Smooth muscle

Smooth muscles are found in most of the body's internal organs. Unlike skeletal muscles, smooth muscles do not have striations. Smooth muscles in the walls of the stomach and intestines move food through the digestive system. Smooth muscles also control the width of the blood vessels and the size of the breathing passages. In all these cases, the smooth muscles contract and relax automatically--that is, we do not consciously control them. For this reason, they are often called involuntary muscles.

Smooth muscles cannot contract as rapidly as skeletal muscles. However, smooth muscles can contract more completely than skeletal muscles, and they do not tire as quickly. Smooth muscles can thus produce powerful, rhythmic contractions over long periods.