Tonus

TONUS, the elastic-viscous properties of muscles, a known degree of involuntary constant muscular tension. There are three different understandings of the term T.: 1) resistance of the muscle to forces stretching it (Rieger, Spiegel), 2) ability of the muscle to long maintain a certain degree of contraction (Foix) and 3) the consistency of the muscle is taken as the criterion of T. In this respect, the muscle is characterized, besides the state of shortening or lengthening, by its elastic and plastic properties. Elasticity is understood as the ability of the muscle to resist deforming forces and to return to its lost shape (contractile T.), while plasticity is the degree of pliability and the tendency of the muscle to long maintain each newly given position, to fix each deformation of its length, thickness, etc. (plastic tonus). Some authors (Ricsser) define plastic T. as the ability of the muscle to change the degree of tension independently of length, while in contractile tonus this is achieved only by shortening and lengthening the muscle. Regarding the role of T. in the life of the organism, some think that it consists mainly in creating favorable conditions facilitating the performance of tetanus (Pie-ron), while others (Rieger) pay special attention to the moment of delay of movement. In general, tonic functions play an enormous role both in maintaining the body and its parts in a certain position, and in movement. Further development of these concepts requires consideration of some peculiarities of T. of smooth musculature and tonic functions of invertebrate muscles. Unlike striated musculature, in which each single contraction is automatically followed by relaxation, the smooth muscle has the property of freezing in a contracted state, and special innervational influences are required for the contracted smooth muscle to relax. Thus, the smooth muscle, besides the ability to shorten and relax, also has the function of long fixation of the contracted state. In the musculature of bivalve shells, these functions are separated also by substrate. The transparent ligament by its contraction closes the valves, while the white one maintains them in the closed state. In snails, the innervation is also separate. The supraganglionic node regulates the clonic functions, while the pedal ganglion has the role of inhibiting T. In the smooth musculature of the spines of sea urchins, we encounter the ability not only to long maintain the contracted state, but also to vary the degree of tension depending on the resistance overcome. This function is denoted by the term "sliding tension" (Uexkull). The tonic function of smooth muscles differs in its peculiarities compared to the tetanus of striated muscles. It does not, like the latter, involve an increase in oxidative processes, is not connected with loss of weight, does not give the typical for tetanus fluctuations of action current, proceeds without formation of lactic acid. In the striated musculature of vertebrates, two types of fibers are distinguished: 1) red fibers, containing hemoglobin, rich in sarcoplasm and granular on cross-section, and 2) those not containing hemoglobin, having fewer granules on cross-section, poor in sarcoplasm. Muscles of the first category are rich mainly in those muscles that produce prolonged and strong contractions. If one compares, for example, the m. pectoralis of a falcon, a goose, and a domestic chicken, it will turn out that red fibers are most numerous in this muscle in the falcon and least in the chicken. In the rabbit we encounter muscles consisting almost entirely of red fibers. Such muscles by their function belong to the tonic category. To the latter can also be attributed such muscles as the t. tenzor tympani. In most animals, however, in the same muscle there are fibers of both kinds. In accordance with this, each muscle has both clonic and tonic functions. Some authors consider it possible to speak of division of functions within a single muscle fiber and attribute to the sarcoplasm the ability to contract extremely long-lasting, while the myofibrils give rapid contractions (Bottazzi). Regarding tonic functions, plasticity is supposedly conditioned by the properties of sarcoplasm, while the fibrils are elastic bodies (Riesser). Others attribute plastic and contractile tonus to different muscle fibers. Still others consider sarcoplasm the carrier of tonic functions in general; plastic tonus supposedly depends, in the opinion of these authors, on the relaxed state of sarcoplasm, contractile on its active state (Langelaan). In parallel with this, evidence is also brought for the separateness of tonic and clonic functions of striated musculature on the basis of other data. It is believed that the former are characterized by the creatine phase of metabolism in the muscle, unlike the lactic acid phase typical for clonic functions (Pekelharing). To this is added that T. is connected with the transformation of free potassium into bound potassium. Potassium supposedly enhances T., while calcium causes clonic functions. Gas exchange and action currents in both types of functions also differ sharply. If some authors find a fundamental difference in gas exchange and action currents in tonic and clonic functions and believe, for example, that the influence of tonic functions on gas exchange equals zero, and action currents are either completely absent or give extremely slow long-wave oscillation of the string, others find only a quantitative difference and reduce negative results to shortcomings of methodology. Thus, there are two fundamentally different viewpoints on this basic question: 1) the mechanism of T. and tetanus is the same; 2) T. and tetanus differ both in metabolism and in their reflection in electrical processes and are even separate by anatomical substrate. Finally, proponents of the latter viewpoint speak of different innervation of these functions of striated muscle. Some (de Boer) in general connect T. with the autonomic nervous system, others (Hunter, Langelaan) believe that only plastic T. depends on the latter, while contractile T. is innervated, in their opinion, by cerebrospinal fibers. There is also a dispute as to whether tonic functions are connected only with the sympathetic system (n. sympathicus) or also with the parasympathetic (n. vagus). In this, proponents of the latter viewpoint differ in that some (de Boer) attribute to the sympathetic nerve an increase in T., to the vagus a decrease, while others (Егапк) on the contrary. It should be noted that recently a theory (Orbeli) is being developed, according to which both contractile and plastic T. are conditioned by cerebrospinal innervation, while the sympathetic innervation has the role of adaptation of the muscle to one or another state of T. in the sense of creating certain favorable conditions ("topotropic" influence of n. sympathici according to the terminology of this school). More and more facts speak in favor of this latter viewpoint. In the whole organism, T. is under the influence of a number of neuro-humoral influences. The result of these is that displacement to which tendons are subjected during their in vivo severance. On the other hand, destruction of the spinal cord or interruption of afferents or efferents of the muscle leads to cessation of this tense state and to flabbiness of the musculature. If in a decapitated frog the posterior roots innervating one of the paws are severed on one side, it will turn out that in the preparation suspended by a hook this paw will hang lifelessly, while the second will be somewhat drawn up. Such tension, caused by constantly incoming along the posterior roots innervational influences, in this case originating from proprioceptors of stretched muscles, is designated as reflex T. (Brondgeest). However, the musculature of spinal animals is still comparatively relaxed even with complete preservation of all parts of the reflex pathway. This already testifies that the innervational influences coming from the brain to the cells of the anterior (and lateral) horns of the spinal cord play a dominant role in creating muscle T. In decerebrated animals, the state of the musculature (especially of extensor groups) becomes extremely tense (see Decerebration, decerebrational rigidity). If in such an animal the posterior roots innervating any limb are severed, rigidity in it sharply weakens (mainly due to exclusion of proprioceptors). This proves the reflex nature of decerebrational rigidity. For the latter to arise, it is necessary that the pyramidal and rubrospinal tracts be interrupted (according to data of some authors only the rubrospinal tracts). Preservation of the cerebellum or its exclusion has no decisive significance. With destruction of the cerebellum, decerebrational rigidity even increases, and with its irritation it somewhat weakens (at least in extensors; in flexors sometimes an increase in T. is observed). Destruction of the labyrinth also causes a decrease in T. of extensors of the decerebrated animal on the corresponding side. Any attempt to bring a limb of such an animal out of its initial position encounters sharp resistance.

In this case, we encounter an increase in contractile T., in the origin of which a dominant role is attributed to the so-called myostatic reflexes (Sherrington). The latter are conditioned by stimuli coming from proprioceptors of stretched muscles. Parallel to this, in decerebration rigidity, the tonic neck and labyrinthine reflexes (Magnus) are sharply expressed, regulating posture and participating in the distribution of T. by means of proprioceptors of the neck and balance apparatuses in the inner ear. Part of them is connected with the cervical region of the spinal cord (neck reflexes), the localization of others (tonic labyrinthine reflexes) is attributed to the pons Varolii, and finally the third ones (Stellreflexe) require the preservation of the red nucleus. A significant role in conditions increasing T. is also played by the so-called postural reactions of decerebrated animals upon irritation of a transverse section at the level of the red nuclei (Graham Brown). In this case, a prolonged, persistent contraction of the muscles extending and turning the head to the same side is obtained, which continues after the cessation of irritation, flexion of the homolateral forelimb and extension of the contralateral one. The contralateral hind limb is flexed, the homolateral one is extended, the back is slightly convex on the side opposite to the irritation. The tail straightens and bends toward the side of irritation. The pyramidal phase functions are in antagonism with this postural reaction. Stimulation of the corticospinal tract ceases the latter. In decerebration that preserves not only the midbrain but also the area of the optic thalamus, an increase in plastic T. is observed. In such decerebrated animals, the extensor and flexor reflexes are sharply expressed, manifesting themselves in the fact that if the attachment points of tendons are brought closer together or moved apart, the muscle always freezes in this new shortened or lengthened position (Sherrington). In these reactions as well, proprioceptors play the main role. The section of the posterior roots innervating one or another limb eliminates its participation in the extensor and flexor reflexes, which proves their reflex nature. If one approaches the question of the conducting pathways and nerve centers of muscle T. and its manifestations (contractility and plasticity) as a whole, much will still be undeveloped (in particular: 1) the role of vegetative centers in the regulation of plastic properties of the muscle, 2) the relationship of the cerebellum to contractile tonus, etc.). Therefore, one still has to be content with certain schemes developed on the basis of pathological data, which are to a significant extent inaccurate in regard to their accuracy.

L. Fidelgolts. Pathology. For the study of tonus in clinical practice, many methods are used. Some of them have only an indirect relation to tonus. Thus, tonus is judged by the condition of tendon reflexes, by the condition of active movements. Particularly included here are the studies of Lewy, who used the technique proposed by Isserlin for recording finger movements (Weiler's apparatus), accurately calculating the speed of active movements in their various phases and determining the occurrence of action currents in agonists and antagonists. Of course, much can be clarified in this way. However, the main significance belongs to other methods: methods for studying muscle resistance and methods for studying muscle resistance during passive movements. For studying muscle resistance, a whole series of apparatuses has been proposed: the apparatus of Exner and Tandler; the apparatus of Noyons and Uexkull, which determines the depth of indentation produced in the muscle by a load supported by an electromagnet when the current is broken; the spring sclerometer of Wertheim-Salomonson, which measures both the depth of indentation and the degree of tension of the spring pressing on the pelotte; the ballistic sclerometer of Noyons, constructed on the principle of a pendulum, the oscillations of which die out the weaker the more solid the object against which the pendulum strikes; the apparatus of Mangold, consisting of a lever with two arms, equipped with a pelotte: the depth of indentation of the muscle produced by this pelotte is determined by the displacement of the arm on a millimeter scale; the apparatus of Gildemeister, which determines the time of impact of a load falling on the muscle. To this same group belongs the method of Lewy, which, however, registers not the resistance of the muscle, but the degree of its thickening during passive movement. The second group of methods, studying tonus in clinical practice, consists of methods for measuring the resistance of muscles during their stretching during passive movements. Uexkull asserts that the state of tension (Sperrang) of muscles corresponds to their resistance. Then resistance is indeed a measure of T. On the contrary, Gildemeister considers the parallelism of both phenomena doubtful, Lewy establishes only a 'certain' relationship between T., resistance, and elasticity of muscles, Spiegel considers resistance only as a 'side effect' and believes that it gives only an approximate idea of the state of the tonic function (Haltefunktion), while the state of resistance to stretching is the actual measure of T. The significance of resistance to stretching as revealing the function of 'internal muscle tension' was already emphasized by Fick. Weizsäcker considers as a measure of tonus that force which is necessary to perform a passive movement at a given speed, and defines the index of T. by the formula I = -^ (K = force, S = speed). In general, it must be admitted that both methods—the method of measuring resistance and the method of measuring resistance to stretching—do not give fully comparable results, since the states they study are qualitatively different, therefore such a comparison should be approached with great caution. It is necessary in this respect to follow the example of Förster, who in characterizing extrapyramidal rigidity considers in separate rubrics the state of plastic T. that gives form to the muscle, the increase in resistance to stretching, and the phenomena of fixation rigidity. Unfortunately, a global characterization of changes in T. is much more common. In clinical practice, first place among all methods belongs to determining the state of tonus by studying resistance to stretching. Instrumental research, requiring the use of complex apparatuses, great expenditure of time, etc., is used very rarely, in essence only for clarifying special pathophysiological questions. As a rule, ordinary manual research is used for diagnosis: the patient is made not to actively tense the muscle groups being examined, not to offer active resistance, and passive movement is performed, determining by one's own sensation the degree of resistance encountered. This research is one of the most difficult among neurological examinations, places great demands on the experience of the researcher, who must be well acquainted with the normal state of T., which is achieved only through great practice, must be able to make the patient refrain from active resistance, must be able to isolate purely functional (neurotic) changes and differentiate them from organic changes, which is especially difficult in those very frequent cases where neurotic changes exist simultaneously with organic changes and mask them to a greater or lesser degree. Changes in T. that interest the clinician can go in two directions: toward lowering (hypotonia, atonia) and toward raising (hypertonia). Since T., as has been shown for a long time, represents a reflex, its lowering or disappearance can be caused primarily by disruptions of the reflex arc at one or another of its points. What role the autonomic nervous system plays in this is not yet fully clarified with certainty, certain participation in the implementation of T. has been attributed by various researchers to both the sympathetic and parasympathetic systems, with a certain probability the topographic function is attributed to the sarcoplasm, while the myofibrillar system must manage only purely motor functions. It is firmly established that damage to both efferents (anterior horn cells, anterior roots, peripheral motor nerves) and afferents (peripheral nerves, posterior roots) of the spinal arc leads to hypotonia or atonia. This condition is revealed, first, by an increase in the range of passive movements in the corresponding joints, and second, by a decrease in resistance. In normal conditions, the muscle being stretched tenses at the beginning of its stretching, as if striving to maintain the original length which it has at rest [inhibition or braking (Bremse) of Rieger], which has very great significance for motor functions, for their smoothness, evenness, for their entire dynamics. In case of damage to the spinal arc (a particularly illustrative example here is provided by cases of posterior radiculitis, tabes dorsalis, where the characteristic paralyzes and amyotrophies of the effector system are absent, masking to a greater or lesser degree the role of the loss of the actual tonigenic moment), the disappearance of this braking finds its clear expression not only in passive movements but also in the voluntary motorics of patients, in the excessive range of their movements, in the classic throwing of the legs at the hip joints when walking, in stamping the ground with the heels, etc.; tabetic ataxia is explained to a large extent by disorders of T., by disappearances of the Rieger mechanism. But hypotonia can also be observed in the absence of a direct interruption of the spinal reflex arc. As is known, strokes affecting the pyramidal system in the region of the internal capsule, etc., lead to hemiplegias which in the initial phases of the disease are usually characterized not by an increase in T., as happens later, but by its falling. Essentially, however, here too the matter concerns damage to the same spinal arc, namely its effector neuron (anterior horn cell), whose function is deeply, although only dynamically, affected in this case due to diaschisis (see). When the phenomena of diaschisis are overcome, hypotonia gives way to hypertonia, which is in essence characteristic of the pyramidal symptom complex. In some cases, however, hemiplegia remains flaccid forever, and here the matter concerns a long-lasting diaschisis due to the weakness of the protoneuron itself (general exhaustion, etc.), which is unable to recover from the shock it has experienced. The same genesis apparently also have flaccid paraplegias arising from high total lesions of the spinal cord (Bastian's law), although besides diaschisis other factors (circulatory disorders, etc.) may also play a role here. Finally, hypotonia can be the result of damage to the cerebellum. The mechanism of the occurrence of hypotonia of this origin is not yet fully clear, experiments in general give largely contradictory results, but the clinical significance of the decrease in T. on the side of cerebellar damage stands beyond any doubt. The second large group of tonus disorders consist of hypertonicities. Experiments with decerebration rigidity show that in the area below the level of the red nuclei there is a strong tonigenic center, influenced by impulses originating from the general proprioceptive system, especially from the proprioceptors of the neck musculature and from the labyrinths (see Magnus-de Kleijn reflexes). The main significance for the regulation of tonus is had, as especially the experiments of Rade-maker show, by the red nuclei. In cuts oral to the red nuclei, no essential changes occur in the T. of animals, whereas the separation of the red nuclei from the underlying parts or the destruction of the Forel's decussation leads to general hypertonia.

Based on these experiments as well as an analysis of the literature relating to clinical practice, Rademaker considers it necessary to recognize the primary importance of the red nuclei in the regulation of T. in humans, disputing traditional views on the significance here of the pyramidal system, pallidum, and substantia nigra: 'The significance of complete or partial destruction of the pyramidal pathway for the occurrence of hypertension in humans has been evaluated too highly up to now', 'the role which the striatum plays in the regulation of tonus in humans' is still unknown, and it is not yet reliably established that it plays any role in this at all', 'the existence of a regulating T. center in the substantia nigra is very doubtful'. How correct this revision of conventional views is, the future will show; for now, the prevailing position is occupied by the doctrine of hypertension as a consequence of lesions of both the pyramidal and extrapyramidal systems. The symptom complexes arising in the one and the other case possess characteristic features and differ very substantially from each other. Hypertension occurring with a lesion of the pyramidal system, or spastic hypertension, is characterized above all by the uneven distribution of its spread in agonists and antagonists, affecting only certain muscle groups (selective type): on the upper extremity, the muscles that depress the shoulder joint, the adductors of the arm, the flexors and pronators of the forearm, and the flexors of the hand and fingers are usually affected by hypertonia; on the lower extremity—the adductors of the thigh, the extensors of the thigh and leg, and the dorsal extensors of the foot. In some cases this formula may be distorted, but the basic principle—the unevenness of hypertonia in antagonists—always remains in force. The next characteristic feature of spastic hypertension is that the increase in resistance to stretching of the muscles is uneven in different phases of passive movement: for example, when flexing the knee, the examiner encounters sharp resistance right from the start, and after overcoming this, further flexion proceeds more or less freely; when extending the forearm, a similar jerky resistance is encountered approximately in the middle phase of movement (around 90°), etc. Furthermore, for spastic hypertension, its 'springy' character is typical: at the end of passive movement, the corresponding segment of the extremity forcefully returns to its original position (rebound phenomenon). Finally, a strong dependence of spastic hypertension on external irritations is characteristic: for example, the lower extremity may exhibit phenomena of the most powerful extensor hypertension of insurmountable force, yet nociceptive irritation (forced flexion of the fingers according to Marie and Foix, a prick to the sole, its thermal irritation) immediately changes the entire picture, shortening of the extremity occurs, i.e., its flexion in the hip, knee, and ankle joints. Also typical is the enhancing effect of the speed of passive movements on hypertonia: the faster the movement, the stronger the tension, while slow passive movement overcomes this tension with much greater ease. Hypertonia observed with extrapyramidal lesions has a completely different character, or rigidity. Here, first of all, there is no selective involvement of certain muscle groups by hypertonia and no contrast between the state of protagonists and antagonists, which are so characteristic of spastic hypertension. Furthermore, the uniformity of resistance in all phases of passive movement, the 'waxy' character of hypertonia, is extremely typical for extrapyramidal rigidity. The 'cogwheel' phenomenon is often encountered, i.e., the overcome tension has a saccadic character. However, the jerks here are also more or less evenly distributed in all phases of passive movement and thus sharply differ from the jerk that is characteristic of spastic hypertension. The rebound phenomenon is absent; the opposite phenomenon is observed: the muscles exhibit the property of adapting to the passive approximation of their attachment points by active tension (adaptive tension) and to remain tonically fixed in this tension (fixation tension, O. Forster). These properties are particularly clearly revealed in the Westphal phenomenon and in the postural reflexes of Thevenard and Foix. The effect of the speed of passive movement on the degree of rigidity is not always noted and in any case does not reach such a degree as in spastic hypertension. Finally, in extrapyramidal rigidity, the dependence of hypertonia on nociceptive irritations, noted in spastic hypertension, is absent.

I. Filimonov. Nervous, psychic, vital tonus—a designation for the functional state in which a person finds himself at a given time. This concept, despite its approximation and, strictly speaking, unscientific nature, has earned the right to wide application as an indicator of the psychophysiological 'energy' of the organism, its readiness for one or another manifestation of healthy activity and for the possibility of overcoming one or another influences. The concept of T. usually also includes moments related to one or another mood, its elevation, excitement, depression [euphoria (see), depression (see)], etc. At the same time, changes in T. are evaluated not only as a pathological phenomenon inherent in various somatic and neuropsychic diseases, but also as a subtle manifestation of the general reactivity of the organism, giving certain fluctuations in various periods of life in completely normal people (for example, age-related changes). In diseases, T. plays a very large role as either a negative or a positive factor. Numerous clinical observations show that when the infectious agent enters an organism with weakened or 'lowered' tonus, the course of the disease often passes significantly more acutely, more severely, accompanied by various complications. It has been noted that during epidemics, fearful, bewildered individuals get sick more often. Similarly, in surgical practice, the preoperative condition of a patient with 'lowered' T. usually causes anxiety in the surgeon, because according to clinical observations, a person with such altered T. is more susceptible to operative shocks or less resistant in the postoperative period. From the neuropsychic point of view, the behavior of a person with 'lowered' T. presents a picture of indecisiveness, fear, hesitation, or of indifference and depression. For certain psychoses and 'psychoneuroses', lowering of T. is a characteristic sign. For example, cyclothymic depression, certain forms of schizophrenia, cerebral arteriosclerosis, etc., among other symptoms, give more or less sharply expressed lowering of neuropsychic T. One or another state of T. depends on endogenous moments, for example, on disturbance of the physiological balance on the basis of insufficient or improper nutrition, on disturbance of sleep, sexual excesses, fatigue, etc. A large role must be attributed to endocrinological moments (for example, sharp lowering of T. in hypothyroidism, etc.). The general state of the autonomic nervous system is of enormous importance. Thus, the basis of the variability of T. lies in psychophysiological, humoral, and neurotrophic influences, and therefore the substrate of T. is purely material, ranging from the type of the entire structure of the organism and nervous system (the asthenic type) to complex biochemical moments. Changes in the behavior of animals (simplest) under the influence of various physiological moments—see Depression in biology. Artificial raising or lowering of T. in humans under the influence of alkaloids (caffeine, morphine, nicotine, strychnine, etc.) vividly indicates the enormous importance of intoxication moments in the origin of pathological changes in tonus. Finally, it should be pointed out that purely psychic factors, both negative and positive, are capable of sharply changing the general picture of neuropsychic tonus either in the direction of depression (psychic trauma, unfavorable situations) or in the direction of elevation (successes, achievements, skillful psychotherapeutic intervention).

L. Brusilov.