L10- Membrane potential: the resting and action potential

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membrane potential
Rose P
Flashcards by Rose P, updated more than 1 year ago
Rose P
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Concentration gradients: Work done at the membrane How much work is done at the membrane? - The amount of work done at the membrane depends on the size of the conc. gradient. - The bigger the concentration gradient, the more work done to seperate ions accross it
Equations for concentration gradient at the membrane for positive ions Conc of ions out(Cout)/conc of ions in(Cin)
Equation for concentration gradient at the membrane for negative ions Conc grad= conc of ions in(Cin)/conc of ions out (Cout)
If the concentration gradient is known, the membrane voltage due to an ion can be found using the Nernst equation Nernst equation= E=58(mv) X log (c)out/(c)in
Resting potential - Typically around -70mV - Principally determined by concs of Na+ and K+ - If the inside of the cell is very negative, it will stop K+ from leaving - If the inside of the cell is very positive, it will prevent Na+ from entering
Equilibrium potential for an ion The equlibrium potential for an ion is the membrane voltage a cell needs to be at in order to prevent movement of that ion down its concentration gradient
Nernst eq to stop K+ from leaving in a physiological concentration Ek= -90mV
Nerst eq in a physiological conc to stop Na from leaving Ena= +50mV
Membrane potential Vm The membrane potential is much closer then Ek than Ena because the membrane is about 50X more permeable to K+ than Na+ - At constant, Vm net flow of ions is zero as the passive leak of K+ out is matched by leak of Na+ in. Therefore, resting potential maintains at -70mV - If a cell becomes more permeable to an ion, then it will move down its electrochemical gradient and will drive membrane potential towards equilibrium potential for than ion.
Driving force on an ion Vm-Eeq (membrane potential-equilibrium potential for a given ion) - Unbalanced forces on a membrane result in resting potential because the membrane is more permeable to some ions.
Permeability and conductance Conductance- amount of charge that moves across the membrane. Depends on conc. gradient and the number of open channels. Permeability- the ease at which ions move across the membrane
Goldman Hodgkin Katz equation The Nernst eq deals w/one ion at a time and makes no assumptions about the permeability of the membrane. The Goldman Hodgkin Katz eq is a modified version and considers relative permeabilities of monovalent ions: Vm= 58Mvlog(Pk(k+out)+Pna(Na+out)/Pk(K+in)+Pna(Na+in)
Action potential: properties of the action potential 1. Triggered by depolarisation 2. Threshold of depolarisation is required to trigger an AP 3. APs propogate without decrement- they have a constant amplitude 4. At peak, the membrane potential Vm, approaches the equilibrium potential Ena 5. After the AP, the membrane is inexcitable during the refractory period
Effect of channels on APs - AP is due to current flow through voltage gated sodium and potassium channels - These channels are either open or closed - The probability of these channels opening/closing is determined by the voltage across the channel. - Channels are voltage dependent - If cell becomes permeable to and ion, that ion will move down its electrochemical gradient, driving Vm down its equilibrium potential - During AP, membrane becomes permeable to sodium first, then potassium
Vm to Ena- positive feedback Depolarisation results in the opening of Na channels, resulting in Na influx, which maintains depolarisation. - Prolonged depolarisation caused sodium inactivation and the peak cycle of AP The opening of K+ channels due to depolarisation results in K+ efflux resulting in repolarisation
Charge separation Charge (Q), measured in coulombs= capactance (c) x voltage (v) Capactance= ability of membrane to store charge
Use of Faraday's constant to express the fraction of a mole required - Each mole of a monovalent ion has 10^5coloumbs of charge - So, fraction of mole required = Charge/10^5 - Very few ions need to be seperated in order to give big biological effects - this gives negligible osmotic consequences
Propogation of action potential - when sodium channels open, the inside of the membrane becomes positive - This allows local current circuit to flow from positive to negative across the axon - Increase of positivity at the 'foot' of the action potential, sufficient to kickstart the opening of more sodium channels, so it propogates along the nerves.
Path of injected current- local circuit in squares
Structure of myelinated nerves - Axons are myelinated by a single schwann cell - the purpose of myelination is to increase the speed of conduction-velocity - Salutatory conduction: The current enters at the nodes of ranvier, depolarises it , flows down the axon, to the next node of ranvier, depolarising it. - This doesn't happen at the speed of light because it takes time for channels to open
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