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3 Fundamentals of the Formation of Biosignals
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intracellular space
CLDS
RLDS
RKT
extracellular space
ce
Na+
ce
K+
ce
Cl−
ci
Na+
ci
K+
ci
Cl−
Fig. 3.6: Schematic representation of a cell membrane with an embedded, open channel protein
(green) and an outward facing transport protein (blue): compared to closed channels, the variable
sum resistance due to the embedded channel and transport proteins RKT is much lower than the
membrane resistance RLDS.
though not exactly identical to the analogue components of electrical engineering,
they deliver quite good predictions about the temporal course of the currents and po-
tentials in the cell. Thus for simplicity, electrical equivalent circuit diagrams are used
as models. They consist of simple electrical components, such as resistors, batteries
and capacitors and thus allow a prediction of the measurement results of an electro-
physiological experiment. According to the model of Hodkin and Huxley¹⁶ electrical
properties of the cell membrane can be represented by the electrical equivalent circuit
(cf. Figure 3.7).
As shown in Figure 3.5 ff. the cell membrane is represented by a combination of
the individual conduction channels for Na+, K+, Cl−(for the nerve cell) and addition-
ally Ca2+ (for the cardiac muscle cell), the membrane capacity CLDS and the respective
intracellular space
CL DS
UL DS
R L DS
R N a+
UN a+
extracellular space
UK +
R K +
UC l−
R C l−
intracellular space
CL DS
UL DS
R L DS
R N a+
UN a+
extracellular space
UK +
R K +
UC a2+
R C a2+
Fig. 3.7: Electrical equivalent circuit based on Hodkin and Huxley’s cell membrane model [29] for
the nerve cell (left) and the cardiac muscle cell (right): Ion channels are represented by the variable
resistances RNa+, RK+ and RCl−(nerve cell, left) and additionally RCa2+ (cardiac muscle cell, right).
The membrane wall is represented by a capacitance CLDS and the leakage current resistance RLDS.
The electrochemical grandient of an ion species A (cf. Figure 3.9) generates a corresponding current
flow IA through the respective ion channels.
16 Alan Lloyd Hodgkin (1914-1998) and Andrew Fielding Huxley (1917-2012), Nobel prize winners of
physiology for their discoveries about the ionic mechanism involved in excitation/inhibition of the
nerve cell membrane.