Chronaxy

CHRONAXY, CHRONAXY OF THE MUSCLE AND NERVE. Dubois-Reymond established that nervous-muscular contraction is caused exclusively by the oscillation of the current strength, not by its absolute value; the change in the oscillation of current strength is the more effective, the faster it occurs; the time of current passage and consequently the amount of electricity and energy at this time is of no significance. As is evident, the time factor is not taken into account. Fick points out that after a certain threshold, the time of current passage plays a large role. Hoorweg, causing muscular contraction by means of a capacitor discharge, came to the conclusion that the time factor in nervous-muscular contractions has a very great significance. Weiss showed that the current strength giving the threshold of contraction decreases if the time of current passage increases, but this happens only up to a certain value, after which one can increase the time arbitrarily, and the current strength causing contraction will no longer change. The time of current passage, the amount of electricity, and the current strength are connected by the following two formulas: it = a + bt; a i = -r + b. In these formulas t is the time of current passage, i is the current strength giving the threshold, and a and b are two constants: a is the quantity constant, b is the current strength constant. This is the threshold of irritation in Dubois-Reymond's law. Weiss showed that the constants a and b can vary depending on the conditions of the experiment, but the ratio a/b remains constant for a given nerve and muscle under all conditions of the experiment. Lapicque, studying the excitability of muscle and nerve, by varying the experiments in various ways, showed that the ratio a/b is the only constant element characterizing excitability. The quotient of the division of the quantity a by the current strength b is time, i.e., excitability is characterized by time, which Lapicque called chronaxy. He established that excitability is characterized by two parameters—the reobase and chronaxy, to which the following empirical definition is given. The reobase, or basic threshold, is the necessary current strength giving the threshold of contraction by means of a prolonged closing of the current. Chronaxy is the time of passage of such a current which gives the threshold of contraction at double the current strength of the reobase. While the reobase depends on the conditions of the experiment, chronaxy changes only from those causes which change excitability, such as temperature. In the very latest time Lapicque introduced the concept of constitutional chronaxy and subordinate chronaxy. The former is a constant value and, with certain reservations, characterizes the muscular state outside connection with the central nervous system. Subordinate chronaxy characterizes the muscle in connection with the central nervous system and therefore is a variable value, changing under the influence of various influences. Under ordinary conditions, various influences compensate each other and a certain average value is obtained. This is the so-called normal chronaxy. In clinic, one usually determines the subordinate (subordinate) chronaxy. Lapicque's experimental data Burgignon transferred to clinic and created a new method for determining nervous-muscular excitability in normal and pathological states, called chronaxymetry. Different muscles and nerves have different chronaxies; consequently, there was a need to compile a table of chronaxies of muscles and nerves of the human body. Measurements showed that chronaxy in the same subject or in different people fluctuates within known limits. The chronaxy of a separate muscle fluctuates between two values—the maximum and the minimum, and on a large material it is found that in most cases it holds around a certain average value. Burgignon preferred to give two figures for chronaxy—one minimum, the other maximum. A nerve has not one chronaxy, but several, depending on those muscles which it innervates; therefore it is more correct to speak not of "chronaxy of a nerve" but of "chronaxy of fibers" innervating a given muscle. With this correction one can say that the chronaxy of a nerve and the muscle to which it approaches is the same—or, formulating otherwise, one can say that the chronaxy of a muscle is the same, regardless of where it is taken—from the motor point or from the nerve. The measurement of chronaxy is expressed in "s", i.e., sigmas, or thousandths of a second. Burgignon establishes that all muscles of our body in relation to chronaxy can be divided into three groups: Group I—with small chronaxy—0.06–0.14; Group II—with average chronaxy—0.16–0.35; Group III—with large chronaxy—0.40–0.70. The reaction of degeneration received its explanation thanks to chronaxy. In a normal state in the muscle and nerve we have the phenomenon of isochronism (the same chronaxy for muscle and nerve). Upon irritation along the length, the same chronaxy is obtained, with partial degeneration heterochronism is discovered, simultaneously the character of contraction becomes less vivid. In the same muscle two chronaxies are obtained—one for the more vivid fibers, the other for the delayed ones. Homogeneity characterizes a normal muscle, heterogeneity characterizes a pathological state, and with partial reaction of degeneration this heterogeneity is greater than with full reaction of degeneration; with the latter fast fibers are not found. Changes in chronaxy during degeneration of nerve and muscle sometimes serve as the first symptom of damage. As a rule, at first a certain decrease in chronaxy is obtained, but it holds for a very short time, in the next moment chronaxy sharply increases. At this time the character of muscular contraction changes from vivid to delayed and flaccid. For some time chronaxy remains at a certain height, the muscle preserves the slowness of its contraction. Subsequently, chronaxy decreases in its value and does not come off from it until the muscle loses the ability to contract. Contractions remain slow. If regeneration does not occur, muscular fibers in the future dissolve and contractions cannot be caused even with a large current strength. During regeneration of the nerve, chronaxy undergoes approximately the same changes in the reverse direction, and in those cases where complete regeneration of the nerve occurs, chronaxy reaches a normal value. Cramps do not change chronaxy. As for contractures, chronaxy is decreased in the contractured muscle and increased in the antagonists. The chronaxy of the nerve changes not at all or changes very little. All forms of contractures change chronaxy in the corresponding muscles, and the chronaxy itself does not depend on what kind of contracture there is—central or peripheral—and in what place the contracture is located. Chronaxy determines only the functional state of the nerve and muscle. Chronaxy establishes the difference between the contractured muscle and the flaccid one and does not change with various hyperkineses. Noting the contracture, it does not indicate the type and character of it. Chronaxy reflects all functional properties of the given tissue. Muscles having one chronaxy have one and the same function and one and the same pathological potentiality. In lesions of the central neuron, as well as in peripheral, chronaxy is linked with function, but not with the cause. For early diagnosis of a number of nervous diseases, sensitive chronaxy, i.e., chronaxy of the sensory nerve, may have significance. Vestibular chronaxy is determined by means of two thin chlorinated silver electrodes, which are inserted into both ears. With a special holder in the form of a helmet they are held in place. Upon passing interrupted direct current, the head tilts towards the positive pole. The reobase is determined, the voltage is doubled, the threshold of head tilting is sought by means of capacitors, and chronaxy is determined. It has been established that each semicircular canal has its own chronaxy. Vestibular chronaxy changes in various mental diseases. Vestibular chronaxy opens new paths and possibilities in the study of the equilibrium of the human body in normal and pathological states.