Spontaneous depolarization is characterized by a spontaneous reduction in membrane potential at the cell membrane. Depolarization serves to transmit electrical impulses from nerve or muscle cells. Thus, the pacemaker position of the sinus node is based on spontaneous depolarization of cardiac muscle cells.
What is spontaneous depolarization?
A spontaneous depolarization is characterized by a spontaneous reduction in membrane potential at the cell membrane. Spontaneous depolarizations occur mainly at the sinus node of the heart. The sinus node is the primary stimulation center of the heart. There, a spontaneous depolarization of the membrane potential occurs once per second. This transmits electrical impulses to the working myocardium of the atria. These initiate the contraction of the heart. The basis of the processes is a resting potential at the cell membranes. There is an electrical potential between the interior and exterior of the cell, which is established by certain ion concentrations. When external stimuli are applied, neurotransmitters are activated, which in turn lead to a short-term reduction in the resting potential. In this process, sodium and calcium ions flow into the cells via ion channels, while potassium ions are transported from inside the cell to the outside. These ion transports are accomplished by ion pumps activated by neurotransmitters. Stimulus transmission in nerve and muscle cells is based on the change in electrical potential due to depolarization and polarization. Thus, after the resting potential is decreased by depolarization and the stimulus is transmitted, the normal resting potential rebuilds.
Function and task
Spontaneous depolarizations are important for excitation of the heart muscle, as mentioned earlier. Spontaneous depolarizations take place constantly at the so-called sinus node to transmit electrical impulses to the heart muscle. This results in contraction of the heart muscle, which leads to the heartbeat. With each heartbeat, several liters of blood are pumped through the body. The sinus node is located in the area of the right ear of the heart. It has a muscular structure with three outgoing fiber bundles. Sympathetic and parasympathetic nervous systems control the heart rhythm via the sinus node. The nodal cells in turn have the ability to depolarize spontaneously. Per minute, they usually give 60 to 80 heartbeats. For the sinus node, the special feature is that immediately after depolarization, repolarization takes place again. The initial potential is thus immediately restored, to be immediately depolarized again. This ensures regular cardiac activity. Responsible for the immediate depolarization are the so-called HCN channels. They open by hyperpolarization and induce the influx of sodium ions. Hyperpolarization refers to an excessive polarization of the cell membrane, which occurs immediately after each depolarization. In addition, HCN channels are modified by cyclic nucleotides. HCN channels play a major role especially for heart and brain to ensure rhythmic activity. However, they are also found in the retina, in the taste buds of the tongue, or in sperm. On the tongue, HCN channels respond to acidic stimuli. In this case, they open immediately, thereby amplifying the acid signal.
Diseases and ailments
Various diseases can occur in connection with spontaneous depolarizations. These include cardiac arrhythmias as well as neurological diseases such as epilepsy. For example, in epilepsy, the depolarization behavior of nerve cells changes. This results in overexcitation, which manifests itself in an epileptic seizure. Very strong discharges can cause disturbances in certain areas of the brain that affect motor function, consciousness, or even thinking. Depolarization properties are also affected by certain drugs. These include muscle relaxants administered for spinal paralysis. This can result in permanent depolarization, causing uncoordinated muscle tremors. However, other drugs can also cause hyperexcitability. If the sinus node is impaired, the so-called sick sinus syndrome can develop. Sick sinus syndrome refers to a number of autonomic cardiac arrhythmias that originate in a malfunction of the sinus node.This can be caused by damage to the sinus node tissue that occurs in the context of coronary artery disease, myocarditis or cardiomyopathy. Sometimes an overdose of antiarrhythmic drugs such as beta blockers is also responsible for sick sinus syndrome. Both tachycardia and bradycardia may occur. Tachycardia is manifested by an excessive increase in heart rate and causes palpitations. In contrast, bradycardia is characterized by a decrease in heart rate. The most severe form of bradycardia is atrial fibrillation. Combined clinical pictures with phases of tachycardia and bradycardia also occur frequently. In these disorders, coordinated depolarizations no longer occur. Individual cardiac muscle cells are discharged independently and involuntarily. Depolarizations can also occur during physiological resting phases if incorrect fiber connections exist. In addition to organic causes of cardiac arrhythmias, incorrect medication dosages as well as hectic activity and stress can also lead to them. Since irregular impulses are generated, often only a pacemaker can support the function of the sinus node. This sends out regular electrical signals that again generate ordered depolarizations. If no clinical symptoms occur, the use of a pacemaker is not always necessary. In some cases, psychotherapeutic measures can help normalize the heart rate and heart rhythm.
