Muscle enables complex movements that are either voluntary—under conscious control—such as turning the pages of this book, or involuntary, such as the contraction of the heart or the peristalsis in the gut. To understand how muscle accomplishes these various activities, you need to know the physiology behind a muscle contraction. This requires a detailed knowledge of the muscle's microscopic anatomy. Of course, muscle contractions will not take place without adequate nervous stimulation or a sufficient supply of ATP, the muscles' fuel. ATP is obtained via cellular respiration, which is accomplished by several different metabolic pathways.
There are three types of muscles:
- Skeletal muscles are attached mainly to the skeletal bones but some are also attached to other structures (such as the eyes for eye movement) and causes movements of the body. Skeletal muscle is also called striated muscle, because of its banding pattern when viewed under a microscope (for clarification, see cardiac muscle below), or voluntary muscle (because muscle contractions can be consciously controlled).
- Cardiac muscle is responsible for the rhythmic contractions of the heart. Cardiac muscle is involuntary—it generates its own stimuli to initiate a muscle contraction. While cardiac muscle also consists of striations, the main characteristic (to differentiate these striations from skeletal muscle) is the presence of intercalated disks.
- Smooth muscle lines the walls of hollow organs. For example, it lines the walls of blood vessels and of the digestive tract, where it serves to advance the movement of substances. A smooth muscle contraction is relatively slow and involuntary.