Muscular System
The muscular system includes those tissues of the body which, by virtue of being composed of contractile tissue, are able to cause some type of movement. The term "contractile" refers to the fact that muscles are made of complex muscle fibers, composed of myofibrils. These myofibrils are composed of filaments which overlap each other in a way that allows them to slide past each other, changing the degree of overlap. This allows the muscle to grow shorter or longer, resulting in movement.
Some muscles are under voluntary control, meaning that an individual can decide to move those muscles. Voluntary muscles are also known as skeletal muscles, because most of these muscles are attached to the bones of the body, and are responsible for moving the skeletal system. Voluntary muscles which are not directly attached to bones include those muscles around the mouth and the anus. While skeletal muscles are under voluntary control, they also may receive some automatic input from areas of the brain. This is certainly true of many of the muscles of respiration (breathing), which are automatically programmed to continue contracting and relaxing, so that an individual does not have to decide to take each breath. There are more than 650 different voluntary muscles in the human body.
A second type of muscle, called smooth muscle, is considered to be under involuntary control. Smooth muscle makes up the muscles of the intestine, the uterus, the blood vessels, and the eye. The intestine contracts and relaxes without an individual even being aware of its actions, allowing food to be churned up and moved along its length. An individual cannot exert control over these muscles; one cannot, for example, decide to speed up the amount of time it takes for a meal to travel down the length of intestine. Similarly, when a woman goes into labor, she cannot prevent her uterus from contracting simply by willing it to stop. Once the correct hormone environment is in place, the uterus will continue contracting entirely without voluntary control. An individual's pupillary muscles contract or relax due to their response to the presence or absence of light, and not due to an individual's desire to have dilated or contracted pupils.
A third type of muscle is cardiac (heart) muscle, which is responsible for the forceful contraction of the heart beat. This type of muscle is primarily involuntary, although individuals have been taught to manipulate heart rate through a process known as biofeedback. Cardiac muscle is amazingly strong and resilient, given its ability to beat continuously over an individual's lifetime. In contrast, skeletal muscle needs periods of rest, a luxury which cardiac muscles neither require nor can afford to take.
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