14.20: Structure and Organization of Smooth Muscles
Smooth muscle tissue is a type of muscle tissue that can be found lining various vital organs in the human body, including the lungs, blood vessels, digestive tract, and respiratory tract. This type of tissue is responsible for regulating the movements of these organs, playing crucial roles in the functioning of various systems, including the vascular, digestive, respiratory, and urinary systems.
Structure of smooth muscle cell
Smooth muscle cells are spindle-shaped with tapering ends and a single nucleus in the center. They typically range from 30 to 200 µm in length and have a diameter of about 5-10 µm. Unlike the striated appearance of skeletal and cardiac muscles, smooth muscle fibers have a uniform, non-striated appearance. This appearance is due to the lack of organized sarcomeres. The contractile proteins actin and myosin, which form the thin and thick filaments, respectively, are arranged obliquely and irregularly along the long axis of the cell. The thin filaments are anchored to the sarcolemma by dense bodies interconnected via the intermediate filament network. Instead of T-tubules, these muscle fibers have caveolae — small pockets of invaginated sarcolemma that help influx Ca2+ ions during contraction cycles.
Smooth muscles can be further categorized into two types: multi-unit and single-unit (visceral) smooth muscle.
Multi-unit smooth muscle
In contrast, multi-unit smooth muscle fibers consist of individual muscle fibers that operate more independently of each other. Each fiber has its own nerve supply, and there are fewer gap junctions between cells. This arrangement allows for more precise and localized control of contraction. Multiunit smooth muscle is found in areas where fine control is necessary, such as in the iris of the eye, where it controls pupil size, and in the walls of blood vessels, where it regulates blood flow and pressure.
Visceral smooth muscle
Visceral smooth muscle is the most common type of smooth muscle. It is found in the walls of hollow organs such as the stomach, intestines, uterus, and bladder. These cells are connected by gap junctions, which allow electrical and chemical signals to pass directly from one cell to another. As a result, when one cell becomes excited and contracts, the signal quickly spreads to neighboring cells, causing the muscle layer to contract as a synchronized unit. This property is essential for functions like peristalsis in the gastrointestinal tract, where coordinated waves of contraction move contents through the system.
