(1) The muscle in a steady partially contracted state caused by the successive flow of nerve impulses.
(2) The amount of tension or resistance to movement in a muscle.
Muscle tonus helps in maintaining body posture. For instance, the muscles holding the jaw keep it in position with a continuous and partial contraction. When a person sleeps or loses consciousness, the muscle tonus declines.
Word origin: tonus » Greek tónos (tone)