Unit 5: Nerve Tissue. Neurons, fibers, fascicles, & ganglia

Transcription

1 81 Unit 5: Nerve Tissue Neurons, fibers, fascicles, & ganglia

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3 Introduction and Objectives The structural and functional unit of nerve tissue is the nerve cell, or neuron. There are over 10 billion of these neurons in the human nervous system. Most neurons have processes called nerve fibers, which are thin extensions of the cytoplasm covered by the neuron cell membrane. Neurons and their processes within the brain and spinal cord are in the central nervous system, while neurons and their processes located outside the brain and spinal cord are in the peripheral nervous system. The study of nerve tissue in this unit will be strictly in the peripheral nervous system. Our study will begin with the histology of the neurons located in the spinal ganglia and then proceed with learning the parts of a nerve fiber, concluding with learning the connective tissue wrappings that package large numbers of nerve fibers into a nerve like the median nerve. There is much to be learned about the histology of the central nervous system as in the difference between the spinal cord, cerebellum and cerebrum. However, our focus will be on the peripheral nervous system. 83 After completing this unit, you should be able to identify and distinguish between: 1. the histological components of a sensory neuron 2. the histological components of a sensory ganglion 3. the histological components of a nerve fiber 4. endoneurium, perineurium, and epineurium 5. nerve axon, myelin sheath, sheath of Schwann (neurolemma) Also, after completing this unit in conjunction with reading a chapter on nervous tissue in a histology textbook and/or hearing a lecture on nervous tissue, you should understand:. 1. the functional relationship between the cell body, axon, and dendrites of a sensory neuron. 2. the difference between myelinated and unmyelinated nerve fibers as to function and structure. 3. the role of myelin in nerve fibers.

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5 Learning the histological components of a sensory neuron 85 Let s begin our study by choosing a specimen of a spinal ganglion. Recall that spinal ganglia are located in the dorsal root of the spinal nerves. Along with the ventral (anterior) root, the dorsal root (posterior) forms a spinal nerve. Sensory nerve axons coming from the peripheral nervous system course through the spinal nerve into the dorsal root and connect to the sensory neurons in the dorsal root (sensory) ganglion. From the cell body of the sensory neurons in the spinal ganglion the axons continue into the spinal cord to both ascend to the brain and synapse with other neurons in the spinal cord. The communication of the sensory nerve fibers with motor nerve fibers in the spinal cord via synapses completes a reflex. Now, in the general histology section under nerve tissue choose: Spinal Ganglion-Hematoxylin-Eosin-Metyhlene Blue Select the 5x image and review the staining properties of this stain before proceeding. Collagen stains greener in this 5x image but will appear blue-green in the higher magnifications. Now scan the 5x image noting ganglion cells, nerve fibers, and the connective tissue capsule of the ganglion. As you can see there are numerous ganglion cells. You are seeing the cell bodies of these cells. They have axon and dendrite processes which also can be found in this image (find and observe the labeled nerve fibers). Select the 20 x image with the labels turned on. In this image two of the many ganglion cells present are labeled. The label lines for these two cells are indicating the cell body of the sensory ganglion neurons. The shape of the cell bodies is rounded, and, indeed, in the intact neuron the cell bodies of sensory ganglion cells are spherical. A single process projects from the cell body and then separates into an axon that extends out into the peripheral nervous system traveling through nerves to finally end in a sensory receptor for touch, pain, temperature, etc. The other subdivision of the process extends into the spinal cord to either synapse with an intermediate or motor neuron at the spinal cord level, or extend into the brain via the spinal cord white matter. Note the labeled nerve fibers which are the processes just described. It is not easy to know which is which, just that these nerve fibers are the processes of the ganglion cell bodies. Supporting cells called satellite cells surround each ganglion cell. Read the text related to the label satellite cells. You now know that these cells lie close to the ganglion cell and are surrounded with a basal lamina outside of which is a delicate network of connective tissue fibers. Now select the 100x image. In this image you can observe the ganglion cell body in more detail. Note the large nucleus with a prominent nucleolus. This along with the large nucleus with extended chromatin (euchromatin) suggests a high level of synthesis. Read

6 86 the text to the labels to learn the components of a spinal ganglion cell body and its surroundings. Before you leave this image change the view to full screen and look for the labeled nerve fiber at the bottom of the image. Note the axon and read the text of the label nerve fiber. Now, in the specimens under nerve tissue choose: Spinal Ganglion-Hematoxylin-Eosin Your objective in studying this specimen is to view the histology of a spinal ganglion, its cells, its nerve fibers, and it connective tissue stained with the routine stain-hematoxylin and eosin. First, in the 5x image, note the introduction of a new term, the perikaryon (peri = round, karyon = nucleus), the cytoplasm around the nucleus or, as you learned while studying the last specimen, the cell body. Note also the appearance of the connective tissue and nerve fibers in this image. Study the 20x and 40x images reviewing the histological structure of the ganglion cell bodies, satellite cells, and nerve fibers. In the 40x image read the text for the label lipofuscin pigment. You can expect to find this kind of pigment in cells of the body that do not divide but get old with us. Their organelles, except for the nuclear material, are renewed. So neurons can live a long time by getting replacement parts. Cardiac muscle cells also accumulate lipofuscin pigment as they age. Finally, I want to bring the labeled nerve fiber in this 40x image to your attention. Again, as you did in the last specimen, observe the appearance of the nerve fiber and read the text. Now we are going to study the histology of a nerve fiber in more detail. Learning the histological components of a nerve fiber A nerve fiber is composed of an axon, a myelin sheath, and a cellular covering called the Schwann sheath (or neurolemma). To study these components go to the general histology section and under nerve tissue choose: Nerve Hematoxylin-Eosin This is a specimen of a nerve cross-sectioned. There are large groups of nerve fibers wrapped by connective tissue wrappings. We will study the connective tissue wrappings later. For now, scan the 5x and 20x images noting the nerve bundles, then, in the 20x image the labeled nerve fibers. In this same image you can observe that most of the rounded profiles, like the labeled nerve fiber, have a dark round center. All the nerve fibers are cut in cross-section. The dark center is the axon and the clear lighter staining region surrounding the axon is the myelin sheath. Now select the 40x image and scan it looking first for the labeled myelinated nerve fiber. Read the related text and recall that the Schwann s sheath is also called the neurolemma. Now find the

7 87 label Schwann s sheath and read the related text. At this point if you have not done so, change the view from microscope to full-screen. Survey the entire image noting the numerous cross-sections of nerve fibers in addition to those labeled. This image is a portion of a nerve fascicle. A nerve fascicle is a collection of nerve fibers bundled by connective tissue. Note the delicate connective tissue amongst the nerve fibers. This is the endoneurium. It is similar to the endomysium connective tissue that surrounds each skeletal muscle fiber. Now turn your attention to the perineurium (look for the label in the upper left portion of the image). Read the related text to this label. Now you have learned that the perineurium is similar to the perimysium of skeletal muscle. Like the perimysium which surrounds large groups of skeletal muscle fibers, perineurium surrounds a large number of nerve fibers. But they differ in that perineurium acts as a physiological barrier between the regions outside of the fascicle and the interior of the fascicle where the delicate nerve fibers are present. Before leaving this image, note the presence of several blood vessels. Nerve fibers require a constant source of oxygen and carbon dioxide exchange. Now return to the 20x image and note that one entire fascicle occupies the image. Note that outside of the fascicle there is also connective tissue that is filling in between the fascicles. This is known as the epineurium which also extends to the very outside and wraps all of the fascicles of a nerve (for example the median nerve). Finally, return to the 5x image and note several more fascicles are now visible for this nerve. It would require even a lower magnification to view the cross-section of the entire nerve. To reinforce what you have learned by studying this hematoxylin and eosin stained nerve specimen now choose a special stain that will highlight the connective tissue wrappings. Under the nerve tissue list choose: Nerve Goldner In this specimen the collagen is stained green. Scan and study the 5x, 40x and 100x images. As you do this, review the components of a nerve fiber and the connective tissue wrappings that organize large numbers of nerve fibers into a nerve. Read all of the text labels. Before concluding this exercise on nerve tissue, I would like to take you back to a sample of smooth muscle that we previously studied so that you can observe nerve fibers one more time for reinforcement. In the general histology section under muscle tissue choose: Skeletal Muscle, transverse Hematoxylin-Methylene Blue In this specimen of skeletal muscle that is cross-sectioned you will be able to observe small fascicles of nerve fibers that are present in skeletal muscle. Skeletal muscle requires direct stimulation of its muscle fibers by nerve fiber endings known as neuromuscular junctions (also

8 88 known as motor endplates). These smaller nerve fascicles consist of branches of the large nerve that brings the innervation to the entire muscle. First quickly scan the 5x image to orient yourself. Then select the 20x image and look for the labeled nerve fiber bundle. The label line ends in the perineurium. Within the fascicle you can observe that each nerve fiber has a central dark staining axon and a surrounding clear region, the myelin sheath. Note also the presence of few nuclei within the fascicle. These would most likely below to Schwann cells or fibroblasts of the endoneurium. You have now completed the exercise on the histology of nerve tissue. You have learned that neurons have 1) a cell body (perikaryon) where the nucleus is located and 2) processes known as axons. Your study of a neuron was focused on learning the histological features of a spinal ganglion cell (sensory ganglion cell). You learned that the ganglion cells are surrounded by supporting cells that are called satellite cells. These cells are actually the same as Schwann cells only they enclose the ganglion cell body. Recall that axons, processes of the cell bodies of neurons, in the peripheral nervous system make up the core of a nerve fiber. The other components of a nerve fiber are the myelin sheath and the Schwann sheath (neurolemma). Finally, you learned that nerve fibers are wrapped and organized by connective tissue (endoneurium, perineurium, and epineurium) in bundles known as fascicles and that groups of fascicles can make up a nerve such as the median nerve.

9 89 Sample Practice Questions: The arrow in this image indicates which of the following structures? A. nerve fiber B. axon C. neuron cell body D. satellite cell E. myelin sheath Structure labeled 1 is which of the following? A. nerve fiber B. Schwann cell nucleus C. endoneurium D. perineurium E. epineurium

10 90 Nerve Section Labeled Structures Viewing the structures you have studied in this lesson in sections stained with special stains other than Hematoxylin and Eosin could be helpful in making your learning complete. In this unit and all others the specimens under each heading will be listed in this way that will help also when you are reviewing to know which specimen and which magnification that certain structures are labeled. Nerve- Staining: Goldner 20X 160X 400X Epineurium Nerve fiber Endoneurium Nerve fiber bundle Axon Axon Adipose tissue Endoneurium Myelinated nerve fiber Vein Epineurium Schwann's cell nucleus Myelinated nerve fiber Myelin Perineurium Perineural cell nucleus Collagenous fiber Perineurium Epineurium Nerve- Staining: Hematoxylin-Eosin 20X 80X 160X Nerve fiber bundle Blood vessel Epineurium Epineurium Epineurium Perineurium Perineurium Perineurium Endoneurium Adipose tissue Nerve fiber Axon Arteriole Myelinated nerve fiber Schwann's sheath Myelin sheath Blood vessel

11 Spinal Ganglion- Staining: Hematoxylin-Eosin-Methylene Blue 91 20X 80X 400X Ganglion cell Ganglion cell Ganglion cell nucleus Nerve fibers Satellite cell Nucleolus Ganglion capsule Nerve fibers Lipofuscin pigment Nucleus Nerve fiber Axon Satellite cell Spinal Ganglion- Staining: Hematoxylin-Eosin 20X 80X 160X Ganglion cell - perikaryon Ganglion cell perikaryon Ganglion cell - perikaryon Ganglion capsule Ganglion cell nucleus Ganglion cell nucleus Adipose tissue Lipofuscin pigment Lipofuscin pigment Nerve Fiber Satellite cell Nerve fiber Satellite cells Axon Nerve fibers Satellite cell nucleus Nucleolus

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