Smooth muscle (named as such because the cells do not have striations) is present in the walls of organs that contain a lumen like the urinary bladder, uterus, stomach, intestines, and in the walls of large vascular tubes, such as the arteries and veins of the circulatory system. The tracts of the respiratory, urinary, and reproductive systems contain smooth muscle. Smooth muscle is also present in the eye, where it functions to change the size of the pupil and in the skin where it causes hair to stand erect in response to low temperatures or fear. Smooth muscle is referred to as an involuntary muscle since is not under voluntary control. In certain locations, such as the walls of visceral organs, stretching the muscle can trigger its contraction (the stretch-relaxation response).
Smooth muscle contraction relies on the presence of Ca++ ions similar to skeletal and cardiac muscle. However, smooth muscle fibers are much smaller in all dimensions than skeletal muscle cells. T-tubules are not required to reach the interior of the cell and therefore not necessary to transmit an action potential deep into the fiber. Smooth muscle fibers have a limited calcium-storing sarcoplasmic reticulum but have calcium channels in the sarcolemma that open during the action potential along the sarcolemma. The influx of extracellular Ca++ ions, which diffuse into the sarcoplasm to reach a protein called calmodulin. The calcium-calmodulin complex activates myosin light chain kinase, an enzyme that phosphorylates myosin and allows it to bind to the actin network within the cell.
Muscle contraction continues until ATP-dependent calcium pumps actively transport Ca++ ions back into the sarcoplasmic reticulum and out of the sarcoplasm. A small amount of calcium remains in the sarcoplasm to maintain muscle tone and keep the cells contracted to a small degree. This is important in certain organs and around blood vessels. Because most smooth muscles must function for long periods without rest, their power output is relatively low, but contractions can continue without using large amounts of energy.
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