Smooth muscles do not have transverse striations like those of skeletal and cardiac muscles. The smooth muscles line the blood vessels, digestive tract, urinary bladder, uterus etc. These are also present in the iris and skin. Smooth muscles are small and spindle shaped. Each muscle has a single nucleus. The cytoplasm of the smooth muscle cells contains actin and myosin filaments arranged in a random manner. Despite the lack of internal organisation of the filaments, there is cross bridge formation between the actin and myosin filaments. The contraction mechanism in smooth muscles is similar to the sliding filament type of contraction as found in the skeletal muscles.
Figure: Smooth Muscles.
In order to contract, smooth muscle need not be stimulated through the nerves. They show spontaneous rhythmic contractions that can vary in both frequency and intensity. This spontaneous activity of smooth muscle, however, can be modified by nerves as well as by hormones such as epinephrine and norepinephrine. A distinctive feature of vertebrate smooth muscle is the slowness of response. Another important property of it is that the smooth muscle can maintain contraction for prolonged periods with very little energy expenditure.