BIOELECTRIC PHENOMENA

From Big Medical Encyclopedia

BIOELECTRIC PHENOMENA — the processes of distribution and transport of electric charges in a live organism caused by presence at the living cells and fabrics of a large number fixed (the loaded groups of bio-macromolecules) and mobile (free ions and electrons) of electric charges. Many substances coming to an organism and removed from it in the course of metabolism are in a type of ions. Redistribution and transport of these charges serve as an origin bioelectric potential (see). The most abrupt gradient of ions is found on cellular membranes (see. Membranes biological ), which are capable to regulate effectively uneven and selective distribution of ions. At the majority of cells the membrane potential called by rest potential depends on a gradient of potassium ions. Its size can be calculated on Nernst's equation:

where R — a gas characteristic; T — absolute temperature; n — a charge of an ion; F — Faraday's number; [To +] vn. — ion concentration of potassium in a cell; [To +] plank beds — ion concentration of potassium in outside solution. Rest potential has the sign «minus» on an inner surface of a membrane and the sign «plus» on external.

In excitable educations (e.g., nervous and muscle cells) the special type of membrane potential — the so-called action potential which, unlike rest potential, represents the spike potential moving in the form of an excitation wave on a surface of a cell with a speed up to several tens meters per second is registered. In the excited site potential gets the return sign (i.e. potential is reversed) and the inner surface is loaded positively, and outside — is negative. Absolute value of potential, as a rule, exceeds the level of rest potential. According to the ionic theory of Hodzhkin (A. L. Hodgkin, 1958) emergence of action potential is caused by selective increase in permeability of a membrane for ions of sodium.

The basic biopower processes in a live organism are connected with biological oxidation (see biological oxidation), in process to-rogo electrons move on a chain of the respiratory enzymes located in an inner membrane of mitochondrions on a gradient of redox potential. According to a hemiosmotichesky hypothesis of Mitchell (P. Mitchell, 1966) electron transfer on a respiratory chain which links are located across a membrane is followed by emergence of membrane potential.

The live organism is not only the generator of bioelectricity, but also the passive conductor of the electric current arising in an organism or enclosed from the outside (see. Conductivity of biological systems ). Conductivity of living cells and fabrics depends on an impedance, i.e. the sum active (ohmic) and reactive (capacity) resistance. Passing of direct electric current through living cells is followed by polarization, edges can be calculated by a formula:

I = (U-P)/R,

where I — current, R — the electric resistance, U — tension of current, P — tension of polarization (see. Polarization ). At a transmission of alternating current tension of polarization decreases with increase of frequency. This phenomenon carries the name of dispersion of conductivity (see. Dispersion ). By B. N. Tarusov in 1938 it was shown that dispersion of conductivity can serve as the sensitive indicator of viability of cells and fabrics.

As a rule, the surface of living cells and fabrics bears a considerable negative charge since ionizable group on the surfaces of cells presented by endgroups of macromolecules of a cellular surface at physiological pH values are ionized. Under the influence of forces of an electrostatic attraction positive ions from solution will be attracted to a surface and to create a double electric layer. A part of energy of a double layer can be measured on the speed of the movement of the loaded cell in electric field. This so-called electrokinetic potential (see Elekt the rokinetichesky phenomena) is calculated on Smolukhovsky's equation: Z = 4πηu/DE, where D — a dielectric constant of the environment, η — a coefficient of viscosity, E — a gradient of tension, u — electrophoretic mobility. In a crust, time electrokinetic potential is used as the indicator of a condition of a cellular surface. In particular, it was shown that mobility of astsitny cells is slightly lower in comparison with normal cells.

Carry to the electrokinetic phenomena also the capacity of the current. In a live organism there is a constant current of liquid. Any fluid movement (e.g., a current of blood on arteries and veins) is followed by emergence of capacity of the current between a liquid phase and a vascular wall. As specific conductivity of blood is big, these potentials are small — no more than 1 — 2 mV. Change of this potential can probably have also diagnostic value.

Electroosmosis which is observed at fluid movement through a porous partition under the influence of external electric field also belongs to the electrokinetic phenomena. In some cases positive and negative abnormal osmose in kidneys is explained by electroosmotic transfer of water.

See also Excitement , Impedance , Polarization , Potentials (physical and chemical).


Bibliography: Katts B. A nerve, a muscle and a synapse, the lane with English, M., 1968, bibliogr.; Kurella G. A. Bioelectric potential, in book: Biophysics, under the editorship of B. N. Tarusov and O. R. Necklace, page 238, M., 1968, bibliogr.; Skulachev V. P. Transformation of energy in biomembranes, M., 1972, bibliogr.

V. F. Antonov.

Яндекс.Метрика