Muscles make up most of the fleshy parts of the body and average 43 percent of the body’s weight. Muscle tissue is classified in three ways based on the tissue’s function, shape, and structure:
Smooth muscle tissue: So-called because it doesn’t have the cross-striations typical of other kinds of muscle, the spindle-shaped fibers of smooth muscle tissue do have faint longitudinal striping. This muscle tissue forms into sheets and makes up the walls of hollow organs such as the stomach, intestines, and bladder.
The tissue’s involuntary movements are relatively slow, so contractions last longer than those of other muscle tissue, and fatigue is rare. Each fiber is about 6 microns in diameter and can vary from 15 microns to 500 microns long. If arranged in a circle inside an organ, contraction constricts the cavity inside the organ. If arranged lengthwise, contraction of smooth muscle tissue shortens the organ.
Cardiac muscle tissue: Found only in the heart, cardiac muscle fibers are cylindrically branched, cross-striated, feature one central nucleus, and move through involuntary control. An electron microscope view of the tissue shows separate fibers tightly pressed against each other, forming cellular junctions called intercalated discs that look like tiny, dark-colored plates. It’s believed that intercalated discs are not just cellular junctions but special structures that help move an electrical impulse throughout the heart.
Skeletal muscle tissue: This is the tissue that most people think of as muscle. It’s the only muscle subject to control by the central nervous system via the somatic nervous system (SoNS). The somatic nervous system (or voluntary nervous system) is the part of the peripheral nervous system that provides the motor innervation needed for voluntary control of body movement via skeletal muscles.
The long, striated cylindrical fibers of skeletal muscle tissue contract quickly but tire just as fast. Skeletal muscle, which is also what’s considered meat in animals, is 20 percent protein, 75 percent water, and 5 percent organic and inorganic materials. Each multinucleated fiber is encased in a thin, transparent cell membrane called a sarcolemma that receives and conducts stimuli.
The fibers, which vary from 10 microns to 100 microns in diameter and up to 4 centimeters in length, are subdivided lengthwise into tiny myofibrils roughly 1 micron in diameter that are suspended in the cell’s sarcoplasm.