Electroexcitability

ELECTROEXCITABILITY, the property of living tissue to undergo changes under the influence of an electric current. Even lower organisms exhibit sensitivity to galvanic current. In highly organized animals, the nervous system is most sensitive to electric current. The ability of living matter to propagate a stimulus received from a current to further points is especially characteristic of the nerve fiber. The conduction of excitation through the nerve cannot be reduced to a simple physical phenomenon. It is based on a complex biochemical process associated with a disturbance of ion equilibrium (see Ion theory of excitation), which leads to a change in the degree of swelling of the nerve membrane and its fibrils. In the study of the excitability of nerves and muscles, galvanic and faradic currents are used. Helmholtz estimated the speed of excitation propagation in the human nerve at 33 m per second. Modern, more precise studies have shown that it reaches 70 m per second in humans and warm-blooded animals. The speed of excitability propagation varies depending on the animal species, the function of the nerve, and external conditions. In animals during winter hibernation, it decreases to 1 m per second. In the medullated nerves of invertebrates, it is particularly low, reaching only one centimeter per second. In the same nerve, different fibers possess different speeds of excitation propagation. At lower temperatures, anesthesia, and after prolonged stimulation, the speed of excitation propagation falls. In experiment, it is possible to cause isolated muscle contraction by curarizing it, thereby preventing current spread through intramuscular nerve endings. A muscle separated from the nerve retains its faradic excitability for over a year. When current passes through a nerve, the latter's excitability changes. This change is called electrotonus. When a direct current is passed through a nerve, increased excitability to electrical, mechanical, and thermal irritants occurs at the cathode (cathodelectrotonus) and decreased excitability occurs at the anode (anodelectrotonus). Between the two poles lies an "indifferent point," in which, when current passes, excitability changes almost not at all. The regularities of excitability were first formulated by Du Bois-Reymond in his "general law of excitation," according to which a motor nerve responds with contraction of the innervated muscle not because of the absolute magnitude of the current density, but because of a change in this magnitude. Thus, for characterizing excitation, the fluctuation of current strength is of primary importance. In contrast to Du Bois-Reymond, Hoorweg emphasizes the importance of time in excitability. "The fundamental law of excitability" of Hoorweg states that nerve excitation does not occur at all from fluctuations in current strength, but that each differentiated excitation is determined by the instantaneous strength of the current. Thus, every excitation is only a function of temporal intensity. The search for temporal constants led to the introduction of the concept of useful time and chronaxia. Useful time (Gildemeister) is understood as the duration of the action of the stimulus at the minimum current strength sufficient to cause a motor effect. Useful time depends on the nature of the object being stimulated, the state of the medium, and the form and strength of the stimulus. The value of useful time is different for different muscles of the same animal and varies from 1 to 2,000 sigma (sigma = 1/1,000 sec). It is different when stimulated by direct current and by capacitor discharges. The minimum direct current strength acting for a long time and sufficient to cause an effect is called the rheobase. The time required for a current of double rheobase to cause an effect is called chronaxia (see). The excitability of the nervous-muscular apparatus has been studied in most detail. The regularities of various muscular responses, depending on the passage of currents of different strength and direction through the nerve, are expressed in Pfliiger's laws of contraction (see Pfliiger's laws). The state of excitation in a nerve or muscle manifests itself as action currents. They occur because any excited point becomes negative relative to newly excited places. To register action currents, a string galvanometer or capillary electrometer is used. Action currents appear upon experimental stimulation of both motor and sensory nerves; moreover, they can be registered under natural conditions as well. Thus, various phases of the respiratory act are reflected in the action currents of the vagus nerve, which can be verified by recording them on the so-called electrovagogram. When muscles are stretched, action currents arise in sensory nerves (Adrian). Action currents also arise in central apparatuses upon stimulation of peripheral devices associated with them. (see Electric current). Bioelectric brain currents can be obtained not only directly from the brain, but also can be detected through bone tissue (Sarkisov). Action currents arise upon excitation not only of the nervous system, but also of various, if not all, cellular elements. This is the basis of the psychogalvanic reflex of Veragut. Any physical stimulus, as well as a psychic emotion, causes a deviation of the galvanometer needle in the subject, with whom it is connected; the deviation is especially sharp in the case where parts of the body with abundant sweating, for example, the palms, are connected. The psychogalvanic reflex can be temporarily extinguished by injection of atropine. In clinic, it is used for differential diagnosis between organic and functional anesthesia. In the former, a deviation of the galvanometer needle does not occur if a stimulus is applied to the anesthetized surface; conversely, with pricks in the area of hysterical anesthesia, the needle deviates. To study the excitability of the labyrinthine apparatus, Babinski proposed a special technique (voltic test or vertigo voltic test). Small electrodes from a galvanic apparatus are inserted into both ear canals of the subject and a current of 2–5 mA is passed. Already with a current of 2 mA, dizziness occurs, the head gradually tilts toward the positive pole, the body begins to sway, and vomiting occasionally appears. With an increase in current to 4 mA, "galvanic nystagmus" occurs toward the cathode. In diseases of the labyrinth and its nervous apparatus, Babinski's test changes in the sense that greater tolerance to the current is observed, and the forms of the general motor and nystagmatic reaction change. In diseases of the cerebellum, Babinski's test is negative. The study of excitability in clinic is usually conducted with special apparatus (pantostats, multistats, boards, etc.), connected to the city network. However, for these purposes, simple apparatus used by the classics of electrophysiology and creators of electrodiagnostics and therapy can be used. The following instruments are necessary for the study of excitability. To study faradic excitability, a faradic apparatus powered by one or two dry or wet cells is needed. To study galvanic excitability, direct current from a galvanic box of 25–30 Leclanche elements with a rheostat and milliammeter is used. The necessary strength of the galvanic current is about 20 mA. In addition to these apparatuses, at least two cords with good insulation and two electrodes are needed: one, larger, indifferent, should be about 10 cm; the other, smaller, active, about 1–2 cm in diameter. The latter is mounted on a handle with a breaker. The study is conducted in such a way that faradic excitability is tested first, then galvanic excitability of the nerve; after this, the study of the excitability of the muscles innervated by this nerve is carried out in the same order. The results of the study serve as the basis for electrodiagnosis (see).