Muscular (contractile) tissue is composed of cells called muscle fibers (Table 4.3). Muscle fibers contain actin and myosin, which are protein filaments whose interaction accounts for movement. The three types of vertebrate muscles are skeletal, smooth, and cardiac.
Skeletal muscle, also called voluntary muscle (Fig. 4.11), is attached by tendons to the bones of the skeleton. When skeletal muscle contracts, body parts such as arms and legs move. Contraction of skeletal muscle, which is under voluntary control, is forceful but of short duration. Skeletal muscle fibers are cylindrical and quite long-sometimes they run the length of the muscle. They arise during development when several cells fuse, resulting in one fiber with multiple nuclei. The nuclei are located at the periphery of the cell, just inside the plasma membrane. The fibers have alternating light and dark bands that give them a striated (striped) appearance. These bands are due to the placement of actin filaments and myosin filaments in the fiber.
Figure 4.11 Skeletal muscle. The cells are long, cylindrical, and multinucleated.
Smooth (visceral) muscle is so named because the arrangement of actin and myosin does not give the appearance of cross-striations. The spindle-shaped cells form layers in which the thick middle portion of one cell is opposite the thin ends of adjacent cells. Consequently, the nuclei form an irregular pattern in the tissue (Fig. 4.12). Smooth muscle is not under voluntary control and therefore is said to be involuntary. Smooth muscle is found in the walls of hollow viscera, such as the intestines, stomach, uterus, urinary bladder, and blood vessels. Smooth muscle contracts more slowly than skeletal muscle but can remain contracted for a longer time. Contractility is inherent in this type of muscle, and it contracts rhythmically on its own. Even so, its contraction can be modified by the nervous system. Smooth muscle of the small intestine contracts in waves, thereby moving food along its lumen (central cavity). When the smooth muscle of blood vessels contracts, blood vessels constrict, helping to regulate blood flow.
Figure 4.12 Smooth muscle. The cells are spindle-shaped.
Cardiac muscle (Fig. 4.13) is found only in the walls of the heart. Its contraction pumps blood and accounts for the heartbeat. Cardiac muscle combines features of both smooth muscle and skeletal muscle. Like skeletal muscle, it has striations, but the contraction of the heart is involuntary for the most part. Also like skeletal muscle, its contractions are strong, but like smooth muscle, the contraction of the heart is inherent and rhythmical. Also, its contraction can be modified by the nervous system. Even though cardiac muscle fibers are striated, the cells differ from skeletal muscle fibers in that they have a single, centrally placed nucleus. The cells are branched and seemingly fused one with the other, and the heart appears to be composed of one large, interconnecting mass of muscle cells. Actually, cardiac muscle cells are separate and individual, but they are bound end-to-end at intercalated disks, areas where folded plasma membranes between two cells contain adhesion junctions and gap junctions. These permit extremely rapid spread of contractile stimuli so that the fibers contract almost simultaneously.
Figure 4.13 Cardiac muscle. The cells are cylindrical but branched.