IRRITABILITY

IRRITABILITY, the property of living organisms or their parts to specifically change their states under the influence of changes in the environment or in other parts of the organism. Very often the same definition is given to the concept of excitability. There is great uncertainty in the question of defining and distinguishing these two concepts. The uncertainty occurs mainly because the intimate nature of I. and excitability is still far from being known. Mangold separates these concepts, considering excitation to be any active change in the processes occurring in a living organism, while he understands I.

IRRITABILITY

as an inactive reaction of changes in the life processes of the organism in response to changes in the 'surrounding environment. Until recently, Max Hartmann , the greatest biologist of our time, pointed to the vagueness of the latest definitions of I. This vagueness and generality of definitions allow them to be applied not only to organisms of any type, but even to phenomena of the inanimate world. Characterizing I. predominantly by negative signs and only on the basis of external similarity of the final reactions of the organism, Hartmann is far from the tendency to excessively generalize or reduce the phenomena of I. to analogies of the inanimate world. On the contrary, relying especially on Blaauw (Blaauw), Hartmann distinguishes special types of I. for protozoa, plants, and for animals possessing a nervous system. The problem of I. here comes into close contact with physiology, but since comparative physiology is still in its infancy and since the study of the most intimate physicochemical processes in phenomena of I. is still little accessible, the general biological analysis of the problem is by no means useless for its development. With all sharpness, the problem of I. and excitability is posed by E. Bauer (Bauer), who considers it unnecessary to create special concepts for I. and excitability and combines them into one. Attaching great importance to the physicochemical analysis of the phenomenon of I., Bauer proclaims the specific biological nature of I., which cannot be explained by usual comparisons with the phenomena of discharge. The external manifestation of I., proceeding not in accordance with the magnitude of the energy of the stimulus, but according to the type of a powerful explosion arising from a tiny spark, forced one to stop with special attention on this feature of I. The mechanistic traditions of 19th-century biology and physiology produced a schematic pseudophysicochemical analogy of I. with discharge instead of investigating the actual physicochemical processes of I. in their biological uniqueness. But along with this, in the plan of physicochemical consideration of I., a special, deep direction was acquired by the problem in the research of J. Loeb, Nernst, Bethe (J. Loeb, Nernst, Bethe), Lazarev and others_k Loeb focused his attention on the electrical properties and the role of salts in the media bathing the cells (sea water, blood). The previously prevailing concepts of semi-permeable membranes forced Loeb to carry out a cycle of works that proved the relativity of semi-permeability, i.e. the possibility of passage through cell membranes not only of water, but also of various salt ions. In this case, phenomena of I. occur only in those cases when the ratio between the concentrations of monovalent ions (sodium and potassium) and divalent ions (calcium and magnesium) has a certain value: ^{Na K} = Const. Thanks to the electrical properties of transfer, ionic relations acquire corresponding values at the boundaries of cell membranes and at the same moment contraction of the muscle fiber occurs. The phenomena of electrical polarization and depolarization, associated with the weakening and strengthening or with selective ionic permeability of cell membranes, allowed Bernstein (Bernstein) to introduce a number of new confirming positions and to explain the results of classical experiments of du Bois-Reymond and Pfluger over currents of rest, currents of action, electrotone, etc. The surface of a cell excited by any stimulus or damaged is equally depolarized and becomes electronegative in relation to the non-excited or undamaged, i.e. polarized surface. At the site of damage or irritation, some ions enter the cell, while in undamaged or non-irritated areas other ions exit outward. The excited or damaged surface becomes more permeable. When connected by a conductor, current flows from the excited (damaged) place to the non-excited (undamaged). The ion-membrane theory of I. was developed simultaneously with Loeb and the physicist Nernst, who approached the question with an attempt to clarify the causes of not only the harmlessness but also the ineffectiveness in terms of irritation of the organism's exposure to currents of enormous frequency (Tesla currents). He came to the conclusion that the only possible change in the cells during irritation consists in a change in the concentration of ions at the boundary of two phases - the surface of the cell and the fluid bathing it. In this case, Nernst established the law -=Const, where i is the current strength, and n is the number of periods of oscillation of alternating current. Thus Nernst showed how with the help of electric current one can change the concentration of ions to the corresponding threshold irritation, and Loeb in his formula indicated which ions and in what relations appear in this biological phenomenon. Bethe and Lazarev expanded and strengthened the concepts of changes in ionic concentrations and ratios of ions on cell membranes and surfaces as direct causes of excitation. But Bethe attributes the main role to the redistribution of H- and OH-ions, and Lazarev, with his indications of ionized products of the photochemical process in the rods and cones of the retina of the eye, etc., believes that besides the great importance of alkaline and alkaline earth cations, it is possible to detect in special cases no less important a role in the process of I. of other ions. According to these concepts, in order to cause a state of irritability, irritation must first of all cause changes in ionic concentrations and ratios at the surfaces of cells, cell fibrils, cell inclusions, etc. In this case, naturally, the strength and duration of irritation must have certain values. In other words, a certain threshold magnitude of the stimulus must be reached. But as soon as a certain concentration and ratio of ions is reached, reversible changes occur in the colloidal state of the protoplasm, expressed in an increase in viscosity, gelatinization, etc., as well as a loosening of the cell membrane and an increase in its permeability. This is basically the general physicochemical initial processes of I. in any cell with any irritant. The specificity of individual types of I. is determined mainly by the subsequent stages of the process, where the biological significance of I. in the system of a particular tissue, organ appears with special force. In the nervous system, muscle, glands, in protozoa, in individual cells (leukocytes, tissue culture), in special plant tissues, etc., I. after the initial 22 ft general features takes on unique features, incidentally much less clarified than the primary links of the phenomenon of irritability. It is characteristic that the primary links of I. are almost exclusively of a physical nature, while the chemical processes of life activity appear later, in the process of reversible restoration of the colloidal properties of the cell, just changed by irritation. Metabolism is therefore negligible in the first phase of I. and intense in the subsequent period, which often extends not only to the process of restoration of ionic ratios and concentrations, viscosity and permeability, but also to the elimination of those changes that occurred as a result of the first and led to the external manifestation of the organ's work (contraction, movement, secretion, etc.). The crude analogy of I. with discharge, with a powerful explosion from a spark, loses its attractiveness and meaning in the process of detailed study of I. The physicochemical analysis of secondary specific differences in the phenomena of I. cannot, however, replace the biological study. In this respect, even such a mechanistic physiologist as Winterstein (Winterstein) was recently forced to make a number of principal concessions in a special article devoted to questions of I. and excitability. The process of evolution of secondary specific differences in the phenomena of I. is deeply biological and has led to the formation of a whole series of special adaptations and conditions for the so-called func., nutritive or trophic, formative and a whole series of other irritations. Very interesting in modern biology are the so-called formative irritations, appearing in the process of development of organisms, when some parts influence the development of other parts of the organism in one way or another. In this sense, the former are considered formative irritants for neighboring areas, where under the influence of formative irritations a certain morphol. structure begins to develop. As classical examples of formative irritation in the neighborhood of tissues, the experiments of Spemann, Lewis (Spemann, Lewis) and a large number of other researchers on the development of the lens of the eye in amphibians from the epidermis only under the conditions of direct contact with the eye cup are cited. Close in meaning, but even more special is the concept of formative-trophic irritations. Here irritation is directed directly at morphogenesis, acting through local excitation of tissues to enhanced nutrition. Speaking about this type of irritation, it should be stated that it often coincides with the func. state of the tissues and therefore manifests itself as func. irritation.

Roux (Roux) has indicated a number of general examples of functional irritation, of which a particularly vivid example is the dependence of the formation of bone struts on the mechanical forces of tension and pressure on bone tissues during the functioning of the limbs. - The clarification of the physical and chemical mechanisms of all these special types of irritability still remains at an extremely primitive stage. The most significant successes have been achieved in the field of the study of endocrine irritants and the mitogenetic factor of cell division. However, the specific problems of biology do not always require such clarification. One must expect that much of what is attributed in biology to phenomena of irritability, especially formative irritability, will upon further study turn out to be phenomena of a completely different character than ordinary physiological irritability.