c. Smooth Muscle. All muscles that are not found in the heart or are not
attached to the skeletal system are called smooth muscles.
(1) Anatomy. The fibers of smooth muscles are elongated and nonstriated.
The size of the fiber varies with the location of the muscle. For example, the smallest
smooth muscles are found in the blood vessels and the largest are found in the
digestive tract. Smooth muscle is responsible for such important functions as
peristalsis, blood pressure, and air volume. Peristalsis is the rhythmic wave-like motion
of the alimentary canal and other tubular organs caused by waves of contraction
passing along the smooth muscle in the tube. Smooth muscle is involved in blood
pressure by altering the diameter of blood vessels. It is involved in the control of air
volume by altering the diameter of the bronchial tubes. Smooth muscle contracts
involuntarily-it is an unconscious act.
(2) Physiology. The same chemical substances are found in smooth
muscle as are found in skeletal muscle. Contraction of smooth muscle tissue occurs by
the activation by ions--just the same as with skeletal muscles: Contraction occurs
during depolarization of the muscle membrane, and it stops after repolarization.
Smooth muscle tissue does not contract as rapidly as skeletal muscle tissue.
Furthermore, the relaxation of the smooth muscle following contraction is likewise
slower than in skeletal muscle. Smooth muscle is capable of maintaining tonic
contractions over a long period of time. Smooth muscle can undergo changes in length
without significant change in tension. This is called stress-relaxation.