Silelihaskoed

Silelihaskoed, or smooth muscle tissue, are non-striated, involuntary muscles found in the walls of many hollow organs. They consist of spindle-shaped cells with a single central nucleus and lack the distinct banding seen in skeletal muscle. Smooth muscle is arranged in sheets or layers, typically with circular and longitudinal orientations; some organs also contain a single-unit (visceral) arrangement where cells are connected by gap junctions to coordinate contractions, while others have a multi-unit arrangement with more independent innervation (as in the iris or certain airways).

Functionally, silelihaskoed contract to propel contents through digestive and other tracts, regulate the passage and pressure of fluids within organs, and maintain baseline tone in vessels and ducts. They are essential for peristalsis in the gastrointestinal system, constriction and dilation of blood vessels, regulation of airway diameter, and uterine contractions during labor and reproductive processes.

Physiology and regulation: smooth muscle contraction is typically triggered by calcium entering from the extracellular space and released from intracellular stores. Calcium binds calmodulin, activating myosin light-chain kinase, which phosphorylates myosin and allows cross-bridge cycling with actin. Contractions are usually slower and can be sustained for longer with lower energy expenditure; smooth muscle can enter a latch state to maintain tone with minimal ATP use. Activity is controlled by the autonomic nervous system, hormones, and local chemical signals, with responses that vary by tissue type.

Locations include the walls of the gastrointestinal tract, respiratory and urinary tracts, blood vessel walls (tunica media), and parts of the reproductive system. Clinical relevance spans motility disorders, vascular regulation issues, bronchial hyperresponsiveness, and labor-related contractions.