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3 Fundamentals of the Formation of Biosignals

Fig. 3.12: Action potential after external excitation of a muscle cell in the ventricle according to the

model of Beeler and Reuter (top) [2], action potential according to the model of Yanagihara for the

sinus node (SA) (middle) [87], and action potential according to the model of Noble for the Purkinje-

fibres (bottom) [54].

The action potential of the cells in the Purkinje-fibres as well as in the muscle fibres

takes much longer: 300–400 ms compared to 3 ms in the large nerve cell of an oc-

topus, which can be described according to the Hodgkin-Huxley-equations. In other

words, the cells of the sinus node produce an oscillation with the highest frequency,

on average at 60 to 80 oscillations periods per minute. The oscillation of the AV node

is slightly lower (40 to 50 per minute), and the cells in the His bundle and in the

Purkinje-fibres have the lowest frequency of about 39 to 40 oscillations per minute.

The associated action potentials are shown in Figure 3.12.

Since the heart does not beat at a combination of these oscillatory frequencies

in the non-pathological case, the sinus node synchronises the AV node and the latter

synchronises the excitation by the His bundle and the Purkinje-fibres. There exists a

certain redundancy; if, for example, the sinus node fails, the heart can still continue to

beat, but at the lower frequency of the AV node. If the AV node also fails, it can still con-

tinue to work with the excitation from the His bundle and the Purkinje fibres, although

the heartbeat is then even lower. The interaction of the individual pacemaker centres

(SA node, AV node, HP complex) can be described by three oscillators, whereby coup-