Early clinical and physiological speculations about a special motor system for mammalian muscle tone appeared to be disproved by the correlation of electrical (motor unit) activity with muscle contraction, and conversely, of electrical silence with the resting state.1,2However, evidence accumulated in the last decade indicates that such a system does, indeed, exist, although not in the sense of a mechanism that produces tonic muscular contraction directly. The efferent innervation of muscle spindle tension receptors by a special group of ventral spinal root fibers was first suggested by the finding that stimulation of the small (gamma) ventral root fibers (comprising about 25% of the total in the cat) produced no further muscle shortening beyond that produced by stimulation of the large (alpha) motor fibers.3Leksell4first directly confirmed this function of the gamma efferent (fusimotor) fibers. The sensitivity of muscle spindle stretch receptors is modulated by the