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In physiology
Physiology

Physiology is the study of the mechanical, physical, and biochemical functions of living organisms. Physiology has traditionally been divided between plant physiology and animal and all living things physiology but the principles of physiology are universal, no matter what particular organism is being studied....
, medicine
Medicine

Medicine is the art and science of healing. It encompasses a range of health care practices evolved to maintain and restore health by the prevention and treatment of illness....
, and anatomy
Anatomy

Anatomy is a branch of biology that is the consideration of the body plan. It is a general term that includes human anatomy, animal anatomy and plant anatomy ....
, muscle tone (aka residual muscle tension or tonus) is the continuous and passive partial contraction of the muscles. It helps maintain posture
Neutral spine

A Neutral Spine or "good posture" refers to the "three natural curves [that] are present in a healthy spine."...
, and it declines during REM sleep. It is not to be confused with the concept of toning
Toning exercises

Toning exercises are physical exercises that aim to develop musculature that is hard, as opposed to flaccid. There is no single exercise to "tone." The best way to tone is to utilize aerobic exercise to reduce body fat and perform weight resistance exercise to build muscle....
 in physical exercise
Physical exercise

Physical exercise is any bodily activity that raises the heart rate above its resting level and enhances or maintains physical fitness and overall health....
.

nscious 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
Balance disorder

A balance disorder is a disturbance that causes an individual to feel unsteady, giddy, woozy, or have a sensation of movement, spinning, or floating....
.

The presence of near-continuous innervation makes it clear that tonus describes a "default" or "steady state" condition.






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In physiology
Physiology

Physiology is the study of the mechanical, physical, and biochemical functions of living organisms. Physiology has traditionally been divided between plant physiology and animal and all living things physiology but the principles of physiology are universal, no matter what particular organism is being studied....
, medicine
Medicine

Medicine is the art and science of healing. It encompasses a range of health care practices evolved to maintain and restore health by the prevention and treatment of illness....
, and anatomy
Anatomy

Anatomy is a branch of biology that is the consideration of the body plan. It is a general term that includes human anatomy, animal anatomy and plant anatomy ....
, muscle tone (aka residual muscle tension or tonus) is the continuous and passive partial contraction of the muscles. It helps maintain posture
Neutral spine

A Neutral Spine or "good posture" refers to the "three natural curves [that] are present in a healthy spine."...
, and it declines during REM sleep. It is not to be confused with the concept of toning
Toning exercises

Toning exercises are physical exercises that aim to develop musculature that is hard, as opposed to flaccid. There is no single exercise to "tone." The best way to tone is to utilize aerobic exercise to reduce body fat and perform weight resistance exercise to build muscle....
 in physical exercise
Physical exercise

Physical exercise is any bodily activity that raises the heart rate above its resting level and enhances or maintains physical fitness and overall health....
.

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
Balance disorder

A balance disorder is a disturbance that causes an individual to feel unsteady, giddy, woozy, or have a sensation of movement, spinning, or floating....
.

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
Extensor muscle

An extensor muscle is any muscle that opens a joint increasing the angle between components of a limb, such as straightening the knee or Elbow-joint and bending the wrist or vertebral column....
 and flexor muscle
Flexor muscle

A flexor muscle is a skeletal muscle whose contraction bends a joint, decreasing the angle between components of a limb, such as bending the knee or Elbow-joint....
s, under normal enervation
Nerve

A nerve is an enclosed, cable-like bundle of Peripheral nervous system axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons....
 maintain a constant tone while "at rest" that maintains a normal posture.

Cardiac muscle
Cardiac muscle

Cardiac muscle is a type of involuntary sarcomere muscle found in the walls of the heart, specifically the wikt:myocardium. Cardiac muscle cells are known as cardiac myocytes ....
 and smooth muscle
Smooth muscle

Smooth muscle is a type of non-striated muscle, found within the tunica media layer of large and small arteries and veins, the urinary bladder, uterus, male and female reproductive tracts, gastrointestinal tract, respiratory tract, the ciliary muscle, and iris of the eye....
, 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
Hypotonia

Hypotonia is a condition of abnormally low muscle tone , often involving reduced muscle strength. Hypotonia is not a specific medical disorder, but a potential manifestation of many different diseases and disorders that affect motor nerve control by the brain or muscle strength....
) or high (hypertonia
Hypertonia

Hypertonia is an upper motor neuron dysfunction marked by an abnormal increase in tightness of muscle tone and a reduced ability of a muscle to stretching ....
) muscle tone. Another form of hypertonia is Paratonia
Paratonia

Paratonia or gegenhalten is classified as a form of hypertonia with an involuntary variable resistance during passive movement . The disease develops during a period of dementia and the degree of effect is dependent upon the disease's progress....
, which is associated with dementia
Dementia

Dementia is the progressive decline in cognition due to damage or disease in the body beyond what might be expected from normal aging. Although dementia is far more common in the geriatric population, it may occur in any stage of adulthood....
.

Tonus in surgery

In ophthalmology
Ophthalmology

Ophthalmology is the branch of medicine which deals with the Eye diseases and Eye surgery of the visual pathways, including the eye, brain, and areas surrounding the eye, such as the lacrimal system and eyelids....
, tonus may be a central consideration in eye surgery
Eye surgery

Eye surgery, also known as orogolomistician surgery or ocular surgery, is surgery performed on the eye or its adnexa, typically by an ophthalmologist....
, as in the manipulation of extraocular muscles to repair strabismus
Strabismus

Strabismus is a condition in which the eyes are not properly aligned with each other. It typically involves a lack of coordination between the Muscles of orbits that prevents bringing the gaze of each eye to the same point in space and preventing proper binocular vision, which may adversely affect depth perception....
. Tonicity aberrations are associated with many diseases of the eye (e.g. Adie syndrome
Adie syndrome

Adie syndrome, sometimes known as Holmes-Adie's syndrome or Adie's Tonic Pupil, is a neurological disorder which affects the pupil of the eye and the autonomic nervous system....
).

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