CONDUCTIVITY OF BIOLOGICAL SYSTEMS — the quantitative characteristic of ability of live objects (fabrics) to carry electric current. Conductivity is inversely proportional to the size of electric resistance of system.
Measurement E. villages use for obtaining information on a functional state biol, fabrics, for identification of inflammatory processes, change of permeability of cellular membranes and walls of vessels at pathology or action on an organism of physical, volumetric and other factors (see. Permeability ). Measurement E. the page is the cornerstone of many methods of assessment of a krovenapolneniye of vessels of bodies and fabrics (see. Reografiya ).
During the giving of potential difference (U) through an object electric current by force (I) flows, size a cut is proportional to conductivity (G): I = GU. The size of conductivity depends on quantity of free electric charges and their mobility. The more the quantity of charges and their mobility, the is more conductivity. In a cell the main free charges are potassium ions and organic annona, and outside of a cell, in intercellular spaces and intercellular lymphs — ions of sodium and chlorine. In biol. liquids (blood, cerebrospinal fluid, urine etc.) conductivity is proportional to contents in them free ions. The connected charges (ionizable group of proteins, lipids, carbohydrates), movement to-rykh it is limited, and large ions with small mobility have no significant effect on size E. page.
Conductivity or resistance of cells, fabrics, bodies and the whole organisms measure at a transmission through them constant or variable current of a sinusoidal form, frequency to-rogo can change from shares of hertz to 10 10 Hz. At measurements on alternating current with a circular frequency ω (ω = 2πf, where f — frequency in Hz) the general resistance of system, or impedance (see), depends on existence of limits of the section in system, on to-rykh there can be an accumulation of charges — polarization (see). Properties of limits of the section (in biol, an object it is hl. obr. various membranes) can be described if to enter a concept of capacity With, resistance a cut of Hs (reactive resistance unlike R — active resistance) depends on frequency, on a cut measurement is performed: Xs = 1 / (ωC).
The general resistance (Z) is equal to the sum of resistance reactive Hc and active R if R and S are connected consistently; at a connection in parallel of R and S the general resistance is calculated by a formula:
For measurement of electric characteristics biol. an object apply metal or coal electrodes (see), to-rye put to an object by means of liquid contact — a thin coat of well carrying out liquid, most often — fiziol. solution. In some cases, e.g. at measurement of conductivity of cytoplasmic membranes, one of electrodes is entered in a cell, and another is brought to a cell outside (see. Microelectrode method of a research ). At measurement E. the page on constant and variable currents of low frequency is important to consider the size of polarization of electrodes as due to electrode polarization true electric parameters biol, an object can differ from measured considerably. The size of polarization of electrodes is defined by density of current, edges depends on the potential difference attached to system, resistance of system, the area of potential electrodes. The more the area of electrodes, the is less density of current and the less distortions brought in measurements by electrodes. Therefore for reduction of density of current use electrodes with a big effective surface, in particular the platinum electrodes covered with platinum black (their spongy covering increases an effective surface of electrodes by 100 — 1000 times). Use and the so-called not polarized electrodes is possible (e.g., calomel, chlorsilver).
For simplification of interpretation of the received results biol. the system (fabrics, suspensions of cells) is often presented in the form by models — ale of the ktrichesky scheme consisting of a set of the active resistance and tanks which are as if equivalents biol. structures of the cells or fabrics participating in carrying out electric current.
Measurement E. the page on a direct current because of high extent of polarization of membranes and electrodes is extremely complicated. At low frequencies of alternating current the most part of current proceeds on intercellular intervals. At increase in frequency of electric current the reactive resistance of capacity falls, the polarizing phenomena decrease. Dependence of resistance and capacity of an object on frequency received the name dispersions (see). At high frequencies the general resistance of system depends only on active resistance of intercellular spaces and cytoplasm.
In medicine and biology E. villages most often investigate in the field of so-called β-dispersion, edges is observed in frequency range 10 2 — 10 8 the Hz is also defined by polarization of limits of the section and heterogeneity of structure of an object. Measurements E. pages showed that in process of increase in frequency conductivity increases, reaching a yield value. At experience and dying off of fabric E increases. page at low frequencies. It is connected with the fact that at dying off of fabric permeability of membranes for ions grows, and they are not border any more, on a cut there can be a polarization. Based on ability of living tissue to polarization, B. N. Tarusov suggested to use as criterion for evaluation of viability of fabric coefficient To, Rn/Rv in number equal to the relation where Rn and Rv — resistance of fabric measured respectively at low and high frequencies; at dying off of fabric he aims at unit. The choice of frequencies for calculation To is defined by the range of β-dispersion:::::::::: to low frequency there corresponds the frequency of the beginning of β-dispersion, high — frequency, at a cut E. the page reaches max. size. E.g., for muscular tissue is 10 3 and 10 6 Hz, blood cells and fatty tissue — 10 4 and 10 7 Hz, skin — 10 2 and 10 4 Hz etc. At high frequencies when active and general resistance do not depend on frequency, the research of relative changes of number of ions in biol is possible. to system, connected with disturbance of a water salt metabolism.
Comparing the Data received at measurement at low and high frequencies it is possible to calculate the volume and an ionic conduction of intercellular spaces and cytoplasm of cells, permeability of membranes for ions, capacity characteristics of a membrane. If measurements are taken in system where intercellular spaces are occupied rather large volume (more than 20 — 30%), e.g. at measurements of conductivity of blood, then for calculation of parameters of a dispersed phase (erythrocytes) use special formulas. Frequencies, on to-rykh dispersion is observed, depend on the size of cells and volume of intercellular spaces. So, dispersion E. the page for blood cells begins at frequencies about several tens kilohertzes, for muscular tissue — several kilohertz, fatty — hundreds of kilohertzes. At a research of electric characteristics of plasma membranes of cells dispersion is found at frequencies about several tens hertz. Electric characteristics of fabrics and bodies at low frequencies depend on heterogeneity of an arrangement of cells and intercellular spaces and a ratio of their volumes. This fact is used in a reografiya and rheoencephalography (see) at a research of changes of a krovenapolneniye of fabric and elastic properties of walls of vessels. Measurement E. the village at low frequencies allows to estimate changes of volume of intercellular spaces, in particular at development inflammations (see). So, at the first stages of inflammatory process the structure of cells changes slightly, and the impedance of cells keeps the size. In process of swelling of cells and reduction of volume of intercellular spaces there is an increase in the general resistance of system. At later stages of development of an inflammation the impedance of system decreases due to increase of permeability of membranes for various ions.
Thus, measurement E. page or an impedance, especially with the broad range of frequencies, it can be used at a permeability survey of cellular membranes and other limits of the section in cells, fabrics, bodies, and standardization (measurement of specific sizes) gives the chance to compare the data obtained by different researchers. Excitement, change of intensity of metabolism and other functions of cells lead to change E. page. Methods of measurement E. villages use for a research of influence on biol. objects of various factors: works (increase in intensity of metabolism leads to increase in permeability of membranes); psychogenic (permeability of skin due to work of sweat glands changes); physical (radiation, ultraviolet radiation, temperature, etc.) and chemical (acids, alkalis, alcohols, etc.), usually accompanied with growth of permeability. Change of permeability of membranes often depends on a dose or concentration of active ingredient. So, salts of copper in small concentration reduce permeability of membranes of muscle cells of leather of a frog, and in concentration more than 10 - 3 M — increase. The research of electric properties of excitable fabrics promoted studying of the mechanism of carrying out excitement on a nerve p a muscle. On the basis of measurements of active resistance, capacity and their dispersion the static capacity of a cellular membrane was calculated (apprx. 1 mkf/cm 2 ) also thickness its lipidic bisloya is for the first time determined. It was found that the specific resistivity of an axoplasm and myoplasm of extracellular liquid only 2 — 3 times higher than resistance whereas resistance of a membrane is tens of thousands times higher. These data formed the basis for emergence of idea of «cable» structure of fiber. Temporary ratios between change of permeability of a membrane for ions and development of action potential — «impedance Spike» are established (see. Bioelectric potential , Nervous impulse ). Research E. the page can be used for assessment of a condition of fabrics at their preservation, and also efficiency of effect of biologically active agents on model systems. In some cases permeability biol. membranes for ions it is accompanied by their permeability for uncharged particles — sugars, amino acids and other connections. Therefore measurement E. the page can be useful during the studying of permeability of membranes and for nonelectrolytes. Research E. the page can find application and in biotechnology for assessment of an optimality of the environment and culture conditions of cells.
See also Conductometry .
Bibliography: Andreyev of V. S. Konduktometricheskiye methods and devices in biology and medicine, M., 1973; Biophysics, under the editorship of B. N. Tarusov and O. R. Necklace, page 186, M., 1968; Grechin V. B. and Borovikova V. N. Slow not electric processes in assessment of a functional condition of a brain of the person, page 22, L, 1982; Gurevich M. I., etc. Impedance reopletizmografiya, Kiev, 1982; Egorov Yu. V. and Kuznetsova G. D. Brain as volume conductor, M., 1976; Slynko P. P. Bases of a low-frequency conductometry in biology, M., 1972; Hassett Dzh. Introduction to psychophysiology, the lane with English, page 53, M., 1981; Electronics and cybernetics in biology and medicine, the lane with English, under the editorship of P. K. Anokhin, page 71, M., 1963; Schwan H. P. Electrical properties of tissue and cell suspensions, Advanc, biol. med. Phys., v. 5, p. 147, 1957.
Yu. A. Zhukov, G. V. Fedorov.