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| Question | Answer |
| Concentration gradients: Work done | - How much work is done at the membrane depends on the size of the concentration gradient. - The bigger the concentration gradient, the more work that has to be done to separate ions across it. |
| Concentration gradients: Equations | For positive ions: Conc of ions out/Conc of ions in For negative ions: Conc of ions in/Conc of ions out |
| Membrane voltage (E) due to an ion: Nernst equation | E= 58xlog (c)out/(c)in |
| Resting potential | - Typically around 70mv - Principally deternined by Na+ and K+ levels - The equilibrium potential for an ion is the membrane voltage a cell needs to be at in order to prevent the movement of that ion down its concentration gradient |
| For physiological solutions | - If the inside of the cell is very negative, it will stop K+ from leaving Ek=-90Mv (to stop k from leaving) - If the inside of the cell is very positive it will prevent Na+ from entering Ena= +50mV to (stop Na from entering) |
| Membrane potential (Vm) | - Vm is much closer to Ek than it is to Ena because the membrane is about 50x more permeable to K+ than it is to Na+. - At constant vm net flow of ions is 0 as the passive leak of K+ is matched by the movement of Na+ in; so resting potential stays at -70mV - If a cell becomes permeable to an ion, it moves down its electrochemical gradient, driving membrane potential towards equilibrium potential for that ion. |
| Driving force on an ion | - Driving force on an ion= Vm (membrane potential)-Eeq (equilibrium potential for a given ion) . - Unbalance forces on a membrane result in resting potential because the membrane is more permeable to certain ions. |
| Permeability and conductance | Conductance- amount of charge that moves across a membrane. Depends on conc. gradient and number of open channels. Permeability- ease at which ions move accross the membrane. Property of the membrane. |
| Goldman Hodgkin Katz equation: a modified nernst equation | - Nernst equation w/one ion at a time and makes no assumptions about the permeability of a membrane. - The Goldman Hodgekin katz equation considers the relevant permeabilities of monovalent ions. |
| Goldman hodgkin katz | |
| Action potential: Properties of an action potential | 1. Triggered by depolarisation 2. Must reach threshold of depolarisation to trigger action potential. 3. Propogate without decrement (constant amplitude) 4. At peak of AP, membrane potentiol (Vm) approaches equilibrium potential (Ena) 5. After the AP, membrane is inexcitable during a refractory peroid |
| Action potential continued: channels | 1. AP is due to current flow through voltage-gated sodium and potassium channels 2. Channels are either open or closed 3. The probability of a channel opening/closing is determined by voltage across the channel. |
| Voltage dependent channels: Mechanisms | - Channels are voltage dependent - I a cell is permeable to an ion it will move down its conc gradient, driving Vm towards equilibrium potential - During an action potential, membrane becomes permeable to sodium first, then potassium |
| Positive feedback | Vm to Ena: - Depolarisation opens sodium channels, resulting in an influx of sodium ions, which causes sodium channels to remain open . Vm to K+: - Depolarisation opens K channels, casuing K+ efflux (k ions move out) and repolarisation |
| Charge seperation | Charge (Q) measured in colombs= capactance(C) X voltage(V) Then the amount needed to seperate charge can be calculated using Faraday's constant: Each mole of a monovalent ion has 10-5 coloumbs of charge, so charge required= charge/10-5 - However, |
| Propogation of action potential | 1. Sodium channels open, inside of the membrane becomes positive 2. Local current circuit can now flow from positive to negative across the axon. 3. Increase in positivity at the 'foot' of the AP, sufficient to kickstart the opening of more sodium channels, allowing AP to propogate along the nerve |
| Propogation of action potential: path of injected current eg at the elbow | |
| Structure of myelinated nerve cells | - Axon is myelinated by a single schwann cell - purpose of myelination is to increase speed of conduction aka velocity - current enters at nodes on ranvier, flows down axon- depolarising it. Next node of ranvier is depolarised- salutatory conduction. - Takes time for channels to open so doesnt happen at the speed of light. |
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