Structure of a skeletal muscle A skeletal muscle consists of a fleshy part, the body or belly, from each end of which protrudes a tendon or aponeurosis which is continuous with the belly of the muscle and which serves as a means of attachment for the muscle to the bony levers. The fleshy part is contractile while the tendon and fan-shaped aponeurosis are nonelastic and excellently adapted to the rôle which they play in muscular mechanics. These muscles are very complex structures. They are composed of a large number of structural units known as muscle fibers. The muscle as a whole is surrounded by a strong connective tissue sheath or fascia (epimysium) from which partitions or septa (perimysium) of this same tissue extend into the body of the muscle, dividing it into larger or smaller bundles, the fasciculi. The fasciculi are composed of a large number of muscle fibers. These are separated, one from the other, by delicate partitions of connective tissue (endomysium) which are continuous with the septa. The epimysium blends with and partly forms the tendon. The muscle fibers contain the contractile units. Since the mechanism of contraction in a skeletal muscle is most probably identical in all of its fibers, an examination of the structural and functional properties of these should give an insight into the mechanism of muscular contraction. For general purposes, we may assume that the action of the muscle as a whole is the composite action of its individual fibers. Structure of a striated muscle fiber The muscle fibers are the ultimate structural units of the muscular system. For the most part they run parallel to each other, but do not usually extend throughout the full length of the muscle. They usually vary in length from thirty to forty millimeters, although in the sartorius muscle they may be two or even three times this length. In diameter, they probably do not exceed 0.02 to 0.03 millimeter. Their ends are somewhat pointed and evidence has been presented recently which seems to show that each muscle fiber is connected either directly or by means of a fine tendinous strand to each end tendon or attachment. In some muscles their course lies parallel, or nearly so, with the longitudinal axis of the muscle. This type of arrangement is termed fusiform. In those muscles where power is of prime importance, a different arrangement of the fibers is found. For this purpose various types of pinniform muscles have been adopted in developmental growth. In unipinniform muscles (peronei) the fibers are arranged parallel to each other, but not to the long axis of the muscle, running diagonally across the belly to their attachment in the tendon. This arrangement affords the necessary increase in the number of fibers by increasing the length of the belly. In this type there is a slight loss of efficiency, but this is partly offset by the economy of the tendon. In bipinniform muscles (rectus femoris), which are feather-like with the tendon running through the center, and in multipinniform muscles (deltoid) even greater power is obtainable. When a muscle fiber is examined under the microscope, it is found to be cylindrical and surrounded by a thin transparent and elastic membrane, the sarcolemma, within which is contained the true muscle substance. Beneath the sarcolemma are nuclei. These are surrounded by a mass of granular material. The muscle fiber, then, may be regarded as a large multinucleated cell . Within the fiber are the true contractile units or elements, the muscle fibrils or sarcostyles. These may be separated longitudinally in a muscle fiber which has been previously fixed in alcohol or formalin. Between the sarcostyles is a more fluid substance, the sarcoplasm. When the muscle fiber is examined under the microscope, especially after suitable histological technique consisting of fixation and selective staining, it presents a series of transverse bands, alternately dim and bright, which give to it the characteristic striated appearance. When examined more carefully under higher magnification, lines extending across the fiber may be seen to divide the bright bands into approximately equal parts. This line was described by Krause and is known as Krause's membrane. It was originally thought by him to be an optical expression of a membrane which was attached to the sarcolemma at its circumference, hence, its name. It is composed of a series of granules laid down in the form of a continuous line. Likewise each dim band is divided by another line known as Hensen's line. The sarcostyles give to the fibers their longitudinal striation. They show the same cross-striation as the fibers and are so placed side by side that the dim and bright bands are arranged in exact apposition so as to give the characteristic transverse striations of the muscle fibers. The sarcostyles thus consist of a series of sarcomeres placed end to end; the muscle fiber consists of a number of sarcostyles placed side by side; the unity of the whole being established by virtue of the connective-tissue framework, cement substance, and the attachments to the tendons. When sarcostyles are separated from the sarcoplasm they are still able to contract. The sarcomere is the ultimate physiological unit of muscular contraction. If we were in a position to state precisely the events which take place in a single sarcomere we could then predict, with sufficient certainty, those which take place in all others and in turn those of the muscle fiber or muscle as a whole. Striated muscle fibers are of two varieties, red and white. The red variety contains relatively more sarcoplasm, the white relatively more sarcostyles. The muscles of all higher animals, including man, are usually mixed but muscles consisting largely, if not entirely, of one or the other of these varieties of fibers may occur. In some, the pale fibers predominate. These are, therefore, termed pale or white muscles. In others, the red fibers predominate and hence they are known as red muscles. The red muscle contracts more slowly than white. Since that time others have shown that all phases of the contractile process are slower and more sluggish in red than in white muscle. The appearance may vary somewhat under certain physiological conditions. Thus, the muscles of an athlete in good physical condition are distinctly redder than those of a "soft" or senile individual. The hypertrophy of muscle in response to training is not due to an increase in the number of fibers, but to a filling out of those already present with sarcoplasm.		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