Muscle enables complex movements that are either under conscious control, such as clicking a computer mouse, 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.

There are three types of muscles:

• Skeletal muscle is attached to bones and causes movements of the body. Skeletal muscle is also called striated muscle (because of its banding pattern when viewed under a microscope) or voluntary muscle (because muscle contraction 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 muscle contraction.
• 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. Smooth muscle contraction is relatively slow and involuntary.