Automation of the heart is a rhythmic contraction of an organ under the influence of impulses arising in it without external stimuli. Automation is inherent in the entire organ and individual parts, but not the heart muscle. There is evidence of this phenomenon - rhythmic contractions of the organ of animals and humans, isolated from everything and taken out of the body.
First-order pacemakers
In determining what is meant by heart automation, it was found that nerve impulses are able to be generated in atypical myocardial cells. If a person is healthy, then this process is observed next to the sinoatrial node due to the difference in cells in properties and structure from other structural components. They are arranged in groups, spindle-shaped, and surrounded by a basement membrane. The second name of these cells is first-order pacemakers (pacemakers). The metabolic processes in them proceed at a high speed, and for this reason the metabolites remain in the intercellular fluid, not having time to endure.
In addition, the characteristic properties are as follows:
- Quite high permeability for calcium and sodium ions.
- A small amount of membrane potential.
Due to the difference in the concentration of sodium and potassium, a slight activity of the sodium-potassium pump is observed.
Heart Automation Study
For quite a long time, the automatism of the heart was not fully explored, even despite the increased interest of scientists in this process. The Stannius ligature method is a well-known series of experiments conducted on the basis of removing parts of a frog’s heart by applying bandages. As a result, it turned out that at least 2 automation centers are present in the organ.
One of them is located in the venous sinus, contributes to the rhythm of contractions, the second is located in the part between the ventricle and atria (it is also called hidden). His work begins only after 1 center is excluded. The muscle of the heart, which is distant from both centers, works - contracts - on its own. Thus, the automation of the human heart is associated with impulses emanating from these centers.
Landerhorf technique
In order to reduce the heart that has been taken out of the body, the Landergorf method is used. The meaning is as follows:
- The heart is cut out and a cannula is inserted into the aorta, which connects to a glass vessel.
- Ringer's solution is poured into the vessel along with glucose, or the addition of defibrinated blood is possible.
- The solution is saturated with oxygen and heats up to a certain temperature (about 48 degrees Celsius).
- The fluid begins to flow under pressure into the aorta, the valves are closed, and the fluid is directed into the coronary arteries, the function of which is to feed the entire organ.
Under such conditions, the organ of an animal or a person is able to work for a long time, this is the automation of the heart. Using this method, you can return the pulses of the heart, which had already stopped a few hours ago. At the beginning of the 20th century, for the first time it turned out to revive the organ of a small child, and only later they restored the work of the heart, which did not function for almost 48 hours. After passing the solution through the vessels, the heartbeat persisted for about 15 hours.
Automation Process Description
Automation of the human heart begins with the diastole phase, its manifestation is the movement of sodium into the cell. The membrane potential is significantly reduced, the value tends to a minimum level of depolarization. The membrane charge decreases, and the slow depolarization of diastole begins. Channels are opened for calcium and sodium in the phase of rapidly proceeding depolarization, ions begin to be actively directed to the cell. As a result of this, the charge first sharply decreases and reaches the zero mark, after which it is replaced by the opposite. Sodium moves until a balance is reached in its ions (electrochemical).
The plateau phase is approaching. Here the movement of calcium continues. The tissue of the heart remains unexcited at this moment. When equilibrium is reached for the corresponding ions, the phase ends and repolarization occurs, which means that the membrane charge returns to its original level.
Heart Automation Nodes
A special place in the complex process is occupied by the nodes of the automation of the heart. The first-order node is called sinoatrial. This is a first-order pacemaker that provides a normal heart rate. Located next to the confluence of the superior vena cava. Its structure is a small number of heart muscle fibers with neural endings. A second-order node is an atrioventricular node. This is a hidden second-order pacemaker. A third-order node is represented by cells of a conducting ventricular system.
All lower-order pacemakers support the frequency of organ contractions if complete heart block is present. At the same time, the ventricular contraction rate approaches the minimum mark, and patients are implanted with an electric type pacemaker, that is, an artificial pacemaker.
Potential development
The potential of the sinoatrial node differs from the usual one in lower amplitude - by 50 mV. In a normal state, potentials appear in the node due to the presence of cells that are first-order pacemakers. Under certain conditions, the remaining cardiac departments also generate nerve impulses when an additional stimulus is turned on, as well as a first-order site is turned off. In this case, pulse generation is observed in the second-order node (frequency about 60 times / min). With irritation in the node, the cells of the bundle of His are excited, the frequency decreases to 30 (third-order pacemakers).
The action potential of all pacemakers is directly proportional to the high membrane permeability for calcium and sodium ions, as well as to a decrease in the permeability of potassium ions.
Gradient Automation
Automation of the heart under the usual conditions of all parts of the system is suppressed by the sinoarterial node, "imposing" its rhythm. For this reason, all the components of the system with their own rhythm are reconstructed to work at a uniform pace. The gradient of heart automation is a phenomenon in which the ability to automate decreases with distance from the place of generalization of impulses, that is, a first-order node.
It is still unknown what causes a sharp change in cell charge that occurs spontaneously. Automation of the heart may be associated with the content of acetylcholine in the pacemakers. Many scientists believe that the phenomenon is due to the peculiarities of metabolic processes in these driver cells, which can change the state of surface membranes.