The heart is made out of cardiac muscle (also known as myocardium), a tissue that is unlike the smooth or striated muscle seen elsewhere in the human body. Striated muscle is the tissue that a person uses to move his or her legs or fingers. Because the individual can control it, it is also known as voluntary muscle.
This tissue has light and dark bands, called striations, which give skel etal muscle yet another name: striated muscle. Smooth muscle, like that in blood vessels, is known as involuntary muscle, because a person cannot direct its movements like he or she can control skeletal muscle. Instead, the autonomic nervous system controls its action.
Falling somewhere in the middle of these two types of tissue is cardiac muscle. Cardiac muscle has the striations seen in skeletal muscle, but it takes its direction from the autonomic nervous system like the smooth muscle does. Unlike either stri ated (also known as skeletal) or smooth muscle, cardiac muscle cells are very closely linked to one another and have fibers that interconnect one cell to the next. As will be shown in the section on electrical activity later in this chapter, this is vital in making the heart beat as a unit. In addition, cardiac muscle does not tire out like skeletal muscle does, and it requires a shorter resting time between contractions. It is easy to assume that skeletal muscle can contract for a very long time, especially when considering how a body maintains muscle tone. A closer look reveals that different groups of skeletal muscle alternately shorten to give the appearance of constant contraction, even when the muscle cells are individually contracting and relaxing. In the heart, conversely, all of the cardiac cells contract at the same time.