20.4 Muscle tissue enables movement.

Most animals move. And muscle tissue, made up of elongated cells capable of generating force by contracting, is responsible for much of that movement. Most muscle tissue cells are packed with protein filaments that slide past one another as they break down ATP, causing the entire cell to shorten and thus the muscle to contract. The action of muscle cells enables organisms to generate force and motion. There are three types of muscle tissue: skeletal, cardiac, and smooth muscle (FIGURE 20-8).

Figure 20.8: Muscle tissue is made up of elongated cells capable of generating force when they contract.

Skeletal muscle (sometimes called voluntary muscle) is usually attached to bones and is responsible for generating most movement we see in animals, including facial expressions and breathing. Skeletal muscles can be under conscious control, such as when you choose to flex your biceps, or unconscious control, such as the muscles that control breathing or moving your eyes around. Muscles account for about 40% of human body weight. The individual skeletal muscle cells are called muscle fibers. The muscle fibers are very long and contain multiple nuclei, and the repeating units of protein filaments in the cells give the fibers a striped, or striated, appearance. Skeletal muscles are controlled by the nervous system, and the individual nerve cells (neurons) connected to each muscle fiber stimulate its contraction.

Cardiac muscle, as the name indicates, is located only in the heart. It is not under conscious control and causes the heart to pump blood through the body. Because of the tremendous amount of energy used by cardiac muscles, constantly contracting throughout our life, the cells contain many more mitochondria than do other types of muscle cells. As with skeletal muscle, cardiac muscle has striations. The cells of heart muscle are fused together and connected by gap junctions (see Section 3-12) that allow the electrical signals that initiate each contraction to pass through.

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Smooth muscle is found in the walls surrounding blood vessels, the stomach and intestines, the bladder, and many other organs and internal tubes within the body. It gets its name from the fact that it lacks obvious striations. Not under conscious control (and so sometimes called involuntary muscle), smooth muscle generates slow, rhythmic contractions that can gradually move food or other substances through the organ or tube or alter blood pressure by increasing or decreasing the diameter of blood vessels. Smooth muscle can generate contractions without nervous stimulation—for example, through the opening and closing of ion channels, such as those controlling the movement of calcium ions.

TAKE-HOME MESSAGE 20.4

Muscle tissue consists of elongated cells capable of generating force when they contract. Skeletal muscle is responsible for generating most of the movement we see in animals. Cardiac muscle causes the heart to pump blood through the body. Smooth muscle, surrounding blood vessels and many internal organs, generates slower contractions that can gradually move food or other substances and alter blood pressure.

Thinking about skeletal, smooth, and cardiac muscles, which are under “conscious control” versus “unconscious control”?

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