All muscles contain the same contractile proteins that enable them to shorten and produce force—actin and myosin (Chapter 10). These proteins are organized into bundles called filaments that interact with one another to cause muscles to shorten.

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Although all muscle fibers have filaments of actin and myosin, the filaments are arranged differently in different types of muscle. Muscles are divided into two broad groups on the basis of their function and appearance under a light microscope (Fig. 37.2). As their name suggests, striated muscles appear striped under a light microscope because the actin and myosin filaments are arranged in a regularly repeating pattern. Striated muscles include skeletal muscles (Fig. 37.2a), which connect to the body skeleton to move the animal’s limbs and torso, and cardiac muscle (Fig. 37.2b), which contracts to pump blood. Skeletal muscle cells are elongated and have many nuclei in each cell, whereas cardiac muscle cells are typically less elongated than skeletal muscle cells, often branched, and contain one or more nuclei per cell.

In contrast to skeletal and cardiac muscle, smooth muscles appear unstriated under the light microscope because the organization of actin and myosin filaments in smooth muscles is irregular (Fig. 37.2c). Smooth muscles are found in the walls of vertebrate and certain invertebrate (squid and octopus) arteries to regulate blood flow, in the respiratory system to control airflow, and in the digestive and excretory systems to help transport food and waste products. Smooth muscles contract slowly compared with cardiac and skeletal muscles. Consequently, bivalve mollusks rely on smooth muscle fibers to keep their shells closed for long periods without having to expend much energy.