noun, plural: smooth muscles
An involuntary, non-striated type of (vertebrate) muscle that are capable of slow rhythmic involuntary contractions
The muscular system encompasses all the muscles of the animal body. There are three types of muscles: skeletal muscles, smooth muscles, and the cardiac muscles. Both the skeletal muscles and the cardiac muscles have striations when viewed under the microscope. In contrast, the smooth muscle lacks striations. This is because of the uniform distribution of myosin filaments in the smooth muscle cell. Apart from the lack of striations, the smooth muscle differs from the other two by the cell shape. The smooth muscle cells are typically spindle-shaped and the nuclei are centrally located. The cells have higher actin/myosin ratio than skeletal muscle cells. They are also capable of contracting to a much smaller fraction of its resting length. Smooth muscles may be sub-divided into single-unit (unitary) smooth muscles and multiunit smooth muscles.
Smooth muscle tissues are found mainly in the walls of the arteries, hair follicles, intestines (particularly the gizzard in birds), urinary bladder, throat, uterus, lungs, and other internal organs. They are responsible for rhythmic involuntary movements of these organs. Their contraction is relatively slower than that of skeletal muscles. Nevertheless, the smooth muscle tissues remain contracted for longer periods than the skeletal muscle tissues. The contractile system and its control resemble those of motile tissue cells (e.g. fibroblasts, leucocytes) and antibodies against smooth muscle myosin will crossreact with myosin from tissue cells, whereas antibodies against skeletal muscle myosin will not.