Peripheral nervous system damage
Injuries or diseases of peripheral nerves supplying specific muscles can also cause muscle atrophy. This is seen in nerve injury due to trauma or surgical complication, nerve entrapment, or inherited diseases such as Charcot-Marie-Tooth disease. Medications
Some medications are known to cause muscle atrophy, usually due to direct effect on muscles. This includes glucocorticoids causing glucocorticoid myopathy or medications toxic to muscle such as doxorubicin. Endocrinopathies
Disorders of the endocrine system such as Cushings disease or hypothyroidism are known to cause muscle atrophy. Pathophysiology
Muscle atrophy occurs due to an imbalance between the normal balance between protein synthesis and protein degradation. This involves complex cell signalling that is incompletely understood and muscle atrophy is likely the result of multiple contributing mechanisms.Mitochondrial function is crucial to skeletal muscle health and detrimental changes at the level of the mitochondria may contribute to muscle atrophy. A decline in mitochondrial density as well as quality is consistently seen in muscle atrophy due to disuse.The ATP-dependent ubiquitin/proteasome pathway is one mechanism by which proteins are degraded in muscle. This involves specific proteins being tagged for destruction by a small peptide called ubiquitin which allows recognition by the proteasome to degrade the protein. Diagnosis
Screening for muscle atrophy is limited by a lack of established diagnostic criteria, although many have been proposed. Diagnostic criteria for other conditions such as sarcopenia or cachexia can be used. These syndromes can also be identified with screening questionnaires.Muscle mass and changes can be quantified on imaging studies such as CT scans or Magnetic resonance imaging (MRI).
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 during disuse is of significant importance.During hibernation, bears spend 4-7 months of inactivity and anorexia without undergoing muscle atrophy and protein loss. A few known factors contribute to the sustaining of muscle tissue. During the summer, 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. 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.