Muscle atrophy,
is defined as a decrease in the mass of the muscle,
it can be a partial or complete wasting away of muscle,
and is most commonly experienced when persons suffer temporary disabling circumstances such as being restricted in movement and/or confined to bed as when hospitalized.
When a muscle atrophies,
this leads to muscle weakness,
since the ability to exert force is related to mass.
Sarcopenia,
initially introduced by Rosenberg,
has been proposed to be an age-related muscle atrophy (pre-sarcopenia) that appears in combination with low muscle strength and/or physical performance condition in the criteria of the European Working Group on Sarcopenia in Older People (EWGSOP).
In general,
sarcopenia including muscle atrophy is frequently observed with aging,
while it has also been shown to be associated with chronic diseases (e.g.,
heart failure,
obstructive pulmonary disease,
diabetes mellitus,
kidney disease,
connective tissue disease,
tuberculosis infection,
other wasting conditions).
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 degradation. 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.
CAUSES
There are two types of muscle atrophy: disuse and neurogenic.
Disuse atrophy is caused by not using the muscles enough.
This type of atrophy can often be reversed with exercise and better nutrition.
People who are most affected are those who:
- Have seated jobs,
health problems that limit movement,
or decreased activity levels.
- Are bedridden
- Cannot move their limbs because of stroke or other brain disease
Neurogenic atrophy is the most severe type of muscle atrophy.
It can be from an injury to,
or disease of,
a nerve that connects to the muscle.
This type of muscle atrophy tends to occur more suddenly than disuse atrophy.
Examples of diseases affecting the nerves that control muscles:
- Amyotrophic lateral sclerosis (ALS,
or Lou Gehrig disease)
- Damage to a single nerve,
such as carpal tunnel syndrome
- Guillain-Barre syndrome
- Nerve damage caused by injury,
diabetes,
toxins,
or alcohol
- Polio (poliomyelitis)
- Spinal cord injury
Other causes of muscle atrophy may include:
- Long-term corticosteroid therapy
- Malnutrition
- Muscular dystrophy and other diseases of the muscle
- Osteoarthritis,
Rheumatoid arthritis
- Several common diseases,
including cancer, AIDS, congestive heart failure, COPD, renal failure,
and severe burns
Although people can adapt to muscle atrophy,
even minor muscle atrophy causes some loss of movement or strength.