Term
| Explain the current clamp method |
|
Definition
| You stick an intracellular electrode into the cell and then the Vm changes in accordance with Ohm's law. |
|
|
Term
| Why is the current clamp method not good for measuring Vm changes with drugs/signal transduction? |
|
Definition
| Because if a drug binds a g protein or whatever and this turns off K+ channels, then the cell could rapidly become less negative (depolarized), which could then open voltage gated K+ channels, or inward rectifiers. SO, you can't control Vm or Im (conductance) |
|
|
Term
| Why is the current clamp method not good for measuring Vm changes with drugs/signal transduction? |
|
Definition
| Because if a drug binds a g protein or whatever and this turns off K+ channels, then the cell could rapidly become less negative (depolarized), which could then open voltage gated K+ channels, or inward rectifiers. SO, you can't control Vm or Im (conductance) |
|
|
Term
| What is the voltage clamp method? |
|
Definition
| Two microelectrodes inserted into the cell. One of them measures Vm, one of them passes current into the cell. There is a command voltage set by the researcher, so you can measure how the voltage is changing. (ie how much current has to be applied to keep the voltage constant). The current passing into the cell through an electrode is equal and opposite to whatever is passing through the cell membrane at all times. |
|
|
Term
| Can the voltage clamp method be used in all cells? |
|
Definition
| No, only those that are large! |
|
|
Term
| What is current? What is the symbol for current? |
|
Definition
| amount of charge moving past per unit time. Im |
|
|
Term
| What is voltage? What is the symbol for the membrane? |
|
Definition
| Vm. This is the difference between two potentials (like on each side of the membrane) |
|
|
Term
|
Definition
| This is like when you use a voltage clamp and you are recording the voltage vs the current (Vm vs Im). The slope is the conductance (g). |
|
|
Term
| The the line on an V-I plot is straight, what does that mean? |
|
Definition
| It means the conductance does not change. |
|
|
Term
| On a V-I plot, which is what axis? |
|
Definition
| I is on y axis. V is on x axis. G is the slope |
|
|
Term
| Where would you find the equilibrium potential on a V-I plot? |
|
Definition
| The Vm where I=zero. This is the voltage where current is nonexistant. (obs, because equilibrium is where stuff doesn't move) |
|
|
Term
| Let's say you are doing voltage clamp with only K+ channels with constant conductance. At equilibrium, I=0 and Vm=-91. Which direction will K+ flow if the Vm becomes -70, -150? |
|
Definition
| If it becomes -70, K+ ions will flow out, because cell became depolarized, and K+ efflux inhibits depolarization. If it becomes -150, K+ will flow in because now the cell is too polarized. |
|
|
Term
| Which version of Ohm's law is used for I-V plots? |
|
Definition
|
|
Term
| What definition can be ascribed to outward and inward currents? |
|
Definition
Outward currents are positive to the Veq. Inward currents are negative to it.
"By definition, outward
currents have positive values, and inward currents have negative values." |
|
|
Term
| What would happen to an I-V plot if you took out half of the K+ channels with a drug? |
|
Definition
| The conductance (g) would go in half. However, the Vk would not change. This translates to a shallower slope, but the same x int. (if you doubled the channels, steeper line, but same intercept. |
|
|
Term
| Why is it that a change in conductance will not change the Vk (or Veq)? |
|
Definition
| Because no matter how many channels you have, the amount of current flowing at equilibrium is still zero. |
|
|
Term
| Would the Y int change if you change conductance? |
|
Definition
| Yes. It is only the Xint that changes |
|
|
Term
| What would an inward rectifier's graph look like? (think Gm on y axis and Vm on x axis |
|
Definition
| Well, at polarized points, the K+ influx keeps the channels open by knocking the Mg+ out of the channels. SO, you'd have very low conductance until close to the point where Vm=Vk. At that point, there would be rapid increase in conductance. |
|
|
Term
| What would an inward rectifier's I-V graph look like in terms of slope? |
|
Definition
| Well, because the conductance changes, the slope would NOT be constant. Somewhere near about -70 or -90 there would be a very rapid increase in slope. At more positive numbers (ie -50) the slope would be pretty low, because conductance is low. |
|
|
Term
| What would the inward rectifier graph look like in terms of x int? |
|
Definition
| The X int would be -91, because that is the point where there is no flow (no current). |
|
|
Term
| What would inward rectifier graph look like in terms of Im? |
|
Definition
| At low values of Vm, Im would be low. This is because negative Im (current) means influx of K+, and at hyperpolarized states (very negative Vm) K+ likes to rush in. [Conversely, when Vm is high--depolarized--the Im would be positive, because K+ wants to efflux to inhibit depolarization. |
|
|
Term
| Tell me about a G vs Vm curve for a voltage gated channel and why |
|
Definition
| Well, you'd have high conductance at high Vm, because this is what would open up a voltage gated K+ channel (the leaflets wouldn't be attracted to the cytosol anymore because its less negative. |
|
|
Term
| Tell me about a I-V curve for voltage gated channel in reference to the slope |
|
Definition
| Well, at high Vm, you'd have a steeper slope. At low Vm, you'd have a slope that is practically flat. |
|
|
Term
| Tell me about I-V plot for voltage gated in reference to the current. |
|
Definition
| Current would be positive for high Vm and zero for low Vm. This is because at high Vm, the channels are open. And they are closed the rest of the time. |
|
|
Term
| How would a voltage gated Na+ channel IV plot look a bit different from K+? |
|
Definition
| Well, there would be a transition from negative to positive current. This is because voltage gated Na+ and Ca+ channels are not unidirectional. Things can flow in either direction. |
|
|
