Muscle Tissue

Barbara M. Vertel, Ph.D.

Histology/Physiology 2005


Muscle Tissue


Reading: Junqueira (2003) Basic Histology, 10th ed. Ch. 10

Gartner & Hiatt (2001). Color Textbook of Histology, 2nd Ed. Ch. 8.

Also refer to Alberts et al. (2002) pp. 961-965; Lodish et al. (2004) pp. 226-7, 811

I. General Introduction


Muscle tissue, one of the four tissue types, is specialized for contraction and body movement. Muscle cells have either a striated or a non-striated appearance, depending upon the arrangement of contractile elements within the cells. Control of contraction is either voluntary or involuntary, based on the type of muscle. Muscle is classified into three main types, skeletal (striated, voluntary), cardiac (striated, involuntary), and smooth (non-striated, involuntary), characterized by structure and function (Figs. 1-6).


II. Skeletal Muscle


Skeletal muscle cells, also called muscle cell fibers, are specialized for rapid, forceful, vountary contractions. They form the muscles of the musculoskeletal system. In addition, they form some muscles that do not move bones, such as the diaphragm, extraocular muscles, and muscles of facial expression. Skeletal muscle tissue is also found in parts of the digestive and respiratory systems.

Skeletal muscle cells have prominent cross-striations in the cytoplasm that are visible by both light and electron microscopy. The cells are large and do not branch. Typical dimensions are 10-100 µm in diameter and 1-40 mm in length. Each cell is multinucleated, with peripherally located nuclei. Thus, the skeletal muscle cell is a syncytium (Fig. 7, 8) many nuclei in a common cytoplasm) that arises during embryonic development by the fusion of mononucleated myoblasts (Fig. 11).




III. Cardiac Muscle


Cardiac muscle cells are found only in the myocardium of the heart and in the walls of the large veins at their junction with the heart. Control of cardiac muscle contraction is involuntary. Like skeletal muscle cells, cardiac myofibers are striated. Thus, striated muscle refers to both skeletal and cardiac muscle. Cardiac myofibers are considerably smaller than skeletal muscle cells and exhibit branching. Typical dimensions are 10-20 µm in diameter and 100-150 µm in length. Only one to two centrally located nuclei are present in each cell. Cardiac myofibers exhibit spontaneous, rhythmic contraction and are joined to each other both structurally and functionally by intercalated disks (Figs. C1-5). Because the gap junctions of the intercalated disks allow for electrotonic coupling between adjacent cells, cardiac muscle functions as a syncytium, but in actuality is not a true syncytium.


IV. Smooth Muscle


Smooth muscle cells are non-striated and thus are very different in appearance from both skeletal and cardiac myofibers (Fig. C24-26). Smooth muscle is an involuntary muscle. The tapered cells have a central bulging nucleus and relatively homogenous cytoplasm. Smooth muscle is found in the walls of many hollow organs, e.g., viscera and blood vessels. The cells are small, with a diameter of 3-8 µm and a length of 15-200 µm.