The muscles of the body are composed of three different types of muscle tissue. In certain characteristics the three types are similar: they are affected by the same type of stimulus, they produce action potentials shortly after being stimulated, they possess the faculty to contract, the force of contraction depends on their initial length, they have the property of maintaining tone, they atrophy due to inadequate circulation, and they hypertrophy in response to increased work. On the other hand, in some other ways they show great differences:
– Smooth muscle is involuntary and forms the walls of the hollow viscera, such as the stomach and bladder, and that of the various tubular systems, such as the circulatory, digestive tract, respiratory apparatus, and reproductive organs. They contract more slowly, are more extensible and capable of more sustained and rhythmic contraction, more sensitive to thermal and chemical stimuli. The contraction of the smooth muscle of a hollow organ causes the emptying of this organ; in the case of the digestive tract, the contents are propelled outward by waves of contraction (peristaltic movements).
– The cardiac muscle is not under the control of the will, it is automatic. It has four basic properties; 1) rhythm, that is, the property of generating periodic impulses; 2) conductivity, that is, the property of transmitting these impulses throughout the myocardium; 3) irritability, that is, the property of responding to stimuli; 4) contractility, that is, the property of contracting in response to a stimulus.
– The striated or skeletal muscle, the one of greatest interest to us, is composed of fibers that present alternating dark and light bands. These fibers are innervated by cranial or spinal nerves and are under voluntary control. This type of muscle contains pain receptors, and its main functions are body movement and maintenance of posture.
Muscles account for 40% to 45% of adult body weight. The voluntary muscular system is composed of approximately 434 muscles, but only 75 pairs are involved in general body posture and movement.
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