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PRINCIPLES OF OPERATION
Axopatch 200B, Copyright 1997-1999, Axon Instruments, Inc.
Inward / Outward Current
In a cell generating an action potential, depolarization is caused by a flow of
positive sodium or calcium ions into the cell. That is, depolarization in this case is
caused by an inward current.
During intracellular current clamping, a depolarizing current is a positive current
out of the microelectrode tip into the interior of the cell. This current then passes
through the membrane out of the cell into the bathing solution. Thus, in
intracellular current clamping, a depolarizing (positive) current is an outward
current.
An inward sodium current flows in some cells after a depolarizing voltage step.
When the cell is voltage clamped, the sodium current is canceled by an equal and
opposite current flowing into the headstage via the microelectrode. Thus it is a
negative current. When two-electrode voltage clamping was first used in the early
1950's, the investigators chose to call the negative current that they measured a
depolarizing current because it corresponded to the depolarizing sodium current.
This choice, while based on sound logic, was unfortunate because it means that
from the recording instrument's point of view, a negative current is hyperpolarizing
in intracellular current-clamp experiments but depolarizing in voltage-clamp
experiments.
To prevent confusion, Axon Instruments has decided to always use current and
voltage conventions based on the instrument's perspective. That is, the current is
always unambiguously defined with respect to the direction of flow into or out of
the headstage. Some instrument designers have put switches into the instruments
to reverse the current and even the command voltage polarities so that the
researcher can switch the polarities depending on the type of experiment. This
approach has been rejected by Axon Instruments because of the real danger that if
the researcher forgets to move the switch to the preferred position, the data
recorded on the computer could be wrongly interpreted. Axon Instruments believes
that the data should be recorded unambiguously.
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