Leanne Booth, age 13, of Jackson, Miss., for her question:
HOW MANY MUSCLES ARE IN THE BODY?
More than 600 muscles can be found in the human body. Usually responding to a signal from the central nervous system, these muscles are able to contract. Muscles force the blood to move through the body, push food through the intestines, allow us to sit and walk and also play an important part in the heart's beat.
There are two main types of muscle: smooth and skeletal. The smooth muscles can be found in the internal organs, blood vessels and digestive system. The skeletal muscles are used to move bones.
Having the characteristics of both the smooth and skeletal muscles is a third kind of muscle: the cardiac or heart muscle.
Cells called muscle fibers can be found in all muscles. Skeletal muscles are made up of long fibers that are surrounded by a sheath called the sarcalemma. Skeletal muscles are supplies with nerves from the central nervous system. They are called voluntary muscles because they are partially under conscious control.
Most skeletal muscles are attached to the skeleton by a tough connective tissue called tendons. When skeletal muscles contract, various bones of the body can move.
Smooth muscles contract more rhythmically and slowly than skeletal muscles. Sheets of smooth muscles arranged in circular and lengthwise patterns can be found in the walls of the stomach and intestines.
Smooth muscles in blood vessels relax and make the vessel openings wide. They then contract to make the vessels narrow.
The body's smooth muscles do not always need direct stimulation by a nerve to work. Hormones are used to make smooth muscles contract automatically.
Since smooth muscles cannot be controlled voluntarily, they are sometimes called involuntary muscles.
Muscle fibers must have energy to work, just as all living cells need energy. Muscle cells get this energy from food. Enzymes in the cell break down the food to release the energy.
Fats and carbohydrates, found in sugars and starches, are the most important food sources for the energy needed by muscles.
A bit of the energy is lost as heat but most of it is captured to make a special high energy substance called adenosine triphosphate, or ATP. This material actually stores the energy and releases it when muscles need energy to work.
Like all living cells, muscle fibers produce wastes. Sometimes wastes collect in the muscle if it works very hard. When this happens, the fibers may lose some of their power to contract. The muscle then becomes tired, or fatigues. It then becomes necessary for the muscle to rest so the body can take away the wastes.