Have you ever wondered how a human being can sustain the body against the force of gravity, which attracts him all the time to the ground?
That’s right … This is only possible because our muscles are always partially contracted. They are in a permanent state of contraction which we call muscle tone. It is due to muscle tone that we have the ability to face gravity and maintain posture.
- NOTE: we must pay attention to the fact that, although the muscle tone is permanent, it is not immutable. It has the ability to respond to changes in body position, being controlled by the nervous system.
Mechanisms to control muscle tone
Among the mechanisms of muscle tone control, we have the stretch reflex:
- Stretch reflex: in orthostasis, antigravity extensor muscles are stretched due to gravity. With this, afferents of the muscle spindles are activated and, consequently, reflex contraction of the muscle itself occurs.
In addition, muscle tone depends on the firing level of the motoneurons, especially when it is necessary to increase the tone of some muscles to prepare for rapid movement.
- In this case, the activation of fusimotor neurons (which innervate intrafusal muscle fibers) indirectly regulates muscle tone and motoneurons directly regulate it. Such control is performed mainly by the medial descending pathways. They are able to regulate axial muscle tone and posture.
In addition to the regulation of muscle tone by modulating the stretch reflexes, the nervous system has reflex chains capable of allowing quick and effective postural adjustments, which are called postural reactions. These can be triggered by several stimuli, mainly those of vestibular and proprioceptive origin.
It should be noted, however, that not all postural reactions are automatic. The cerebral cortex influences voluntary postural adjustment movements. For example, we can mention the peculiarities of a goalkeeper’s posture that, when preparing to defend a penalty, increases the tone of the leg muscles while keeping them slightly flexed. This helps in the defense, as it is easier to throw on the ball thrown by the player of the opposing team.