tonus /tō″nəs/ [Gk, tonos, stretching] , 1. the normal state of balanced tension in the body tissues, especially the muscles. Partial contraction or alternate contraction and relaxation of neighboring fibers of a group of muscles hold the organ or the part of the body in a neutral functional position without fatigue. Tonus is essential for many normal body functions, such as holding the spine erect, the eyes open, and the jaw closed. Also called muscle tone. 2. the state of the body tissues being strong and fit. Also called tone.
