Smooth muscle is found in walls of hollow organs and structures of the body that need to be able to contract without conscious input from the brain. Examples include the stomach, intestines, womb, bladder and blood vessels, and it also controls the opening and closing of the pupils of the eye. Generally, smooth muscle has to be able to contract continuously at a low level of intensity. The structure of smooth muscle enables it to function in this manner and differentiates it from other types of muscle found in the heart and musculature of the arms, legs, back and trunk.
Arrangement of the Cells
As smooth muscle cells have an elongated shape, they can fit together in an arrangement in which the middle portion, which is the widest, tucks in next to the thin end of the neighboring cells. Some of the cells have ends that are split in two. Some smooth muscle cells contract individually, whereas others contract as a group. The cells typically form a sheet wrapping around a hollow center; when they contract, they make the hollow center constrict. For example, when smooth muscle sheets surround an artery, contraction makes the interior of the artery smaller, and relaxation helps the artery dilate.
The nucleus of a smooth muscle cell, which contains the DNA, is located in the center of the cell, where the cell is widest. In the vicinity of the nucleus are various organelles, which are individual structures with specific cell functions. Examples of organelles found in the center of the cell include mitochondria -- the energy powerhouses of the cells -- and rough endoplasmic reticulum, where newly formed proteins are stored. Golgi apparatus, where proteins are modified, are also in this area, along with polyribosomes, which build proteins.
Unlike other types of muscle, smooth muscle does not have a stripy appearance, or striations, when viewed under the microscope. This is why this type of muscle is called "smooth." All muscles need a skeleton of fibers that can contract and relax, and in all muscle types, proteins called actin and myosin form this skeleton. When you can see stripes under the microscope, it means the actin and myosin are arranged in a specific, regular pattern called myofibrils, which appear as stripes. According to "Wheater's Functional Histology," smooth muscle does not have myofibrils. Instead, the actin and myosin are arranged in a lattice shape throughout the cell. This lattice shape means that as the cells contract, they get smaller and less elongated in all directions. This allows the sheet of muscle that contains them to wrap tightly and evenly around a hollow center of an artery, for example.
Smooth muscle cells do not have a special junction where the contraction impulse from a nerve ending attaches. Instead, the nerve endings have little swellings on their ends that are near the smooth muscle cell. These swellings release nerve signals that interact directly with the nearby cell to increase or decrease contraction. The cells that can contract on their own do not need to communicate with one another, but other cells that contract as a group do need to be able to communicate. The way signals move among groups of smooth muscle cells is via junctions between the cells called "gap junctions." Typically, the cells that communicate with one another tend to produce long and slow contractions, such as in the digestive tract. The smooth muscle cells that can react individually are responsible for the faster smooth muscle reactions, such as the pupil of the eye rapidly shrinking in diameter to protect bright light from dazzling you.