Smooth muscles are in the iris (pupil contraction), skin (hair muscles that cause goose bumps), walls of vessels and hollow internal organs: bronchi, lower part of esophagus, stomach, gallbladder, intestine, reproductive and urogenital organs, glands and their ducts. Connective tissue is made up of proteins: collagen and elastin. Average smooth muscle cell is 25-50 µm long and 2-5 µm wide. Actin and myosin are not organized in sarcomeres so no striations are visible under light microscope. Smooth muscle cells are pretty non-organized and meet each other in different angles. In vessels they are arranged circumferentially and they change vessel’s diameter by contracting. Smooth muscles are innervated by sympathetic and parasympathetic (autonomous) nerves. It takes about 500 ms for a smooth muscle to reach peak contraction.


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