Purpose
Unconscious nerve impulses maintain the muscles in a partially contracted state. If a sudden pull or stretch occurs, the body responds by automatically increasing the muscle's tension, a reflex which helps guard against danger as well as helping to maintain balance. Such near-continuous innervation can be thought of as a "default" or "steady state" condition for muscles. There is, for the most part, no actual "rest state" insofar as activation is concerned. Both the extensor and flexor muscles are involved in the maintenance of a constant tone while "at rest". In skeletal muscles, this helps maintain a normal posture.
Although cardiac muscle and smooth muscle are not directly connected to the skeleton, they also have tonus in the sense that although their contractions are not matched with those of antagonist muscles, the non-contractile state is characterized by (sometimes random) enervation.
Read more about this topic: Muscle Tone
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