Tonus is the active state of the nerve centers and associated peripheral structures, ensuring the maintenance of any function at an optimal level.
Depending on the location of the tonic excited peripheral muscle formations, there is a difference in skeletal muscle tone, blood vessel tone, stomach, intestinal tone, etc.
Usually, changes in the tone of the nerve centers cause changes in the activity of the corresponding peripheral organs, which is one of the ways to regulate the vital functions of the body. For example, a decrease in heart rate can be achieved either by increasing the tone of the nucleus of the vagus nerve, or by weakening the tone of the sympathetic centers.
The tone of the nerve centers and related peripheral formations depends on three main factors: from the afferent excitations to the nerve centers, which increase or, on the contrary, weaken the degree of tonic excitation of these centers, from the effects on the nerve centers of various humoral stimuli and from the interaction between the centers.Go
So, for example, the tone of skeletal muscles depends primarily on the release of continuous afferent excitations from the proprioceptors of the muscles themselves to the spinal cord motoneurons. Deafferentation of the muscle leads to a sharp decrease in its tone.
In the regulation of muscle tone plays an important role a number of parts of the brain. These include the reticular formation of the brain stem, the vestibular nuclei of the medulla oblongata, the structure of the midbrain (red nuclei, substantia nigra), the cerebellum , the striopallidary system and, finally, the motor cortex of the hemispheres.
Only due to the combined activity of all the formations listed above a very complicated process of regulation and redistribution of muscle tone can occur. With the defeat of any of these structures, you can observe numerous types of pathologies of muscle tone. For example, in the event of damage to the red nuclei of the midbrain, hypertonic phenomena and rigidity of the extensor muscles develop. With the defeat of the striopallidar system, hypotonia of various muscle groups develops, with the defeat of the motor areas of the cerebral cortex of the hemispheres - increased tone (spasticity), etc.
In the activities of the central formations that ensure the regulation of muscle tone, humoral factors such as adrenaline, carbonic acid , etc., also play an important role.
However, in a more demonstrative form, the tonogenic value of these humoral stimuli is manifested in the activity of the vasomotor center, which provides a certain tone to the peripheral vessels (see Blood pressure).