Types of Muscle

Human muscle tissue is classified into the following categories: skeletal striated muscle and noncardiac visceral striated muscle,^[1] smooth muscle, and cardiac striated muscle. These categories are based on the location, function, and structure of the tissue.

Skeletal striated muscle (skeletal muscle) is the most abundant of the muscle types in the human body. It is responsible for appendicular and axial skeletal movements. In addition to moving skeletal structures, skeletal muscles are also responsible for moving the eyes, tongue, deep fascia, and skin of the face. Skeletal muscle cells possess “stripes” or striations. These striations (A band and I band) are the result of differences in light passage and absorption in the tissue. Skeletal muscle is also termed voluntary muscle in that contractions can be initiated via the “voluntary” or somatic nervous system.

Cardiac muscle is limited to the heart and exhibits a banding pattern quite similar to that of skeletal muscle. Cardiac muscle is commonly called a functional syncytium in that excitation of one cardiac muscle cell will quickly cause the excitation of all adjacent cells via the intercalated discs found between the cells.

Smooth muscle is the only type that does not possess striations. Smooth muscle is typically found within the hollow visceral organs and blood vessels of the body and is functionally subdivided into “single unit” or “multi-unit” smooth muscle, depending on how easily action potentials may spread from one cell to another.

Skeletal Muscle

Skeletal muscle fibers demonstrate the following histological characteristics:

• Cells are long and cylindrically shaped.
• Cells are multinucleate.
• Nuclei are peripherally located.
• When viewed in longitudinal section, skeletal muscle cells demonstrate a regularly arranged banding pattern.

As you examine the following photomicrographs, it is important to keep in mind that skeletal muscle should be the reference by which you judge all other muscular tissue. If you know the histological characteristics of skeletal muscle (in both cross and longitudinal sections), you can then use these characteristics to identify all other unknown muscle specimens.

Skeletal Muscle – Cross Section

Figure 6-1 is a cross-sectional view of skeletal muscle. Because skeletal muscle cells are cylindrical in shape, you will see very little size variation between the individual myofibers, a major histological feature of this tissue. Although this cross section does not demonstrate the multinucleate nature of skeletal muscle fibers, it clearly demonstrates that the nuclei are peripherally located, another major histological feature of skeletal muscle when viewed in cross section.

Skeletal muscle has a rather complex connective tissue investment. The external surface of the entire muscle is covered with connective tissue, termed epimysium, which is not visible in this section. The epimysium then extends into the interior of the muscle as perimysium, which divides the skeletal muscle into fascicles. The endomysium is found surrounding each individual skeletal muscle cell, external to the basal lamina.

Figure 6-2 is a higher magnification of Figure 6-1 and demonstrates the perimysium, endomysium, and accompanying slender fibrocytes.

As you center your attention on the individual myofibers, you will see the peripherally located nuclei. In addition, you can distinguish myosatellite (satellite) cells. Myosatellite cells are stem cells that may divide and form additional myofibers following muscle injury. Myosatellite cells are found beneath the basal lamina of skeletal muscle cells and external to the sarcolemma (muscle cell membrane). They may be distinguished from fibrocytes by their more slender nuclei.

Skeletal Muscle – Longitudinal Section

Figures 6-3 and 6-4 demonstrate skeletal muscle in longitudinal section. The banding pattern is clearly evident, as are the peripheral nuclei. Note how the banding pattern in skeletal muscle is “in register” (lined up) from one cell to another. This is a major histological feature of skeletal muscle when viewed in longitudinal section.

Nuclei of fibrocytes located within the perimysium are also evident in Figure 6-3.

Cardiac Muscle

Cardiac muscle, like skeletal muscle, is striated; however, that is where the histological similarities end. As you examine the following photomicrographs of cardiac muscle, keep in mind the following histological differences between skeletal and cardiac muscle:

• Cardiac muscle cells are considerably smaller than skeletal muscle fibers. The exception to this rule is Purkinje fibers, which will be studied in Chapter 8, the Cardiovascular System.
• Cardiac muscle cells branch frequently and therefore demonstrate a greater size variation when viewed in cross section.
• Cardiac muscle cells (except for Purkinje fibers) have a single, centrally located nucleus.
• Cardiac muscle cells possess specialized cell junctions called intercalated discs, and these are visible in longitudinal sections of cardiac muscle.

Figure 6-5 is a photomicrograph of cardiac muscle in longitudinal section. Note that cardiac muscle usually has only one centrally located nucleus per cell. In addition, note the sectioning artifacts at the periphery of this photomicrograph.

Figure 6-6 clearly demonstrates the centrally located nuclei and the intercalated discs found between adjacent cardiac muscle cells. Intercalated discs are specialized junctions that facilitate the spread of electrical impulses from one cardiac muscle cell to another. They also anchor cardiac muscle cells to one another, thereby allowing the spread of contraction forces.

Compare this figure to skeletal muscle in Figure 6-4. Note:

• How cardiac muscle cells branch
• That the banding pattern of cardiac muscle is not as
  prominent as that of skeletal muscle
• That the banding pattern between individual cardiac muscle
  cells is not “in register,” as was seen in skeletal muscle

These three characteristics are all major histological features of cardiac muscle when viewed in longitudinal section.

You will also note, as you compare Figures 6-4 and 6-6, that cardiac muscle exhibits fibers running in several different directions. This results in the presence of cross, longitudinal, and oblique sections on the same slide because of the helical arrangement of cardiac muscle within the heart. This can be a useful identification tool.

Figure 6-7 is a photomicrograph of cardiac muscle in cross section. This figure clearly demonstrates two major histological features of cardiac muscle when viewed in cross section:

• Note that the nuclei in cardiac muscle cells are centrally located.
• In addition, this tissue demonstrates an increased variation in cell size compared to skeletal muscle in cross section (see Figure 6-1). This increased size variation is due to the branching of cardiac myocytes.

Smooth Muscle

As you examine the following photomicrographs of smooth muscle, you must compare smooth muscle to skeletal and cardiac muscle. As you make these comparisons, you will see the following similarities and differences:

• Smooth muscle, unlike cardiac and skeletal muscle, is not striated.
• Like cardiac muscle, smooth muscle has a single, centrally located nucleus.
• Smooth muscle cells, unlike skeletal and cardiac muscle, are fusiform in shape. Therefore they will demonstrate the greatest amount of size variation when viewed in cross section.
• Smooth muscle cells have long, football-shaped nuclei.
• The lateral borders of smooth muscle cells are quite difficult to see in hematoxylin and eosin preparations.
• Smooth muscle cells are much smaller than cardiac or skeletal muscle cells.

Figure 6-8 was taken from the smooth muscle found within the muscularis externa of the ileum of the small intestine. You can see smooth muscle in longitudinal section in this photomicrograph. (Note the sectioning artifacts at the periphery of this photomicrograph.)

When viewed in longitudinal section, smooth muscle cells are long and cigar shaped, appearing almost to interdigitate with each other. This reflects the overlapping of the individual smooth muscle cells.

Smooth muscle, when viewed in cross section, presents a considerable variation in cell and nucleus size. In addition, because of the shape of smooth muscle cells and the resulting plane of section, the number of cells possessing visible nuclei will also vary considerably throughout the field.

Figure 6-9 is a photomicrograph of smooth muscle in longitudinal and cross section taken at a higher magnification. Note the artifacts between the two layers of smooth muscle in this photomicrograph.

References

Wareham AC, Whitmore I: A comparison of the mechanical properties of oesophageal striated muscle with skeletal muscles of the guinea pig, Plflugers Arch 395:312-317, 1983.

Whitmore I: The ultrastructure of oesophageal striated muscle in the guinea pig and marmoset, Cell Tiss Res 234:365-376, 1983.

Whitmore I: 2006 Terminologia histologica. International terms for human cytology and histology, Philadelphia, Lippincott Williams & Wilkins.

Whitmore I, Gosling JA, Gilpin SA: A comparison between the physiological characteristics of urethral striated muscle in the guinea pig, Pflugers Arch 400:40-43, 1984.

Whitmore I, Notman JA: A quantitative investigation into some ultrastructural characteristics of guinea pig striated muscle, Ann Anat 153:233-240, 1987.