Postural tonus

The most significant role of skeletal-muscle tonus as a factor in bodily activity is the part it plays in the maintenance of posture. Posture is that nicety of adjustment between the various parts of the body to fit the new position taken as in sitting, standing, running, and reclining. In each position, the muscles must assume a new tonic state (posture) in addition to the more obvious contractions used for the change of position. Smooth continuity between normal movements in all kinds of muscular activity is dependent upon a mechanism which correlates these two types of response. When the position of the body changes, as during exercise, the tonus of its entire musculature must be modified so that each part may make a harmonious contribution to the activity as a whole. The importance of this postural or plastic tonus is indicated by the fact that when the underlying mechanism is disturbed the movements become incoördinated and ataxic.

Postural tonus depends upon the integrity of the reflex arcs and upon a steady but continuous bombardment of impulses into the muscles through the motor nerves from the central nervous system. An increase or decrease in the number of nerve fibers, and consequently the number of muscle fibers involved at any one time, leads to a greater or lesser state of tonic contraction. The afferent impulses upon which the posture of the body as a whole is dependent may arise in the muscles themselves (proprioceptors) or in the semicircular canals or from the cerebrum circuited through the cerebellum. Those from the muscles are influenced by the position of the parts with relation to the body as a whole, those from the semicircular canals by the position of the entire body in space. These are reinforced by the coördinating influence exerted by the cerebellum which also exercises or reinforces a general tonic effect. Thus, with the cord transected (spinal animal), an animal will have lost a large part of its power to maintain postural tonus. Simple myotatic reflexes are still present and are sufficient for certain simple movements of a reflex type. These are not adequate, however, for the maintenance of the postural tonus necessary for standing, walking, running, or support of the body.

There are two types of reaction which, when the proper stimuli arise, are accompanied by changes in postural tonus of the skeletal muscles. If one of a pair of antagonistic muscles normally employed in supporting the weight of the body against gravity is suddenly stretched, it responds with an increased development of tonus and comes to a new and shorter length. Its antagonist, on the other hand, is inhibited and comes to a new and greater length. Reflexes of this type involve the monomuscular are (myotatic reflex). In the other type of reaction the reverse is true. The tonus of the muscle may, from the same stimulus, give way almost suddenly. On releasing the tension applied, the muscle will be found to have taken on a new state of tonus and consequently a new length.

Just what conditions determine which response shall predominate is not known. The tension exerted by the muscles upon their points of insertion is maintained approximately constant. In progressive movements, as walking, postural adjustments must not only be made rapidly but with exactness. The stimulus which sets up a contraction of the extensors will inhibit the tonus existent in the flexors. A shortening reaction in one set of extensors is also associated with a simultaneous extending reaction in those of the opposite side or limb and vice versa. This condition is to be considered as an adaptation to the alternate responses which normally take place, as in walking and running. This kind of coördination is an example of reciprocal inhibition of opposite muscles and limbs.