Muscular System

The muscular system is composed of all those body tissues capable of contracting and relaxing and, therefore, of producing movements in its body parts. Muscles are able to produce movement by converting chemical energy into mechanical energy. This conversion occurs due to the ability of muscles to contract and relax. Animals use muscles not only to move about but to operate their many necessary internal processes such as blood circulation.

Since animals cannot make their own food as plants do, they must be able to move about to locate things to eat. Movement is therefore a trait shared by all animals, from a one-celled amoeba to a killer whale. Animals are able to move because they have a support framework (a jointed skeleton) that is moved by muscles, which push bones together and pull them apart. Muscles are made up of long fibers of contractile tissue, meaning they have the ability to contract or grow shorter by means of a chemical change, as well as relax or return to their normal length. Muscles are rich in blood vessels that bring them food and oxygen and take away their wastes. The harder the muscles work, the more blood is transported to them.

TYPES OF MUSCLES

There are three types of muscle found in the muscular system: skeletal muscle, smooth muscle, and cardiac muscle. Although all three contract in basically the same manner, they look very different when examined under a microscope.

Skeletal Muscle. Skeletal muscle is attached to bones and can be said to make up the flesh of an animal. It is also called striated muscle since it is made up of long, striped muscle fibers. Skeletal muscle is a voluntary muscle because it can be consciously controlled. A person can, therefore decide when and how to move his or her arms or legs to walk or run, or to move facial muscles to smile or frown. Most skeletal muscles are attached to bones and are connected at both ends by a tough, connecting tissue called a tendon. Tendons can be felt in the forearms near the wrist, in back of the leg near the knee, and in the back of the ankle. Skeletal muscles move bone by acting in pairs. One muscle, called the flexor, contracts and pulls the bones connected by a joint together. Another muscle, called the extensor, pulls, or moves the bones apart. Muscles, such as the flexor and extensor that work together but in opposing ways, are called an antagonistic pair.

Smooth Muscles. Smooth muscle is a second type of muscle, which actually appears smooth under a microscope. It makes up the walls of many

organs inside the body, including the digestive tract, reproductive organs, bladder, arteries, and veins. Since it does not respond to a person's will or command—that is, it cannot deliberately be controlled—it is called involuntary muscle. Blood is pushed through the veins, and the lungs expand whether a person wants them to or not. Smooth muscles contract like any muscle, and although they do so more slowly than skeletal muscle, they can maintain these contractions for a longer period of time.

Cardiac Muscle. A third type of muscle is cardiac muscle. As its name implies, it is found only in the heart and it is responsible for the strong, regular contractions known as the heartbeat. Like smooth muscle, cardiac muscle is involuntary, but unlike any other type of muscle, cardiac muscle contracts independently of any nerve supply. This means that cardiac muscle contracts and relaxes automatically according to its own, built-in rhythm. Cardiac muscle is amazingly strong and resilient, given that it has to beat continuously over an individual's lifetime. Unlike skeletal muscle, which requires regular periods of rest, cardiac muscle neither requires—nor is allowed—to take a rest.

WHY MUSCLES CONTRACT

The actual contraction or shortening of any muscle occurs because a muscle contains two proteins, actin and myosin, that form long threads called filaments. These filaments are arranged in parallel stacks, like over-lapping decks of cards. When a muscle gets a nerve signal to contract or shorten, the actin filaments physically slide over the myosin filaments and overlap them. The more they overlap, the more the muscle shortens. When the filaments slide back again, the muscle relaxes.

The human body has more than 600 muscles, and in the adult male, muscles make up about 40 percent of the total body mass. Although skeletal muscle fiber cannot replace itself by cell division after an organism is born, it can increase in size through exercise, which is important for healthy, strong muscles. Regular physical activity creates an increase in the number of blood vessels, which means the muscle receives more nutrients and oxygen. Muscles that are not used for a long period of time undergo a wasting process called atrophy.

[See alsoHeart ]