Ohm's Law

 

For electrophysiology, perhaps the most important law of electricity is Ohm's law. The potential difference between two points linked by a current path with a conductance G and a current I is:

 

U = I.R (units: volts) = Ohm's Law

 

This concept applies to any electrophysiological measurement, as illustrated by the two following examples:

 

(1) In an extracellular recording experiment:

the current I that flows between parts of a cell through the external resistance R produces a potential difference Delta V,which is usually less than 1 mV. As the impulse propagates, I changes and, therefore, Delta V changes as well.

 

 

 

In extracellular recording, current I that flows between points of a cell is measured as the potential difference ("IR drop") across the resistance R of the fluid between the two electrodes.

 

(2) In a voltage-clamp experiment:

 

when N channels, each of conductance g, are open, the total conductance is N . g.

The electrochemical driving force Delta V (membrane potential minus reversal potential) produces a current N.g.DeltaV.

As channels open and close, N changes and so does the voltage-clamp current I.

Hence, the voltage-clamp current is simply proportional to the number of open channels at any given time.

Each channel can be considered as a g conductance increment.