THE TERM "tonus" applied to skeletal muscle is gradually disappearing from use. Sherrington's summation at the conclusion of the International Neurological Congress in 19311 indicated the present day concept of skeletal muscle "tonus" as purely a simpler form of contraction activated largely by proprioceptive reflexes. However, the term is still retained with reference to the activity of the striated muscles forming the voluntary anal and urethral sphincters. In describing the mechanism of defecation, Cowgill2 stated that "... both the internal and external sphincter are normally in tonus." Denny-Brown and Robertson,3 in their study of micturition, stated that it is "only when vesical contraction has commenced and has reached a degree sufficient to be accompanied by opening of the internal sphincter that relaxation of the tone of the external sphincter, sufficient to allow the passage of fluid, occurs."
Hoefer4 showed that the normal striated muscles of the extremity
NEWMAN HF. TONUS OF THE VOLUNTARY ANAL AND URETHRAL SPHINCTERS. Arch NeurPsych. 1949;61(4):445-448. doi:10.1001/archneurpsyc.1949.02310100109010