The functioning of the 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 the nerve impulse is the formation of a specific wave of excitation, called the action potential.
What is this?
An action potential is commonly called a wave of excitation moving 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 positively charged). The generated wave contributes to a change in the properties of the ion channels of the cell, which leads to the recharging of the membrane. At the moment when the action potential passes through the membrane, there is a short-term change in its charge, 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.
When its formation is disturbed, many diseases develop, which makes the determination of the action potential necessary ina complex of diagnostic and treatment measures.
How is an action potential formed and what is characteristic of it?
Research history
The study of the occurrence of excitation in cells and fibers was started quite a long time ago. The first to notice its occurrence were biologists who studied the effects of various stimuli on the frog's exposed tibial nerve. They noticed that when exposed to a concentrated solution of table s alt, muscle contraction was observed.
In the future, research was continued by neurologists, but the main science after physics that studies the action potential is physiology. It was physiologists who proved the existence of an action potential in heart cells and nerves.
As we delved deeper into the study of potentials, the presence of the resting potential was also proved.
From the beginning of the 19th century, methods began to be created to detect 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.
Action potential mechanism
The formation of excitation occurs due to changes 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, resulting in the flow of sodium ions into the cell. Because of thisthe charges outside and inside the cell change (the cytoplasm is charged positively, 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, the charge is reversed due to the current inside the cell of negatively charged chloride ions. The initial levels of charge outside and inside the cell are restored, 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 AP there will obey somewhat different laws.
PD phases
The course of an action potential can be divided into several phases.
The first phase proceeds until a critical level of depolarization is formed (a passing action potential stimulates a slow discharge of the membrane, which reaches a maximum level, usually around -90 meV). This phase is called the prespike. 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 membrane depolarization (fast), and the descending part means repolarization.
Third phase - negative trace potential - shows trace depolarization (transition from the peak of depolarization to the state of rest). Caused by the entry of chloride ions into the cell.
At the fourth stage, the phase of positivetrace potential, the charge levels of the membrane return to the original.
These phases determined by 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 simply be an inactive organ, but due to the propagation of the wave through all the cells of the heart, it contracts, which helps to push blood through the vascular bed, enriching all tissues and organs with it.
The nervous system also could not normally perform its function without an action potential. Organs could not receive signals to perform a particular function, as a result of which they would simply be useless. In addition, the improvement in the transmission of a nerve impulse in nerve fibers (the appearance of myelin and intercepts of Ranvier) 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 also formed in many other cells, but in them it plays a role only in the performance of the cell's specific functions.
Rise of an action potential in the heart
The main organ whose work is based on the principle of action potential formation is the heart. Due to the existence of nodes for the formation of impulses, the work of this organ is carried out, the function of which is to deliver blood to the tissues andauthorities.
The action potential in the heart is generated at the sinus node. It is located at the confluence of the vena cava in the right atrium. From there, the impulse propagates along the fibers of the conduction system of the heart - from the node to the atrioventricular junction. Passing along the bundle of His, more precisely, along its legs, the impulse passes to the right and left ventricles. In their thickness are smaller pathways - Purkinje fibers, through which excitation reaches every cell of the heart.
The action potential of cardiomyocytes is compound, i.e. depends on the contraction of all cells of the heart tissue. In the presence of a block (a scar after a heart attack), the formation of an action potential is disturbed, which is recorded on the electrocardiogram.
Nervous system
How is PD formed in neurons - cells of the nervous system. Everything is done a little easier here.
External impulse is perceived by outgrowths of nerve cells - dendrites associated with receptors located both in the skin and in all other tissues (resting potential and action potential also replace each other). Irritation provokes the formation of an action potential in them, after which the impulse goes through the body of the nerve cell to its long process - the axon, and from it through the synapses to other cells. Thus, the generated wave of excitation reaches the brain.
Feature of the nervous system is the presence of two types of fibers - covered with myelin and without it. The occurrence of an action potential and its transmission in those fibers where there is myelin,carried out much faster than in demyelinated.
This phenomenon is observed due to the fact that the propagation of AP along myelinated fibers occurs due to “jumps” - the impulse jumps over the myelin sections, which, as a result, reduces its path and, accordingly, accelerates its propagation.
Resting potential
Without the development of the resting potential, there would be no action potential. The resting potential is understood as 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 charged inside). The resting potential shows the difference between the charges inside and outside the cell. Normally, it ranges from -50 to -110 meV. In nerve fibers, this value is usually -70 meV.
It is due to the migration of chloride ions into the cell and the creation of a negative charge on the inside of the membrane.
When changing the concentration of intracellular ions (as mentioned above), PP replaces PD.
Normally, all 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 carry out their activities.
Significance of research on resting and action potentials
Resting potential and action potential allow you to determine the state of the body, as well as individual organs.
Fixation of the action potential from the heart (electrocardiography) allowsdetermine 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 occurrence of these potentials in the atria displays the P wave, and the spread of excitation in the ventricles - the R wave).
As for the electroencephalogram, the occurrence of various waves and rhythms on it (in particular, alpha and beta waves in a he althy person) is also due to the occurrence of action potentials in brain neurons.
These studies allow timely detection of the development of a particular pathological process and determine almost 50 percent of the successful treatment of the original disease.
