Sléttvöðvafrumur

Sléttvöðvafrumur, also known as smooth muscle cells, are a type of muscle cell found in the walls of internal organs and structures throughout the body. Unlike skeletal muscle cells, which are under voluntary control, or cardiac muscle cells, which form the heart, smooth muscle cells are involuntary. This means their contraction and relaxation are regulated by the autonomic nervous system, hormones, and local factors, rather than conscious thought.

These cells are characterized by their elongated, spindle shape, with a single, centrally located nucleus. The cytoplasm of a smooth muscle cell is filled with contractile filaments, primarily actin and myosin, which are arranged in a less organized manner compared to skeletal muscle. This arrangement allows for sustained, slow contractions that are crucial for functions like moving food through the digestive tract (peristalsis), regulating blood flow by constricting or dilating blood vessels, and emptying the bladder.

Smooth muscle can be found in the walls of hollow organs such as the stomach, intestines, uterus, bladder, and blood vessels. They also form the iris of the eye, controlling pupil size, and are responsible for the erection of hairs (piloerection). The coordinated action of smooth muscle cells allows for a wide range of essential physiological processes that maintain homeostasis and bodily function.