Excitation conduction and contraction

The electrical excitation of the heart muscle is effected by a cardiac conduction system, which, as with smooth muscles, is based on the presence of spontaneously discharging (depolarizing) pacemaker cells. The first instance of this system is the so-called sinus node, the primary pacemaker. Here, the heart rate is set at about 60 to 80 beats per minute for healthy people.

From the sinus node, the excitation is transmitted to the muscles of the two atria. These contract and transmit the excitation to the AV node, which is located between the atria and the ventricles. After a moment of delay in this node, the excitation is finally transmitted to the heart muscle of the ventricles via the His bundle, the Tawara legs and finally the Purkinje fibers.

This transmission also occurs via gap junctions and not via special nerve fibers. The excitation causes the chambers of the heart to contract and thus empty the blood that remains in them into the adjacent vessels. So you can distinguish between two different phases of each heartbeat: There is the diastole, in which the heart muscles of the chambers relax and the cavities fill with blood.

This is always followed by systole, in which the muscle cells of the heart chambers tense up and build up such a high pressure that the blood can finally be pumped out of the heart. If there are short-term fluctuations in blood pressure (for example, if you suddenly stand up after lying down for a long time and the blood pressure drops relatively sharply due to the fact that the blood initially sinks into the legs), the heart muscle can generally adjust its activity itself first, without having to switch on the brain stem or the autonomic nervous system. This is achieved by the so-called Frank-Starling mechanism, which is based on the prefilling of the heart and the afterload, i.e. the pressure in the downstream vessels into which the blood is to be pushed.