Bioelectric potentials

Bioelectric potentials (currents) - electrical phenomena observed in the cells at rest and during physical activity.
The emergence in living cells of the electric potential and the resulting currents associated with physico-chemical properties of cell membranes and components of cytoplasm (amino acids, proteins, ions). Between the outer surface of the cell membrane and the internal content of the cells there is always the potential difference, which is created due to different concentrations of ions K+, Na+, Cl - inside and outside of cells and different permeability for them the cell membrane. This potential difference is called the "current peace", or membrane potential, and an average of 60 to 90 mW.
Upon excitation of a living cell changes the original membrane potential, due to a change in membrane permeability and the movement of ions. In excitable cells of tissues (muscle, nerve) these processes can occur at very short intervals of time (milliseconds), and known by the current actions." Its value can reach 120 mV. For the discharge of currents from individual cells of the body will popolous special glass and metal microelectrodes with bare ends (1 - 2 MK). Registration of bioelectric potentials by using oscilloscopes and cathode different chernilnikova devices with the use of biopotential amplifiers. Cm. also Electroencephalography, Electrocardiography, Electromyography, the Electroretinography.

Bioelectrical phenomena (bioelectric potentials, currents) - electrical processes characteristic of living tissues.
Bioelectrical phenomena are open Galvani (A. L. Galvani), Matteucci (C. Matteucci). First hypotheses about the nature of bioelectrical phenomena nominated du Bois-Reymond (that is, Du Bois-Reymond) and Hermann (Hermann L.). C. Y. chagovets developed the theory of bioelectrical phenomena on the basis of modern physical chemistry of electrolyte. A detailed study of bioelectrical phenomena became possible only with the creation of the appropriate measuring equipment (cathode and slavnye oscilloscopes in combination with highly sensitive electronic amplifiers). Nowadays methods, allowing to enter the electrodes deep into the individual cells of living tissue and register their electrical activity. Such studies show that between the outer and inner surfaces of the cell membrane there is a constant potential difference, with outer surface toward the interior has a positive charge. The amount of such "transmembrane" potential difference is a few tens of millivolts; it is referred to as "the membrane potential" or "building peace". Changes in building peace in different functional States cells have names: "action potentials", "synaptic potentials", "generating capacities", "secretory potentials", etc.
The basis of constant electric polarization surface of the cell membrane is the uneven distribution of inorganic ions (especially potassium, sodium and chlorine) between the cytoplasm of a cell and its environment. In the cytoplasm of the cell there is a significant excess of potassium ions with a relatively small number of ions of sodium and chlorine (the so-called ion asymmetry). In a dormant condition cells of the stationary distribution of ions and supported in different cell membrane permeability to various ions activity of the active protoplasmatic mechanisms, pumping certain ions out of the cell, or, conversely, pulling them inside her,so - called potassium-sodium "pump"or "pump". In detail the mechanism of activity of this "pump" are not yet clear, but found that it is associated with metabolic energy sources cells, particularly with the system of splitting macroergic. phosphorous compounds. Ions of sodium and chlorine affect the resting potential only at low concentrations of potassium ions outside the cell.
The most accurate determination of the potential difference existing at cell surface, can be carried out microelectrode method, when one of the collecting electrodes (glass introduction with the tip diameter less than 0.5 MK) entered into the cell, and the second electrode is surrounding a cell tissue. In mammals, for example, the resting potential nerve cells is 60 to 80 mV, striated muscle fiber - 80-90 mW, heart - 90-95 mV.
When the stimulation of the cells changes ion permeability its surface membrane, resulting appear short-term movement of ions across the membrane. These ion currents are caused by changes of the electric polarization of the cell membrane.
The mechanism of occurrence of the most common forms of active electric reaction - action potential, associated with propagating wave excitation,can be presented as follows. A necessary condition for the occurrence of contagious excitation is to decrease the level of building peace (depolarization) to a certain value (true excitation threshold cells). The mechanism of excitation of a membrane depolarization universal; this excitation occurs not only electric, but also for all other types of irritation, including adequate stimuli receptor endings. When depolarization reaches a critical level (different for different types of cells), is rapidly developing a short-term increase of ion permeability of cell membranes for such ions, which are in a dormant condition have difficulty passing through the membrane.
The nature of these changes is unknown; it is determined that in most cases ion, which starts particularly easy to pass through the cell surface, is sodium. The positive charges of ions moving into the cell, not only eliminate the resting potential, but even for a short time so pervert transmembrane potential difference that the outer surface of the membrane becomes negative in relation to its inner side. The result of these processes on the surface of the cell is created longitudinal potential difference - Horny its plot is negatively charged against unexcited. Arising in connection with this ring electric (ion) currents between the plot of excitation and neighboring unexcited plots ("current activities") are the cause depolarization unexcited sites up to a threshold level, which provides for the distribution of wave excitation by the cell.
The amplitude of the action potential and its duration in the nerve cells of mammals are 100-110 mV and 1-2 MS, striated muscle fibres - 110-120 mV and 3-5 msec. However, the potential actions at the heart muscle fiber is extremely long. In each cell of the amplitude of the action potential in normal conditions of constant regardless of the conditions of irritation ("all or nothing"); however, if the resting potential for some reason, extremely reduced, and the amplitude of the action potential starts to decrease or generation of the latter is impossible ("katolicheskaya depression or inactivation"). After basic "high voltage" part of the action potential (peak) occur more insignificant in the amplitude of the oscillations of the electric polarization cell surface (so-called trace potentials). The severity of their rather varies for different types of cells.
In most cases, immediately after the peak develops trace depolarization, changing the trace by hyperpolarization, which, for example, nerve cells soma reaches 100 msec.
The development process of excitation corresponds to a certain heat production, which is well understood on a stand-alone nerve and is in stage I (initial thermogenesis) 7·10-8 cal per 1 g of the nerve impulse, and in stage II (deferred thermogenesis) significantly exceeds this value.
In most cases, capacity development actions precede intermediate forms of electrical activity. One of such forms is the local potential arising from the near-threshold irritation and different from the action potential gradually dependence on the strength of irritation (i.e. on the value of the calling it depolarization of the cell membrane. In the case of synaptic transmission of excitation occurs special form of local capacity - postsynaptic potential (SRP), which is also characterized by gradualist and the ability to stack with other similar potentials. The SRP is the result of a specific reaction postsynaptic membrane cells (i.e. the part of the membrane which borders synaptic end of the axons of other cells) on the action allocated synaptic terminals of the mediator. Depending on the nature of the mediator, and, apparently, and on the characteristics of those receptor groups postsynaptic membrane that are with him react, the SRP can be expressed changes of electric polarization membranes in different directions.
In some cases membrane depolarized; if depolarization reaches a threshold, it generates the usual action potential. Such SRP are exciting (ITSP) and associated with the activities of special excitatory synaptic endings. In other cases, the electric polarization of a membrane is increased, and the emergence of potential actions accordingly difficult; such SRP be inhibitory (TPSP) and are the basis of synaptic inhibition.
With adequate stimulation of nerve endings in the receptors of the external energy is transformed initially in gradually the depolarization of the membrane (the generating capacity), which directly leads to the development of the spreading of nerve impulses, if irritation reaches a threshold. Features of generating potentials well-studied at the mechanoreceptors, the simplest photoreceptors and other Secretory processes are also related to the appearance on the cell membrane glandular cells gradually secretory potentials, which can have a different direction and duration, depending on the nature of secretory activity.

Bioelectric potentials of individual cells can be summarized in electric reactions of a tissue or organ. Because the fabric is a conductor of electricity (of the second kind), these reactions may be registered even if the location of the discharge electrodes at some distance from the body (such as on the skin). In normal conditions the resting potential of a cell is not found in healthy tissues, because the outer surface of each of them is isopotential and has the potential difference only in relation to internal content of the cell. However, if tissue damage, destroying one way or another shell cell parts, then this "alteromonas" section in all cases will be negatively charged against the undamaged parts of the same fabric.
If a sufficiently large number of cells of the tissue studied excited at the same time and generate action potentials or other electrical reaction, an electrical reaction can be registered from whole cloth (nerve, muscle, cancer, brain segment, and so on). As if the damage, and at excitation amplitude of oscillations is much less than on the membrane of a separate cell, in connection with bridging extracellular currents intercellular fluid. In all cases the electrode is in contact with an excited tissue, will be negative to the electrode located on unexcited part. If the excitation pulse in turn passes under one and the other discharge electrodes, it will be registered two-phase current action; if the second electrode be positioned so that the wave excitation could not reach, registered oscillation will be single-phase. In heterogeneous in cellular composition of tissue appear more complex types of electric reactions, which can consist of a large number of oscillations. For example, the action potential of the nerve trunk, containing fibers with different speed, if to take him at some distance from the place of irritation when nerve impulses in various types of fibers already able to significantly diverge over time.
From the very complex structure of formations, such as the Central nervous system, electrical vibrations are discharged all the time, even in the absence of special irritation. This is caused by the continuous flow pulsation from a large number of uncontrolled sources, the possibility of prolonged circulation of nervous impulses in complex chains of neurons, as well as the generation of the rhythmic pulses of nerve cells under the influence of chemical and other factors. The electrical activity of the relevant parts of the brain appears rhythmic variations are very small amplitudes (several tens of microvolts), reflecting the General functional state of neurons in the appropriate area of the brain (although the mechanisms of appearance of rhythmic oscillations of the potential and changes of rhythm in different functional States of the brain is not yet clear). The currents, allocated from the surface of the bark of the big hemispheres, was named electroencephalogram; they register and study for physiological and clinical studies of brain activity (see Electroencephalography).
If irritation various sensor systems (receptors, afferent nerves) in certain areas of the brain are registered characteristic of bioelectric answers (so-called evoked potentials). They have precise localization at the various levels of the Central nervous system and reflect some specific parameters of excitable systems (latency period). By registration of evoked potentials study the ways of transmission of touch impulsaciu and their cortical projection (localization). Widely used registration total electric reactions of the heart muscle (electrocardiography), skeletal muscle (electromyography), retina (electroretinography).