Skeletal muscle was a relatively late evolutionary development, and muscle cells are highly specialized for rapid and efficient contraction. The long, thin muscle fibers of skeletal muscle are actually huge single cells that form during 918 Chapter 16: The Cytoskeleton development by the fusion of many separate cells. The large muscle cell retains the many nuclei of the contributing cells. These nuclei lie just beneath the plasma membrane (Figure 16–31). The bulk of the cytoplasm inside is made up of myofibrils, which is the name given to the basic contractile elements of the muscle cell. A myofibril is a cylindrical structure 1–2 μm in diameter that is often as long as the muscle cell itself. It consists of a long, repeated chain of tiny contractile units— called sarcomeres, each about 2.2 μm long—which give the vertebrate myofibril its striated appearance (Figure 16–32).
Each sarcomere is formed from a miniature, precisely ordered array of parallel and partly overlapping thin and thick filaments. The thin filaments are composed of actin and associated proteins, and they are attached at their plus ends to a Z disc at each end of the sarcomere. The capped minus ends of the actin filaments extend in toward the middle of the sarcomere, where they overlap with thick filaments, the bipolar assemblies formed from specific muscle isoforms of myosin II (see Figure 16–27). When this region of overlap is examined in cross section by electron microscopy, the myosin filaments are arranged in a regular hexagonal lattice, with the actin filaments evenly spaced between them (Figure 16–33). Cardiac muscle and smooth muscle also contain sarcomeres, although the organization is not as regular as that in skeletal muscle.
Figure 16–31 Skeletal muscle cells (also called muscle fibers). (A) These huge multinucleated cells form by the fusion of many muscle cell precursors, called myoblasts. Here, a single muscle cell is depicted. In an adult human, a muscle cell is typically 50 μm in diameter and can be up to several centimeters long. (B) Fluorescence micrograph of rat muscle, showing the peripherally located nuclei (blue) in these giant cells. Myofibrils are stained red. (B, courtesy of Nancy L. Kedersha.)
Figure 16–30 The force of a single myosin molecule moving along an actin filament measured using an  optical trap. (A) Schematic of the experiment, showing an actin filament with beads attached at both ends and held in place by focused beams of light called optical tweezers (Movie 16.4). The tweezers trap and move the bead, and can also be used to measure the  force exerted on the bead through the filament. In this experiment, the filament was positioned over another bead to which myosin II motors were attached, and the optical tweezers were used to determine the effects of myosin binding  on movement of the actin filament.
These traces show filament movement in two separate experiments. Initially, when the actin filament is unattached to myosin, thermal motion of the filament produces noisy fluctuations in filament position. When a single myosin binds to the actin filament, thermal motion decreases abruptly and a roughly 10-nm displacement results from movement of the filament by the motor. The motor then releases the filament. Because the ATP concentration is very low in this experiment, the myosin remains attached to the actin filament for much longer than it would in a muscle cell. (Adapted from
C.
Rüegg et al., Physiology 17:213–218, 2002. With permission from the American Physiological Society.)