BIOELECTRIC POTENTIALS

The amplitude of the PD of most nerve and muscle fibers is approximately the same: 110-120 mv. The duration of PD varies widely: in warm-blooded animals, the duration of PD of the nerve fibers that most rapidly conduct excitation is 0.3-0.4 msec, in the fibers of the heart muscles - 50-600 msec. In plants, the cells of freshwater algae hara PD lasts approx. 20 sec. A characteristic feature of PD that distinguishes it from other forms of cell response to irritation is that it obeys the "all or nothing" rule, i.e. occurs only when the stimulus reaches a certain threshold value, and a further increase in the intensity of the stimulus no longer affects either the amplitude or the duration of the PD, the action potential is one of the most important components of the excitation process. In the nerve fibers, it provides the conduction of excitation from the senses. endings (receptors) to the body of the nerve cell and from it to the synaptic endings (see Synapses) located on various nerve, muscle or glandular cells. Entering the effector endings, PD causes the release (secretion) of a certain portion of the specific. chemical substances, so-called mediators that have an exciting or inhibitory effect on the body. cells. In muscle fibers, the spreading PD causes a chain of physico-chemical reactions underlying the process of muscle contraction. PD is carried out along nerve and muscle fibers by so-called local currents, or currents of action, arising between the excited (depolarized) and adjacent resting areas of the membrane (see Excitation). The action currents are recorded by conventional extracellular electrodes; in this case, the curve has a two-phase character: the first phase corresponds to the arrival of the PD under the near electrode, the second - under the far electrode (Fig. 4).

3. Changes in the sodium and potassium conductivity of the nerve fiber membrane during the generation of the action potential (I). Changes in conductivity are proportional to changes in permeability for Na+ (II) and K+ (III).

As a result of such an explosive circular process, the so-called regenerative depolarization, there is a distortion of the membrane potential, characteristic of PD. The increase in permeability for Na+ is very short-lived and is replaced by its fall (Fig. 3), and consequently, a decrease in the flow of Na+ into the cell. The permeability for K+, in contrast to the permeability for Na+, continues to increase, which leads to an increase in the flow of K+ from the cell. As a result of these changes, the PD begins to fall, which leads to the restoration of the PP. This is the mechanism of PD generation in most excitable tissues. There are, however, cells (muscle fibers of crustaceans, nerve cells in a number of gastropods, some grow cells), in which the ascending phase of PD is caused by an increase in membrane permeability not for Na+ ions, but for Ca2+ ions. The mechanism of PD generation in the muscle fibers of the heart is also peculiar, which is characterized by a long plateau in the descending phase of PD (Fig. 2, c). The inequality of the concentrations of K+ and Na4+ (or Ca2+) ions inside and outside the cell (fiber) is supported by a special mechanism (so-called "sodium pump"), pushing Na+ ions out of the cell and pumping K+ ions into the protoplasm, requiring energy expenditure, which is drawn by the cell in metabolic processes.

Fig. 1. The scheme of measurements of the resting membrane potential using an intracellular glass microelectrode (M). The second electrode(S) is placed in the washing cell liquid.

Action potential (PD). All stimuli acting on the cell cause, first of all, a decrease in PP; when it reaches critical. values (threshold), there is an active propagating response - PD (Fig. 2). During the ascending phase of PD, the potential on the membrane is briefly distorted: its inner. the side charged at rest electronegatively acquires a positive potential at this time. Having reached the top, the PD begins to fall (the descending phase of the PD), and the potential on the membrane returns to a level close to the initial one - PP. Complete recovery of the PP occurs only after the end of the trace fluctuations of the potential - trace depolarization or hyperpolarization, the duration of which usually significantly exceeds the duration of the peak of the PD. According to the membrane theory, the depolarization of the membrane caused by the action of an irritant leads to an increase in the flow of Na+ into the cell, which reduces the negative. the potential is internal. the sides of the membrane - increases its depolarization, which, in turn, causes a further increase in permeability to Na+ and a new increase in depolarization, etc.

Ext. the side of the membrane is charged electronegatively with respect to the outer one (Fig. 1). PP is due to the choice. the permeability of the resting membrane for K+ ions (Yu. Bernstein, 1902, 1912; A. Hodgkin and B. Katz, 1947). The concentration of K+ in the protoplasm is about 50 times higher than in the extracellular fluid, therefore, diffusing from the cell, the ions carry positive charges to the outer side of the membrane, while the inner. the side of the membrane that is practically not permeable to large organics. anions, acquires negate. potential. Since the permeability of the membrane at rest for Na+ is about 100 times lower than for K+, the diffusion of sodium from the extracellular fluid (where it is the main cation) into the protoplasm is small and only slightly reduces the PP caused by K+ ions. In skeletal muscle fibers, C1-ions diffusing into the cell also play an important role in the occurrence of the resting potential. The consequence of PP is the resting current recorded between the damaged and intact sections of the nerve or muscle when the discharge electrodes are applied. The membranes of nerve and muscle cells (fibers) are capable of changing ionic permeability in response to shifts in the membrane potential. With an increase in PP (hyperpolarization of the membrane), the permeability of the surface cell membranes for Na+ and K+ decreases, and with a decrease in PP (depolarization) it increases, and the rate of changes in permeability for Na+ significantly exceeds the rate of increase in membrane permeability for K+.

There are the following osn. types of BP nerve and muscle cells: resting potential, action potential, excitatory and inhibitory postsynaptic. potentials, generator potentials.

Resting potential (PP, resting membrane potential). In living cells at rest between int. there is a potential difference (PP) of the order of 60-90 between the contents of the cell and the external solution, which is localized on the surface membrane.

Fig. 2. Action potentials recorded using intracellular microelectrodes: a - giant squid axon; 6 - skeletal muscle fiber; b - fibers of the dog's heart muscle; 1 - ascending PD phase; 2 - descending phase; 3 - trace hyperpolarization (a) and trace depolarization (b).

BIOELECTRIC POTENTIALS, electrical potentials arising in tissues and individual cells of humans, animals and plants, are the most important components of the processes of excitation and inhibition. The research of B. P. is of great importance for understanding physico-chemical and physiological processes in living systems and is used in the clinic with diagnostics. purpose (electrocardiography, electroencephalography, electromyography, etc.).BIOELECTRIC POTENTIALS, electrical potentials arising in tissues and individual cells of humans, animals and plants, are the most important components of the processes of excitation and inhibition. The research of B. P. is of great importance for understanding physico-chemical and physiological processes in living systems and is used in the clinic with diagnostics. purpose (electrocardiography, electroencephalography, electromyography, etc.).

The first data on the existence of BP ("animal electricity") were obtained in the 3rd quarter. 18 c. when studying the nature of the "blow" inflicted by some fish with electric organs during defense or attack. The beginning of Ital's research dates back to the same time. physiologist and physician L. Galvani, who laid the foundation of the doctrine of B. P. Long-term scientific. the dispute (1791-97) between L. Galvani and physicist A. Volta about the nature of "animal electricity" ended with two major discoveries: facts about the existence of bioelectric phenomena in living tissues were obtained and a new principle of obtaining electricity was discovered. current with the help of dissimilar metals - a galvanic element (voltaic column) was created. The correct assessment of Galvani's observations became possible only after the application of sufficiently sensitive. electrical measurement. devices - galvanometers. The first such studies were conducted by ital. physicist C. Matteucci (1837). Systematic study of B. P. was started by him. physiologist E. Dubois-Raymond (1848), who proved the existence of BP in nerves and muscles at rest and when excited. But he failed (due to the large inertia of the galvanometer) to register fast, lasting thousandths of a second fluctuations of BP. when conducting impulses along nerves and muscles. In 1886 it. physiologist Yu . Bernstein analyzed the form of the action potential; the French scientist E. J. Marei (1875) used a capillary electrometer to record fluctuations in the potentials of a beating heart; the Russian physiologist N. E. Vvedensky used (1883) a telephone to listen to rhythmic discharges of impulses in a nerve and muscle, and the gol L. physiologist V. Einthoven (1903) introduced a clinic into the experiment. this string galvanometer is highly sensitive. and a low-inertia device for registering electricity. currents in the tissues. Hence, the contribution to the study of BP was made by Russian physiologists: V. V. Pravdich-Neminsky (1913-21) was the first to register an electroencephalogram, A. F. Samoilov (1929) investigated the nature of neuromuscular transmission of excitation, and D. S. Vorontsov (1932) discovered the trace vibrations of BP accompanying the action potential in nerve fibers. Further progress in the study of BP was closely related to the success of electronics, which allowed to apply in physiology. in the experiment, electronic amplifiers and oscilloscopes (works of amer. physiologists G. Bishop, J. Erlanger and G. Gasser in the 30-40s of the 20th century.). The study of B.P. in individual cells and fibers, it became possible with the development of microelectrode technology. The use of giant nerve fibers of cephalopods, ch. mod. squid, was of great importance for elucidating the mechanisms of BP generation. The diameter of these fibers is 50-100 times larger than that of vertebrates, it reaches 0.5-1 mm, which allows microelectrodes to be inserted into the fiber, various substances to be injected into the protoplasm, etc. The study of the ionic permeability of the membrane of giant nerve fibers allowed the English physiologists A. Hodgkin, A. Huxley and B. Katz (1947-52) to formulate a modern membrane theory of excitation.

BIOELECTRIC POTENTIALS, electrical potentials arising in tissues and individual cells of humans, animals and plants, are the most important components of the processes of excitation and inhibition. The research of B. P. is of great importance for understanding physico-chemical and physiological processes in living systems and is used in the clinic with diagnostics. purpose (electrocardiography, electroencephalography, electromyography, etc.).