| the steady reflex contraction that resides in the muscles concerned in maintaining erect posture. Tonus has its basis in the positional interactions of the muscle and its accompanying nerve structure; for example, a muscle holds the body (mandible) in a given position, and the awareness of this position is constantly being relayed by the sensory approaches to the cortex. Any change in position or contractility of the muscle that affects its tonus is immediately relayed by the sensory apparatus for readjustment. Also called tone. |