Axon Axopatch 200B, Theory Of Operation

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PRINCIPLES OF OPERATION • 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 pote n tial (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.