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Movement and its regulation, on a microscopic and/or macroscopic scale, are essential for human life. Muscle, composed of actin, myosin, and a variety of structural and regulatory proteins, is one of the main cell/tissue types involved in this movement. Skeletal, cardiac, and smooth muscles offer a coordinated and regulated means to move the human body from place to place; interact with its surroundings; keep nutrients flowing to and waste products flowing from various cells; or move nutrients, blood, lymph, and other molecules. Specialized proteins and the means to provide energy for these processes have evolved for particular tissues and functions.

Microtubules with tubulin, dynein, and kinesin molecules and associated structures such as cilia, flagella, centrioles, basal bodies, centromeres, and mitotic spindles provide another important mechanism for a variety of cell movements and internal cell functions that affect all types of cells and allow the division of cells. Intermediate filaments (IFs) also serve several essential roles of cell motility. Nonmuscle cells utilize all of these mechanisms—actin/myosin, microtubule/dynein/kinesin, and IFs—to achieve a wide array of functions throughout the human body.


Muscle is a tissue that specializes in transforming chemical energy into mechanical work or movement. The muscular system comprises all the individual anatomic muscles. Muscle tissue is derived from the mesodermal layer of embryologic germ cells and is divided into three main types: skeletal muscle, cardiac muscle, and smooth muscle (Figure 12-1). The structure of these different types of muscles is similar but the architecture and regulation are often very different, making their functions unique. Nonmuscle cells are a fourth type that utilizes many of the same proteins and processes for motility.

Figure 12-1.

A–C. The Three Major Types of Muscle. Light micrographs of each type, accompanied by labeled drawings. A. Skeletal muscle is composed of large, elongated, multinucleated fibers that show strong, quick, voluntary contractions. B. Cardiac muscle is composed of irregular branched cells bound together longitudinally by intercalated disks and shows strong, involuntary contractions. C. Smooth muscle is composed of grouped, spindle-shaped cells with weak, involuntary contractions. The density of intercellular packing seen reflects the small amount of extracellular connective tissue present. [Reproduced with permission from Mescher AL: Junqueira’s Basic Histology Text and Atlas, 12th edition, McGraw-Hill, 2010.]

There are two major proteins involved in contraction in all types of muscle: actin and myosin. These proteins convert chemical energy into mechanical work through an interaction with adenosine triphosphate (ATP). The triggering and control of this process are different for each muscle type. Skeletal, cardiac, and smooth muscles have a repeating unit called the sarcomere, containing the muscle fibers composed of actin and myosin and accessory proteins (see below). Surrounding these muscle fibers is the equivalent of a plasma membrane called the sarcolemma. Within ...

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