Successive Contraction

SUCCESSIVE CONTRACTION, a muscle contraction occurring after the cessation of a process of inhibition in the nerve centers innervating the given muscle, or a contraction observed not during the actual action of a stimulus, but after its cessation. This phenomenon was described by Sherrington under the name of "rebound contraction," and the process in the central nervous system which causes the appearance of successive contraction received from him the name of "successive induction." It refers to the fact that a center falling into a state of inhibition under the influence of afferent impulses finds itself, after the cessation of these impulses, in a state of increased excitability or even falls into a state of excitation. A typical example is the contraction of extensor muscles after the cessation of the contraction of flexors. The contraction of the latter causes inhibition in the extensor apparatus, and the disappearance of these inhibitory impulses leads to successive contraction. In its most acute form, the phenomenon of successive contraction manifests itself precisely in the extensors after the cessation of the flexor contraction that inhibited them. The reverse process (contraction of flexors after the cessation of extensor contraction) is expressed less sharply. The regulation of respiration established by Hering and Breuer likely belongs to this same class of phenomena: irritation of the sensory endings in the lungs and respiratory muscles during exhalation increases the excitability of the inhalation apparatus, and conversely, the activity of the latter "switches" the respiratory innervation to the execution of exhalation. Successive contraction of muscles is thus merely an expression of the state of nerve centers altered in the process of reflex coordination. These phenomena are described with the greatest clarity only for the lower sections of the brain (spinal cord, spinal cord with the medulla oblongata, midbrain, and diencephalon, up to the red nuclei exclusively). It is highly probable, however, that the central activity underlying successive contraction also has significant importance in normal life during the performance of well-practiced movements. In particular, the mechanism of walking apparently includes the contraction of the extensors of one leg after the cessation of its flexion and the increased excitability of the flexors of the symmetrical limb after the cessation of the flexion of the limb that contracted in the previous moment (the homologous muscles of the limbs are antagonists just as the flexors and extensors of one limb are; it must be remembered, however, that the grouping of muscles into synergists and antagonists is functional, not anatomically predetermined). For humans, Bethe and Kast demonstrated the existence of successive contraction during the rapid alternation of flexion and extension of a limb amputated according to Sauerbruch. It must also be remembered that instead of successive contraction, one can have increased excitability of a center after the cessation of inhibition, which facilitates the execution of a reflex opposite to the one that, occurring in the previous moment, created the conditions for inhibition—subsequent excitation (excitation after inhibition) in the given centers. A number of reflexes will then appear as "diphasic" (Weizsacker). Closely adjoining successive contraction (in any case, externally) are the cases described by Sechenov of increased muscle contraction not at the moment of strong irritation of an afferent nerve, but after the cessation of this irritation. It is possible to think that the actual stimulus, due to extreme intensity, causes phenomena of depression, and the traces of this stimulus turn out to be optimal in strength for evoking an effect. Similar cases of excitation after inhibition are also described in the physiology of conditioned reflexes (Frolov, Rikman). G. Konrad.