Section 5: Muscular Tissue

Ronald A. Bergman, Ph.D., Adel K. Afifi, M.D., Paul M. Heidger, Jr., Ph.D.
Peer Review Status: Externally Peer Reviewed

Plates

Muscle fibers are elongated cells with distinctive shapes specialized for shortening or contraction. These contractile fibers provide the means of movement for minute body hairs, air in respiration, ingested food and liquid, reproductive cells, blood and lymph, and small and large parts of the body. Muscle permits appropriate responses to external and internal stimuli as well as every form of communication by the individual with the external environment.

Muscular contractions, which may be coarse or extremely refined and are graded between fast and slow, are controlled by the nervous system, which is devoted in large measure to these essential activities. Muscle fibers in vertebrates may be classified structurally as nonstriated, plain, or smooth; striated cardiac; or striated skeletal. A broadened classification, which considers function, follows: smooth, involuntary; striated cardiac, involuntary; and striated skeletal, voluntary. The structural/functional classification indicates whether the contractile activity is under intentional or autonomic control.

Another functional consideration is concerned with the ability of smooth and cardiac muscle to contract spontaneously in the absence of a nerve supply (myogenic contraction). The contractile activity of involuntary muscle is normally regulated by the autonomic (sympathetic and parasympathetic) nervous system. Striated skeletal muscle fibers are totally dependent upon the nervous system, however, for both their structural integrity and function. Each striated skeletal muscle fiber is supplied with a nerve fiber ending on a specialized region of the cell membrane or sarcolemma, the subneural region of the motor end plate. If the nerve supply to a skeletal muscle is interrupted, the component muscle fibers will atrophy rapidly (denervation atrophy). If muscle is worked, it increases in size and strength; if it is not used, it will also atrophy (disuse atrophy).

Smooth muscle fibers are generally small fusiform fibers that vary from about 15 to 200 µm in length and from 3 to 10 µm in diameter. Each muscle fiber possesses a single, elongated nucleus, which characteristically becomes shorter and broader and may coil when the muscle fiber contracts. Smooth muscle fibers may occur singly, as in the scrotum (tunica dartos); in small bundles or fascicles associated with hair follicles (arrector pili muscle); in well-defined, layered sheets that are coiled, as in muscular arteries, or arranged in two thick layers at right angles to each other, as in the intestines; or with an additional layer in an irregular pattern, as in the stomach, bladder, and uterus. Branched smooth muscle fibers can be found in the nipple of the mammary glands and in the enclocardium of the atrium of the heart.

Smooth muscle fibers usually contract slowly but are capable of sustained contractile activity. Most of the smooth muscle fibers of the gastrointestinal and genitourinary tracts are linked to each other by specialized surface membrane (sarcolemmal) contacts (gap junctions or nexus), which transmit electrical excitatory stimuli from cell to cell. The gap junction can be seen in the introductory plates of this book. This structural/functional arrangement permits large numbers of smooth muscle fibers to be activated sequentially by a minimal nerve supply. The excitatory nerve impulse is transmitted to a smooth muscle fiber, which conducts it over its surface, across the gap junction to another fiber, which passes it on, resulting thereby in a sustained and coordinated contraction (peristalsis) over long distances.

Cardiac muscle fibers are generally larger than smooth muscle fibers and appear cross-striated when stained or examined with polarized light. Cardiac muscle fibers are joined serially end to end and characteristically branch to unite with adjacent fibers. Cardiac muscle fibers form a functional but not a protoplasmic syncytium. The junctional site between fibers is called the intercalated disc. The intercalated disc is composed of two important components: the adhesion plate (desmosome) between adjacent cells and the gap junction (or nexus), which allows the electrical excitatory impulse to be transmitted from cell to cell in the same way as in smooth muscle, resulting in a synchronized coordinated contraction relaxation cycle essential to normal heart function. The branched cardiac fibers possess one or two nuclei, which are centrally located. The contractile substance of the cardiac fiber is organized into subunits called myofibrils, which are cross-striated. The cross striations will be discussed in relation to striated skeletal muscle. The myofibril characteristic of cardiac and skeletal muscle is not seen in smooth muscle, although the myofilaments of which the myofibril is composed are found in all three muscle fiber types. Myofilaments cannot be resolved by the light microscope, although Brücke (1858) postulated their existence based on polarization microscopy data analysis and Kölliker (1888) suggested that the hypothetical myofilaments were composed of the newly discovered protein myosin (Kühne, 1864).

Striated skeletal muscle fibers vary in length between 2 and 25 cm, depending upon the muscle. The diameter of a single muscle fiber is also variable but is usually between 10 and 100 µm. Normal mature striated skeletal muscle fibers are irregularly polygonal in shape (Bowman, 1840), whereas developing fibers are small and round; pathologic muscle fibers tend to be round or sharply angular and usually abnormally small. The multinucleated skeletal muscle fibers, unlike smooth and cardiac muscle fibers, are not structurally or functionally uniform. Two or more distinct muscle fiber types have been identified in man and other species by light and electron microscopy, histochemistry, and functionally. Characteristically, the nuclei of skeletal muscle fibers are located peripherally adjacent to the outer limiting membrane or sarcolemma. The usual (normal) position of skeletal muscle nuclei and the discovery of the sarcolemma is credited to Bowman (1840). The nuclei of developing muscle and of cardiac muscle are typically centrally located within the muscle fiber (Bowman, 1840).

In certain skeletal muscle fibers, namely the red or slow contracting muscle fibers, the nuclei may be found scattered throughout the sarcoplasm. Based upon structural/functional studies, living muscle fibers that appear red are designated, in man, as Type I muscle fibers. These muscle fibers contain many mitochondria (Kölliker, 1857), they store and utilize lipid (droplets) metabolically, and they are red in color and contract slowly. One type of red fiber requires only a single stimulus, whereas a second type requires multiple stimuli to initiate a contraction. The former is designated a slow twitch muscle fiber, and the latter as slow tonic muscle fiber. Lorenzini (1678) first noted color differences in muscles; some are red in color, others are white. Kühne (1850) analyzed the intrafiber pigment and reported its similarity to hemoglobin. Lankester (1871) noted that although red muscles were slow in contracting, they were the most active and strongest and capable of sustained contractile activity. He also contrasted red pigeon breast muscle with white chicken breast muscle and, with both birds, capacity for sustained flight. Gunther (1921) introduced the term myoglobin for the red (intrafiber) pigment. Most whole muscles are a mixture of red and white muscle fibers, which vary in number for any particular muscle; these are seen histologically in thin sections.

Muscle fibers that appear white contain few mitochondria, store and utilize glycogen (Bernard, 1855), are essentially devoid of myoglobin, and contract rapidly but fatigue quickly. They are normally larger than red fibers. In man, these are designated as Type II muscle fibers.

In cross section, skeletal muscle fibers are seen to be composed of numerous small aggregates (1 to 2 µm) of contractile substance, the myofibrils. Myofibrils are composed of myofilaments (Hall, Jakus, Schmidt, 1946). Huxley (1954) has shown two types of myofilaments: a thick, A band myofilament 1.6 µm in length; and a thin, 1 band myofilament 1.0 µm in length, which extends from the Z line into the A band for a variable distance.

In longitudinal section, the muscle fiber and the myofibril appear cross-striated (Leeuwenhoek, 1674). The darkly stained segments, 1.6 µm in length (Krause, 1868), is designated the A band (anisotropic band), which is a region of high refractive index and is birefringent when examined with a polarizing microscope (Brücke, 1858). Alternating with the A bands is a lightly staining region of variable length, the I band (isotropic band), which is a region of low refractive index (Brücke, 1858). The I band is bisected by a thin dark-staining Z line. When a muscle fiber contracts, the band pattern changes with the A bands moving toward each other and meeting at the Z line (Bowman, 1840); the I bands disappear, and the middle of the A band becomes dark. The band pattern changes can be precisely related to the movement of myofilaments in relation to each other. Additional structural/functional details of the contractile process will be considered in the legends of the plates of this section.

In other sections of this atlas, muscle can be seen in the following plates:

Smooth muscle: Plates 34, 35, 83, 152, 153, 154, 155, 156, 158, 159, 188, 190, 192, 193, 199, 200, 201, 204, 207, 225, 226, and 305.

Striated muscle: Plates 9, 14, 159, 177, 178, 179, 180, 188, and 208.

Cardiac muscle: Plates 148, 149 and 150.

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