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(1) A phasic reaction of living tissue to a stimulus of specified duration and strength by which the tissue’s excitability, conductivity, and normal processes of excitation are temporarily suppressed. The phenomenon of parabiosis was demonstrated and a theory was developed by N. E. Vvedenskii in 1901 using the neuromuscular system of a frog.
As some stimulus, for example, an electric current, is applied to a nerve a gradually developing, phasic change in the reactive properties of the nerve arises at the site of stimulation. In the first phase—the transformational phase—a weak stimulation of an area of the nerve that is located proximally to the parabiotized area results in roughly the same type of muscle contractions as is the case with strong stimulation. With both strong and weak stimuli, the amplitude of contraction is reduced in comparison with the amplitude that results from a nerve in which no parabiosis has been effected. In the second phase, which is called Paradoxical, strong stimuli to the region that is proximal to the parabiotic region result in weaker tetanic contractions than do weak stimuli. In the third phase—the inhibitory phase—both strong and weak stimuli when applied to an area of the nerve that is located distally to the parabiotic area do not produce any contractions at all. Irreversible changes and atrophy of the nerve result from continued application of the parabiotizing stimulus. When the stimulus is removed, the nerve gradually returns to the initial state through a reverse sequence of the phases of parabiosis.
Parabiosis is accompanied by a permanent decrease in the ability of a nerve to recover after stimulation (seeLABILITY). The excitability and conductivity of a nerve are qualitatively distinct in each phase of parabiosis, which reflects phasic changes in the electrical potential of the stimulated area. Vvedenskii regarded all phases of parabiosis as different manifestations of a single process—excitation. He characterized parabiosis as a stationary type of excitation; he further postulated that parabiosis was the normal form of excitation in the early stages of evolution. D. N. Nasonov and his co-workers showed that the phenomenon of parabiosis can be attributed to reversible changes of cytoplasmic proteins, changes that are similar to the initial phases of protein denaturation. These theories about the mechanism of parabiosis were eventually supported by research on the mechanisms of excitation and inhibition alternation in the central nervous system and by research on higher nervous activity in general.
I. P. Pavlov showed that as internal inhibition in the cerebral cortex arises, a fourth phase of parabiosis is observed—the ultraparadoxical phase. During this phase positive stimuli produce a negative effect, while negative stimuli produce a positive effect. Discovery and elucidation of the phenomenon of parabiosis revealed the genetic unity that underlies the processes of excitation and inhibition and indicated the interconnection between excitability and conductivity.
(2) An experimental method in physiology by which the circulatory and lymphatic systems of two or more organisms are artificially joined. Parabiosis is used in physiological experiments for studying mutual humoral influences between two animals. The technique received widespread acceptance after the work of the German scientists F. Sauerbruch and M. Heyde was publicized in 1908. Parabiosis is also used for studying rejection of tissue and organ transplants and for investigating the action of hormones and other metabolites.
REFERENCESUkhtomskii, A., L. Vasil’ev, and M. Vinogradov. Uchenie o parabioze. Moscow, 1927.
Vvedenskii, N. E. “Vozbuzhdenie, tormozhenie i narkoz.” Poln. sobr. soch, vol. 4. Leningrad, 1953.
Nasonov, D. N. Mestnaia reaktsiia protoplazmy i rasprostraniaiushcheesia vozbuzhdenie, 2nd ed. Moscow-Leningrad, 1962.
I. V. ORLOV and V. V. SHERSTNEV