|
( Smooth muscle)
Smooth muscle is a type of non-striated muscle, found within the tunica media layer of large and small arteries and veins, the bladder, uterus, male and female reproductive tracts, gastrointestinal tract, respiratory tract, the ciliary muscle, and iris of the eye. The glomeruli of the kidneys contain a smooth muscle-like cell called the mesangial cell. Smooth muscle is fundamentally different from skeletal muscle and cardiac muscle in terms of structure, function, excitation-contraction coupling, and mechanism of contraction. Smooth muscle fibers are spindle-shaped, and, like striated muscle, can contract and relax. In the relaxed state, each cell is spindle-shaped, 20-500 micrometers in length, and 2-10 micrometers wide.[1] There are two types of smooth muscle arrangements in the body multi-unit and single-unit. The single-unit type, also called unitary smooth muscle, is far more common. Whereas the former presents itself as distinct muscle fibers that are usually activated by their own nerve fibers, the latter operate as a single unit and are arranged in sheets or bundles. Unitary smooth muscle is also commonly referred to as visceral smooth muscle because it is found in the walls of the viscera, or internal organs, of the body, including the intestines, ducts such as the bile ducts, ureters and oviducts and most blood vessels.[2] Unitary smooth muscle can be further divided into phasic and tonic. Smooth muscle cells have, in general, single nuclei and a plethora of mitochondria. The cells are arranged in sheets or bundles and connected by gap junctions which connect the cells chemically. In order to contract, the cells contain actin filaments and a contractile protein called myosin. Whereas the filaments are essentially the same in smooth muscle as they are in skeletal and cardiac muscle, the way they are arranged is different. Some regulatory proteins also differ, and there are specific smooth muscle isoforms of actin and myosin. The smooth muscle cell contains less protein than a typical striated muscle cell and much less myosin. The actin content is similar, so the ratio of actin to myosin is ~61 in striated muscle and ~151 in smooth muscle.[citation needed] Smooth muscle does not contain the protein troponin, rather (calmodulin takes on the regulatory role in smooth muscle) and caldesmon and calponin are significant proteins expressed within smooth muscle. As non-striated muscle, the actin and myosin are not arranged into distinct sarcomeres that form orderly bands throughout the muscle cell. However, there is an organized cytoskeleton consisting of the intermediate filament proteins vimentin and desmin, along with actin filaments. Actin filaments attach to the sarcolemma by focal adhesions or attachment plaques and attach to other actin filaments via dense bodies (acting much like Z-lines in striated muscle). Evidence indicates that smooth muscle myosin is not bipolar with a central bare zone as in striated muscle, but is either side-polar or row-polar, and has no bare zone. Some smooth muscle preparations can be visualized contracting in a spiral corkscrew fashion, and contractile proteins can organize into zones of actin and myosin along the axis of the cell.
|
Smooth muscle Subcategories
Smooth muscle Articles
|
|
