Answer
Smooth muscles are different from skeletal muscles in different ways:
1, They are more responsive to stretch
2. They can maintain contractions (tension longer than skeletal muscles.
3, They comparatively less energy than skeletal muscles to generate the same tension
4, They contract more slowly than skeletal muscles.
These qualities are very useful in places where smooth muscle are found and in facilitating the
functions they perform.
Smooth muscles are found in the walls of the cardiovascular organs, in the walls of the GI organs, in the respiratory tract, in the urinary bladder, in the uterus, in the os of the cervix and duodenal and anal sphincter and in the iris of the eye. In the blood vessels the smooth muscles contract continuously with varying force to maintain vascular tone and to control blood flow. Contraction in sphincters like the anal sphincter and the cervix of the uterus are strong and prolonged strong. Smooth muscle structure and physiology which facilitate the formation of latch cross bridges enable the maintenance of these long periods of contraction.
The energy metabolism in smooth muscle enable them to maintain prolonged contraction utilizing much less ATP ( 1/10th to 1/100th) than what would be required by skeletal muscles to maintain the same strength of contraction for the same time.
Work Step by Step
Smooth muscles contract in response to stimuli from the autonomic nervous system. One of the special characteristics of smooth muscles is their ability to contract for prolonged periods.
Alhough smooth muscle cells have no striations, they do have actin and myosin (thin and thick filaments) molecules. Calcium is supplied both from the sarcoplasmic reticulum (SR) and from the extracellular fluid by way of caveoli. Crossbridge formation is regulated by Calmodulin (there is no troponin). Calcium that gets into the muscle fiber is bound to Calmodulin. The Calmodulin-Calcium complex activates a myosin kinase enzyme which splits ATP into ADP and Pi. The phosohate binds to, and activates myosin heads . The myosin heads then attach to actin filaments and pull them along. As the thin filaments slide past the tick filaments,this action pulls on the intermediate filaments in the sarcoplasm, causing the entire muscle fiber to contract.
Smooth muscle fibers do not have much Ca++ stored in their SR membranes, but under the impulse of ATP their sarcoplasm can open channels that allow Ca++ to diffuse into the cell(fiber) to bind with Calmodulin and trigger contractions. After the contraction the ATP-Ca++ pump actively removes Calcium from the cell into the ECF. But a low concentration of Calcium remains, and that is enough to maintain muscle tone. Moreover, even as Ca++ is being removed, and myosin is being dephosphorylated, latch bridges are formed between myosin heads and actin without the need for ATP. This permits the maintenance of tension or tone in cardiovascular organs and other visceral organs with minimal expenditure of energy and without running the risk of muscle fatigue.
