Muscle

Muscle cells, formed from the mesoderm early in development, are the "workhorses" of the vertebrate animal. The distinguishing characteristic of muscle cells, the one that makes them unique, is the relative abundance of actin and myosin micro filaments within them. These micro filaments are present as a fine network in all eukaryotic cells, but they are far more abundant in muscle cells. In muscle cells the actin micro filaments are bunched together with thicker filaments of myosin into many thousands of fiber called myofibrils. The myofibrils shorten when the actin and myosin filaments slide past each other. The shortening of these myofibrils can cause the muscle cell to change shape. Because there are so many filaments all aligned the same way in muscle cells, a considerable force is generated when all of the myofibrils shorten at the same time. Vertebrates possess three different kinds of muscle cells: smooth muscle, striated muscle, and cardiac muscle.

Smooth muscle

Smooth muscle was the earliest from of muscle to evolve, and it is found throughout the animal kingdom. Smooth muscle cells are long and spindle-shaped, each cell containing a single nucleus. The interiors of smooth muscle cells are packed with actin-myosin myofibrils, but the individual myofibrils of a cell are not aligned into organized assemblies. Smooth muscle tissue is organized into sheets of cells. In some tissues the muscle cells contract only when they are stimulated by a nerve or hormone, and then all of the cells contract together as a unit. An example are the muscle found lining the walls of many vertebrate blood vessels and those which 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 muscle

Skeletal muscle art the muscles associated with the skeleton. They are also called striated muscles because they are obviously marked with lines. Striated muscle cells are produced during development by the fusion of several cells at their ends to from a very long fiber. Each muscle cell or muscle fiber still contains all of the original nuclei pushed out to the periphery of the cytoplasm. Each striated muscle is a tissue made up of numerous individual muscle cells that act as a unit. These striated muscle cells represent a distinct improvement in muscle cell organization, as compared with that found in the smooth muscle cells. Imagine a large raft being towed upstream by many small canoes, each canoe bound to the raft by its own towline. This is analogous to the contraction of smooth muscle, each smooth muscle cell participating individually in the contraction of the muscle.

Cardiac muscle

The vertebrate heart is composed of striated muscle fibers arranged very differently from the fibers of skeletal muscle. Instead of very long multi nucleate cells running the length of the muscle, heart muscle is composed of chains of single cells, each with its own nucleus. These chains of cells are organized into fiber that branch and interconnect, forming a latticework. This lattice structure is critical to how heart muscle functions. Heart contraction is initiated at one location by the opening of trans membrane channels that admit ions into the muscle cells there, altering the charge of their membranes. This change in membrane charge is called depolarization. When two cardiac muscle fibers touch one another, their membranes make an electrical junction. As a result, the electrical depolarization of the initial fiber initiates a wave of contraction throughout the heart, with the wave of depolarization rapidly passing from one fiber to another across these junctions. For this reason, a mass of heart muscle tends to contract all at once, rather than gradually.