PRINCIPLES OF OPERATION
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97
Chapter 8
Positive Potential
The term positive potential means a positive voltage at the headstage input with
respect to ground.
Transmembrane Potential
The transmembrane potential (V
m
) is the potential at the inside of the cell minus the
potential at the outside. This term is applied equally to the whole-cell membrane
and to membrane patches.
Depolarizing / Hyperpolarizing
The resting V
m
value of most cells is negative. If a positive current flows into the
cell, V
m
initially becomes less negative. For example, V
m
might shift from an
initial resting value of -70 mV to a new value of -20 mV. Since the absolute
magnitude of V
m
is smaller, the current is said to depolarize the cell (i.e., it reduces
the "polarizing" voltage across the membrane). This convention is adhered to even
if the current is so large that the absolute magnitude of V
m
becomes larger. For
example, a current that causes V
m
to shift from -70 mV to +90 mV is still said to
depolarize the cell. Stated simply, depolarization is a positive shift in V
m
.
Conversely, hyperpolarization is a negative shift in V
m
.
Whole-Cell Voltage and Current Clamp
Depolarizing / Hyperpolarizing Commands
In whole-cell voltage clamping, a positive shift in the command voltage causes a
positive shift in V
m
and is said to be depolarizing. A negative shift in the command
voltage causes a negative shift in V
m
and is said to be hyperpolarizing.
Transmembrane Potential
vs
. Command Potential
In whole-cell voltage clamp, the command potential controls the voltage at the tip
of the intracellular voltage-recording microelectrode. The transmembrane potential
is thus equal to the command potential.
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