THE LIVING WORLD
Unit Six. Animal Life
22. The Animal Body and How It Moves
22.5. Muscle Tissue Lets the Body Move
Muscle cells are the motors of the vertebrate body. The distinguishing characteristic of muscle cells, the thing that makes them unique, is the abundance of contractible protein fibers within them. These fibers, called myofilaments, are made of the proteins actin and myosin. Vertebrate cells have a fine network of these myofilaments, but muscle cells have many more than other cells. Crammed in like the fibers of a rope, they take up practically the entire volume of the muscle cell. When actin and myosin slide past each other, the muscle contracts. Like slamming a spring-loaded door, the shortening of all of these fibers together within a muscle cell can produce considerable force. The process of muscle contraction will be discussed later in this chapter.
The vertebrate body possesses three different kinds of muscle cells: smooth muscle, skeletal muscle, and cardiac muscle (table 22.4). In smooth muscle, the myofilaments are only loosely organized (seen under 1 in the table). In skeletal and cardiac muscle, the myofilaments are bunched together into fibers called myofibrils. Each myofibril contains many thousands of myofilaments, all aligned to provide maximum force when they simultaneously slide past each other. Skeletal and cardiac muscle are often called striated muscles because, as you can see under 2 and 3 in the table, their cells appear to have transverse stripes when viewed in longitudinal section under the microscope.
TABLE 22.4. MUSCLE TISSUE
Smooth muscle cells are long and spindle-shaped, each containing a single nucleus. However, the individual myofilaments are not aligned into orderly assemblies as they are in skeletal and cardiac muscles. Smooth muscle tissue is organized into sheets of cells. In some tissues, smooth muscle cells contract only when they are stimulated by a nerve or hormone. Examples are the muscles that line the walls of many blood vessels and those that make up the iris of the vertebrate eye. In other smooth muscle tissue, such as that found in the wall of the gut, the individual cells contract spontaneously, leading to a slow, steady contraction of the tissue.
Skeletal muscles move the bones of the skeleton. Skeletal muscle cells are produced during development by the fusion of several cells at their ends to form a very long fiber. Each of these muscle cell fibers still contains all the original nuclei, pushed out to the periphery of the cytoplasm. Looking at figure 22.6, you can see how the nuclei are positioned to the outside of the muscle fiber. Each muscle fiber consists of many elongated myofibrils, and each myofibril is, in turn, composed of many myofilaments, the protein filaments actin and myosin. Myofibrils and myofilaments have been pulled out from the muscle cells in figure 22.6 so you can see the levels of organization in the muscle fiber. The key property of muscle cells is the relative abundance of actin and myosin within them, which enables a muscle cell to contract. These protein filaments are present as part of the cytoskeleton of all eukaryotic cells, but they are far more abundant and more highly organized in muscle cells.
Figure 22.6. A skeletal muscle fiber, or muscle cell.
Each muscle is composed of bundles of muscle cells, or fibers. Each fiber is composed of many myofibrils, which are each, in turn, composed of myofilaments. Muscle cells have a modified endoplasmic reticulum called the sarcoplasmic reticulum that is involved in the regulation of calcium ions in muscles.
The vertebrate heart is composed of striated cardiac muscle in which the fibers are arranged very differently from the fibers of skeletal muscle. Instead of very long, multinucleate cells running the length of the muscle, heart muscle is composed of chains of single cells, each with its own nucleus (refer back to the photo under 3 in table 22.4). Chains of cells are organized into fibers that branch and interconnect, forming a latticework. This lattice structure is critical to the way heart muscle functions. Each heart muscle cell is coupled to its neighbors electrically by tiny holes called gap junctions that pierce the plasma membranes in regions where the cells touch each other. Heart contraction is initiated at one location by the opening of transmembrane channels that conduct ions across the membrane. This changes the electrical properties of the membrane. An electrical impulse then passes from cell to cell across the gap junctions, causing the heart to contract in an orderly pulsation.
Key Learning Outcome 22.5. Muscle tissue is the tool the vertebrate body uses to move its limbs, contract its organs, and pump blood through its circulatory system.