Muscular System | Research & Encyclopedia Articles

Independent movement is a unique characteristic of animals. Most animal movement depends on the use of muscles. Together, muscles and bones make up what is known as the musculoskeletal system. This combination provides protection for the body's internal organs and allows for many kinds of movement. Whether the movement is as simple as opening the eyes or as complex as flying, each is the result of a series of electrical, chemical, and physical interactions involving the brain, the central nervous system, and the muscles themselves.

Muscle is the flesh, minus the fat, that covers the skeleton of vertebrate animals. Muscles vary in size and shape and serve many different purposes. Large leg muscles such as hamstrings and quadriceps control limb motion. Other muscles, like the heart and the muscles of the inner ear, perform specialized involuntary functions. Despite the variety in size and function, however, all muscles share similar characteristics.

At the highest level, the entire muscle is composed of many strands of tissue called fascicles. These are the strands of muscle that can be seen in red meat or chicken. These strands are made up of very small fibers. These fibers are composed of tens of thousands of threadlike myofibrils, which can contract, relax, and lengthen.

The myofibrils are composed of up to ten million bands laid end-toend called sarcomeres. Each sarcomere is made of overlapping thick and thin filaments called myofilaments. The thick and thin myofilaments are made up of contractile proteins, primarily actin and myosin.

Muscles are categorized as either voluntary or involuntary. The muscles that animals can deliberately control are known as voluntary muscles. Those that cannot be controlled by the animal, such as the heart, are called involuntarymuscles. Vertebrates also possess several different types of muscle tissue: cardiac, smooth, and striated or skeletal.

The muscle types are classified on the basis of their appearance when viewed through a light microscope. Striated muscle appears striped (striated) with alternating light and dark bands. Smooth muscle lacks the alternating light and dark bands.

Cardiac muscle makes up the wall of the heart, which is called the myocardium. In humans the heart contracts approximately seventy times per minute and can pump nearly 5 liters (4.5 quarts) of blood each minute. The fibers of the heart muscle are branched and arranged in a netlike pattern. The involuntary heart contraction is stimulated by an electrical impulse within the heart itself at the sinoatrial node.

Smooth muscle cells are organized into sheets of muscle lining the walls of the stomach, intestines, blood vessels, and diaphragm, and parts of the urinary and reproductive systems. The smooth muscle contractions push food through the digestive system, regulate blood pressure by adjusting the diameter of blood vessels, regulate the flow of air in the lungs and expel urine from the urinary bladder. These body functions are involuntary and controlled by the autonomic nervous system.

Skeletal muscle, which is muscle tissue attached to bones, makes up a large portion of an animal's body weight— sometimes between 40 and 60 percent. Skeletal muscles move parts of the skeleton in relation to each other. They contain abundant blood vessels that transport oxygen and nutrients, nerve endings that carry electrical impulses from the central nervous system, and nerve sensors that relay messages back to the brain. Skeletal muscles are responsible for the conscious or voluntary movements of the trunk, arms and legs, respiratory organs, eyes, and mouth-parts of the animal. They are used for such actions as running, swimming, jumping, and lifting.

These distinctive muscle types can be observed throughout the evolution of vertebrates, however the arrangement of muscles varies according to differing environmental and survival needs. In fish, for example, most of the skeletal muscles fan out from either side of the backbone. Muscle makes up nearly 60 percent of the fish's body and nearly all of it is involved in moving the tail and spine.

As vertebrates evolved and adapted to life on land, the down-the-spine muscle arrangement began to change. More muscle power was needed for moving the limbs. Limb muscles became both bigger and longer. Some muscle fibers in a frog's hind legs can be nearly a quarter as long as the frog's body, which is proportionately much longer than the muscles in many fish. More muscles developed in the chest to be used for breathing, as vertebrates began spending more time on land. In mammals, this led to the development of the diaphragm, an involuntary muscle that helps to bring air into the lungs.

Nerves connect the spinal column to the muscle. The place where the nerve and muscle meet is called the neuromuscular junction. Inside the muscle fibers, a signal from the nervous system stimulates the flow of calcium, whichcauses the thick and thin fibers (myofibrils) to slide across one another. When this occurs, the sarcomere shortens, which generates a force. The contraction of an entire muscle fiber results when billions of sarcomeres in the muscle shorten all at once.

The "sliding-filament theory" suggests that these thin and thick filaments become linked together by molecular cross bridges, which act as levers to pull the filaments past each other during the contraction of the muscle fiber. Myosin molecules have little pegs, called cross bridges, that protrude from the thick filament. During contraction, another molecule, called actin, appears to "climb" across these bridges.

Movement occurs in all animals, including those without highly developed musculoskeletal systems. Nearly all groups of animals, including relatively simple organisms such as jellyfish and flatworms, have rudimentary muscle fibers that are specialized to move parts of the body. The number of muscles is not necessarily related to the size of the organism or the presence of a skeletal system. For example, a caterpillar may have 2,000 separate muscles compared with some 600 muscles in the human body.

Movement in invertebrates is caused by the same contractile proteins, actin and myosin, that function in the muscles of vertebrates. This primitive muscle tissue is triggered into action by nerves, hormones, or the builtin rhythm of the organism.

Simple protozoans such as the Ameoba, can either contract or extend their one-celled body in any direction. Other protozoans move by means of contractile fibers contained in cilia and flagella. Cilia are minute, hairlike, projections that stick out from the cells of some animals. Cilia allow protozoa to move freely through their aquatic environment. Another adaptation is the flagellum (pl., flagella), a whiplike structure found in sponges. A flagellum moves by a beating pattern that mimics a snakelike undulation.

Both smooth and striated muscle are present in invertebrate animals ranging from cnidarians to arthropods. Flatworms have muscle fibers in three directions, the contraction of which will move the body in multiple planes much like a human tongue. The body wall of earthworms contains both an outer and an inner layer. Contraction of the outer layer causes the body to lengthen and the action of the inner layer shortens it, producing the wiggling motion of the worm.

The only invertebrates without this layered arrangement of muscle tissue are the mollusks, crustaceans, and insects. They do, however, have many separate muscles, varied in size, arrangement, and attachments, that move the body segments and the parts of the jointed legs and other appendages. These muscles are fastened to the internal surfaces of the exoskeleton. Clams and other bivalve mollusks use strong muscle contractions to keep their shells tightly shut at high tide. Once the shell-closing muscles have contracted, they can remain tightly shut for hours without tiring.

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"Muscle." Encyclopedia Britannica Online. 1994-2001. <http://members.eb.com/>.

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