MEMBRANE POTENTIALS AND - PowerPoint Presentation

1. MEMBRANE POTENTIALS ANDACTION POTENTIALS

2. MEMBRANE POTENTIALElectrical potential exists across the cell membraneThis potential could be changed in two directionsMade more positive - depolarizationMade more negative – hyper polarizationSome tissues change this potential on their ownCalled excitable tissues/cellsElectrical potential changes across CM used for normal functioning ofNervous system, muscles, heart etc.- 90mV-90mV0mV+50mV-120mVDEPOLARIZATIONHYPER POLARIZATION

3. ESTABLISHMENT OF MEMBRANE POTENTIALConcentration difference of ions across CM Differential diffusion capabilities of different ions Important ions in development of membrane potential Sodium, Potassium and ChlorideSodium and potassium are primary ions in the development of membrane potentialRapid changes in Permeability to sodium and potassium but not for chloride

4. Na+Na+Na+Na+Na+Na+k+k+k+k+k+PO4-PO4-PO4-PO4-Protein-Protein-Protein-Protein-Cl-Cl-Cl-Cl-Cl-PO4-Na+Na+k+k+k+k+A + ion conc. gradient from inside to outside causes –ve potential inside because +ve ions move out down the conc. GradientLeave several –ve non diffusible ions (Ex.PO4) and molecules (proteins) insideCELLMEMBRANEoutsideinside

5. RESTING MEMBRANE POTENTIALRMP of large nerve cells is - 90mvInside negative w.r.t. outsideSodium potassium pump builds up a conc. gradient for sodium and potassium across CM moves 3 sodium ions out for every 2 potassium ions moved in establishes a negative potential of 4mv / (-4mv) inside the cellSodium and potassium leak channels in CMMore permeable to potassium than sodiumSo more potassium leaks into ECF Because of potassium leakage inside potential becomes -94mvSmall amounts of sodium equal to + 8mv leak into cells Net EMF = -4 + -94 + 8 = - 90mv

6. ACTION POTENTIALComplete reversal of RMP Inside of the cell becomes positive w.r.t outside-90mv+ 50mv- 110mv0 mvResting Membrane – PolarizedLarge –ve potential insideSlight depolarizationA few Na channels in CM openSodium leakage further depolarizes MP to reach threshold for APOpening of large number of sodium channelsMore Na+ leaks in causing Complete reversal of MPClose Na & open K channelsMP more –ve than RMP Threshold PotentialDEPOLARIZATIONREPOLARIZATION operate Na-K pumpReestablish NRMPNRMPNo Na+ influx but K + efflux+ve after potential

7. VOLTAGE GATED SODIUM CHANNELSTwo gatesOuter – activation gateInner - inactivation gateAt NRMP (-90mv) Activation gate closedInactivation gate openAt threshold potential for APActivation gates openSodium permeability increased 500-5000 XSodium pours into the cellThreshold potential closes inactivation gates slowly~1 – 5 /10000th of a second laterMeanwhile AP generated due to influx of sodiumInactivation gates open again only after NRMP is reestablished

8. VOLTAGE GATED POTASSIUM CHANNELSOnly one gate on the inside of the cellDepolarization opens these gates slowly Open simultaneous with closure of sodium inactivation gates Decreased entry of sodium and increased exit of potassium re-polarizes the cell after depolarization

9. ROLE OF OTHER IONS AND ION PUMPS IN ACTION POTENTIALNon diffusible anions inside cellsPhosphates, sulfates, proteinsCreate –ve potential inside when +ve ions move outCalcium pump in all cells Pumps calcium outCalcium leak channelsSodium Potassium pumpCalcium in ECFChloride ion

10. OTHER IONS AND ION PUMPS IN AP – CALCIUM LEAK CHANNELSPresent in CM of all cellsSlow channels Numerous in smooth and cardiac muscle Responsible for depolarizationAlso allow small amounts of sodium to leak in

11. OTHER IONS AND ION PUMPS IN AP – SODIUM POTASSIUM PUMPSmall number of AP do not disturb sodium potassium gradient across CMBut cells generate 105 to 5x106 APserious disturbances in sodium potassium gradientsSodium potassium pump corrects these disturbancesActivity of Na-K pump ∞ (conc. of sodium in ICF)3

12. OTHER IONS AND ION PUMPS IN AP – CALCIUM IN ECFConcentration determines the voltage at which sodium channel gates openHypocalcemiaSodium gates open at much lower positivityIncreased excitabilityHypercalcemiaSodium gates open at higher positivityDecreased excitability

13. OTHER IONS AND ION PUMPS IN AP – CHLORIDE IONLeaks through CM just as Na & KNernst potential for maintaining 4mEq/L of chloride in ICF is -90mvSo at NRMP no net diffusion of chloride

14. PROPAGATION OF ACTION POTENTIALAP generated at any point excites adjacent portionsAP propagated in both the directions from originDepolarization propagated to a distance of ~3mmNewly depolarized areas generate still other new areas of depolarizationOnce generated AP covers entire CM or not at allAll or none principle

15. ACTION POTENTIAL WITH PLATEAUDelayed re-polarization after depolarizationDepolarized state is maintained near the peak of AP for many millisecondscardiac & smooth musclesCauses for AP with plateausodium leak causes spike part Calcium leak causes PlateauCalcium leak channels slow open at start of re-polarizationPotassium channels very slow open only at the end of plateau Plateau

16. REFRACTORY PERIODA new AP cannot occur in a cell until is re-polarization after an AP because inactivation gates of sodium cannot be opened till when NRMP is reestablishedTwo typesAbsolute refractory period No AP what ever may be the strength of 2nd stimulusRelative refractory period A second stronger than previous stimulus may generate an AP

17. ACCOMMODATIONWhen the depolarization occurs slowly over several seconds rather than milliseconds Inactivation sodium gates close Sodium influx is stopped without complete reversal of NRMP Potassium gates open Cell enters into a refractory period without generating AP

18. RHYTHMICITY OF EXCITABLE TISSUESRepetitive self induced generation of APCardiac muscle, most smooth muscles & some neuronsHigh NRMP is the reason