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Related to Tonus: clonus, muscle tonus


The degree of muscular contraction when not undergoing shortening. Also known as muscle tone.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



in physiology, a continuous excitation of nerve centers and muscle tissue that is not accompanied by fatigue.

In tonus of the nervous centers, areas of the brain or spinal cord send continuous impulses along the corresponding efferent nerves, and for prolonged periods maintain a given functional state of organs and tissues. The most important types of tonus are tonus of the centers of the vagus nerve and tonus of the sympathetic nervous system, both of which regulate the heart’s activity, and tonus of the vasomotorial centers.

Muscle tonus is a prolonged contraction of the muscles that maintains the body’s position (tonus of the skeletal muscles), that maintains pressure in the digestive organs, bladder, and uterus, and that maintains blood pressure (tonus of the smooth muscles).

Tonus may be contractile or plastic. With contractile tonus, considerable tension develops in the muscles, particularly the skeletal muscles. The action potential of the muscles intensifies, and there is an increase in the metabolic rate. In the muscles of invertebrates and of some lower vertebrates, contractile tonus is of the tetanus type and consists of very slow and infrequent overlapping waves of contraction. In the skeletal muscles of vertebrates, contractile tonus is maintained through alternating contractions of muscle fibers.

With plastic tonus, the tension developed by the muscle is moderate but can be maintained for a long time without fatigue or a significant increase in the metabolic rate. The muscle becomes capable of stretching considerably without an increase in its elasticity. Plastic tonus involves prolonged continuous excitation in the muscle, a phenomenon first studied and described by N. E. Vvedenskii. Resistance to tensile exertion during plastic tonus is achieved not so much through the increasing elasticity of the muscle as by means of viscous resistance, that is, internal friction. After elimination of the tensile force, the muscle does not return to its original size but remains more or less elongated; excitation is needed to return it to its original length.

Tonus of the skeletal muscles is associated with the state of the motor neurons of the spinal column. The state of these neurons depends on impulses originating in higher centers, as well as on receptors in the muscles and tendons. An increase in the afferent impulses from muscle spindles increases the activity of the motor neurons of the spinal cord and is one of the factors leading to intensification of the reflex tonus of the skeletal musculature. The level of excitability of certain types of motor neurons that help maintain muscle tonus is regulated by the reticular formation of the brainstem.

Muscle tonus is maintained in the body through the participation of various sections of the central nervous system. Tonic muscular tension is measured by tonometers. Disruption of the normal activity of nerve centers may be accompanied by intensification of tonus (hypertonia) or by decrease in tonus (hypotonia and atonía). For example, if the brainstem is cut at the boundary between the diencephalon and the mesencephalon in mammals, there is significant intensification of plastic tonus. If the brainstem is cut at the level of the mesencephalon, there is a sharp intensification of contractile tonus, a condition known as decerebrate rigidity. Analogous conditions, as well as atonia, also develop in certain diseases of the central nervous system.


Orbeli, L. A. Lektsii po fiziologii nervnoi sistemy, 3rd ed. Moscow-Leningrad, 1938.
Ukhtomskii, A. A. Sobr. soch., vol. 4. Leningrad, 1954.
Zhukov, E. K. Issledovaniia o tonuse skeletnykh myshts. Leningrad, 1956.
Iusevich, Iu. S. Elektromiografiia tonusa skeleinoi muskulatury cheloveka v norme ipatologii. Moscow, 1963.
Fiziologiia myshechnoi deiatel’nosti, truda i sporta. (Rukovodstvo po fiziologii.) Leningrad, 1969.
Bendall, J. Myshtsy, molekuly i dvizhenie. Moscow, 1970. (Translated from English.)


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
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Em normotensos sedentarios, o barroreflexo cardiovagal esta mais fortemente relacionado a complacencia arterial das carotidas, sendo que, o efeito positivo do exercicio esta associado a melhoras na complacencia das arterias (Brum e colaboradores, 2000; Monahan e colaboradores, 2001), contudo e possivel que o treinamento aumente a complacencia pela reducao do controle cronico exercido pelo tonus simpatico-adrenergico (Chen e Bonham, 2010; Goto e colaboradores, 2003).
In this study, patients with incontinence and impaired anal tonus were operated by sphincteroplasty and fistulectomy.
When comparing the values found only with the IOPI instrument, there was a greater tongue pressure in the anterior position, possibly suggesting a higher tonus at the apex of the tongue and not in the posterior region of it, despite the greater muscular concentration in the posterior region (31).
Comparative data presented in Table 2 suggest that patient 1 with left insular ischemic stroke had a parasympathetic predominance (higher values of RMSSD, NN50, pNN50 and HFnu), while patient 2 with right insular ischemic stroke had a more pronounced sympathetic tonus (lower values of RMSSD, NN50, pNN50 and HFnu, higher LF/HF ratio).
Loss of tonus and rarely myoclonic jerks may accompany.
(3) Na literatura, foi descrito que, durante o sono, ha um aumento na resistencia nasal causado pela posicao supina e pela diminuicao do tonus muscular.
A taquicardia sinusal e provocada por aumento da atividade simpatica e, quando e encontrada ao repouso em animais higidos, ocorre frequentemente em decorrencia de excitacao e aumento do tonus simpatico (LESSA et al., 2012).
Rather than relying on random motor unit firing of each muscle to produce a steady tonus (a tetanic contraction in which repeated muscle twitches are smoothed out by random activation), a more phasic contraction between the muscles may allow for less variability of the average vocal fold length, and therewith the average [F.sub.0].
In one case report hypotension and bradycardia occurred in acute myocardial infarction with ST elevation and with normal CAG.7 It was argued that prolonged vagal tonus may contribute to such clinical manifestation.
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O acompanhamento do trato reprodutivo, por meio de ultra-sonografia, Midray 2200vet[R] acoplado a um transdutor transretal linear de 5,0 MHz, auxiliou na avaliacao da dinamica folicular ate a ovulacao e posterior observacao das caracteristicas dos orgaos reprodutivos, nos respectivos dias de coleta sanguinea e avaliacao reprodutiva, tais como: tamanho, morfoecogenicidade de corpo luteo, tonus uterino, dissipacao de edema e cervix dos animais em estudo.