The work of organs and tissues of our body depends on many factors. Some cells (cardiomyocytes and nerves) depend on the transmission of nerve impulses generated in special cell components or nodes. The basis of a nerve impulse is the formation of a specific excitation wave, called the action potential.
What it is?
The action potential is called an excitation wave that moves from cell to cell. Due to its formation and passage through 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 positive). 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 pathways of the heart.
If his education is disturbed, many diseases develop, which makes the determination of the action potential necessary in the complex of therapeutic and diagnostic measures.
How is the action potential formed and what is characteristic of it?
Research history
The study of the occurrence of excitation in cells and fibers was begun a long time ago. The first to notice it was the biologists who studied the effects of various stimuli on the bare tibial nerve of a frog. They noticed that when exposed to it with a concentrated solution of edible salt, muscle contraction was observed.
Further research was continued by neurologists, however, the main science after physics, studying the action potential is physiology. It was physiologists who proved the presence of action potential in heart cells and nerves.
As we went deeper into the study of potentials, the presence of resting potential was also proved.
From the beginning of the 19th century, methods began to be created to fix the presence of these potentials and measure their value. Currently, the fixation and study of action potentials is carried out in two instrumental studies - the removal of electrocardiograms and electroencephalograms.
Action potential mechanism
The formation of excitation occurs due to a change 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 higher than in the cytoplasm. Changes caused by the action potential contribute to a change in the charge on the membrane, as a result of which the flow of sodium ions into the cell is determined. Because of this, the charges outside and inside the cell change (the cytoplasm is positively charged, and the external environment is negatively charged.
This is done to facilitate the passage of the wave through the cell.
After the wave has been transmitted through the synapse, reverse charge recovery occurs due to the current of negatively charged chlorine ions into the cell. The initial levels of charge are restored outside and inside the cell, which leads to the formation of a resting potential.
Periods of rest and arousal alternate. In a pathological cell, everything can happen differently, and the formation of PD there will obey slightly different laws.
PD phases
The course of the action potential can be divided into several phases.
The first phase proceeds to the formation of a critical level of depolarization (a slow membrane discharge is stimulated by the passing action potential, which reaches the maximum level, usually it is about -90 meV). This phase is called pre-spike. It is carried out by entering 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 depolarization of the membrane (fast) and the descending part means repolarization.
The third phase - negative trace potential - shows trace depolarization (transition from a peak of depolarization to a state of rest). Due to the entry of chlorine ions into the cell.
At the fourth stage, the phase of the positive trace potential, the charge levels of the membrane return to the initial one.
These phases, due to the action potential, strictly follow one after the other.
Action potential functions
Undoubtedly, the development of the action potential is important in the functioning of certain cells. Excitation plays a major role in the work of the heart. Without it, the heart would be just an inactive organ, but due to the wave propagating through all the cells of the heart, its contraction occurs, which contributes to the pushing of blood along the vascular bed, its enrichment of all tissues and organs.
The nervous system also could not normally perform its function without the action potential. Bodies could not receive signals for the performance of a particular function, as a result of which they would be simply useless. In addition, improving the transmission of a nerve impulse in nerve fibers (the appearance of myelin and Ranvier's interceptions) made it possible to transmit a signal in a matter of fractions of a second, which led to the development of reflexes and conscious movements.
In addition to these organ systems, the action potential is formed in many other cells, but in them it plays a role only in the cell performing its specific functions.
The emergence of action potential in the heart
The main body, whose work is based on the principle of the formation of action potential, is the heart. Due to the existence of impulse formation nodes, the work of this organ is carried out, the function of which is to deliver blood to tissues and organs.
The generation of action potential in the heart occurs in the sinus node. It is located at the confluence of the vena cava in the right atrium. From there, the impulse propagates through the fibers of the cardiac conduction system - from the node to the atrioventricular junction. Passing along the bundle of His, more precisely, on its legs, the impulse passes to the right and left ventricle. In their thickness there are smaller paths - Purkinje fibers, through which excitation reaches every heart cell.
The action potential of cardiomyocytes is composite, i.e. Depends on the contraction of all cells of the heart tissue. If there is a block (scar after a heart attack), the formation of the action potential is disrupted, which is recorded on the electrocardiogram.
Nervous system
How is PD formed in neurons - cells of the nervous system. Everything is a little easier here.
An external impulse is perceived by the processes of nerve cells - dendrites associated with receptors located both in the skin and in all other tissues (the resting potential and the action potential also replace each other). Irritation provokes the formation of an 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 wave of excitement reaches the brain.
A feature of the nervous system is the presence of two types of fibers - coated with and without myelin. The emergence of the action potential and its transmission in those fibers where there is myelin occurs much faster than in demyelinated ones.
This phenomenon is observed due to the fact that PD propagation through myelinated fibers occurs due to “jumps” - an impulse jumps over areas of myelin, which as a result reduces its path and, accordingly, accelerates the propagation.
Resting potential
Without the development of the rest potential, there would be no action potential. By the resting potential, we mean the normal, unexcited state of the cell, in which the charges inside and outside its membrane are significantly different (that is, the membrane is positively charged outside and negatively inside). The resting potential shows the difference between charges inside and outside the cell. Normally, it is normally between -50 and -110 meV. In nerve fibers, this value is usually -70 meV.
It is due to the migration of chlorine 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), PP replaces PD.
Normally, all the cells of the body are in an unexcited state, so the change of potentials can be considered a physiologically necessary process, since without them the cardiovascular and nervous systems could not have carried out their activity.
The significance of the study of the potentials of rest and action
The resting potential and action potential make it possible to determine the state of the organism, as well as individual organs.
Fixing the action potential from the heart (electrocardiography) allows you to determine its condition, as well as the functional ability of all its departments. If you study a 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 P wave, and the spread of excitation in the ventricles indicates the R wave).
With regards to the electroencephalogram, the appearance of various waves and rhythms (in particular, alpha and beta waves in a healthy person) on it is also due to the appearance of action potentials in brain neurons.
These studies allow us to timely identify the development of a particular pathological process and determine almost 50 percent of the successful treatment of the underlying disease.