Muscle
There are three types of muscle tissue found in the human body: skeletal, cardiac, and smooth. They differ structurally and functionally.
Skeletal, or striated, muscles are anchored to bone and are required for control of joint movements and maintenance of joint position. Skeletal muscles constitute 40-45% of the body's weight. A skeletal muscle is divided into cylindrical muscle cell fibers. These fibers are in turn divided into smaller contractile units called sarcomeres, which consist of long parallel chains. The number of sarcomeres contracting correlates to the strength and speed of the contraction.
When the muscle is stimulated by a motor neuron to contract, calcium is released from storage areas in the muscle. The calcium binds to the myosin and exposes the actin binding site. The myosin binds to the actin and rotates, the rotation drawing the filaments together. When a number of filaments bind and move at the same time, contraction of the muscle occurs. Magnesium bound to a usable energy source, adenosine triphosphate (ATP), causes the muscles to relax by breaking the actin myosin bond. Without magnesium-ATP, the muscle will become rigid and inextendable. This condition is commonly seen shortly after death and is called rigor mortis.
There are three types of skeletal muscle fibers which differ in their ability to receive oxygen, in the way they create energy, and in the levels of efficiency with which they use energy:
Slow twitch-resistant fatigue fibers are well supplied with oxygen and nutrients, have slow speeds of contraction, and do not go into fatigue. They are extremely efficient users of energy. These muscles are commonly used for maintaining posture and slow repetitive movements.
Fast twitch-resistant to fatigue fibers have rapid speeds of contraction, and will undergo fatigue only if the contraction is prolonged. They are specialized for rapid repetitive movements, as in walking. Fast twitch-fatigable fibers contract very rapidly, are powerful, but fatigue quickly. Muscle fatigue is caused by a lack of ATP which is what these muscles use as an energy source; if there is no ATP available, glucose is used instead. This conversion, though requiring no oxygen, produces lactic acid. The buildup of lactic acid can lead to muscle damage. The acid is removed from the muscle by the bloodstream and converted back to glucose in the liver.
There are three major types of muscle fiber arrangement: longitudinal, pennate, and radiate. The arrangements are involved in the strength of the muscle. The more fibers in an area, the greater the potential strength of the muscle.
A shortening contraction occurs when the muscle tension is greater than the external force. The muscle will shorten in length and the bones will be pulled closer together; the movement is in the opposite direction of the external force.
A lengthening contraction occurs when the muscle tension is less than the external force. The muscle lengthens and the bones move apart; the movement is in the direction of the external force, and the movement is slower than the force would normally produce.
Isometric contractions occur when muscle tension is equal to the external force; no change in muscle length or bone placement occurs. Isometric contractions hold the position of the joint in opposition to an external force.
Cardiac muscles have striations, but they are much fainter than skeletal muscles. These muscles have the unusual ability to contract rhythmically without nervous stimulation. They are linked in such a way an impulse can spread from one fiber to another, ensuring all cells will be excited in unison.
Smooth muscles line internal organs and vessels. They are not striated like skeletal muscles, consisting of small spindle-shaped cells. A smooth muscle contracts and relaxes more slowly than a striated muscle, and is capable of more sustained contractions. Usually the muscles are arranged in bundles encircling the hollow organ, thus allowing the organ to squeeze the contents forward, as in peristalsis.
References
Eckert, R. & D. Randall. 1983. Animal Physiology, second edition W. H. Freeman and Company, San Francisco. 830 pp.
Gray, H. 1977. Gray's Anatomy. Crown Publishers, Inc, New York. 1257 pp.
Rahlman, J. & J.L. Smith. 1981. Ucla Kinesiology 14 Human Neuromuscular Anatomy. Academic Publishing Service. L A. 490 pp.
Van Amerongen, C. The Way Things Work; Book Of The Body. New York: Simon and Schuster, 1979.
Vander, A. J., J. H. Sherman, et. al. 1980. Human Physiology The Mechanisms of The Body Functions. Mcgraw-Hill. New York. 724 pp.
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