Spinal muscular atrophy

Proximal spinal muscular atrophy (SMA) is an autosomal recessive disease caused by a genetic defect in the SMN1 gene, which encodes SMN, a protein widely expressed in all eukaryotic cells. SMN is apparently selectively necessary for survival of motor neurons, as diminished abundance of the protein results in loss of function of neuronal cells in the anterior horn of the spinal cord and subsequent system-wide muscle wasting (atrophy).

Spinal muscular atrophy manifests in various degrees of severity, which all have in common progressive muscle wasting and mobility impairment. Proximal muscles and lung muscles are affected first. Other body systems may be affected as well, particularly in early-onset forms.

SMA is the most common genetic cause of infant death.

The term spinal muscular atrophy is used as both a specific term for the genetic disorder caused by deficient SMN, and a general label for a larger number of rare disorders having in common a genetic cause and slow progression of weakness without sensory impairment caused by disease of motor neurons in the spinal cord and brainstem – see spinal muscular atrophies for a comparison chart.

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, 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. Modern medicine's understanding of the quick onset of muscle atrophy is a major factor behind the practice of getting hospitalized patients out of bed and moving about as active as possible as soon as is feasible, despite sutures, wounds, broken bones and pain.

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.

Muscle

Muscle is a soft tissue found in most animals. Muscle cells contain protein filaments of actin and myosin that slide past one another, producing a contraction that changes both the length and the shape of the cell. Muscles function to produce force and motion. They are primarily responsible for maintaining and changing posture, locomotion, as well as movement of internal organs, such as the contraction of the heart and the movement of food through the digestive system via peristalsis.

Muscle tissues are derived from the mesodermal layer of embryonic germ cells in a process known as myogenesis. There are three types of muscle, skeletal or striated, cardiac, and smooth. Muscle action can be classified as being either voluntary or involuntary. Cardiac and smooth muscles contract without conscious thought and are termed involuntary, whereas the skeletal muscles contract upon command. Skeletal muscles in turn can be divided into fast and slow twitch fibers.

Muscles are predominantly powered by the oxidation of fats and carbohydrates, but anaerobic chemical reactions are also used, particularly by fast twitch fibers. These chemical reactions produce adenosine triphosphate (ATP) molecules that are used to power the movement of the myosin heads.

Atrophy

Atrophy is the partial or complete wasting away of a part of the body. Causes of atrophy include mutations (which can destroy the gene to build up the organ), poor nourishment, poor circulation, loss of hormonal support, loss of nerve supply to the target organ, excessive amount of apoptosis of cells, and disuse or lack of exercise or disease intrinsic to the tissue itself. In medical practice, hormonal and nerve inputs that maintain an organ or body part are said to have trophic effects. A diminished muscular trophic condition is designated as atrophy.

Atrophy is the general physiological process of reabsorption and breakdown of tissues, involving apoptosis. When it occurs as a result of disease or loss of trophic support due to other disease, it is termed pathological atrophy, although it can be a part of normal body development and homeostasis as well.

Normal development

Examples of atrophy as part of normal development include shrinking and the involution of the thymus in early childhood, and the tonsils in adolescence. In old age, effects include, but are not limited to, loss of teeth, hair, thinning of skin that creates wrinkles, weakening of muscles, loss of weight in organs and sluggish mental activity.

SMA

SMA may refer to:

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