The neural control of skeletal muscles differs significantly from that of smooth muscles. A skeletal muscle fiber has only one junction with a somatic nerve fiber, and the receptors for the neu-rotransmitter are located only at the neuromuscular junction. By contrast, the entire surface of smooth muscle cells contains neu-rotransmitter receptor proteins. Neurotransmitter molecules are released along a stretch of an autonomic nerve fiber that is located some distance from the smooth muscle cells. The regions of the autonomic fiber that release transmitters appear as bulges, or varicosities, and the neurotransmitters released from these varicosities stimulate a number of smooth muscle cells. Since there are numerous varicosities along a stretch of an autonomic nerve ending, they form synapses "in passing"—or synapses en passant—with the smooth muscle cells. This was described in chapter 9 (see figure 9.9) and is shown in figure 12.35.
Test Yourself Before You Continue
Explain how cardiac muscle differs from skeletal muscle in its structure and regulation of contraction. Contrast the structure of a smooth muscle cell with that of a skeletal muscle fiber and discuss the advantages of each type of structure.
3. Describe the events by which depolarization of a smooth muscle cell results in contraction and explain why smooth muscle contractions are slow and sustained.
Distinguish between single-unit and multiunit smooth muscles.
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