160
Biomedical Signal and Image Processing
is always negative. For instance, the muscle has a rest transmembrane potential of
approximately −90 mV, and the nerve axon has on average a transmembrane potential
of −70 mV.
8.3 ELECTRIC CHARACTERISTICS OF CELL MEMBRANE
Considering the fact that the cell membrane has charges on either side and that there
is an ion current flowing through the membrane, the cell membrane can be regarded
as an electric circuit. As with any electric conductor, there will be electric resistance
and capacitance to identify. One can also consider the Nernst potential as a battery in
the cell membrane circuit. These electric characteristics allow modeling the cellular
activities using its equivalent electronic circuits.
We start this section by defining the membrane resistance.
8.3.1 MEMBRANE RESISTANCE
The membrane forms a resistance for each of the ions passing through it. The membrane resistance is defined as the inverse of the conductance, which, in this case, represents the ease with which the ions can pass through the channels in the membrane.
The conductance is in fact a function of the ion concentration and the ion flow. If the
conductance for one individual ion channel is G i , the total resistance of the membrane
of one single cell involves all the N individual channels summarized as follows:
G = ∑ G i
(8.6)
N
The conductance is related to the permeability but is not interchangeable. Since the
resistance, R, is defined as the reciprocal of the conductance, the value can now be
recalculated as
1
1
R = =
(8.7)
G
∑
N
G i
i=1
Equation 8.7 equates to a set of parallel channel resistors in a cell circuit. The unit
for resistance is Ohm (Ω).
8.3.2 MEMBRANE CAPACITANCE
As mentioned earlier, the fact that there are ions dissolved in the intra- and extracellular liquid lined up on either side of the membrane makes the membrane a capacitor.
The charges are separated by the thickness of the membrane, which is only a few
molecular chains thick and is in the order of 7.5 nm.
The capacitance is defined in Equation 8.1. Generally speaking, the capacitance
of a typical cell membrane is relatively high. This is due to the fact that the thickness
Biomedical Signal and Image Processing
is always negative. For instance, the muscle has a rest transmembrane potential of
approximately −90 mV, and the nerve axon has on average a transmembrane potential
of −70 mV.
8.3 ELECTRIC CHARACTERISTICS OF CELL MEMBRANE
Considering the fact that the cell membrane has charges on either side and that there
is an ion current flowing through the membrane, the cell membrane can be regarded
as an electric circuit. As with any electric conductor, there will be electric resistance
and capacitance to identify. One can also consider the Nernst potential as a battery in
the cell membrane circuit. These electric characteristics allow modeling the cellular
activities using its equivalent electronic circuits.
We start this section by defining the membrane resistance.
8.3.1 MEMBRANE RESISTANCE
The membrane forms a resistance for each of the ions passing through it. The membrane resistance is defined as the inverse of the conductance, which, in this case, represents the ease with which the ions can pass through the channels in the membrane.
The conductance is in fact a function of the ion concentration and the ion flow. If the
conductance for one individual ion channel is G i , the total resistance of the membrane
of one single cell involves all the N individual channels summarized as follows:
G = ∑ G i
(8.6)
N
The conductance is related to the permeability but is not interchangeable. Since the
resistance, R, is defined as the reciprocal of the conductance, the value can now be
recalculated as
1
1
R = =
(8.7)
G
∑
N
G i
i=1
Equation 8.7 equates to a set of parallel channel resistors in a cell circuit. The unit
for resistance is Ohm (Ω).
8.3.2 MEMBRANE CAPACITANCE
As mentioned earlier, the fact that there are ions dissolved in the intra- and extracellular liquid lined up on either side of the membrane makes the membrane a capacitor.
The charges are separated by the thickness of the membrane, which is only a few
molecular chains thick and is in the order of 7.5 nm.
The capacitance is defined in Equation 8.1. Generally speaking, the capacitance
of a typical cell membrane is relatively high. This is due to the fact that the thickness