Biomedical Engineering Reference
In-Depth Information
the inside has become more positive or the outside of the cell has become more negative. Any negative
change in
V
m
is called a
repolarization
. Any time the membrane voltage drops below
V
rest
m
is known as
hyperpolarization
.
V
rest
Hyperpolarization
Figure 2.2:
Depolarization, repolarization, and hyperpolarization of
V
m
.
2.1.2 Cellular Currents
The
transmembrane current
,
I
m
, is a measure of the movement of ions across the membrane and is typically
measure in units of
μA/cm
2
. By definition, a positive
I
m
is positive charge leaving the cell. Therefore,
a positive
I
m
will repolarize the cell membrane voltage. The transmembrane current is the sum of four
currents
I
m
=
I
cm
+
I
ion
+
I
syn
−
I
stim
.
(2.2)
The
capacitive current
,
I
cm
, is a result of the natural capacitance of the cell membrane. Recall that a
capacitor is nothing more than an insulator sandwiched between two conductors. The cell membrane
is composed of lipids which are a natural isolator. The intra and extracellular solutions are nothing
more than salty water and therefore are good conductors. We will examine the role of the membrane
capacitance,
C
m
, in more detail in Sec. 2.1.4.
The
ionic current
,
I
ion
, is the primary way for ions to cross the membrane.
I
ion
may be composed
of many different currents (carrying different ions) summed together. We will consider
I
ion
in greater
detail in Ch. 3.
The
synaptic current
,
I
syn
, is any one of hundreds of specialized currents that allow neurons to
communicate chemically across a synapse. We will consider
I
syn
in Ch. 6.
The
stimulus current
,
I
stim
, is an externally applied current and will be considered in more detail
in Sec. 2.2. Note that the sign of
I
stim
is opposite of the other currents.
2.1.3 Membrane Circuit Analog
It is customary to use circuit analogies to describe the relationship between voltages and currents. In
Fig. 2.3, the top node represents the inside of the cell and the bottom node represents outside of the cell.
