Pacing potential is the action potential of the pacemaker cells in the heart. It is a prerequisite for a regular heartbeat and thus elementary for cardiac function.
What is the pacemaker potential?
Pacing potential is the action potential of the pacemaker cells in the heart. The normal heart rate at rest in a healthy person is between 50 and 100 beats per minute. This pulse is generated in special cells of the heart tissue. These are located in bundles in the sinus node. The sinus node is therefore also called the first pacemaker. It is located in the atrium of the heart in the area of the orifice of the superior vena cava. Together with the AV node, the Purkinje fibers, the Tawara bundles and the His bundle, the sinus node forms the excitation system of the heart. The rhythm created by the sinus node is also called sinus rhythm. If the sinus node fails, the other parts of the excitation formation system can also produce a heart rhythm. However, this usually fails more slowly than the sinus rhythm. The requirement for the heartbeat is the pacemaker potential of the pacemaker cells in the excitation formation system.
Function and task
Nerve cells and also the cells of the heart muscle have a constant resting potential. This ranges from -100 to -50 mV, depending on the cell type. In most nerve cells, a voltage difference of about -70 mV is present. The cell interior is thus negatively charged compared to the cell exterior. This resting potential is of fundamental importance for the conduction of excitation in the nerves, for the transport of substances through the cell membrane and for the control of muscle contractions. When a stimulus hits the corresponding cell, an action potential is formed. After this action potential, i.e. after an increase in the membrane potential, repolarization to the original resting potential occurs. Only when a stimulus hits the cell again can depolarization and thus an action potential be triggered again. The pacemaker cells, however, do not have a constant resting potential and are thus capable of spontaneous and autonomous excitation. In the pacemaker cells, excitation formation occurs through a so-called spontaneous depolarization. This depolarization is followed by the repolarization phase of the action potential up to a maximum diastolic potential (MDP). Subsequently, a continuous depolarization develops up to the threshold potential. Once again, an action potential develops. The excitation triggered by the action potential ultimately travels via the atrial muscles to another pacemaker center, the AV node. This is also a potential pacemaker. If the sinus node fails, it too can cause action potentials to maintain cardiac activity. However, as long as the sinoatrial node is functioning, the AV node merely transmits excitation to the His bundle, which is divided into a right and a left ventricular bundle. The ventricular bundle, also called the tawara bundle, runs toward the apex of the heart, where it branches off into the Purkinje fibers. With the help of this excitation conduction, both the atria and the ventricles can contract effectively so that blood can be pumped to the pulmonary or systemic circulation. Pacing potential is influenced by the autonomic nervous system. The sympathetic nervous system provides positive chronotropy. This means that the heart beats faster under the influence of the sympathetic nervous system. The reason for this is the neurotransmitter noradrenaline. The hormone adrenaline also activates the sinus node. The parasympathetic nervous system, on the other hand, has a negative chronotropic effect. It influences the pacemaker potential via the neurotransmitter acetylcholine.
Diseases and disorders
Various dysfunctions of the sinus node and the other pacemakers in the heart can result in cardiac arrhythmias. Cardiac arrhythmias resulting from sinus node dysfunction are grouped under the term sick sinus syndrome. The syndrome predominantly affects people over the age of 50. A complete failure of the sinus node is called sinus arrest. In this case, there is no pacemaker potential left. If no subordinate pacemaker area steps in, acute cardiac arrest occurs. If too many undirected electrical excitations run from the sinus node via the atria, rapid and disordered contractions of the walls occur.The frequency in this atrial fibrillation is between 350 and 600 contractions per minute. As a result, an irregular pulse develops. In the untreated state, this pulse is much too fast. As a result, the heart cannot work in a balanced manner, so that heart failure develops within a few days in predisposed patients. Due to the altered blood flow, more thrombi form in the atria. As a result, the risk of suffering a life-threatening embolism increases. However, a disturbance in the pacemaker potential can also result in bradycardia. In this case, the heart rate drops below 60 beats per minute. Patients with bradycardia may be symptom-free or suffer from reduced performance or even fainting. In the presence of heart failure, bradycardia can have fatal consequences. The opposite of bradycardia is tachycardia. This cardiac arrhythmia is also caused by a disturbance in the pacemaker potential. Tachycardia is defined as a sustained accelerated pulse of more than 100 beats per minute. Since the cause of a disturbed pacemaker potential is found in the atrium, this type of tachycardia is also referred to as supraventricular. Tachycardia is perceived by most patients as a racing or racing heart. The pulse is irregular and varies in strength. The irregular pulse can lead to a lack of blood flow to the heart or even to heart failure. The affected patients suffer from shortness of breath or angina pectoris. Dizziness may occur at very high pulse rates. Some patients also lose consciousness. Characteristically, many patients experience an increased urge to urinate after a tachycardia. Nowadays, disturbances in the pacemaker potential of the sinus node can be eliminated quite easily on a permanent basis with a pacemaker.
