There are three major types of muscle, and their structure reflects their function. Skeletal and cardiac muscle cells are called striated muscle because of the very regular arrangement of their intracellular contractile units, sarcomeres, at the light microscope (LM) and electron microscope (EM) levels. This regular arrangement imparts a cross-striated (or striped) appearance. Such an arrangement is not seen in smooth muscle cells. Skeletal muscle is also called voluntary muscle, because its contraction is under conscious neural control. In contrast, cardiac and smooth muscles are called involuntary muscles because their contractions are either spontaneously generated or are under the control of the autonomic nervous system.
Each of these three types of muscle has a characteristic appearance in both cross and longitudinal sections. You should be able to recognize each type of muscle in both planes of section.
In longitudinal sections of skeletal muscle (Slide 58, odd-numbered slide boxes), observe the following :
Cross striations can be seen, and are due to the structure of the sarcomere. A sarcomere consists of the structures between two Z lines. You should recognize: (Using your microscope and glass slides may help to see these fine structures)
It is admittedly difficult to see the Z line and especially the H zone with the light microscope. However, they can be seen clearly on some areas of almost all slides, and it is just necessary to do some looking around for a favorable area on your slide. Here are some good examples showing cross striations: #058L Webscope #058L Webscope
In slide 59, stained with trichrome (even slide boxes) or H&E (odd slide boxes), three layers of connective tissue sheaths are visible. This is a transverse section of an entire fetal forearm, and contains many elements in addition to the muscle, such as nerves and tendons, both of which are, unfortunately, similarly bundled into fascicles and therefore easily confused with muscle. So, make sure you’re looking at the muscles Image Image
Cardiac muscle will be studied in the wall of the ventricle of the heart. In comparison with skeletal muscle, note the following differences. Cardiac muscle cells branch and form a three-dimensional network. These branch points can sometimes be seen in your sections, and you should also note that the muscle fibers are less parallel than in skeletal muscle.
MAKE SURE YOU CAN DIFFERENTIATE BETWEEN CARDIAC AND SKELETAL MUSCLE IN BOTH LONGITUDINAL AND TRANSVERSE SECTIONS!
Smooth muscle may be studied using slide #29 #029-1 smooth muscle Webscope or slide #169, #169 Webscope, both in the intestine. To find the muscle layer, look at the at slide at the lowest power (this is about the same as looking at the glass slide with the naked eye). The purple layer is largely the epithelium and the lamina propria filled with plasma cell and lymphocytes. Next to that you see a lighter region of connective tissue (the submucosa you looked at to see loose connective tissue and fibroblasts), then a darker pink region which is made up of the two layers of smooth muscle you want to look at. Slide 29 is a cross section of the intestine, so the inner, circular layer of muscle will have cells oriented longitudinally (or, in places, the cells may appear to be oriented more obliquely). Move further out to see the outer sheet of smooth muscle, which runs longitudinally along the intestine, and will therefore be seen in cross section.
Look at slide #155, which is a longitudinal section of the GI tract at the gastro-esophageal junction, to see more smooth muscle in various planes of section. The smooth muscle in the esophagus (the part lined with a stratified, non-keratinizing squamous epithelium) #155 Webscope is organized in the “classic” inner circular and outer longitudinal arrangement. However, the stomach (the part lined by a columnar epithelium) #155 Webscope has an inner oblique layer (seen mostly as longitudinal here), a very prominent middle circular layer, and a sometimes less obvious outer longitudinal layer. Don’t worry knowing about the specific layers or being able to tell esophagus from stomach. However, you should definitely be able to identify smooth muscle in any plane of section (tranverse, longitudinal, or even oblique). In this particular slide, both the hematoxylin and eosin staining are quite intense, which should help you to see the cytoplasm more clearly, especially when the muscle is cut in cross section.
Now, look at slide #250 and see if you can distinguish between small fascicles of smooth muscle and collagen fibers in the lamina propria (this task will be easier if you look first at the trichrome-stained section, which stains the muscle pink(ish) and the collagen blue) #250-2 Webscope . It’s more challenging to make this distinction in the H&E-stained section #250-1 Webscope. You should note that smooth muscle is pink, wheras collagen is a bit more orange-red. Also, smooth muscle tissue is mostly cellular (and therefore more nuclei are present), whereas the connective tissue is mostly extracellular collagen fibers with fewer cells. The table below compares the differences in the morphology of the three types of muscle.
|Skeletal Muscle||Cardiac Muscle||Smooth Muscle|
|Prominet cross- striations||Prominet cross- striations||No cross- striations|
|Fiber diameter large||Fiber diameter medium||Fiber diameter small|
|Nuclei usually peripheral||Central nucleus||Central nucleus|
|No intercalated discs||Intercalated discs||No intercalated discs|
|Longitudinal striation (myofibrils)||Longitudinal striation||No longitudinal striati|
Skeletal Muscle (longitudinal section, low magnification). Find the skeletal muscle nuclei and note their peripheral location. Note the intimate contact between capillaries and muscle cells and be sure you can tell where one muscle cell or fiber stops and another begins (you can see parts of four fibers in this picture). Make sure you know which is the longitudinal axis of the cell. Identify sarcomeres, A bands, I bands, Z lines and H zones. Note that, as you saw at the LM level, the individual myofibrils do not line up perfectly across the fiber.
Skeletal Muscle (cross section, low magnification). Note location of muscle fiber nuclei. You can see cross sections of A bands (darker) and I bands (lighter) side by side in the same cell because of the fact that the myofibrils don’t line up perfectly. Identify the approximate outline of a single myofibril.
Skeletal Muscle (longitudinal section). Identify a sarcomere. Relate the sarcomeric structure seen in the LM to the structure seen here. Note that there is also lots of glycogen in the region between the two myofibrils in this picture, a storage form for glucose (which is metabolized to provide energy for muscle contraction). At the border of the I and A-bands, note triads consisting of a central T (transverse) tubule and flanking cisternae of the sarcoplasmic reticulum.
Cardiac Muscle (Intercalated Disc, longitudinal section). Note the somewhat irregular course of the intercalated disc. In this preparation, the I bands are very short, indicating that the sarcomere is in a contracted state. Review the types of junctions present in an intercalated disc and their functions.
Cardiac Muscle (longitudinal section). Note central location of muscle nuclei. Note the “stacks” of mitochondria between myofibrils. Cardiac muscle is even richer than skeletal muscle in mitochondria (again, important for energy production). An intercalated disc is present in the upper left region of the picture.
Study the orientation of the smooth muscle cells in the intestinal muscularis externa. The micrograph will help you understand the pattern, which arises from the inner circular layer and outer longitudinal layer of smooth muscle cells. Without the knowledge in which direction the intestinal epithelium is located, it is not possible to discriminate between the two sublayers of the muscularis externa.
Smooth Muscle (cross section). Here you can see the filaments in cross-section, appearing as dots. Also, the dark areas, which are membrane-associated, are called dense plaques and are sites of filament attachment.