Voltage Clamp Answers

1.

If resistance decreases and current stays the same, then voltage also decreases so that the result of V/R stays the same - I. So if R decreases and I is constant, V decreases.

2.

In this case, V is held constant while R is decreasing. If R decreases, then I must increase to keep V constant. So if V is constant while R is decreasing, then I must increase.

3.

If you can measure I and V, then R can be easily calculated by re-arranging the equation to:

4.

This is the same as question 2. If voltage is constant and resistance decreases, what happens to current? Current must increase if voltage remains constant when resistance deceases. You can see this in the figure on the right - ion channels open and positively charged ions can flow into the cell.

5.

In questions 3 & 4, you changed the voltage across the membrane and held it there. This is called a "Voltage Clamp." The resistance changed because of the opening of channels, which allowed ions to move. The amount of current it takes to hold the membrane at this new voltage (against the flow of these ions) is directly related to the number of channels opened.

That is, when channels open due to this change in voltage, ions will move across the membrane. It takes an equal amount of current in the opposite direction to hold the voltage at the new potential. By injecting this current with a current elctrode and measuring the current injected you can estimate how many channels are opened.

(Note: It may seem a little bit confusing that the figure above shows current going into the electrode but it's described as "injecting" current. This is because of the way electrical current is defined, i.e. as the movement of positive charges. In the membrane, positive ions are moving into the cell, so current is coming in. The current electrode is injecting electrons, and since electrons are negative charges, current in the electrode is defined as going into the current electrode - the opposite direction of the flow of negative charges. So this outward electrical current balances the inward ionic current.)