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Types of Smooth Muscle

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There are two functional categories of smooth muscle called multiunit and single-unit types (fig. 11.21). Multiu-nit smooth muscle occurs in some of the largest arteries and pulmonary air passages, in the piloerector muscles of the hair follicles, and in the iris of the eye. Its innervation, although autonomic, is otherwise similar to that of skeletal muscle—the terminal branches of a nerve fiber synapse with individual myocytes and form a motor unit. Each motor unit contracts independently of the others, hence the name of this muscle type.

Single-unit smooth muscle is more widespread. It occurs in most blood vessels and in the digestive, respiratory, urinary, and reproductive tracts—thus, it is also called visceral muscle. In many of the hollow viscera, it forms two or more layers—typically an inner circular layer, in which the myocytes encircle the organ, and an outer longitudinal layer, in which the myocytes run lengthwise along the

Figure 11.21 Smooth Muscle Innervation. (a) Multiunit smooth muscle, in which each muscle cell receives its own nerve supply. (b) Single-unit smooth muscle, in which a nerve fiber passes through the tissue without synapsing with any specific muscle cell.

Saladin: Anatomy & I 11. Muscular Tissue I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

434 Part Two Support and Movement organ (fig. 11.22). The name single-unit refers to the fact that the myocytes of this type of muscle are electrically coupled to each other by gap junctions. Thus, they directly stimulate each other and a large number of cells contract as a unit, almost as if they were a single cell.