Other Tensile Structures
The myosin and actin are the contractile parts of continuous chains of tensile structures that stretch both across and between smooth muscle cells.
The actin filaments of contractile units are attached to dense bodies. Dense bodies are rich in α-actinin, and also attach intermediate filaments (consisting largely of vimentin and desmin), and thereby appear to serve as anchors from which the thin filaments can exert force. Dense bodies also are associated with β-actin, which is the type found in the cytoskeleton, suggesting that dense bodies may coordinate tensions from both the contractile machinery and the cytoskeleton.
The intermediate filaments are connected to other intermediate filaments via dense bodies, which eventually are attached to adherens junctions (also called focal adhesions) in the cell membrane of the smooth muscle cell, called the sarcolemma. The adherens junctions consist of large number of proteins including α-actinin, vinculin and cytoskeletal actin. The adherens junctions are scattered around dense bands that are circumfering the smooth muscle cell in a rib-like pattern. The dense band (or dense plaques) areas alternate with regions of membrane containing numerous caveolae. When complexes of actin and myosin contract, force is transduced to the sarcolemma through intermediate filaments attaching to such dense bands.
During contraction, there is a spatial reorganization of the contractile machinery to optimize force development. part of this reorganization consists of vimentin being phosphorylated at Ser56 by a p21 activated kinase, resulting in some disassembly of vimentin polymers.
Also, the number of myosin filaments is dynamic between the relaxed and contracted state in some tissues as the ratio of actin to myosin changes, and the length and number of myosin filaments change.
Smooth muscle cells have been observed contracting in a spiral corkscrew fashion, and contractile proteins have been observed organizing into zones of actin and myosin along the axis of the cell.
Smooth muscle-containing tissue needs to be stretched often, so elasticity is an important attribute of smooth muscle. Smooth muscle cells may secrete a complex extracellular matrix containing collagen (predominantly types I and III), elastin, glycoproteins, and proteoglycans. Smooth muscle also has specific elastin and collagen receptors to interact with these proteins of the extracellular matrix. These fibers with their extracellular matrices contribute to the viscoelasticity of these tissues. For example, the great arteries are viscolelastic vessels that act like a Windkessel, propagating ventricular contraction and smoothing out the pulsatile flow, and the smooth muscle within the tunica media contributes to this property.
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