The human body requires muscle contractions to complete many of the functions of everyday life. There are three types of muscle in the body: skeletal, cardiac, and smooth muscle. Skeletal muscle is found in the large muscles of the body and is also called voluntary muscle. Cardiac muscle is the muscle that makes up the human heart and is not under voluntary control. Smooth muscle is found in the blood vessels and in organs like the stomach. These various types of muscles all follow similar principles of contraction.
Anatomy of a Muscle
According to the book "Textbook of Medical Physiology" by Arthur Guyton and John Hall, about forty percent of the human body is skeletal muscle. Muscles contain individual muscle fibers that are grouped together and surrounded by connective tissue. Groups of muscle fibers are bound to one another and become dense groups called fasciculi, which are further bound with other fasciculi to become whole muscles, according to "Anatomy and Human Movement." The individual muscle fibers feature nerve connections to motor neurons that run within the muscles.
Muscle Fiber in Detail
Individual muscle fibers have a cell membrane called a sarcolemma, and have thousands of tiny filaments called myofibrils. These myofibrils are made up of smaller filaments, called actin and myosin filaments, according to Guyton and Hall. The relationship of the actin and myosin form a cross bridge, and when they are stimulated, the filaments attach to each other and cause a sliding mechanism. According to "Textbook of Medical Physiology," this sliding mechanism is how a muscle contraction occurs.
Neurology of Muscle Stimulation
The neurology of how the contraction occurs begins with the impulse. The impulse, which is called an action potential, travels down the nerve and when it reaches the nerve ending, it causes the release of a chemical called acetylcholine. This neurotransmitter causes sodium ions to be released and they travel to the adjacent nerve receptor on the muscle fiber. This causes an action potential inside the muscle to begin, at which point the muscles release calcium ions, which cause the actin and myosin to slide alongside each other, activating the sliding mechanism. Within a fraction of a second, the calcium ions are absorbed and the sliding mechanism ceases. The muscles will stay this way until a separate action potential is generated, according to "Textbook of Medical Physiology."
Muscle Action
According to "Anatomy and Human Movement," when the muscle is stimulated, the muscle contracts, which brings the ends closer together. This causes tension in the muscle and, when combined with many muscle groups, can cause movement.
Considerations
A muscle contraction requires a very complex process of chemical and neurological steps in order to cause the desired movement. For all movements, an action potential is required; it must be present to initiate the process. If a muscle loses its nerve supply, it is not able to receive the signals needed and will begin to atrophy immediately.
References
- "Anatomy of Human Movement: Structure and Function Fifth Ed."; N. Palastanga, D. Field, R. Soames; 2006
- "Textbook of Medical Physiology Tenth Edition"; A.C. Guyton, J.E. Hall; 2000
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