Muscle is a tissue composed of cells or fibers, the contraction of which produces
movement in the body. There are three types of muscles:

1 - Smooth muscle - controlled by the autonomic nervous system; may either be generally inactive or then respond to neural stimulation or hormones or may be rhythmic.

2 - Cardiac muscle - found in the heart, acts like rhythmic smooth muscle, modulated by neural activity and hormones.

3 - Skeletal muscle - move us around and responsible for most of our behavior; most attached to bones at each end via tendons.

The muscle is composed of subunits called fascicles. Fascicles are bundles of individual muscle fibers. Each fiber is one elongated cell that may extend for the length of the muscle. Each muscle fiber cell has several nuclei (unlike most cells, which have only one), and is segmented into distinct sectional bands. Within each muscle cell are numerous myofibrils, which also extend for the length of the muscle cell. Sarcomeres are the basic contractile subunit of myofibrils.

Muscle growth is a specialized form of protein synthesis. As we saw above, a steroid hormone (testosterone) enters the muscle cell by diffusing directly across the cell membrane, combines with a receptor in the cell and then stimulates gene transcription and protein formation via the DNA -> mRNA -> tRNA -> protein pathway. Specific receptors and genes are involved.

Muscles In The Body


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