Muscle tissue is a composite tissue which is composed of (1) muscle cells, specialized for contraction, (2) connective tissue, (3) nerve fibers and (4) blood vessels. Since not all of the contractile needs of the body are the same, three different types of muscle are used by the body. The types differ in both their structural organization and function. Structurally, muscles are either striated (striped) or smooth (no stripes) in appearance. Functionally, muscles are either voluntary or involuntary in their control. The types of muscle include:
All three muscle types are composed of elongated cells, often referred to as muscle fibers, and contain intracellular contractile elements, called myofibrils. The myofibrils are basically composed of two filamentous, contractile proteins, actin and myosin. The degree of intracellular organization of these contractile filaments is greatest in skeletal muscle and simplest in smooth muscle. All three types of muscles are wrapped by fibroconnective tissue, which maintains the structural integrity and carries the associated blood vessels and nerve fibers.
In cross sectional sections of skeletal muscle, the supporting connective tissue elements are most clearly seen. The arrangement and terminology is as follows:
The cells of skeletal muscle are among the largest in the body. This tissue is a syncytium, a tissue produced by the fusion of a number of cells, producing multinucleated structures called myotubes. Nuclei are always found at the periphery of the cells (Skeletal Muscle 3). Within the myotubes, bundles of contractile proteins are organized into myofibrils giving the tissue a striated appearance. The striation are produced by the alignment of the contractile fibers of myofibrils, producing an alternating pattern of light (I=isotropic) and dark (A=anisotropic) bands, defining the contractile units, the sarcomere (Skeletal Muscle 4).
Unlike skeletal muscle, cardiac muscle is composed of single branching cells, connected together by specialized junctions called intercalating discs. The discs are visible as darkly staining lines, more prominent than the myofibrillar striations (Cardiac Muscle 1). The nuclei are centrally located and surrounded by a perinuclear zone containing synthetic organelles (Cardiac Muscle 2). The cardiac muscle of the heart is surrounded on the outside by a thick layer of fibroelastic connective tissue, the epicardium. Under this layer the cells of the ventricular impulse conducting system, the purkinje fibers, are found (Cardiac Muscle 3). Purkinje fibers containing central nuclei, decreased amounts and peripherally located myofibrils, and a glycogen rich (empty appearing) cytoplasm (Cardiac Muscle 4).
Smooth muscle is found in the walls of hollow tubes of the body. In most cases it is present in two layers: surrounding the tubular structure (circular) and running parallel (longitudinal) with the hollow tube. Therefore, two different orientations will be observed will be observed in most sections, a cross-sectional and longitudinal cut (Smooth Muscle 1). This tissue is composed of single cells which are elongated and have a centrally placed nucleus. The cytoplasm stains eosinophilic and appears homogeneous (Smooth Muscle 2). An occasional dense body may be visible in some cells.
See if you can identify the following types of muscle tissue: