The mechanism of the postural contraction (tonus) of skeletal muscle.

Abstract

Those who find inadequate the belief that the sympathetic nervous system controls the tonus of skeletal muscle have been confronted by the difficulty of offering an alternative view satisfactory to themselves and to their opponents. It is the purpose of this communication to suggest a mechanism of tonic contraction compatible with the all-or-none principle of muscular activity. The present conception takes due account of the long-sustained character of postural reactions; it recognizes the lengthening and shortening reactions, and it conforms with the well-recognized cooperative interaction between voluntary and tonic responses. Finally, it takes into account the probable function subserved by the sympathetic nerve supply of skeletal muscle

To examine tonic reactions of skeletal muscle, it is essential to simplify experimental conditions to the greatest possible extent. Consequently, if we utilize the muscles of a decerebrate preparation, which are admittedly tonic, we must exclude all possible extraneous reflex influence. Magnus 1 has found that decerebrate rigidity continues to exist in preparation, the brain-stem of which has been sectioned just above the vestibular nuclei and in which the VIIIth cranial and the I, II, and III cervical nerves on both sides have been severed. The rigidity of a decerebrate preparation continues to exist after complete denervation of the skin in any given limb, and in any given extensor muscle, after section of all nerves except the nerve supplying that muscle. If one examines the vastocrureus muscle of a decerebrate preparation; the skin and all other muscles of both hind extremities of which have been denervated, one finds that so long as the muscle is stretched the rigidity continues to exist. Detach the tendon and allow the muscle to shorten a few millimeters, and the “rigidity” vanishes.