The work of the organs and tissues of our body dependsfrom many factors. Some cells (cardiomyocytes and nerves) depend on the transmission of nerve impulses generated in special components of cells or nodes. The basis of the nerve impulse is the formation of a specific excitation wave, called the action potential.
The action potential is usually called the waveexcitation, moving from cell to cell. Due to its formation and passage through the cell membranes, a short-term change in their charge occurs (normally the inner side of the membrane is negatively charged and the outer side is positively charged). The formed wave contributes to a change in the properties of the ion channels of the cell, which leads to recharging of the membrane. At the moment when the action potential passes through the membrane, a short-term change in its charge occurs, which leads to a change in the properties of the cell.
The formation of this wave underlies the functioning of the nerve fiber, as well as the system of ways of holding the heart.
If there is a violation of its education, many diseases develop, which makes the determination of the action potential necessary in a complex of medical and diagnostic measures.
How is the potential of action formed and what is characteristic for it?
The study of the occurrence of excitation in cells andfibers was started quite a long time ago. His first appearance was noticed by biologists who studied the effect of various stimuli on the bare tibial nerve of a frog. They noticed that when exposed to a concentrated solution of edible salt, muscle contraction was observed.
Further studies were continuedneurologists, but the main science after physics, studying the potential of action - physiology. It was physiologists who proved the existence of an action potential in the cells of the heart and nerves.
With the deepening of the study of potentials, the presence and potential of rest was proved.
Since the beginning of the 19th century, methods have been created,allowing to fix the presence of these potentials and measure their magnitude. Currently, the fixation and study of action potentials is carried out in two instrumental studies - the removal of electrocardiograms and electroencephalograms.
The formation of excitation is due tochanges in the intracellular concentration of sodium and potassium ions. Normally, the cell contains more potassium than sodium. The extracellular concentration of sodium ions is much greater than in the cytoplasm. Changes caused by the action potential contribute to a change in charge on the membrane, resulting in a current of sodium ions inside the cell. Because of this, the charges vary outside and inside the cell (the cytoplasm is charged positively, and the external environment is negative.
This is done to facilitate the passage of the wave through the cell.
After the wave was transmitted through the synapse,there is a reverse charge recovery due to the current inside the cell of negatively charged chloride ions. The initial levels of the charge are restored outside and inside the cell, which leads to the formation of a resting potential.
Periods of rest and excitement alternate. In a pathological cell, everything can happen differently, and the formation of PDs there will be subject to somewhat different laws.
The flow of the action potential can be divided into several phases.
The first phase proceeds until a criticalthe level of depolarization (the passing action potential is stimulated by a slow discharge of the membrane, which reaches a maximum level, usually about -90 mEv). This phase is called a pre-spike. It is carried out due to the entry of sodium ions into the cell.
The next phase - the peak potential (or spike), forms a parabola with an acute angle, where the ascending part of the potential means the membrane depolarization (fast), and the descending part - repolarization.
The third phase - the negative trace potential - shows the trace depolarization (the transition from the peak of depolarization to the state of rest). Due to the entry of chloride ions into the cell.
At the fourth stage, the phase of the positive trace potential, the charge levels of the membrane return to the original one.
These phases, due to the action potential, strictly follow one after another.
Undoubtedly, the development of the action potential hasimportant in the functioning of certain cells. In the work of the heart, excitation plays a major role. Without it, the heart would simply be an inactive organ, but by spreading the wave across all cells of the heart, its reduction occurs, which promotes blood circulation along the vascular bed, enriching it with all tissues and organs.
The nervous system also could not be normalperform its function without the potential action. The organs could not receive signals to perform a particular function, as a result of which they would be simply useless. In addition, the improvement of the transmission of nerve impulse in nerve fibers (the appearance of myelin and the interceptions of Ranvier) made it possible to transmit the signal in a matter of a fraction of a second, which caused the development of reflexes and conscious movements.
In addition to these systems of organs, the action potential is also formed in many other cells, but in them it plays a role only in the cell's performance of its specific functions.
The main body, whose work is based onprinciple of the formation of the action potential, is the heart. Due to the existence of nodes for the formation of pulses, the work of this organ is performed, the function of which is to deliver blood to tissues and organs.
Generation of action potential in the heart occursin the sinus node. It is located at the point of confluence of the hollow veins in the right atrium. From there, the pulse propagates through the fibers of the conduction system of the heart - from the node to the atrioventricular junction. Passing through the bundle of His, or rather, by his legs, the impulse passes to the right and left ventricles. In their thickness there are smaller paths of conduct - Purkinje fibers, through which excitation reaches each heart cell.
Potential action of cardiomyocytes iscompound, i.e. depends on the reduction of all cells of the heart tissue. If there is a block (a scar after a heart attack), the formation of the action potential is broken, which is fixed on the electrocardiogram.
How does it form a PD in neurons - cells of the nervous system. Here everything is a little easier.
External impulse is perceived by the processes of the nervouscells - dendrites associated with receptors located both in the skin and in all other tissues (rest potential and action potential also change one another). Irritation provokes the formation of the action potential in them, after which the impulse through the body of the nerve cell goes to its long process - the axon, and from it through the synapses - to other cells. Thus, the generated excitation wave reaches the brain.
A feature of the nervous system is the presencetwo types of fibers - coated with myelin and without it. The occurrence of the action potential and its transmission in those fibers where there is myelin, is much faster than in demyelinated.
This phenomenon is observed due to the fact thatdistribution of PD on myelinated fibers occurs due to "jumps" - the pulse jumps the myelin patches, which as a result reduces its path and, accordingly, speeds up the spread.
Without the development of a resting potential, there would be noaction potential. The potential of rest is understood as the normal, unexcited state of the cell, under which the charges inside and outside the membrane are significantly different (that is, the outside of the membrane is positively charged and inside it is negative). The rest potential shows the difference between the charges inside and outside the cell. Normally, it is from -50 to -110 meV. In nerve fibers this value is usually -70 meV.
It is caused by the migration of chloride ions into the cell and the creation of a negative charge on the inner side of the membrane.
When changing the concentration of intracellular ions (as mentioned above), the PP replaces the PD.
Normally, all cells of the body are inunexcited state, so the change of potentials can be considered a physiologically necessary process, since without them the cardiovascular and nervous systems could not perform their activities.
Potential of rest and action potential allow to determine the state of the organism, as well as individual organs.
Fixation of the action potential from the heart(electrocardiography) allows you to determine its state, as well as the functional capacity of all its departments. If you study the normal ECG, you can see that all the teeth on it are a manifestation of the action potential and the subsequent resting potential (respectively, the appearance of these potentials in the atria reflects the tooth P, and the propagation of excitation in the ventricles is the tooth R).
As for the electroencephalogram, then on itthe emergence of various waves and rhythms (in particular, alpha and beta waves in a healthy person) is also due to the occurrence of action potentials in neurons of the brain.
The given researches allow to reveal in time the development of this or that pathological process and cause practically up to 50 percent of the successful treatment of the initial disease.</ p>>