Flashcards
L10- Membrane potential: the resting and action potential
Cards in this deck (20)
- 1Concentration gradients: Work done at the membraneHow 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 - 2Equations for concentration gradient at the membrane for positive ionsConc of ions out(Cout)/conc of ions in(Cin)
- 3Equation for concentration gradient at the membrane for negative ionsConc grad= conc of ions in(Cin)/conc of ions out (Cout)
- 4If the concentration gradient is known, the membrane voltage due to an ion can be found using the Nernst equationNernst equation= E=58(mv) X log (c)out/(c)in
- 5Resting 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 - 6Equilibrium potential for an ionThe 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
- 7Nernst eq to stop K+ from leaving in a physiological concentrationEk= -90mV
- 8Nerst eq in a physiological conc to stop Na from leavingEna= +50mV
- 9Membrane potential VmThe 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. - 10Driving force on an ionVm-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. - 11Permeability and conductanceConductance- 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 - 12Goldman Hodgkin Katz equationThe 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) - 13Action potential: properties of the action potential1. 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 - 14Effect 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 - 15Vm to Ena- positive feedbackDepolarisation 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 - 16Charge separationCharge (Q), measured in coulombs= capactance (c) x voltage (v)
Capactance= ability of membrane to store charge - 17Use 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 - 18Propogation 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. - 19Path of injected current- local circuitin squares
- 20Structure 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