Tonus


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tonus

[′tō·nəs]
(physiology)
The degree of muscular contraction when not undergoing shortening. Also known as muscle tone.

Tonus

 

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.

REFERENCES

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.)

V. G. ZILOV

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