Effects of Aging on Muscle Tissue

Effects of Aging on Muscle Tissue

Nearly all types of muscle tissue develop from the embryonic mesoderm cells known as myoblasts, which fuse to form the myotubes of skeletal muscle fibers. Skeletal muscle fibers begin contracting at approximately the 7th week of embryonic develop-ment. The somatic nervous system begins to control muscle fibers and the amount of fast and slow con-tractile fibers is determined. Very early in the devel-opment, the myoblasts that produce cardiac and smooth muscle cells develop gap junctions instead of fusing. The satellite cells of skeletal muscle help to repair injured fibers. They allow limited regenera-tion of dead skeletal muscles, but this ability lessens with age. Cardiac cells only divide at a rate that is less than other muscle types. When the heart mus-cle is injured, its repair usually involves scar tissue. Smooth muscles regenerate well, with their cells dividing continually throughout life.

Muscular development occurs in a pattern, from head to toe and from proximal to distal directions. For example, infants are able to lift their heads long before they can walk. Gross movements develop before fine movements. By the middle of adolescence, humans reach total neural control of their muscles, which can be improved by athletic activities. Females’ skeletal muscles make up about 36% of their body mass. Males’ skeletal muscles make up about 42%, which occurs because of the effects of testosterone. In both sexes, however, body strength per unit of muscle mass is the same. Strenuous exercise increases mus-cle enlargement in males more than females, again because of testosterone. Skeletal muscle is highly resistant to infection because of its rich blood supply.

Connective tissue in the skeletal muscles increases with aging, yet the number of muscle fibers decreases. Muscles are more sinewed (stringy) than before. By the age of 30, a gradual loss of muscle mass begins, known as sarcopenia. This atrophy of muscle is controlled by the same factors that influence muscle growth: enzymes, hormones, transcription factors, and others. Skeletal muscle fibers become smaller in diameter as the number of myofibrils decreases. Body weight and muscle strength begin to decline as skeletal muscles form so much of the body mass. Blood flow to active muscles does not increase as rapidly with exercise.

By the age of 80, muscle strength has usually decreased by half. This condition, called flesh wasting, is potentially dangerous to the elderly since they expe-rience more falls. As the cardiovascular system ages, nearly all body organs, including muscles, receive less nutrients. The development of atherosclerosis often results in intermittent claudication, a circulatory con-dition that restricts blood delivery. The individual experiences severe leg muscle pain during walking and must rest more often. However, regular exercise, even later in life, helps to reverse sarcopenia. The use of weights helps to rebuild muscle mass and increase muscle strength. Daily walking helps to improve ­neuromuscular function and overall health. Smooth muscle experiences much less problems with aging. Gastrointestinal smooth muscle continues to alter motility in order to cleanse the body of irritants and infections.