3.2 Electrophysiology of the Heart

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3.2.1 General Excitation of Muscle Cells

The heart muscle pumps blood through the heart-circulatory-system, which is distrib-

uted throughout the body in pulse waves to supply the cells with the required nutri-

ents and oxygen for metabolism. Without oxygen, the cells could not fulfill their tasks

in the body. Because this transport is so important, the heart contains autonomous

pacemaker cells that control the excitation of the heart muscles on their own, without

the need for a connection to the central nervous system in the brain. For example, if

the heart were removed from the body and stored in a suitable nutrient fluid, it could

continue to beat without external stimulation from outside. Two types of heart cells

are distinguished:

1.

The cardiac muscle cell, like other muscle cells, can generate an action potential

when they are in turn excited by an external action potential. When excited, they

contract and reduce the size of the anterior and main chamber of the heart. The

resulting increase in internal pressure leads to the opening of the heart valves

and to the ejection of blood. An example of such an action potential is shown by

Figure 3.12. Unlike other muscle cells such as from skeletal muscles, the force with

which they contract does not depend on the strength of the excitation. A heart

muscle therefore does not contract more forcefully when the electrical stimulus

increases. So the all-or-nothing-principle [49] applies here, in contrast to the arm

muscle, whose strength can vary depending on the physical exertion.

2.

The pacemaker cells can also generate action potentials on their own without

external stimulation, as they do not have a constant resting potential. Rather,

after the relaxation phase, the potential grows independently until the excitation

threshold is reached and a new action potential is triggered. These cells are very

common:

–

in the sinus node (SA node) – in the right upper atrium,

–

in the atrioventricular node (AV node) – just before the transition from the

atrium to the main ventricle,

–

in the His bundle – in the ventricular limb after the AV node between the main

chambers and

–

in the Purkinje fibres – in the left and right walls of the main chamber after

the end of the ventricular thigh.

Because of their specific tasks, the action potentials of the heart cells show consid-

erable differences from the action potentials of the other nerve cells in the body, the

description of which is possible by the Hodgkin-Huxley-equations.

In the group of pacemaker cells, the cells in the SA node generate the clock-

determining action potentials for the entire heart. The other pacemaker cells syn-

chronise to this. After a delay, their action potential reaches the cells in the AV node

and from there to the cells in the His bundle and the Purkinje-fibres (HP complex).

The action potential coming from the sinus node is the quickest (see Figure 3.12).