Reading: RR&K, Chapter 11 (pages: 256-266, 271-280, and 283-284)
SLIDE 56i small intestine, rat (H&E)
(RR&K p441, Fig. 16.1; p445, Fig 16.12; p487, Fig. 1,2,3)
As previously noted, the intestine has two smooth muscle layers arranged perpendicular to each other (i.e., inner circular and outer longitudinal). Between these smooth muscle layers, find individual ganglion cells which contribute to the Auerbach's plexus also known as the myenteric plexus. These cells have a distinct large round nucleus with nucleolus and abundant Nissl bodies. What are Nissl bodies? Occasionally you will see the nucleus of a satellite cell in juxtaposition to the nerve cell body. Additional groups of ganglion cells may be found between the inner circular muscle layer and the connective tissue of the submucosa. These groups form the Meissner's plexus.
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SLIDE 43a cochlea, guinea pig (H&E)
(RR&K p792, Fig. 1; p794, Fig 1)
The spiral ganglion is situated inside the modiolus, at the base of the bony spiral lamina, in the cochlea of the inner ear. Observe nerve cell bodies and try to find the satellite cells. Within the ganglion cells, note the large round nucleus with distinct nucleolus and abundant Nissl bodies. Spiral ganglion is a sensory ganglion. Remember that these neurons are bipolar and do not have synapses in this ganglion. Dendrites of ganglion cells pass to the organ of Corti (to be discussed in Lab 16), which is the auditory receptor organ. Axons form the cochlear nerve (the acoustic part of cranial nerve VIII), which terminates within the brain. Is the cochlear nerve myelinated?
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SLIDE 37b peripheral nerve (toluidine blue)
(RR&K p265, Fig. 11.8)
This thin plastic slide demonstrates nerve cytology particularly well. The round blue structures are myelinated axons. The delicate connective tissue around each axon (nerve fiber) is the endoneurium. Bundles of axons (nerve fascicle) are surrounded by a layer of contractile cells collectively called the perineurium. A group of nerve fascicles form the anatomical nerve which is surrounded by dense connective tissue called the epineurium. After orienting yourself in terms of the axons and myelin sheath (the atlas may be helpful), select areas where the tissue is well stained and study it at high power (the axonal cytoplasm does not stain well with toluidine blue). Under oil immersion it is possible to identify mitochondria which appear as very fine rod-like bodies within the axon. The myelin sheaths are the most deeply-stained structures in the field and are often thrown into prominent folds. Identify the Schwann cells that produce the myelin sheath. In longitudinal section, identify nodes of Ranvier. These nodes are interuptions of the myelin sheath due to gaps between adjacent Schwann cells. The Schmidt-Lantermann clefts will appear as paired symmetrical oblique slits in the myelin. Often several clefts can be seen between successive nodes of Ranvier. Try to find nerves that have been longitudinally sectioned. In cross section, the myelin sheath appears as a thick boundary around pale-stained axon. Note the wide range of sizes of nerve fibers. Identify Schmidt-Lantermann clefts. Can you identify unmyelinated nerves? The endoneurium will contain blood capillaries necessary to supply nutrients to the Schwann cells. You should be able to differentiate a capillary from a myelinated axon. Remember a capillary has an endothelial cell, while a myelinated axon has a Schwann cell.
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SLIDE 37a peripheral nerve, longitudinal section (H&E)
(RR&K p293, Fig. 1,2,3,4)
The cytoplasm of the axon is eosinophilic while the myelin sheath is unstained. Why did the myelin sheath stain basophilic in SLIDE 37b, but remains unstained here? Identify the nodes of Ranvier. The numerous nuclei present are either Schwann cells or fibroblasts of the endoneurium. The perineurium contains one or two layers of eosinophilic cells. These contractile cells stain red because of the actin present. Outside the perineurium is the epineurium with multiple layers of fibroblasts. The artificial separation of perineurium from epineurium on this slide is due to slide preparation.
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SLIDE 37c peripheral nerve, cross section (H&E)
(RR&K p293, Fig. 1,2,3,4)
This slide contains many nerves in cross section. Note the different sizes of myelinated nerve fibers within a single nerve. Each nerve fiber contains a Schwann cell (with or without a myelin sheath) around an axon. The pale regions around many of the eosinophilic axons are myelin sheaths. The nuclei found juxtaposed to the sheath are Schwann cells. The eosinophilic staining between nerve fibers is due to the cytoplasm of the Schwann cells and cells of the endoneurium. In addition to fibroblasts , the endoneurium also contains blood capillaries necessary to supply nutrients to the Schwann cells. Do not confuse a capillary with a myelinated axon. Observe the important relationships of the nerve fibers to the endoneurium, perineurium and epineurium. Compare the staining characteristics of the perineurial cells with smooth muscle cells and epineurial cells (fibroblasts) on this slide.
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SLIDE 42c skin, human fingertip (H&E)
(RR&K p401, Fig. 1,2,3)
This slide represents a section of the skin on the tip of the finger. The two sensory nerve endings can be easily identified in the routine examination of this H&E stained section. The first type of nerve ending is the Pacinian corpuscle, groups of which can be identified under low power well below the epidermis. They are large structures which, at first glance, have the appearance of a sliced onion. The second type of nerve endings is the Meissner's corpuscle, which is found just beneath the epidermis in regions called dermal papillae. You will need to use the 40X objective lens to identify them. This slide also shows many small peripheral nerves. Identify these nerves, determine if they are myelinated, and locate the epineurium, perineurium, and endoneurium.
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The Peripheral Nervous System (PNS) is vital in allowing us to recieve signals and also to respond to them. This unit is an overview of the basic functional units of the PNS.