Do you like to dance? Most of us do, or we may simply enjoy watching good dancers. The grace and coordination involved in dancing result from the interaction of many of the organ systems, but the one you think of first is probably the muscular system.

There are more than 600 muscles in the human body. Most of these muscles are attached to the bones of the skeleton by tendons, although a few muscles are attached to the undersurface of the skin. The primary function of the muscular system is to move the skeleton. The muscle contractions required for movement also produce heat, which contributes to the maintenance of a constant body temperature. The other body systems directly involved in movement are the nervous, respiratory, and circulatory systems. The nervous system transmits the electrochemical impulses that cause muscle cells to contract. The respiratory system exchanges oxygen and carbon dioxide between the air and blood. The circulatory system brings oxygen to the muscles and takes carbon dioxide away.

You may recall from Muscle Tissue article that there are two other types of muscle tissue: smooth muscle and cardiac muscle. Before you continue, you may find it helpful to go back to Muscle Tissue and review the structure and characteristics of skeletal muscle tissue.

Even our simplest movements require the interaction of many muscles, and the contraction of skeletal muscles depends on the brain. The nerve impulses for movement come from the frontal lobes of the cerebrum. The cerebrum is the largest part of the brain; the frontal lobes are beneath the frontal bone. The motor areas of the frontal lobes generate electrochemical impulses that travel along motor nerves to muscle fibers, causing the muscle fibers to contract.

For a movement to be effective, some muscles must contract while others relax. When walking, for example, antagonistic muscles on the front and back of the thigh or the lower leg will alternate their contractions and relaxations, and our steps will be smooth and efficient. This is what we call coordination, and we do not have to think about making it happen. Coordination takes place below the level of conscious thought and is regulated by the cerebellum, which is located below the occipital lobes of the cerebrum.

Except during certain stages of sleep, most of our muscles are in a state of slight contraction; this is what is known as muscle tone.

When sitting upright, for example, the tone of your neck muscles keeps your head up, and the tone of your back muscles keeps your back straight. This is an important function of muscle tone for human beings, because it helps us to maintain an upright posture. For a muscle to remain slightly contracted, only a few of the muscle fibers in that muscle must contract. Alternate fibers contract so that the muscle as a whole does not become fatigued. This is similar to a pianist continuously rippling her fingers over the keys of the piano—some notes are always sounding at any given moment, but the notes that are sounding are always changing. This contraction of alternate fibers, muscle tone, is also regulated by the cerebellum of the brain.
Muscle fibers need the energy of ATP (adenosine triphosphate) in order to contract. When they produce ATP in the process of cell respiration, muscle fibers also produce heat. The heat generated by normal muscle tone is approximately 25% of the total body heat at rest. During exercise, of course, heat production increases significantly.

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