Muscle, one of the four basic tissues, is highly specialized for contractility. The fundamental unit of muscle is the muscle cell (usually called a muscle fiber or just “fiber”) which shortens in its long axis upon stimulation. The contraction occurs when two types of filaments, actin and myosin, interact with each other to interdigitate, thus pulling the two ends of the fibers closer together. The three kinds of muscle are smooth muscle, skeletal muscle, and cardiac muscle. The latter two are called striated muscle because of microscopically visible cross striations exhibited by the fibers. Smooth muscle lacks cross striations.
Slide 31: Ileum
On slide 31, Ileum (H&E), identify smooth muscle fibers in the relatively thick muscle coat (muscularis). The muscle fibers are arranged in two well-defined layers: An inner circular and an outer longitudinal. Between the two layers of muscle is a thin layer of connective tissue that varies in thickness and contains a plexus of nerve fibers and also parasympathetic ganglion cells known as the myenteric plexus (of Auerbach).
In the outer muscle layer, where the smooth muscle fibers are cut in longitudinal section, observe the shape of the smooth muscle cell (fiber), its size, the amount of sarcoplasm, and the position of the nucleus. Note the elongated shape of the smooth muscle nuclei as compared to nearby fibroblast nuclei in the connective tissue septa. With medium power, scan this outer muscular layer and notice how the smooth muscle is arranged in the form of broad sheets or bands.
Now look at the inner muscle layer where the smooth muscle fibers are cut transversely. Observe that the nucleus is not always seen. Why? When it is seen, notice how it varies in size in relation to the diameter of the fiber. Note especially the position of the nucleus within the fiber.
Slide 121: Urinary Bladder
On slide 121, Urinary bladder (H&E), notice how the smooth muscle fibers of the thick muscle coat are arranged as interlacing bundles. Each bundle is supported by moderately dense connective tissue. The arrangement increases the efficiency of expelling urine quickly as well as increases the ability of the organ to withstand the force of distension as urine accumulates within the bladder.
Slide 20: Skeletal Muscle
On slide 20, Skeletal muscle (H&E), is one of the muscles in the larynx whose fibers appear more loosely arranged because of edema in the connective tissues; therefore, individual fibers can be seen more easily. The muscle fibers are sectioned in various planes. Keep in mind the large size (diameter) of skeletal muscle fibers as compared with smooth muscle fibers.
Look at the muscle fibers in longitudinal section. Note: The fibers are long and approximately uniform in diameter; cross-striations are visible varying in distinctness in different fibers; fibers are multinucleated with nuclei at the periphery of the fiber; a thin sarcolemma (plasmalemma) covers the fibers.
Select a well-stained longitudinal fiber in which cross-striations are readily visible with low power (10 x). Study the striations. At 40 x power the dark A bands and the light I bands can be distinguished, and the thin Z line in the I band can sometimes be seen. While the A band remains constant in width, the I band width varies according to the state of contraction: Very narrow if muscle was contracted at time of fixation, or wider if muscle was fixed in a relaxed or post-contraction state. An additional band, the H band (a lighter area within the A band) is rarely visible with the light microscope. The segment between two Z-lines is a sarcomere - the function unit of all striated muscle (skeletal and cardiac). With H&E, the Z-lines appear as fine red lines.
In cross section, note the uniform diameter of the fibers and the peripherally located nuclei. In a good cross section, the myofibrils appear as pinpoints. Oblique sections are neither cross nor longitudinal and characteristics are not always well demonstrated. Note the fine loose connective tissue between muscle fibrils (endomysium). It is easily seen here.
Slide 8: Intervertebral Disc
Slide 8, Intervertebral disc (H&E). Some of the deep back muscle fibers attach directly to the periosteum by means of short tendons or by an apparent continuity between muscle and the connective tissue of the periosteum.
Slide 75: Heart and Purkinje Fibers
Slide 75, Heart and Purkinje fibers (H&E), shows a portion of the left ventricle with its epicardium and endocardium, the aorta, coronary artery and vein. Purkinje fibers are present in the subendocardial areas. This slide demonstrates the major characteristics of cardiac muscle tissue: branching myocardial fibers, cross-striations, intercalated discs, and a centrally placed nucleus within each muscle fiber.
Myocardial fibers are seen cut longitudinally, obliquely, and transversely. The reason for this orientation is that the muscle fibers of the heart course spirally within the walls of the heart. The spiral arrangement permits efficient filling and expulsion of the blood. In longitudinal sections of muscle observe: the cross-striations of the fibers; size, shape, and location of the nucleus, the clear area at either end of the nucleus and the myofibrils within the sarcoplasm. Note how the intercalated discs (darkly stained lines) extend either straight across the fiber or in step-wise fashion.
Between the myocardial fibers is seen loose connective tissue of the endomysium containing numerous capillaries. The rich vascularity of heart muscle is emphasized.
Locate the lower right-hand part of the large section (interventricular septum) on low power. The thin tissue covering most of the upper and lower margins of the section is endocardium. The endocardium includes the single layer of endothelial cells plus underlying connective tissue With medium and high power find the Purkinje fibers immediately adjacent to the upper and lower endocardium. The latter are modified cardiac fibers specialized to conduct impulses and form part of the impulse-conducting system of the heart. Purkinje fibers are lighter-stained (H&E), contain fewer myofibrils (in cross-section the myofibrils are found to be limited to the periphery of the fiber), and have more sarcoplasmic space surrounding each nucleus than is the case for regular myocardial fibers. Nuclei are more rounded but often appear irregular in outline. Look for longitudinal and cross-sections of the Purkinje fibers.
Slide 91: Cardiac Muscle
On slide 91, Cardiac muscle (Gomori’s), the cross-striations of the myocardial fibers and the intercalated discs are more prominently displayed. The intercalated discs stain as a black line. Between the myocardial fibers the capillaries and other vessels are readily seen; the black bodies within the vessels are primarily erythrocytes. Note the strands of blue-staining connective tissue which accompany the blood vessels and which also comprise part of the endocardium and epicardium. See sketch on next page for location of Purkinje fibers.