Automation of the heart is a rhythmic contraction of an organ under the influence of impulses that arise in it without the influence of stimuli from the outside. Automation is inherent in the whole organ and individual parts, but not in the heart muscle. There is evidence of this phenomenon - the rhythmic contractions of the organ of animals and humans, isolated from everything and taken out of the body.
First order pacemakers
When defining what is meant by automatism of the heart, it was found that nerve impulses can be generated in cells of atypical myocardium. If a person is he althy, then this process is observed near the sinoatrial node due to the difference in properties and structure of cells from other structural components. They are clustered, 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 ininterstitial fluid, not having time to be taken out.
In addition, the characteristic properties are as follows:
- Quite high permeability for calcium and sodium ions.
- Small membrane potential.
Due to the difference in the concentration of sodium and potassium, there is a slight activity of the functioning of the sodium-potassium pump.
Research on automatism of the heart
For quite a long time, the automatism of the heart has not been fully investigated, even despite the increased interest of scientists in this process. The Stannius ligature method is a well-known cycle of experiments based on the removal of some parts of the frog's heart by applying bandages. As a result, it turned out that there are at least 2 centers of automation in the organ.
One of them is located in the region of the venous sinus, contributes to the rhythmization of contractions, the second is located in the part between the ventricle and the atria (it is also called hidden). His work begins only after 1 center is excluded. The heart muscle, which is distant from both centers, works - contracts - independently. Thus, the automaticity of the human heart is associated with impulses emanating from these centers.
Landergorf method
In order to reduce the out-of-body heart, the Landergorf method is used. The meaning is:
- The heart is cut out and a cannula is inserted into the aorta, which is connected to a glass vessel.
- The vessel is pouredRinger's solution along with glucose, or possibly adding defibrinated blood.
- The solution is saturated with oxygen and heated to a certain temperature (about 48 degrees Celsius).
- Fluid begins to flow under pressure into the aorta, the valves close, and the fluid is directed to 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 automatism of the heart. Using this method, it is possible to bring back the impulses of a heart that has already stopped a few hours ago. At the beginning of the 20th century, for the first time, it was possible to revive the organ of a small child, and later they restored the work of the heart, which had not functioned for almost 48 hours. After passing the solution through the vessels, the heartbeat persisted for about 15 hours.
Description of the automation process
The automatism of the human heart begins with the phase of diastole, its manifestation is the movement of sodium into the cell. In this case, the membrane potential decreases significantly, the value tends to the minimum level of depolarization. The membrane charge decreases, and a slow depolarization of the diastole begins. Channels for calcium and sodium open in the phase of rapidly flowing depolarization, ions begin to actively move towards the cell. As a result, the charge first sharply decreases and reaches zero, after which it is replaced by the opposite. Sodium moves until equilibrium is reached in its ions (electrochemical).
The plateau phase is coming. Here the movement of calcium continues. The tissue of the heart remains unexcitable at this moment. When equilibrium is reached for the corresponding ions, the phase ends and repolarization occurs, which means the return of the membrane charge to its original level.
Knots of automatism of the heart
A special place in the complex process is occupied by the nodes of the automatism of the heart. The node of the first order is called the sinoatrial node. It is a first-order pacemaker that ensures a normal heart rate. It is located near the confluence of the superior vena cava. Its structure is a small number of heart muscle fibers with neural endings. The node of the second order is called the atrioventricular node. It is a hidden second-order pacemaker. The node of the third order is represented by the cells of the conducting ventricular system.
All lower order pacemakers maintain the organ's rate of contraction if complete heart block is present. At the same time, the frequency of ventricular contractions approaches the minimum mark, and patients are implanted with an electrical type pacemaker, that is, an artificial pacemaker.
The emergence of potentials
The potential of the sinoatrial node differs from the usual one by a smaller amplitude - by 50 mV. In the normal state, potentials appear in the node due to the presence of cells that are pacemakers of the first order. The rest of the heart departments, under certain conditions, also generate nerve impulses when an additionalstimulus, as well as turning off the node of the first order. In this case, the generation of pulses in the node of the second order is observed (the frequency is about 60 times/min). When stimulated in the node, the cells of the His bundle are excited, the frequency decreases to 30 (third-order pacemakers).
The action potential of all pacemakers is directly proportional to high membrane permeability to calcium and sodium ions, as well as to a decrease in the permeability of potassium ions.
Automatic gradient
The automatism of the heart under normal conditions of all parts of the system is suppressed by the sino-arterial node, "imposing" its own rhythm. For this reason, all components of the system, with their own rhythm, are reorganized to work at the same pace. Gradient of automatism of the heart is a phenomenon in which the ability to automate decreases with distance from the place of generalization of impulses, that is, the node of the first order.
It is still unknown what causes the abrupt change in cellular charge that occurs spontaneously. The automatism 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 the metabolic processes in these driver cells, which are able to change the state of the surface membranes.
