There are three types of muscle tissue in the body:

• Smooth (visceral) muscle. This makes up the walls of the hollow organs and the walls of ducts, such as the blood vessels and bronchioles. This muscle operates involuntarily and is responsible for peristalsis, the wavelike movements that propel materials through the systems.

• Cardiac muscle. This makes up the myocardium of the heart wall. It functions involuntarily and is responsible for the pumping of the heart.

• Skeletal muscle. This is attached to the bones of the skeleton and is responsible for voluntary movement. It also maintains posture and generates a large proportion of body heat. All of these voluntary muscles together make up the muscular system.

The discussion that follows describes the characteristics of skeletal muscle, which has been the most extensively studied of the three types of muscle tissue.

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