Muscle atrophy

Muscle atrophy
Muscle atrophy. Image courtesy of Professor Peter Anderson DVM PhD and published with permission © PEIR, University of Alabama at Birmingham, Department of Pathology

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List of terms related to Muscle atrophy

Muscle atrophy refers to a decrease in the size of skeletal muscle, which occurs in a variety of settings. Atrophy may or may not be distinct from "sarcopenia", which is the loss of muscle seen in the aged.

When a muscle atrophies, it necessarily becomes weaker, since the ability to exert force is related to mass.

Clinical settings of atrophy

There are many diseases and conditions which cause a decrease in muscle mass, known as atrophy, including: inactivity, as seen when a cast is put on a limb, or upon extended bedrest (which can occur during a prolonged illness); cachexia - which is a "body-wasting" syndrome that is a co-morbidity of cancer and Congestive Heart Failure; Chronic Obstructive Pulmonary Disease; burns, liver failure, etc. Other syndromes or conditions which can induce skeletal muscle atrophy are congestive heart disease, liver disease, and starvation.

Causes

Causes by Organ System

Cardiovascular	No underlying causes
Chemical/Poisoning	No underlying causes
Dental	No underlying causes
Dermatologic	No underlying causes
Drug Side Effect	Pergolide
Ear Nose Throat	No underlying causes
Endocrine	No underlying causes
Environmental	No underlying causes
Gastroenterologic	No underlying causes
Genetic	No underlying causes
Hematologic	No underlying causes
Iatrogenic	No underlying causes
Infectious Disease	No underlying causes
Musculoskeletal/Orthopedic	No underlying causes
Neurologic	No underlying causes
Nutritional/Metabolic	No underlying causes
Obstetric/Gynecologic	No underlying causes
Oncologic	No underlying causes
Ophthalmologic	No underlying causes
Overdose/Toxicity	No underlying causes
Psychiatric	No underlying causes
Pulmonary	No underlying causes
Renal/Electrolyte	No underlying causes
Rheumatology/Immunology/Allergy	No underlying causes
Sexual	No underlying causes
Trauma	No underlying causes
Urologic	No underlying causes
Miscellaneous	No underlying causes

Causes in Alphabetical Order

• Pergolide

• Disease B

• Disease C

Other muscles diseases, distinct from atrophy

During aging, there is a gradual decrease in the ability to maintain skeletal muscle function and mass. This condition is called "sarcopenia". The exact cause of sarcopenia is unknown, but it may be due to a combination of the gradual failure in the "satellite cells" which help to regenerate skeletal muscle fibers, and a decrease in sensitivity to or the availability of critical secreted growth factors which are necessary to maintain muscle mass and satellite cell survival.

In addition to the simple loss of muscle mass (atrophy), or the age-related decrease in muscle function (sarcopenia), there are other diseases which may be caused by structural defects in the muscle (muscular dystrophy), or by inflammatory reactions in the body directed against muscle (the myopathies).

Pathophysiology

Muscle atrophy occurs by a change in the normal balance between protein synthesis and protein degradation. During atrophy, there is a down-regulation of protein synthesis pathways, and an activation of protein breakdown pathways. The particular protein degradation pathway which seems to be responsible for much of the muscle loss seen in a muscle undergoing atrophy is the "ATP-dependent, ubiquitin/proteasome pathway. In this system, particular proteins are targeted for destruction by the ligation of at least four copies of a small peptide called ubiquitin onto a substrate protein. When a substrate is thus "poly-ubiquitinated", it is targeted for destruction by the proteasome. Particular enzymes in the ubiquitin/proteasome pathway allow ubiquitination to be directed to some proteins but not others - specificity is gained by coupling targeted proteins to an "E3 ubiquitin ligase". Each E3 ubiquitin ligase binds to a particular set of substrates, causing their ubiquitination.

Diagnostic Findings

• 
  Muscle Wasting Left Hand: Atrophic flattening most prominently seen in the thenar and hypothenar muscles, resulting from generalized denervation.

Potential treatment

Muscle atrophy can be opposed by the signaling pathways which induce muscle hypertrophy, or an increase in muscle size. Therefore one way in which exercise induces an increase in muscle mass is to downregulate the pathways which have the opposite effect.

See also

• Muscle weakness
• Muscle dystrophy

External links

• Muscular+atrophy at the US National Library of Medicine Medical Subject Headings (MeSH)