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Nervous System

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Slide 1: A pseudounipolar neuron has single fiber which leaves the cell body and then branches to form a central (dendrite) and a peripheral (axon) fiber. The white spaces around the perikarya are shrinkage artifacts. Dorsal root ganglion - metal impregnation.  

Slide 2: Bipolar neurons. A bipolar cell of inner nuclear layer of retina of a goldfish has been demonstrated by Procion yellow injection. You will understand limits of the short bipolar neuron better after we study the Eye and Ear.  

Slide 3: Multipolar neurons- sympathetic ganglion. Note the very incomplete appearing satellite layer. Groups of fibers intermingle and do not form the neat bundles of processes seen in the DRG. Note the large number of dendritic processes and brown "age" pigment (globules in the cytoplasm). This is "wear and tear" pigment, a build-up of residual bodies during aging, especially in nervous tissue and muscle which are generally long-lived non-dividing cells. Recall that lipofuscin granules are derived from secondary lysosomes. Human - Bodian stain.  

Slide 4: Meissner's corpuscle (not to be confused with Meissner's plexus in the digestive tract). These are found in the connective tissue papillae of the dermis of skin. This is a touch receptor. It has a sensory nerve ending which is wound around a CT scaffold and surrounded by a connective tissue capsule. It is found very close to the basal surface of the skin epithelium. The free surface of the epithelium (epidermis) is at the top of the field. The nuclei [visible] are those of fibroblasts of the CT scaffold and capsule. The nerve process is not seen in this preparation. H & E.  

Slide 5: Meissner's corpuscle. Skin. Silver impregnation. Here the nerve ending is seen because of the Ag staining but the capsule cannot be seen. Note that the free surface of the skin is at the top of the field. The copper brown staining of the epidermis and the tan staining of the dermal CT are non-specific.  

Slide 6: Two Pacinian corpuscles, low power. In the center of each is an axon (not seen without special staining); the capsule is formed by concentric layers of connective tissue cells and fibers. The appearance is that of a sliced onion. It is a deep pressure receptor. Pancreas, H & E.  

Slide 7: Muscle spindle in cross section. This is a stretch receptor as in the knee jerk response. There are two types of inrafusal fibers found within a connective tissue capsule: nuclear bag and nuclear chain fibers. Both of them have afferent nerve fibers ending on them which are fired when the muscle is stretched. Basically, the action potential is carried along the afferent fiber to the spinal cord, and in the simplest situation it makes a synaptic connection with an efferent fiber in the motor horn. These efferent or motor fibers end on the extrafusal (normal striated muscle cells) fibers outside the connective tissue capsule causing them to contract. In real life, however, the situation is generally much more complicated than this with motor fibers going to some of the intrafusal fibers to fine-tune the system, and occasional association neurons being involved within the spinal cord and perhaps even the brain. For more information, read the section in the text. Locate the intrafusal and extrafusal fibers, connective tissue capsule, and myelinated nerve. H & E.  

Slide 8: Muscle spindle - x.s. Low power TEM. Note intrafusal fibers, and and part of one extrafusal fiber (to the bottom right). Also locate the myelinated nerve fibers within and outside of the capsule. These latter especially may be either motor or sensory.  

Slide 9: Olfactory mucosa, H & E. Bipolar neurons are found within the pseudostratified columnar epithelium with their nuclei in the center of the epithelium and not really delineated from the other nuclei in this preparation. Note the bundles of unmyelinated axons and dilated blood vessels in the connective tissue as well as Bowman's glands.  

Slide 10: Sensory endings on tendon. Note the skeletal muscle cells. This is another "stretch" receptor, which is involved with position of joints in space (proprioception).  

Slide 11: The neuromuscular junction is the area of close apposition between a nerve ending (motor end plate) and a skeletal muscle cell where the nerve exerts its control over contraction by the muscle cell. Here each of the examples shown forms several motor end plates (at least 12 can be seen). This is a very thick preparation of whole muscle cells teased apart and stained for the neural elements. Muscle cell striations are apparent in only a few areas.  

Slide 12: Nerve bundles, cross and obliquely sectioned with epineurium and perineurium visible. The perineurium surrounds each nerve bundle (fascicle) and the epineurium forms the dense connective tissue that forms the external coat of nerves and holds the fasicles together. This image shows the typical wavy appearance of the fibers (there is a retraction/contraction of the nerve when it is cut) as well as the connective tissue ensheathment. H & E.  

Slide 13: Nerve bundles - x.s., Mallory's trichrome stain (collagen - blue; fibroblasts - red; myelin - yellow sometimes; axons - red). Note the epineurium, perineurium, and endoneurium. Endoneurium is a delicate layer of connective tissue which surrounds each nerve fiber (it is visible here because the collagen is stained blue). Perineurium surrounds each nerve bundle(fascicle), and epineurium surrounds a collection of nerve bundles. Locate axons (axis cylinders) and myelin ghosts. Besides fibroblasts and other connective tissue cells, to what cell types would nuclei within this nerve bundle belong? ANS: endothelial cells and Schwann cells. The Schwann cell is the ensheathing cell of the PNS.  

Slide 14: A small nerve bundle - Mallory's trichrome stain. Locate the blue staining wisps of collagen in the endoneurium and neurokeratin (the non-extracted protein and lipid of the myelin ie. an artifact of L.M. preparation techniques). It usually appears as red streaks extending from the axon to the edge of the myelin "ghost".  

Slide 15: Nerve bundles, H & E. Locate the epineurium holding several nerve fasicles together and the perineurium surrounding each fasicle.  

Slide 16: Three unmyelinated nerve bundles in the olfactory mucosa. H&E. Note the absence of myelin. Do the nuclei within this bundle belong to the same cell types as in myelinated nerve bundle? ANSWER: Yes. Even non-myelinated fibers are ensheathed by Schwann cells (see textbook). The dark staining structures are glands of the olfactory mucosa and if you are reviewing this at the end of the course find and identify the three blood vessels in the field. (vein, large venule, small venule).  

Slide 17: TEM xs of a small unmyelinated nerve. The nerve fibers are enveloped in the cytoplasm (here colored yellow - no we can't take color EM's yet but Dr. Robbins' trusty highlighter did the job) of a Schwann cell (whose nucleus is not in this section). Note the external lamina and endoneurial collagen fibers as well as a fibroblast process. Within the nerve cell processes are abundant microtubules, some mitochondria. Several of the processes also contain transmitter vesicles.  

Slide 18: Two mixed nerve bundles and adipose tissue. Plastic-embedded section. Note the perineurium. Which are the myelinated fibers? (Hint: they are large and there are three clearly identifiable ones).  

Slide 19: Obliquely sectioned myelinated nerves near striated muscle cells (intercostal nerve and muscle). Rat. Plastic-embedded. Toluidine blue stain.  

Slide 20: Longitudinal section of nerve. As usual the myelin is partially extracted. Locate neurokeratin, axis cylinders, nodes of Ranvier. The collagen of the perineurium is blue. Why? (Ans: Mallory's trichrome stain).  

Slide 21: A very low power image showing the C.T. covering. Are the nuclei central? No - look carefully. Satellite cells (supporting cells) lie around each perikaryon and there is connective tissue in the background  

Slide 22: Sympathetic ganglion. Note position of nuclei in cells, lipofuscin granules, nerve fibers. Metal impregnation. Silver stain.  

Slide 23: Two small parasympathetic ganglia in connective tissue, very thin capsule.  

Slide 24: Ciliary ganglion. Multipolar neurons - note the large number of processes and eccentric nuclei (made more apparent by shrinkage artifact). Note satellite cells. Doesn't appear as organized as in DRG. Compare to DRG. Cajal stain.  

Slide 25: Myenteric plexus found between the two muscle layers of the muscularis externa layer of the digestive tract. Note the eccentric nuclei and position between the smooth muscle layers. Plastic-embedded section.  

Slide 26: Isolated neurons (a really small ganglion!) in tongue. Why is it not sympathetic? What kind of muscle is this? (Ans: Parasympathetic because it is in an end organ - you can tell this because of the skeletal muscle in field. Note: There is also a small nerve bundle in cross-section (myelinated? mixed?) in this field.  

Slide 27: A very small nerve bundle near skeletal muscle cells that are obliquely sectioned (note striations). 0.5 μm plastic-embedded section, toluidine blue stain.  

Slide 28: This low power image shows the meninges, spinal cord in cross section and both dorsal roots and their ganglia. In the spinal cord note the grey and white matter, central canal, meninges, ventral median fissure, dorsal sulcus, ventral roots. Note the continuity of the dura matter with perineurium of the spinal nerve. Cracks in the white matter on the left are artifact. Monkey. Trichrome stain.  

Slide 29: Higher mag of one of the dorsal root ganglia. In the DRG note organization into groups of perikarya and bundles of fibers alternating with each other. Nuclei are light staining and centrally located, These neurons are pseudounipolar. Note the large round nuclei of satellite cells surrounding ganglion perikarya. Occasional capsule cells (fibroblasts) just outside the satellite cell layer. White spaces are mostly extracted myelin. Find some nodes of Ranvier.  

Slide 30: Low power of the lumbar spinal cord. The ventral side is to the bottom. White matter is peripheral and the H-shaped gray matter is central (only half the H is seen). Look for the central canal: what is its lining called? (Ans: the ependyma.) Note deep ventral median fissure and identify the dorsal surface. Where would the lower motor neuron perikarya be located? (Ans: in the ventral horn grey matter). Myelin stain.  

Slide 31: Spinal cord. Mallory stain. Meninges (in right upper corner), white matter and grey matter. The strands of collagen fibers from the pia into the white matter accompany thin walled (collapsed) blood vessels. Note the extracted myelin "doughnuts" around axon cross sections in the white matter and the different sized multipolar perikarya of the grey matter. Red blobs are stain artifact. What is the neuropil composed of? ANSWER: Cell processes, glia and blood vessels.  

Slide 32: Anterior (ventral) horn perikarya. Gray matter. Nissl stain. Pick out dendrites, and an axon hillock of a neuron. How can you tell the neuroglial cells apart? (Ans: Astrocytes have large pale nuclei; oligodendrocytes - smaller darker nuclei; microglia - smallest darkest nuclei (these are difficult to detect in normal tissue). What are Nissl bodies? Ans: Large stacks of RER with many free polysomes between the cisternae.  

Slide 33: Two neuronal perikarya of the spinal cord are surrounded by neuropil. The stain (Kluver-Barrera for the purists) contains cresyl violet which stains the Nissl bodies and nucleoli and Luxol fast blue which stains myelin. Thus the extent and complexity of the meshwork of nerve cell processes can be appreciated. All that is seen of the glial cells is their nuclei.  

Slide 34: Gray matter. Neuropil. EM. Thinly myelinated fibers in anterior horn. Would axons contain rough ER? (Ans: No).  

Slide 35: Astrocytes (fibrous) of white matter, metal impregnation. Hard to tell this is white matter although if you look carefully you can seen some myelin ghosts around fibers. Star-like shape of astrocytes. Note perivascular feet ending on blood vessels. What functions may they have? Monkey - Cajal.  

Slide 36: Microglia - metal impregnation. Few processes. What is origin and function of these cells? ANS: These are the only neuroglial cells that are not derived from ectoderm. They are mesodermal in origin just as any other macrophages and presumably get into the nervous system from the blood.  

Slide 37: Oligodendrocyte - white matter. Silver impregnation to show processes. How does this cell form myelin? How many axons can it myelinate? ANS: Processes wrap around several fibers. Note difference with PNS, where Schwann cell can wrap around several unmyelinated fibers but only one myelinated fiber (jelly roll theory of myelination is OK for a Schwann cell, not for an oligo).  

Slide 38: An oligodendrocyte (perineuronal) is seen towards the top of the image. The large cell with the processes is a neuron. Note its pale nucleus and prominent nucleolus. The little cell (mostly the nucleus) pressed up against it is an oligodendrocyte. Of course, oligos also send out processes which surround fibers. Note the myelinated fibers in the neuropil. Brain stem - paraphenylene diamine stain, mouse. Plastic-embedded, 0.5 μm section.  

Slide 39: Ependymal cells line the ventricles and central canal. Spinal cord. H&E. Cells are ciliated but it's difficult to see here.  

Slide 40: The choroid plexus is found in the lateral, 3rd and 4th ventricles. Thin roofs formed by ependymal cells are invaded by blood vessels which then protrude into the ventricles. Cerebrospinal fluid (CSF) is secreted here. Review the circulation of CSF in the text from choroid plexus to arachnoid villi. H&E.  

Slide 41: Higher power Note the blood vessels and the cuboidal epithelium.  

Slide 42: Cerebrum, very low power. Human, silver impregnation. Note the gyri, sulci, grey and white matter, pia and arachnoid. The white matter appears dark here because its bundles of cell processes are darkly impregnated.  

Slide 43: A higher power of part of the cerebrum. The long cell processes perpendicular to the surface of the organ as well as pia matter arachnoid and blood vessels (mostly delimited by shrinkage spaces around them) are clearly seen.  

Slide 44: Cerebral cortex, H&E . Neurons are arranged in vertical columns (usually similar functions, e.g., motor areas, sensory areas, etc.) and horizontal layers (usually different types of neurons). There are six layers of gray matter peripherally; (DON'T BOTHER LEARNING THEM NOW); the white matter of the cortex is central to this. What is the major motor neuron in the cerebral cortex?

ANS: Pyramidal cell. Note: the surface of the cerebrum (not seen) is to the left and the white matter to the right.  

Slide 45: Cerebrum. Note the shape of the pyramidal cells and their long processes. The pia (not visible) would be to the left of the field.  

Slide 46: A different preparation showing the impregnation of only a few pyramidal cells with their processes. Are the very long processes axons? (No, these are the apical dendrites). Do these long processes travel towards the cortical surface? Where are the axons? Which direction do axons travel?  

Slide 47: Pyramidal neuron. TEM. Where is its axon hillock? (Ans: not in this section). Note that the cell body cell has the usual organelles. Where in the field is the neuropil? (Ans- all around the perikaryon). Of what does it consist? (Ans: neuronal processes, glial cells and blood vessels).  

Slide 48: Note the 2 large folds (lobules) and smaller folds within lobules (folia). Identify the pia matter (closely invests brain).
Question: Between which meningeal layers is CSF found?
Ans: Pia and arachnoid (subarachnoid space).

Slide 49: Cerebellum, metal impregnation. Note how densely the white matter stains.

What is staining?
ANSWER: Neurofibrils (collections of neurofilaments within fibers).

Which cells are in the different layers?
ANSWER: (Outer) molecular layer: basket & stellate cells.
Intermediate: Purkinje cells (white areas in this layer are shrinkage artifact).
Granular layer: granule and Golgi Type II cells.

Slide 50: Cerebellum, H&E. Note the density of cells in the granule cell layer, the pia, the paucity of cells in the molecular layer, as well as the Purkinje perikarya. Only a small amount of white matter is seen in this field.  

Slide 51: Cerebellum, metal impregnation. The ramifying dendritic trees of the Purkinje cells are clearly shown although this is not from a human brain.  

Slide 52: Cerebellum. Find the small axon from this Purkinje cell (on the opposite pole from dendrites, i.e. towards the right).  

Slide 53: Review layers of cerebellum. All four are seen here (there is some white matter in the corner of the slide). Plastic-embedded,1.5μm section, mouse, toluidine blue stain.  

Slide 54: Locating degenerating neuron cell processes is a way of determining where they go. Note beaded appearance of the dark degenerating fiber. Spinal cord. Metal impregnation. What change would you expect to see in the cell bodies of these neurons on a TEM level? (Ans: eventually death).  

Slide 55: Tracts in white matter of spinal cord. Long black fibers are normal but beaded fibers are degenerating. Metal impregnation.

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