Our bodies are made up of about 40 percent muscle and contain over 600 muscles in total. Approximately 30 muscles are located in the face and are used to express emotions. Muscles are elastic, rubber band-like structures made of protein. A single muscle may be composed of tens of thousands of muscle fibers. Each of the three muscle types--cardiac, skeletal and smooth--has important functions. The unique attributes of muscles allow them to perform the jobs needed to keep our hearts beating and enable us to walk, talk and breathe.
Cardiac
Cardiac muscles, found in the heart walls, are striated, involuntary muscles. Striated means they have dark and light cross-fibers that give them a textured or striped appearance. Involuntary muscles such as those in the heart are controlled by the central nervous system. The brain sends a message to the heart to contract without our conscious thought. The heart muscles turn over energy-producing molecules called adenosine triphosphate (ATP) very quickly in order to beat approximately 100,000 times per day. They contract to pump oxygen-rich blood out of the heart and relax to let carbon dioxide-rich blood back in for recycling.
Skeletal
Skeletal muscles make up the musculoskeletal system and are so called because they connect to bones. This striated muscle tissue can be contracted or relaxed voluntarily. Skeletal muscles can be further divided according to their predominant type of fiber--fast- or slow-twitch muscle fibers. They vary in their onset of fatigue and means of producing ATP. Type I fibers are slow-twitch, oxidative (oxygen-using) fibers with many mitochondria (energy-producing cells) and capillaries, slow contraction rates and fatigue-resistant properties. Type IIA fibers are fast-twitch fibers with many mitochondria and capillaries and are resistant to fatigue. Type IIB fibers have few mitochondria and capillaries and high amounts of glycogen, or stored carbohydrates. Type IIB fibers, such as those in the arm muscles, exhibit fast muscle contraction and fatigue easily.
Smooth
Smooth muscle tissue is found inside blood vessels and in the stomach, intestines and bladder. These muscles are usually involuntary, though there are exceptions. For example, smooth muscle tissue allows us to hold in urine. The same muscles contract to allow urination. Smooth muscles appear to be smooth because they are not striated and occur in sheets or layers. The digestive system utilizes smooth involuntary muscles to move food from the stomach into and through the intestines. Smooth muscle tissue is used in such actions as reading, vomiting and giving birth.
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