New Chapt 17-Smooth muscle(1)

New Chapt 17-Smooth muscle(1) - Chapter 17 Smooth Muscle We...

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193 Chapter 17 Smooth Muscle We now need to very briefly consider smooth muscle. Although we will see that the basic contractile mechanisms are the same in that they involve the interaction of actin and myosin, smooth muscle differs from skeletal muscle in several important respects. First, smooth muscle fibers are much smaller than skeletal muscle fibers. Each smooth muscle fiber is a single cell having a single nucleus. The cells are spindle-shaped, about 2-20m in diameter, and 10-100 times as long as they are wide (Fig. 1). Fig. 1. Three major classes of vertebrate muscle, showing the morphology (left) and relationships between electrical and mechanical activity (right panel). A: skeletal muscle fibers have a striated appearance. They are cylindrical and multinucleated. These fibers have brief action potentials and brief contractions. B: Cardiac muscle cells are also striated but are not innervated by a motor neuron. Fibers are joined end-to-end by gap junctions that allow electrical current to flow from one cell to the next. Both the action potentials and contractions of these cells are prolonged. C: Smooth muscle fibers are small and spindle shaped, each with a single nucleus. They are joined laterally by gap junctions into electrically coupled groups. The innervation is diffuse. The transmitter is released into spaces from varicosities (swellings) along an autonomic nerve and diffuse to many fibers. While the action potentials are brief, the resultant contractions are slow and long lasting. Secondly, smooth muscle fibers are electrically coupled to each other through gap junctions. Bundles of smooth muscle fibers coupled in this manner actually form functional units (Fig. 1). Thirdly, smooth muscle fibers lack striations (hence the name, smooth muscle)--the characteristic organization of actin and myosin filaments into sarcomeres present in skeletal and cardiac muscle is absent. Actin and myosin filaments are still present in smooth muscle fibers,
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194 but there are no Z lines and the contractile proteins are not organized as sarcomeres. Rather the actin filaments are anchored to dense bodies, which are functionally similar to Z lines of skeletal
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This note was uploaded on 09/19/2011 for the course BIO 365R taught by Professor Draper during the Spring '08 term at University of Texas.

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New Chapt 17-Smooth muscle(1) - Chapter 17 Smooth Muscle We...

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