14.22: Functions of Smooth Muscles
Smooth muscles are an important type of muscle tissue that plays a vital role in the involuntary movements of internal organs. For example, they help regulate the movement of food through the gut and the flow of blood through the circulatory system.
Function of visceral smooth muscles
Visceral smooth muscle is found in the walls of all hollow organs, except the heart, and is a key player in the involuntary movements that drive the functioning of these internal organs. This tissue is arranged in layers of tightly packed fibers that communicate via gap junctions, enabling them to contract synchronously as a single unit. For example, smooth muscle in the intestine is organized into two primary layers: the longitudinal and circular layers. The longitudinal layers run along the length of the intestine, and their contraction shortens the overall length of the intestinal tract. Meanwhile, the circular layers wrap around the circumference and facilitate constriction and dilation of the inner cavity. The alternating contraction and relaxation of these layers result in peristalsis that helps propel the contents and ensures thorough mixing and breakdown, optimizing nutrient absorption and digestion.
Function of multi-unit smooth muscles
While many organs rely on the coordinated activity of visceral smooth muscle, certain areas, like the eye and walls of large arteries, contain multi-unit smooth muscle fibers. These fibers operate independently, each with its own nerve supply, allowing for precise and localized control. In large pulmonary arteries, for instance, multi-unit smooth muscles adjust the diameter of the lumen to direct blood flow to well-ventilated regions of the lung, enhancing oxygenation and adjusting to the body's changing needs.
Overall, smooth muscles in various organs demonstrate incredible adaptability and precision in their functions. For example, in the respiratory system, smooth muscles adjust the airway diameter, increasing or decreasing airflow in response to the body's oxygen demands. In the vascular system, these muscles regulate blood pressure and flow by altering the diameter of blood vessels.
