Muscle tone
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Overview
Muscle tone (aka residual muscle tension or tonus) is the continuous and passive partial contraction of the muscles. It helps maintain posture, and it declines during REM sleep. Note that muscular tone is not defined as muscular shaping or the aspect of general Human physical appearance.
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.
The presence of near-continuous innervation makes it clear that tonus describes a "default" or "steady state" condition. There is, for the most part, no actual "rest state" insofar as activation is concerned.
In terms of skeletal muscle, both the extensor muscle and flexor muscle use the term tonus to refer to the "at rest" or normal enervation that maintains current positions of bones.
Cardiac muscle and smooth muscle, although not directly connected to the skeleton, also have tonus in the sense that although their contractions are not matched with those of antagonist muscles; their non-contractive state is characterized by (sometimes random) enervation.
Pathological tonus
Physical disorders can result in abnormally low (hypotonia) or high (hypertonia) muscle tone. Another form of hypertonia is Paratonia, which is associated with dementia.
Tonus in surgery
In ophthalmology, tonus may be a central consideration in eye surgery, as in the manipulation of extraocular muscles to repair strabismus. Tonicity aberrations are associated with many diseases of the eye (e.g. Adie syndrome).