The typical human body has over 600 muscles, according to "Atlas of Human Anatomy" by Frank Netter. Three types of muscle exist in the human body. Skeletal muscles are primarily responsible for moving the body and generating heat. Cardiac, or heart, muscle pumps blood throughout the body. Smooth muscle allow vital organs, such as the intestines, liver and kidneys to work without thinking about them. Overall, the main functions of the muscular system include movement, muscle tone and heat generation.
Movement
Skeletal muscles are responsible for the movement of most body parts, such as the arms, legs and neck. Contractions of skeletal muscles cause movement, as described by Dr. Elaine Marieb and Dr. Katja Hoehm in "Human Anatomy and Physiology". For a contraction to start, two proteins contributing to muscle contraction, called actin and myosin, bind to each other. An energy source for the body, called ATP (adenosine-5'-triphosphate), occurs when actin and myosin bind. After ATP is made, myosin separates from actin, slides down the actin protein, and binds to another site further down on the actin protein. This sliding and reattachment causes the muscle to become taught.
Blood moves throughout the body by the cardiac muscle. Heart contractions or heartbeats, set the pace of blood flow throughout the body. The electrical impulse that initiates a heart contraction begins at the sinoatrial (SA) node or intrinsic pacemaker. From the SA node, the electrical impulse spreads down the heart through special networks of cells and causes the heart to contract.
Smooth muscle is also involved in movement, such as pushing food through the digestive tract. Although the contraction mechanism of sliding myosin and actin proves similar in smooth muscle as in skeletal muscle, contractions occur more slowly in smooth muscle.
Muscle Tone
Skeletal muscles never fully relax, as they always remain slightly tense. This tension makes the muscles ready for action. Muscle tone includes the continual and partial contraction state of muscles, and it contributes to posture. For example, muscle tone helps keep the torso and head erect when sitting, standing or walking. The body automatically sends nerve impulses to maintain muscle tone.
Heat Generation
Skeletal muscles release heat when they work. Muscle cells use much ATP during muscle contractions, and most of this ATP turns into heat, as described in detail by Dr. Carlson and Dr. Wilkie in "Muscle Physiology." Shivering causes fast muscle contractions that produce heat. To prevent overheating, the body gives off heat through the skin and glands in the skin produce sweat.
References
- "Atlas of Human Anatomy"; Frank Netter; 2006.
- "Human Anatomy and Physiology"; Elaine Marieb and Katja Hoehm; 2007.
- "Muscle Physiology"; Francis Carlson and Douglas Wilkie; 1974.
