Muscle atrophy

Muscle atrophy is defined as a decrease in the mass of the muscle; it can be a partial or complete wasting away of muscle. When a muscle atrophies, this leads to muscle weakness, since the ability to exert force is related to mass. Muscle atrophy results from a co-morbidity of several common diseases, including cancer, AIDS, congestive heart failure, COPD (chronic obstructive pulmonary disease), renal failure, and severe burns; patients who have "cachexia" in these disease settings have a poor prognosis. Moreover, starvation eventually leads to muscle atrophy. Disuse of the muscles will also lead to disuse atrophy while neurogenic atrophy is muscle atrophy that results from damage to the nerve that stimulates the muscle. Also, time in a less than 1 g environment without exercise will lead to atrophy.

Clinical settings of atrophy
There are many diseases and conditions which cause a decrease in muscle mass, known as atrophy, including: Dejerine Sottas syndrome (HSMN Type III), 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 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 liver disease, and starvation.

Quality of life
Muscular atrophy decreases quality of life as the sufferer becomes unable to perform certain tasks or worsen the risks of accidents while performing those (like walking). Muscular atrophy increases the risks of falling in conditions such as IBM (inclusion body myositis). Muscular atrophy affects a major number of elderly.

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 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.

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.

One important rehabilitation tool for muscle atrophy includes the use of functional electrical stimulation to stimulate the muscles. This has seen a large amount of success in the rehabilitation of paraplegic patients.

Since the absence of muscle-building amino acids can contribute to muscle wasting (that which is torn down must be rebuilt with like material), amino acid therapy may be helpful for regenerating damaged or atrophied muscle tissue. The branched-chain amino acids or BCAAs (leucine, isoleucine, and valine) are critical to this process, in addition to lysine and other amino acids.

In severe cases of muscular atrophy, the use of an anabolic steroid such as methandrostenolone is administered to patients as a potential cure. A novel class of drugs, called SARM (selective androgen receptor modulators) are being investigating with promising results. They would have fewer side-effects, while still promoting muscle and bone tissue growth and regeneration. These claims are, however, yet to be confirmed in larger clinical trials.

Quantification
A CT scan can distinguish muscle tissue from other tissues and thereby estimate the amount of muscle tissue in the body.

Fast loss of muscle tissue (relative to normal turnover), can be approximated by the amount of urea in the urine. The equivalent nitrogen content (in gram) of urea (in mmol) can be estimated by the conversion factor 0.028 g/mmol. Furthermore, 1 gram of nitrogen is roughly equivalent to 6 gram of protein, and 1 gram of protein is roughly equivalent to 4 gram of muscle tissue. Subsequently, in situations such as muscle wasting, 1 mmol of excessive urea in the urine (as measured by urine volume in litres multiplied by urea concentration in mmol/l) roughly corresponds to a muscle loss of 0.67 gram.

Hibernation
Inactivity and starvation in mammals lead to atrophy of skeletal muscle, accompanied by a smaller number and size of the muscle cells as well as lower protein content. In humans, prolonged periods of immobilization, as in the cases of bed rest or astronauts flying in space, are known to result in muscle weakening and atrophy. Such consequences are also noted in small hibernating mammals like the golden-mantled ground squirrels and brown bats.

Bears are an exception to this rule; species in the family Ursidae are famous for their ability to survive unfavorable environmental conditions of low temperatures and limited nutrition availability during winter by means of hibernation. During that time, bears go through a series of physiological, morphological and behavioral changes. Their ability to maintain skeletal muscle number and size at time of disuse is of significant importance.

During hibernation, bears spend four to seven months of inactivity and anorexia without undergoing muscle atrophy and protein loss. There are a few known factors that contribute to the sustaining of muscle tissue. During the summer period, bears take advantage of the nutrition availability and accumulate muscle protein. The protein balance at time of dormancy is also maintained by lower levels of protein breakdown during the winter time. At times of immobility, muscle wasting in bears is also suppressed by a proteolytic inhibitor that is released in circulation. Another factor that contributes to the sustaining of muscle strength in hibernating bears is the occurrence of periodic voluntary contractions and involuntary contractions from shivering during torpor. The three to four daily episodes of muscle activity are responsible for the maintenance of muscle strength and responsiveness in bears during hibernation.