EXCITEMENT — the response of live protoplasm to external influence by change of character or intensity of the processes proceeding in it; in narrower sense — it is physiological process, the Crimea nek-ry living cells (nervous, muscular, ferruterous) answer external influence. Ability of fabric to response is called excitability (see). It is caused by active change of the cell activity using internal margins of energy for its implementation century; irritants (see) are only the factor starting these changes.
In the course of evolution nek-ry living cells had an ability to answer irritation with active reactions, the main component to-rykh specific change is physical. - chemical and biochemical, properties of a superficial cell membrane. At metazoans of V. is the main function of cells of nervous tissue.
Nervous cell (see) has property of carrying out V. from that site where it for the first time arose, to other sites, and also V.'s transfers on other cells, i.e. can transmit signals from one structures of an animal organism to others.
The nervous cells generating V. are available already for the lowest coelenterates animals (e.g., for hydras). Complication of a nervous system in the course of evolution happened due to change of a structure and function of nervous cells, emergence of a set of shoots, on the Crimea V. can extend to considerable distances and involve in activity a large number of other cells, accelerations of development of V. and improvement of the mechanism of its carrying out hl. obr. due to repeated increase in quantity of nervous cells and complication of bonds between them. Excitement and genetically related braking (see) are at the same time a basis of all types of nervous activity, including and mental. Is the data carrier about properties of irritants and means of regulation of functions of bodies and systems of an organism century. Century of muscle and ferruterous cells is the factor starting specific activity of these cells — reduction, secretion.
It is closely connected with development of the electric phenomena century. For the first time existence of electric current in a live organism was established by the ital. scientist L. Galvani (1791) who called it «animal electricity». In 1838 the ital. scientist Matteuchchi (S. of Matteucci, 1811 — 1868) showed that the muscle of a frog at reduction generates the electric current sufficient for irritation of a nerve of other nerve-muscle preparation, and E. Du Bois-Reymond in 1848 opened the characteristic reaction accompanying V. — so-called action potential (see Bioelectric potential). In 1850 Helmholtz for the first time measured rate of propagation of V. in a nerve of a frog. Bernstein (J .Bernstein, 1868) by means of the differential rhetome invented by it confirmed the data obtained by G. Helmholtz and defined the eclipse period, building-up period and recession of action potential. Then these measurements were taken by means of the capillary electrometer on heart (E. Marey, 1876), a nerve [F. Gotch, 1899] and a skeletal muscle (A. F. Samoylov, 1908). More precisely registration of action potential was carried out by J. Erlange rum and G. Gasser in 1924 by means of the cathode oscillograph.
For the first time V.'s theory was offered by E. Du Bois-Reymond. He was a hypothesis of existence in live formations of special molecules its basis, to-rye it called electromotor; each such molecule, according to the author, consists of two half: one has positive charge, and another — negative. At rest of a molecule are located on a surface of a cell so that positive charges are directed outside, and negative — inside. At irritation of a molecule turn around the lateral axis, and negative charges appear outside, as creates action potential.
Germann (L. Hermann, 1879) put forward so-called is scarlet the iterative theory, according to a cut electric potential arises only at the time of damage of fabric; before any electric potentials live to the tkanena have. Germann's theory as well as Du Bois-Reymond's theory, had no physical. - chemical justifications.
V. Yu. Chagovets (1896) for the first time used the theory of electrolytic dissociation for an explanation of electric potentials in living tissues. He assumed that V. is connected with strengthening of a metabolism and formation of the acids dissociating on ions, diffusion to-rykh from the excited site of a cell leads to emergence of potential difference between the excited and unexcited site. However the sizes of potentials received in experiences appeared much more those, to-rye shall be according to Chagovts's assumptions. P. P. Lazarev (1916) offered the so-called ionic theory; he considered as the mechanism B. change of a ratio in cytoplasm one - and bivalent cations. In a crust, time these theories are of only historical interest.
For understanding of essence of V. development by Ostvald (W. Ostwald, 1890), Overton had paramount importance (E. Overton, 1902) ideas of existence on a surface of a cell of a semipermeable membrane capable to detain one ions and to pass others. On the basis of these representations Bernstein (J. Bernstein, 1912) developed the membrane theory of emergence of electric potentials in live educations, according to a cut on a cell membrane there is a constant potential difference (membrane potential, or rest potential). Bernstein assumed that at V. the membrane loses the semipermeable properties owing to what in the excited site there is a disappearance of rest potential. The provision of the theory of Bernstein about existence of a pas of a surface of living cells of the semipermeable membranes bearing on themselves a constant of potential difference is a basis of modern ideas of V.
Odnako's essence views of the nature of ionic processes on this membrane significantly changed that is connected with data acquisition that the based membrane of a pronitsayem not only for potassium ions as Bernstein considered, but also for ions of sodium, to-rye would have to arrive constantly through a membrane in a cell (see. Membrane equilibrium ). Dee nomas (V. of Dean, 1941) and A. Hodzhkin (1951) made the assumption that the cell membrane possesses the transport system capable to transfer ions of sodium from a cell against an electrochemical gradient due to energy of a metabolism (see Transport of ions). At the same time it was shown that in the course of V. the potential difference on both sides of a membrane does not disappear, and only polarity of a charge of a membrane changes, i.e. the outer surface of a membrane becomes negatively charged in relation to internal.
The explanation of this phenomenon was offered A. Hodzhkin, Katts (V. to Katz) and A. Huxley (1952). At excitement there is not the general increase in permeability of a membrane, and selective increase in permeability only for ions of sodium (in nek-ry membranes — for calcium ions). These ions, concentration to-rykh out of a cell are much higher, than inside, begin to diffuse with high speed inside, transferring positive charges through a membrane and recharging it. Such assumption was validated by a number of experiments, and in particular direct measurement of ionic currents of a cell membrane during V.
Naiboley exact data on ionic currents of a membrane during V. are obtained by method of a so-called voltage clamp on a membrane. By means of this method it is shown what originally at V. arises short-term, the entering current directed in a cell. Then it is replaced by longer leaving current. Initial, entering, the current connected with the movement through a membrane of ions of sodium causes a recharge of a membrane. It disappears if the cell is in the beznatriyevy environment. Also calcium ions participate in nek-ry cells in creation of the entering current. The leaving current is result of the movement from a cell of potassium ions. It promotes bystry recovery of initial electric polarization of a cellular membrane.
The being of reorganizations in a membrane, to-rye is created by opportunities for emergence of the ionic currents described above, is yet not a clear link in the mechanism B. Apparently, the movement of ions through a membrane happens on system of channels (time) diameter in several angstrom, to-rye in the based state are closed, but open at the moment on time matching change of electric field in a membrane up to the certain critical size — a threshold (see. Permeability ).
Change of electric field in a membrane can be received a transmission through it electric current of the corresponding direction. Therefore electric current is a universal irritant for excitable fabrics. Century at the same time arises in the field of the cathode of the irritating chain where the irritating current is directed from within a cell outside and therefore reduces the size of membrane potential. Change of polarization of a cellular membrane at a transmission of external electric current received the name of physical electro-tone. The depolarization of a membrane arising in the field of application of the cathode is designated as a catelectrotonus, and opposite process, i.e. increase in membrane potential in the field of application of the anode — as anelektroton. Electrotonic changes on a surface of a cell are followed by its corresponding changes excitabilities (see).
If membrane potential already in a nek-swarm of degree is reduced, then achievement of threshold value of depolarization is facilitated; on the contrary, at increase in membrane potential, i.e. hyperpolarization of a cellular membrane, achievement of a threshold is at a loss. These changes of excitability are designated as «physiological» electrotone (see. Electrotonic phenomena). Nek-ry researchers described opposite changes of excitability and polarization in the sites of fabric remote from the location of the cathode and the anode of the irritating current (so-called perielektroton). However the perielektrotonichesky phenomena have non-constant character.
Since the cell membrane has electric capacity, to-ruyu shall load the irritating current, efficiency of the last depends not only on size, but also on duration of its transmission. The graph of irritant action of current from its duration (Weiss's curve) has an appearance, characteristic of this fabric, and can be used for more detailed characteristic of her excitability. L. Lapik suggested to define only nek-ry characteristic points of this curve. It designated the threshold force of the long irritating current as rheobase, and the minimum time of action of current in 2 rheobases, V., necessary for emergence — as a chronaxia (see. Hronaksimetriya ).
At okoloporogovy irritation of V. can be gradualny and not extending. However at stronger irritations process in a membrane becomes self-sustaining, and the arising action potential has a constant (see. „Everything or nothing“, law). At this V. begins to extend on a cell with constant speed due to irritant action of the local electric currents arising between the unexcited and excited its site. Rate of propagation of V. in cross-striped muscle fibers makes from 1 to 4 m/s, and in nerve fibrils fluctuates from 1 to 130 m/s, being in proportional dependence on diameter of fiber or its myelination (see. Nerve fibrils ). V.'s duration in various cells also considerably varies, changing from 1 — 2 to tens of ms.
Constancy of amplitude of action potential is the important factor providing high reliability of signaling in nervous cells. Coding of information on these signals happens by means of change of frequency and the sequence of distribution of impulses, and also their spacing in system of nervous cells.
V.'s emergence in fabrics is followed by a short-term phase of total loss of excitability (absolute a refrakternost Duration of an absolute refractory phase approximately matches duration of action potential and makes for cross-striped muscle fiber of 2,5 — 4 ms, for thick nerve fibrils of 0,4 ms, for som of a nervous cell of 2,5 — 4 ms. Then excitability is gradually recovered (relative refrakternost) to initial level and can exceed this level (an ekzaltatsionny phase).
After these fast changings in nek-ry cells the additional long period of decrease in excitability matching after hyperpolarization of a membrane is observed. The specified cyclic changes of excitability lead to the fact that V.'s frequency cannot increase infinately; with high frequencies of irritation there occurs transformation of a rhythm of the arising V. (see. Lability ).
At long action of the irritating agent there is a counteraction of excitable fabric in the form of decrease in excitability, a cut received the name fiziol, adaptations (see. Adaptation , physiological adaptation). In particular, at gradual increase of the irritating current it can not cause V., despite achievement of considerable intensity — a so-called vkradyvaniye, or accommodation (see).
Excitability can be reduced or completely eliminated with action of a number of factors. One of them block system of the channels providing ionic currents in a membrane (neurotoxins, nek-ry drugs, two - and trivalent cations), others cause permanent depolarization of a membrane which as it was shown for the first time to B. F. Verigo (1888), also suppresses excitability.
H. E. Vvedensky considered such permanent depolarization as a special form of not extending V. (see. Parabiosis ).
Since an indispensable condition of emergence of V. is change of initial polarization of a membrane (membrane potential), under natural conditions there is an activation of the special mechanisms transforming external irritation to electric current. In receptor structures manifestation of such transformation is so-called generator potential, to-ry represents local depolarization of a cellular membrane. In synoptic connections between nervous cells the chemical substance (mediator) emitted by the terminations of an axon of one cell interacts with receptor groups of a postsynaptic membrane of other cell that also leads to local depolarization of a membrane (exciting postsynaptic potential). At achievement of threshold size of depolarization the extending impulse is created. In neuromuscular connection the local reaction of a membrane of muscle fiber created by a mediator received the name of potential of a trailer plate.
V.'s emergence is connected with emergence not only electric reaction, but also with development difficult structural and biochemical, changes. There is a disturbance of ionic balance between cytoplasm and Wednesday: the cell gains a nek-swarm quantity of ions of sodium and loses the corresponding quantity of potassium ions. This disturbance activates the fermental systems of transport of ions through a membrane recovering the broken balance. There is a change of heat production and oxygen consumption that is also a characteristic sign of excited state.
Researches on the isolated nerves showed that the oxygen absorption making from 30 to 100 mm at rest 3 on 1 g of fabric, at irritation increases on average by 24%. Heat production at V. is divided into two phases — initial and detained (lasting several minutes after the termination of irritation). During initial heat generation it is allocated apprx. 8×10 - 6 kcal on 1 g of nervous tissue; general heat generation much more (7×10 - 5 kcal) so directly during V. it is generated apprx. only 10% of heat.
The chemical analysis showed that nervous cells contain a number of the enzymes participating in processes of aerobic and anaerobic exchange and processes of phosphorylation; activity of these enzymes changes at Century.
At V.'s carrying out points out fall of the contents of ATP and change of the ATF-aznoy level of activity in a nerve in operating time power consumption. Data that V.'s carrying out is followed by chemical reactions are obtained, in the course to-rykh arise free radicals (see) and that intensity of these reactions is regulated by natural inhibitors — antioxidants (see). It is shown also that education of free radicals happens hl. obr. in fosfoinozitida, and the phosphotidylcholine fraction of phospholipids has anti-oxidizing activity.
Is known of ultrarestructurings in nerve fibril at V. a little; in particular, V.'s carrying out is followed by conformational changes in proteinaceous molecules, change of double refraction, etc.
Studying of processes of V. is carried out by means of various fiziol., biochemical, biophysical, and other methods. Broad use has a tracer technique.
See also Lability .
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P. G. Kostkzh, O. R. Necklace.