Key Concepts
Vocabulary: smooth muscles, skeletal muscles, flexor extensor, cardiac muscle, tendinitis, hernia
The Muscular System
What Muscles Do
Main Idea: The muscular system allows for voluntary and involuntary movements.
Like rubber bands, muscles are elastic; they stretch to allow a wide range of motion. This elasticity allows muscles to move the bones or organs to which they are attached. In this way, your muscular system allows you to move.
You might think that muscles work only when you do things such as pick up an object, catch a ball, or walk across a room. Some muscles in your body, however, are always at work. Even when you are sleeping, muscles help you breathe, make your heart beat, and move food through your digestive system. These involuntary processes occur without your knowing it. At other times, such as when you play the piano, make a dash toward first base, or shoot a basketball, you are using muscles that are under conscious, or voluntary, control. You are aware that you are controlling them. Without the use of both voluntary and involuntary muscles, you would not be able to perform these functions.
How Muscles Work
Muscles consist of long, fibrous cells that can shorten and stretch to make muscles move.
A muscle is made up ofhundreds oflong cells called muscle fibers. Major muscles in the body are made up ofhundreds of bundles of these fibers. When these bundles are stimulated by nerve impulses, or signals, they contract, or shorten. When they relax, the bundles extend, or stretch. Some nerves stimulate many muscle fibers, especially large muscles such as your calf muscle or your biceps. In other areas, such as your eyes, a single nerve may provide impulses to only two or three muscle fibers.
Types of Muscles
The body contains three types of muscle tissue: smooth muscle, skeletal muscle, and cardiac muscle.
Smooth muscles are muscles that act on the lining ofthe body’s passageways and hollow internal organs. These muscles can be found in the digestive tract, the urinary bladder, the lining of the blood vessels, and the passage- ways that lead into the lungs. Smooth muscles are involuntary muscles.
Skeletal muscles are muscles attached to bone that cause body movements. Skeletal muscle tissue has a striated, or striped, appearance under a microscope. Most ofyour muscle tissue is skeletal, and almost all skeletal muscles are under voluntary control. Skeletal muscles often work together and perform opposite actions to produce a movement. One muscle contracts while the other muscle relaxes. An example of this can be seen in Figure 14.6,which shows the biceps and triceps muscles of the upper arm. To bend and straighten your arm at the elbow, these muscles have opposite jobs. The flexor is the muscle that closes a joint. In this example, the biceps is the flexor. The extensor is the muscle that opens a joint. In this case, the triceps is the extensor. When the biceps contracts, the triceps extends and the joint closes. When the biceps extends, the triceps contracts and the joint opens. Identify other opposing skeletal muscles that appear in The Skeletal Muscles illustration, Figure 14.8.
Cardiac muscle is a type ofstriated muscle that forms the wall ofthe heart. Cardiac muscle is involuntary and is responsible for the contraction of your heart. The heart contracts rhythmically about 100,000 times each day to pump blood throughout your body.
Caring for Your Muscles
Main Idea: Eating a healthy diet and getting regular exercise will help you care for your muscular system.
Physical activity will keep your muscles strong and healthy. Muscles that remain unused for long periods of time will atrophy, or decrease in size and strength. Muscle tone is the natural tension in the fibers of a muscle. The following tips can help you care for your muscular system and maintain muscle tone:
Get regular exercise.
Eat high protein foods to build muscle.
Practice good posture to strengthen back muscles.
Use proper equipment and wear appropriate clothing to protect muscles during any physical activity.
Warm up properly and stretch before exercising, and cool down after exercising to prevent injury.
Understanding Muscular Problems
Main Idea: Caring for the muscular system can help prevent health problems and injuries.
Your muscles might be sore after strenuous activity, such as an all-day hike or bike ride. Although it can be painful, muscle soreness is usually temporary. However, other problems of the muscular system can be more serious. The recovery time varies according to the type and severity of the injury or disease.
Bruises are areas of discolored skin that appear after an injury, usually a blow to the body. The injury causes the blood ves- sels beneath the skin to rupture and leak, resulting in a bruise. Large bruises can be treated with an ice pack to reduce initial swelling.
Muscle strains or sprains result when muscles are stretched or partially torn from overexertion. Apply ice to strains to reduce swelling, and rest the affected area.
Tendinitis, or the inflammation of a tendon, can be the consequence of injury, overuse, or natural aging. Treatment includes ultrasound or anti-inflammatory medication to reduce pain and swelling.
A hernia occurs when an organ or tissue protrudes through an area ofweak muscle. Hernias commonly occur in the abdomen from straining to lift a heavy object. Surgery is usually recommended, and may be required’ to repair hernias.
Muscular dystrophy is an inherited disorder in which skeletal muscle fibers are progressively destroyed. There is no cure, but with early detection, muscle weakness can be delayed through exercise programs.
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