Basic Contractile Unit

Actin and myosin are the major contractile proteins in all smooth muscles. As in striated muscle, these proteins are arranged in two sets of filaments: actin in thin filaments and myosin in thick filaments. Although there are many similarities between the smooth muscle contractile proteins and their counterparts in striated muscle, there are also qualitative as well as quantitative differences. Smooth muscle actin is a globular protein with a molecular weight of approximately 50,000 Da. However, there are two, perhaps three, isoforms in smooth muscle, and they all differ from the predominant ones found in striated muscle. Most of the actin is polymerized into thin filaments, but some appears to be present in globular form. Each molecule of smooth muscle myosin consists of two myosin heavy chains and two pairs of light chains; however, each of these monomers differs in isoform from those found in skeletal muscle. Myosin molecules are thought to be arranged in the thick filaments in the same way as those in skeletal muscle, with crossbridges extending to make contact with the actin filaments.

Although thick and thin filaments are present, their arrangement in smooth muscle cells is different from that seen in striated muscle (Fig. 1). The most striking difference is that, in smooth muscle, filaments are not organized into sarcomeres, thus giving a homogeneous appearance under the light microscope. There are many more thin filaments than thick, with the ratio being closer to 10:1 than to the 2:1 seen in skeletal muscle. Also, not every thin filament is in close proximity to a thick filament. In fact, there may be two or more populations of thin filaments: those associated with thick filaments and those associated with other actin-binding proteins and the cytoskeleton. Thin filaments are attached to elements of the cytoskeleton, but these

FIGURE 1 (A) Schematic longitudinal view of the sarcomeric structure of striated muscle. (B) Schematic longitudinal view of the filaments of smooth muscle. (C) Schematic cross-sectional view of the filaments of smooth muscle.

attachments bear little anatomic resemblance to the Z disks found in striated muscle. Most common are thin filaments anchored to protein structures known as dense bodies.

Even though thick and thin filaments are not arranged in sarcomeres, the basic contractile model given for striated muscle—the sliding of one filament over the other due to crossbridge cycling—is thought to hold for smooth muscle. The lack of a rigid structure may account for some of the quantitative differences seen in contractions of smooth muscle compared to those of striated muscle (see below).

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