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Muscle

Muscle cells are specialized for contractility, and they can be classified according to their microscopic appearance into 3 categories:

  1. Smooth muscle which is found primarily in tubular organs.
  2. Skeletal muscle which is generally attached to bones.
  3. Cardiac muscle which is found in the wall of the heart.

Objectives

  1. To be able to distinguish muscle from other tissues, e.g., connective tissue.
  2. To distinguish between the 3 types of muscle.
  3. Identification of the connective tissue arrangements of skeletal muscle.

Smooth Muscle

The smooth muscle cell is spindle-shaped with an elongated centrally located nucleus. If contraction of the cell has occurred the nucleus may be coiled. With ordinary fixation, its cytoplasm appears almost homogenous. Smooth muscle cells may occur as solitary fibers such as those found in the spleen capsule or prostate stroma. More generally, they are grouped together in bundles called fascicles. These fascicles may be isolated (e.g., arrector pili muscles) or they may be aggregated bundles or sheets around tubular organs or vessels. Blood vessels maybe found between the fascicles.

Locate the smooth muscle in slides #56 or #57 (intestine). The muscle coats of the intestinal wall are arranged in an inner circular layer (nearest the columnar epithelium) around the lumen of the intestine, and an outer longitudinal layer. Therefore, if the outermost muscle coat is seen in longitudinal section, the tissue has been cut in the longitudinal plane, and if the outer fibers are seen in cross-section, the tissue has been cross-sectioned. Determine which way the muscle bundles have been cut on slides #56 and #57 to decide whether the piece of gut was cut in longitudinal or in cross section. Determine the position of the nucleus within the cell. Are striations present in the cytoplasm?

Skeletal Muscle

Skeletal muscle fibers slides #49 (tongue) are striated and contain many nuclei. In individual fibers cut in perfect cross-section, find: (1) the peripherally located nuclei just under the sarcolemma (or plasma membrane); (2) the myofibrils cut in cross-section which give a stippled appearance. What cell elements are present in the light space between myofibrils? Now find fibers in longitudinal section and observe the cross-striations under high magnification (40X). The fiber is filled with parallel myofibrils that extend the length of the cell, and have alternate light (I) and dark (A) bands which you should identify. The cross-bandings of adjacent myofibrils are aligned with each other so that the banding is seen across the whole fiber.

   

To study skeletal muscle-associated connective tissue, examine slide #25 (a section of rib with bone marrow). The intercostal muscles are present, cut mostly in cross-section. The delicate connective tissue (c.t.) surrounding individual fibers is the endomysium which contains capillaries. The more substantial c.t. surrounding the various muscle fascicles is the perimysium and the connective tissue on one side of the section, continuous with that over the outer surface of the bone, is the epimysium, the sheath of the entire muscle. The c.t. spaces in the muscles are large in these preparations due to muscle shrinkage. Scan the muscle tissue and locate a muscle spindle. In most slides, the spindles will be found in the perimysium. These are seen in cross-section as a group of 3 or 4 very small muscle fibers contained within a capsule of flattened squamous cells, the perineural epithelium.

   

Examine an EM of part a skeletal muscle cell. A sarcomere is shown here. You should understand the molecular components of the contractile apparatus and the mechanism of contraction.

   

The EM at left shows a grazing section of sarcoplasmic reticulum and elements of the T-tubular system at either end of the sarcomere (coincident in this case with the I-bands).The bottom photo is at lower magnification and shows a series of triads (arrows indicate the T-tubules).

   

Locate the nerves in the perimysium on slide #25 (rib, muscle) These will appear as round, eosinophilic structures containing numerous nuclei. The majority of these nuclei are in Schwann cells but some fibroblasts are also present. There are many axons, but individual ones will not be discernable due to their tight packing. Surrounding the nerve bundle is the perineurium.

   

The myelin sheath is formed by Schwann cells and the nuclei that you see probably belong to these cells.

   

Study the EM of a neuromuscular junction. You should be able to recognize the pre-terminal nerve ending and the muscle fiber. Note the junctional folds with basement membrane. How does this "synapse" work?

Cardiac Muscle

Slides #21 (odd boxes) and #23 should be used to study cardiac muscle. Find longitudinal sections of cardiac fibers, where the branching of fibers and the striations of the myofibrils may be seen. The nuclei are located centrally within the muscle fiber and not at the periphery as in skeletal muscle. This can also be observed in cross-sections. Intercalated discs usually appear as areas of increased staining density crossing the fibers. They are easier to see if you stop down the diaphragm but may be hard to locate on some slides. Study the E.M. appearance of cardiac muscle. You should be able to recognize the ultrastructure and understand its functional significance and be able to distinguish between electron micrographs of cardiac and skeletal muscle.