrelative to the resting potential. Membrane potential (mV) Time The resting membrane potential (at -70 mV) is polarized. “Polarized”meansthattheoutsideandinsideofacellhave a different net charge. The membrane is said to be depolarized when its potentialislessnegative thantherestinglevel. The membrane is repolarized when the potential returns toward the resting value. Themembraneishyperpolarized whenthepotentialis morenegativethan therestinglevel. Changes in Membrane Potential  Graded Potentials Short-lived, local changes in membrane potential Decrease in intensity with distance Their magnitudevariesdirectlywith the strength of the stimulus Sufficientlystronggradedpotentialscan initiateaction potentials Graded Potentials Figure 11.10  Graded Potentials Can only travel short distances Voltagechanges ingradedpotentialsare gradual Currentquicklyspreadsanddisappearsdue to the leaky plasma membrane  TermsDescribingthe MembranePotential Potential=potential Thevoltagedifferencebetweentwo difference points. Membrane potential The voltage difference between the =transmembrane potential insideandoutsideofa cell. Equilibrium potential The voltagedifferenceacrossa membranethat producesa flux of a givenionspeciesthatisequalbut oppositetotheflux due tothe concentrationgradientof that same ion species.  TermsDescribingthe MembranePotential Resting membrane potential Thesteadytransmembranepotential ofa cell thatisnot producing an = restingpotential electric signal. Abriefall-or-none depolarizationof Action potential the membrane, reversing polarity in neurons;ithasa thresholdand refractoryperiod and is conducted Threshold potential without decrement. Themembranepotential atwhichan actionpotential is initiated.  A small region of a membrane has been depolarized by a stimulus, – Opens membrane channels – produces a potential less negative than adjacent areas. – inside the cell, positive charge will flow through the intracellularfluidawayfrom thedepolarizedregionand toward the more negative, resting regions of the membrane. – outside the cell, positive charge will flow from the more positive region of the resting membrane toward the less positive regions just created by the depolarization.  Thus, it produces a decrease in the amount of chargeseparation(i.e.,depolarization)in the membrane sites adjacent to the originally depolarizedregion,andthesignalismovedalong the membrane.  Dependingupontheinitiatingevent,graded potentialscanoccurineither a depolarizingor a hyperpolarizingdirection. Membrane potential (mV) Suchexperimentsshowthat gradedpotentials (a) can be depolarizing or hyperpolarizing, (b) can vary in size. *Therestingmembranepotentialis-70 mV.  Charge islostacrossthemembranebecausethe membraneispermeabletoionsthroughopen membrane channels. As aresult,themembranepotentialchanges decreases by the distance from the initial site.  Because the electrical signal decreases with distance, graded potentials can function as signals only over very short distances. If additional stimuli occur before the graded potential has died away, these can be added to the depolarization from the first stimulus. This process is termed summation. Graded potentials are the only means of communication used by some neurons. They play very important roles in the initiation and integration of long-distance signals by neurons and some othercells.  Action Potential Neurons communicate over long distances by generatingandsendingan electricalsignal calledanerveimpulse,or actionpotential. outside ATP inside Whenastimulusappliedto themembranein a restingpotential,whathappens??? Disturbed by the stimulus  Action Potentials (APs) A brief reversal of membrane potential with a totalamplitudeof 100mV Action potentials are only generated by muscle cells and neurons They do notdecrease in strength over distance They aretheprincipalmeans ofneuronal conduction Anactionpotentialintheaxonofaneuronisa nerve impulse – is very rapid – all-or-none – mayoccur atarate of 1000persecond – some cells have plasma membranes capable of producing action potentials. – Voltage-dependentionchannelsinthemembraneare the basis for APs. – The propagation of action potentials is the mechanism used by the nervous system to communicate over long distances.  Resting State Na+and K+channelsareclosed Leakage accounts forsmallmovements ofNa+ and K+ EachNa+channelhastwovoltage-regulated ga te s – Activation gates – closed in the resting state – Inactivation gates – open in the resting state Figure 11.12.1  Depolarization Phase Na+permeabilityincreases;membranepotential reverses VoltagegatedNa+channelsareopened, butK+ are closed Threshold –a critical level of depolarization (-55 to -50 mV) Atthreshold, depolarization becomes self-generating  Repolarization Phase Sodium inactivation gates close MembranepermeabilitytoNa+declinesto resting levels As sodium gates close, voltage-sensitive K+ gates open K+exitsthecelland internal negativity oftheresting neuron is restored  Hyperpolarization Potassium gatesremainopen, causingan excessiveeffluxof K+ This efflux causes hyperpolarization of the membrane (undershoot) The neuron is insensitiveto stimulusand depolarization during this time Figure 11.12.4  Action Potential: Role oftheNa-KPump Repolarization – Restorestherestingelectrical conditionsof the neuron – Doesnotrestorethe restingionicconditions Ionicredistributionback torestingconditions isrestored bythe sodium-potassiumpump  TheActionPotential: An Overview The action potential is a large change in membrane potential from a resting value of about -70 mV to a peak of about +30 mV, and back to Theactionpotentialresultsfroma -70 mV again. rapidchangein the permeabilityoftheneuronalmembranetoNa+ and K +. The permeability changes as voltage-gated ion channels open and close. Action potentials are rapid, large alterations in the membrane potential during which time the membrane potential may change 100mV,from-70to 30mV,andthen repolarizetoits resting membrane potential.  Whatisresponsibleforthechangeinmembrane permeability during the action potential? Althoughcalled“action”potential,it isNOT anactive (energy-consuming) event for the cell. It is purely a passive event. It is due to diffusion of ions! It is dependent on – ionic electrochemical gradients (Na+, K+) and – the membrane’s permeability. Excitable cells have “fickle(unstable)” cell membranes…they keep changing their permeabilities. Whatdetermines themembrane’s permeability atany moment? Answer:GATED ion channels—These allow SIMPLE DIFFUSION of ions down their electrochemical gradients 37  IonicBasisoftheAction Potential Theactionpotentialisinitiatedbyatransient changeinmembraneionpermeability,which allowsNa+andK+ionstomove down their concentration gradients. Ionic Basis of the Action Potential  Ionic Basis of the Action Potential When a stimulus applied in the membrane of the cell  Ionic Basis of the Action Potential VoltagegatedNa+channels areopenimmediately, andK+channels openslowly Ionic Basis of the Action Potential Voltage gated Na+ channels close Ionic Basis of the Action Potential Channels of APs  1. PHASE: In the resting state, The leak channels in the plasma membranearepredominantly those that are permeableto K+ions. Very few Na+ion channels are open. Therestingpotentialis close to the K+ equilibrium potential.  The action potential beginswithdepolarization of the membrane in response to a stimulus. Thisinitialdepolarization opens sodium channels, which increases the membrane permeability to sodium ions   Phase 2 Moresodiumionsmove into the cell. Thecellbecomesmore and moredepolarizeduntil athreshold(2)isreached totrigger the action potential. This is called the  Threshold and Action Potentials Threshold–membraneisdepolarizedby15 to 20mV Establishedby the totalamountofcurrent flowing throughthe membrane Weak (subthreshold)stimuliarenotrelayed intoaction potentials Strong(threshold)stimuliarerelayedinto action potentials All-or-nonephenomenon–actionpotentials either happen completely, or not at all   Phase 3 Afterthethresholdpotentialis reached, voltage-gated sodium channelsopen(3). The membrane potential overshoots,becomingpositiveon the inside and negative on the outside of the membrane. In this phase, the membrane potential approaches but does not quitereachthesodiumequilibrium potential (ENa=60 mV).   Phase 4 Atthe peakofthe actionpotential(4),Na+ permeabilityabruptly decreases and voltage- gatedpotassium channels open.   Phase 5 The membrane potential begins to rapidly repolarize (5) toitsresting level.   Phase 6 Afterthe sodiumchannels haveclosed,some ofthe voltage-gated potassium channelsarestillopen,and innervecellsthereis generally a small hyperpolarization(6)ofthe membrane potential beyond therestinglevelcalledthe after hyperpolarization.   Phase 7 Once the voltage-gated potassium channels close, the restingmembranepotentialis restored (7). Na+/K+pumprestorepotential to -70mV in 1-2 msec. Chloride permeability does not change during the action potential.  0: Between -70 to -40 mV Na+ channels open & Na + ionsfloodinside. 1:At-40mV,Voltage-gated Na+channelsopen&Na+ 50mV ionsfloodinside. 2:At+50mV,Na+ channels 0 close&K+ channelsopenso that K + ions flood outside. 3:Voltagedecreasesto -90 -50mV mV &K+ channelsclose 4:Na+ /K+ pump restores -100mV potentialto-70mV in 1- 2msec. Resting membrane Potential Na+ concentrated on outside. K+ concentrated on inside Depolarization Begins + Nagatesopen + andNabeginstoflowrapidly intotheaxon Depolarization Continues Na+continuestoflowrapidlyintothe axon Kgates + open andKbeginsto + flowslowlyoutoftheaxon Depolarization Peaks Na+ channels close and Na+stops flowing into the axon K+has onlyjuststartedtoleave theaxon Na+and K+are now both briefly concentrated on the inside of the axonresultingintheinsidebeingpositiverelativethe outsideofthe axon Hyperpolarization Begins + TheNachannelsclose,theNa+pumpforcestheNa+outof the axon, back to where it started. K+channels start to close. Because positive ions are both concentrated on the outside of the axon, the outside is now more positive than when the axon is at rest. In other words, the inside is more negative than resting. Axon Returns to The Resting State Na+ has been pumped back outside K+ has been pump back inside  Manycellsthathavegradedpotentialscannot formaction potentialsbecausetheyhaveno voltage-gated sodium channels.  References Medival physiology- Guyton Human physiology-Wander