elasticity
The muscles all begin the actual process of contracting shortening when a protein called actin is pulled by a protein called myosin. This occurs in striated muscle skeletal and cardiac after specific binding sites on the actin have been exposed in response to the interaction between calcium ions Ca++ and proteins troponin and tropomyosin that shield the actinbinding sites. Ca++ also is required for the contraction of smooth muscle, although its role is different: here Ca++ activates enzymes, which in turn activate myosin heads. All muscles require adenosine triphosphate ATP to continue the process of contracting, and they all relax when the Ca++ is removed and the actin-binding sites are re-shielded. A muscle can return to its original length when relaxed due to a quality of muscle tissue called elasticity. It can recoil back to its original length due to elastic fibers. Muscle tissue also has the quality of extensibility; it can stretch or extend. Contractility allows muscle tissue to pull on its attachment points and shorten with force. Differences among the three muscle types include the microscopic organization of their contractile proteinsactin and myosin. The actin and myosin proteins are arranged very regularly in the cytoplasm of individual muscle cells referred to as fibers in both skeletal muscle and cardiac muscle, which creates a pattern, or stripes, called striations. The striations are visible with a light microscope under high magnification see Figure 10.2 . Skeletal muscle fibers are multinucleated structures that compose the skeletal muscle. Cardiac muscle fibers each have one to two nuclei and are physically and electrically connected to each other so that the entire heart contracts as one unit called a syncytium . Because the actin and myosin are not arranged in such regular fashion in smooth muscle, the cytoplasm of a smooth muscle fiber which has only a single nucleus has a uniform, nonstriated appearance resulting in the name smooth muscle . However, the less organized appearance of smooth muscle should not be interpreted as less efficient. Smooth muscle in the walls of arteries is a critical component that regulates blood pressure necessary to push blood through the circulatory system; and smooth muscle in the skin, visceral organs, and internal passageways is essential for moving all materials through the body.