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Lab 5. Muscle & Nerve

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 cells are also called fibers. They are striated and contain multiple nuclei owing to fusion of myoblasts during their formation. On slide # 49 (tongue) locate fibers cut in perfect cross-section and find: 1) the peripherally located nuclei just uder the sarcolemma (plasma membrane); 2) the myofibrils cu 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 and dark 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.

 

The dark and light bands reflect the sarcomeric structure of striated muscle. The dark bands are A-bands (anisotropic) and the light bands are the I-bands (isotropic). The I-band is divided into two halves by the Z-disc of band. A sarcomere extends from one Z-disc to the next (about 2.5 µm). You should understand the protein make-up of these regions and understand what happens when the muscle contracts. At the center of the A-band is a lighter region lackinga ctin, the H-band. Running through the center of this H-band is a dark line called the M-line.

You should familiarize yourself with the appearance of the other muscle cell organelles including the sarcoplasmic reticulum and the T-tubular system.

 

 

Large muscles consist of fascicles that are composed of many individual fibers or cells. Each fiber is surrounded by thin collagenous C.T. called endomysium. This is continuous with a denser C.T., perimysium, that envelopes a group of fibers to make a fascicle. Surrounding a bundle of fascicles is a thicker C.T. called epimysium. 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 C.T. surroundsing 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 he section, continuous with that over the surface of the bone, is the epimysium, the sheath of the entire muscle. The C.T. spaces in the muscle are large in these preparations due to shrinkage. Scan the muscle tissue at low power and located a muscle spindle. In most slides, the spindles will be found in the perimysium. They are seen in cross-section as a group of 3 -4 very small muscle fibers contained within a capsule of flatteded squamous cells, the peri-neurial epithelium. The spindle is a sensary organ that is responsive to stretch. Its function will be discussed elsewhere in tbe Block.

 

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.

 

Nervous Tissue

The nervous system is divided into central (brain and spinal cord) and peripheral nerve systems. Nerve cells, or neurons, consist of a cell body and an axon of variable length. Smaller processes called dendrites extend from the cell body and contact other neurons. At tht tip of the axon multiple brances extend out to contact nerves or muscles . A collection of neurons is called a fascicle. A bundle of fascicles make up an anatomical nerve.

Neurons are surrounded by cells that protect and support them. These are called glial cells and differ between central and peripheral nerves and between one neuron and another. In the central nervous system the predominant type is called an oligodendrocyte. Each oligodendrocyte can envelope multiple neurons. Peripheral neurons are surrounded by Schwann cells. Both myelinated and unmyelinated axons are present in eaach nerve fascicle. In the peripheral nervous system one Schwann cell can surround many unmyelinated axons. However, each myelinated axon is surrounded by a single Schwann cell. The myelin sheath is formed by oligodendrocytes or Schwann cells wrapping around the nerve axon many times with essentially no cytoplasm between the wrapped plasma membrane. This plasma membrane has a modified composition and contains specific protesin and a large proportion of lipids. The effect of the myelin sheath is to increase the speed of impulse conduction along the axon. Various diseases result from a defect in myelination and in multiple sclerosis the myelin is subject to auto-immune atack.

Slide 13 (even) contains a cross-section of a multifascicular, human peripheral nerve stained with H & E. Each sascicle is outlined by a thin perineurial sheath and sits in a mass of epineurial collagenous connective tissue containing blood vessels, fibroblasts and fat cells. Within each fascicle are numerous axons, both myelinated and unmyelinated, embedded in a fine connective tissue called endoneurium.

At 40X observe the numerous numerous myelinated axons. These are the the dots or darker staining, central regions within a circular, cart-wheel structure. This appearance is an artefact of shrinkage during specimen preparation. In life, the axon is closely enveloped by the myelin sheath. Non-myelinated axons will also be present but hard to distinguish. Both myelinated and non-myelinated axons are surrounded by delicate endoneurium composed of fine collagen fibers.Nuclei within the fascicle belong to fibroblasts and Schwann cells. Blood vessels are also present.

Slide 13 (even) also has a longitudinal section of peripheral nerve. The fascicles appear as pale, fibrous regions with elongated nuclei (generally parallel to the nerve fibers). Perimysium is seen as coarser C.T. separating the bundles. The appearance of the sections can vary depending on the fixation.

Slide 13 (odd) contains a cross-section of a peripheral nerve stained with osmium. This stain, used extensively in electron microscopy, binds to lipid-rich structures such as membranes. The stain appears as a brown or black deposit. Again notice the nerve fascicles embedded in epineurial tissue. Fascicles varyconsiderably in diameter. The dark black structures are fat cells. Within the perineurium the myelinated axons are seen as dark circles with a white center (the axon). Notice the variation in thickness of the myelin sheaths .

 

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5

 

Electron microscopy provides a clearer view of the relationship between nerve axons and the support cell. The freeze-etch image (1) shows several nerve bundles surrounded by epineurium. Note the orientation of the blood vessels. A higher magnification of a myelinated axon (2) shows the Schwann cell with its nucleus surrounding the axon. The plasma membrane of the Schwann cell is "rolled" around multiple times to form the myelin sheath. At higher magnification (3) the layers of plasma membrane are visible along with neurofilaments and occasional mitochondria. Unmyelinated axons (4) are grouped in clusters surrounded by plasma membrane and cytoplasm of a Schwann cell. A single Schwann cell can thus envelope multiple axons. Notice the absence of the multi-layered membrane seen for myelinated axons.

 

Where autonomic nerves innervate muscles a specialized structure, the neuromuscular junction, is formed (5). Synaptic vesicles are clustered in this region of the axon (the nerve terminus) as are mitochondria. The axon with its Schwann cell forms a series of irregular folds that interdigitate with similar folds of the muscle cell membrane. The folds are synaptic clefts. Acetylcholine receptors are concentrated in this region of the muscle cell membrane.