Diseases of and Disorders of the Muscular System
Given proper exercise, muscles can react to stimuli quickly and powerfully, and are said to possess tone. As a result of excessive use, muscles may hypertrophy, that is, increase in size because of an increase in size of the individual muscle cells. As a result of prolonged disuse, muscles may atrophy, or diminish in size, and become weaker. In certain diseases, such as various forms of paralysis, the muscles may atrophy to such a degree that they are reduced to a fraction of their normal size.
Muscular atrophy refers to a decrease in size of muscles. Individual muscle fibers decrease in size, and there is a progressive loss of myofibrils.
Disuse atrophy is a muscular atrophy that results from lack of muscle use. People with limb in casts, bedridden people, or those who are inactive for other reason experience disuse atrophy in the muscles that are not used. Disuse atrophy is temporary if a muscle is exercised after it is taken out of a cast. However, extreme disuse of a muscle results in a muscular atrophy in which there is a permanent loss of skeletal muscle fibers and the replacement of those fibers by connective tissue.
Muscular Dystrophy is a crippling disease characterized by gradual wasting of skeletal muscle. The clinical course is progressive, with increased weakness and diminution in muscle mass and function until the patient is confined to a wheelchair, remissions do not occur. Four clinical forms of the disease are recognized, based on pattern of inheritance, age when symptoms are first noted, and distribution of the muscles earliest involved.
In the Duchenne form of the disease, symptoms usually are noted before age five. The muscles first affected are those of the pelvis and trunk, resulting in spinal deformity and a waddling gait. Wasting of almost all muscle groups may be advanced by the late teens. Death may result from respiratory weakness or from involvement of the heart muscle. Because inheritance of this form is by an X-linked recessive mechanism, virtually all patients are boys.
The facio-scapulo-humeral form of the disease affects both sexes equally and results in weakness and wasting of the shoulder girdle and upper arms. It is usually noted around the onset of puberty. The characteristic weakness of the facial muscles may occasionally be seen during the first years of life. Most patients remain ambulatory until an advanced age.
The limb-girdle type of the disease affects both sexes. Muscles of either the shoulder or hip girdle, or both, may be involved. The disease may begin early or late in life, and usually the course is slow. In the stages most of the muscles of the body may be affected.
In myotonic muscular dystropy, delayed relaxation of the muscles accompanies wasting and weakness. Cataracts of both eyes may occur, and reproductive functions may be disturbed. The age of onset and severity of symptoms are variable, but the disease tends to occur earlier and is more severe in later generations of an affected family. Myotonic muscular dystrophy affects both sexes, but children of affected mothers are more likely to inherit a severe form of the disease than children of affected fathers.