Threshold

THRESHOLD, THRESHOLD STIMULATION, the minimal strength of stimulation capable of causing a reaction. The classical concept of threshold is based on the principle that any stimulation affecting excitable tissue must reach a certain intensity to elicit a reaction. As long as this intensity is not reached, the stimulation causes no physiological changes whatsoever. However, this view contradicts the firmly established fact that subthreshold stimulations (those below the threshold) change the state of excitable tissue. Such weak stimulations, while not causing a reaction by themselves, are capable of summation, that is, of producing an effect when repeated. This is possible only if they leave certain traces in the excitable tissue. This phenomenon was named by Ch. Richet as latent summation, and Lapicque even gave it mathematical expression. It is most sharply expressed in the central nervous system. The concept of threshold is essentially incompatible with the idea of subthreshold stimulations, since in that case one would have to consider as threshold the strength of stimulation that begins to leave in the tissue the aforementioned trace. However, the effects of subthreshold and suprathreshold stimulations are essentially different. Suprathreshold stimulations cause excitation. Subthreshold stimulations only change the tissue's sensitivity to incoming stimulations—they sensitize it. Therefore, while associating threshold with the appearance of excitation, one must preserve alongside it the concept of subthreshold stimulations. Threshold is considered a measure of excitability; this is incorrect if by excitability one means the capacity of a given tissue for excitation, since in that case changes in threshold should go parallel with changes in the intensity of the reaction to stimulation of the same strength. In reality this does not happen, and it is possible for threshold to rise under various conditions while the reaction decreases, although the strength of the applied stimulations remains the same, and vice versa. Threshold represents an independent constant that, among other factors, characterizes the physiological state of excitable tissue. It is a linear quantity. It can be called excitability if the latter is understood as the limiting value of a physical or chemical factor at which this factor causes a reaction. It is extremely difficult to find a method of measuring stimulation that would give the same expression of threshold intensity for different stimulations. The most general expression would be threshold energy; however, it is not always possible to determine the energy of the stimulus. But even in cases where we deal with only one kind of stimulation, we obtain different threshold values depending on the method of measurement. For example, in mechanical stimulation by a falling weight, one can seek threshold by changing the height of fall at constant weight or by changing the weight at constant height. In both cases we change the kinetic energy acting on the excitable system. Nevertheless, the value of threshold energy will be different in both methods of measurement. All this makes threshold a constant that is extremely inexact and dependent on experimental conditions. Particular efforts have been devoted to developing a method for determining threshold for electrical stimulation, the most commonly used in physiology and medical practice. Dubois-Reymond asserted that electrical current stimulates only at the moment of its onset and disappearance, while the time of current flow plays no role. Moreover, he believed that the stimulating effect of current is connected with the change in current density (that is, the ratio of current strength to the cross-section of the conductor). According to this view, with instantaneous switching on of current, threshold will be determined only by current strength, since the cross-section of the excitable organ will remain constant during the experiment. Therefore, for a long time threshold was determined by the strength of the stimulating current. However, it was soon established that for the stimulating effect of current not only strength but also the time during which it flows through the tissue are important. Then the determination of threshold by current strength became insufficient. The works of Hoorweg, Waller, Tsibulsky, and Zanietovsky emphasized the importance of energy as the quantity determining the stimulating effect of current. More recently, Lapicque, using Nernst's law, gave theoretical justification for the connection between current energy and its stimulating effect. Weiss proposed as the quantity characterizing the stimulating effect of current the amount of electricity. Cremer even proposed a special unit, the ampere-coulomb, for expressing the threshold value of the amount of electricity. The question of which of these two viewpoints is more correct remains open. Along with the determination of threshold energy and threshold amount of electricity, in practice the determination of threshold current strength and threshold voltage continue to be used. Since both energy and amount of electricity include current strength (voltage), resistance, and time, in experimental conditions where both resistance and time of flow of the stimulating current through the object are constant, current strength gives, as a first approximation, relative values of threshold. It is more convenient to use voltage, since this value does not depend on resistance.

A. Magnitsky. The concept of 'threshold, threshold stimulation' is applied not only to the physiology of excitable tissues but also to the mechanics of development. The term was introduced into this science by M. Zavadovsky (1924) with respect to the development of sexual characteristics under the influence of sex hormones. The fact was established of the non-simultaneous disappearance of sexual characteristics when the activity of the sex gland ceases, as well as the gradual appearance of these characteristics during regeneration of the gland after its partial extirpation. An attempt at quantitative study of threshold stimulation was made by Ilyin (1926) in the analysis of the coloring of an ermine rabbit. The pigmentation of these rabbits depends on the ambient temperature, with a decrease in temperature leading to the growth of black hairs in place of white ones (back and sides), and an increase in temperature leading to the growth of white hairs in place of black ones (ears, nose, legs, tail). According to Ilyin's definition, the threshold stimulation of tissue for the pigment-forming reaction should be understood as the maximum temperature at which the tissue is still able to form pigment, or the minimum temperature at which non-pigmented hairs grow. For different parts of the ermine rabbit's body, these threshold temperatures are different. Thus, for the side and back, threshold stimulation corresponds to temperatures of +1° to +2°, for the front legs +14°, for the hind legs +16°, for the ear +25° to +27°, and for the tail and nose +29°. Different thresholds of stimulation have been established for the organs of a tadpole transforming into a frog with respect to the thyroid hormone (Blyakher), and for various areas of the pigment system in the skin of an axolotl with respect to the pituitary hormone (Vorontsov). The latter author established that the difference in coloring of black and white races of axolotls is connected with the difference in the height of the threshold of stimulation of their skin with respect to the pituitary hormone. Specifically, the skin of the white axolotl has a higher threshold of stimulation than that of the black one. The study of the effect of hormones on the formative process from the standpoint of the threshold amount of active hormone raised the question of the applicability to the mechanics of development of the 'all-or-none' law (see). Pezard, in experiments with partial castration of roosters, concluded that even 1/100 part of the male sex gland remaining in the body is capable of maintaining normal development of the comb. On the basis of his experiments, Zavadovsky comes to the conclusion that the 'all-or-none' law is not applicable to formative processes. For different degrees of development of characteristics related to the activity of endocrine factors, different threshold amounts of hormonal stimulation may be important.