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| Pregunta | Respuesta |
| 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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