Smooth muscle falls into two general categories, visceral smooth muscle and multi-unit smooth muscle. Visceral smooth muscle fibers line internal organs such as the intestines, stomach, and uterus. They also facilitate the movement of substances through tubular areas such as blood vessels and the small intestines. Multi-unit smooth muscles function in a highly localized way in areas such as the iris of the eye. Contrary to contractions in visceral smooth muscle, contractions in multi-unit smooth muscle fibers do not readily spread to neighboring muscle cells.
Smooth muscle is unstriated with innervations from both sympathetic (flight or fight) and parasympathetic (more relaxed) nerves of the autonomic nervous system. Smooth muscle appears unstriated under a polarized light microscope, because the myofilaments inside are less organized. Smooth muscle fibers contain actin and myosin myofilaments which are more haphazardly arranged than they are in skeletal muscles. The sympathetic neurotransmitter, Ach, and parasympathetic neurotransmitter, norepinephrine, activate this type of muscle tissue.
The concentric arrangement of some smooth muscle fibers enables them to control dilation and constriction in the intestines, blood vessels, and other areas. While innervation of these cells is not individual, excitation from one cell can spread to adjacent cells through nexuses which join neighbor cells. Smooth muscle cells have a small diameter of about 5-15 micrometers and are long, typically 15-500 micrometers. They are also wider in the center than at their ends. Gap junctions connect small bundles of cells which are, in turn, arranged in sheets.
Within hollow organs, such as the uterus, smooth muscle cells are arranged into two layers. The outer layer is usually arranged in a longitudinal fashion surrounding the inner layer which is arranged in a circular orientation. Many smooth muscles are regulated by hormones in addition to the neurotransmitters of the autonomic nervous system. In addition, contraction of some smooth muscles are myogenic or triggered by stretching as in the uterus and gastrointestinal tract.
Smooth muscle differs from skeletal and cardiac muscle in its energy utilization as well. Smooth muscles are not as dependent on oxygen availability as cardiac and skeletal muscles are. Smooth muscle uses glycolysis to generate much of its metabolic energy.
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