Types of Contractile Tissues in the Body

The contractile tissues of the body arc composed of cells which are able to exert tension by decreasing their length. In the human body there are three principal types of contractile tissue: skeletal muscle, heart muscle and smooth muscle, each of which has certain distinct structural and functional features. The characteristic result of the shortening of muscle is movement, either of parts of the skeleton (skeletal muscle), or of the contents of hollow organs (heart muscle and smooth muscle). Skeletal muscle is characterized structurally by the presence of distinct cross striations; hence, it is frequently called striated muscle. It contracts and relaxes much more rapidly than do the other types of muscle and normally becomes active only in response to stimulation from the central nervous system. Smooth muscle has no cross striations; it contracts and relaxes very sluggishly and is capable of contracting in the absence of stimulation from the central nervous system, although its activity may be increased or decreased by extraneous nerves. It is usually found in the walls of hollow organs or tubes such as the digestive tract, the blood vessels, the ureters and urinary bladder, and the uterus. When the muscle cells contract, the volume of these hollow organs is decreased and their contents are moved onward. Heart muscle is intermediate between skeletal muscle and smooth muscle, both structurally and functionally; like skeletal muscle its cells are cross striated, and like smooth muscle it is capable of contracting in the absence of extraneous nervous stimulation. Unlike skeletal or smooth muscle, heart muscle cells are not separated from one another, but form a continuous network or syncytium, so that all its fibers contract at each beat of the heart.

Structure of Skeletal Muscle

A skeletal muscle, such as the biceps, is composed of thousands of muscle fibers bound together with connective tissue. Each fiber is an elongated cell varying in length from 1 to 40 mm. The thickness of the fibers varies from 10 to 100 μ or more (1 μ=0.001 mm.); apparently the thickness depends not on the length of the fiber, but on the type of animal and the particular muscle. In a given animal the more primitive muscles, such as those of the eye, have thinner fibers. Fibers of varying diameter may be found in the same muscle, perhaps indicating different amounts of usage since the thickness of fibers is known to increase under the influence of strenuous muscular activity.

Each muscle fiber is covered by a thin structureless membrane, the sarcolemma. The fiber itself consists of two parts: (1) a protoplasmic mass, the sarcoplasm, and (2) very thin cross-striated fibrils, the myofibrils, which are arranged parallel to one another in the sarcoplasm. A great deal of discussion has centered around the existence of myofibrils in living muscle cells (many workers claiming that they arc artifacts resulting from killing and staining procedures) and their possible role in the contraction process. Their existence in living muscle cells is now apparently well established and most authorities attribute to them a fundamental role in the contraction process. If a single fiber from a fresh muscle is examined under the microscope both longitudinal and transverse striations are seen. The transverse striation of the muscle fiber results from the fact that each myofibril consists of alternate light and dark segments and the corresponding segments of adjacent fibrils lie at the same level, forming light and dark bands passing completely across the fiber. During contraction complex changes occur in the position and relative thickness of these light and dark bands. These changes, representing reorientation of molecules in the myofibrils, are believed to be the fundamental basis of contraction.

The cytoplasm of the muscle fiber which fills the spaces between the myofibrils is called sarcoplasm. The relative amount of sarcoplasm in the fiber varies. In some fibers it is more abundant and contains pigment granules (muscle hemoglobin) which give it a reddish appearance (red muscle), while in others it is less abundant and the fiber is paler (white muscle). Red muscles are capable of slow, powerful contractions and are not easily fatigued. The diaphragm and the extensor (postural) muscles are composed predominantly of red fibers. White fibers are specialized for speed rather than strength of contraction and predominate in the flexor muscles. In man both types of fibers enter into the composition of every muscle; the relative proportions varying according to the function of the particular muscle. Recently it has been found that if the tendon of a red muscle is cut and then sewed to the tendon stump of a pale muscle (i.e., forced to take over the function of a pale muscle), its hemoglobin content and resistance to fatigue gradually diminish. This indicates that the appearance and endurance of a muscle are largely the result of the type of work it must perform.