Download 5 Nervous Tissue Lab 2011

Molecules and Cells
2011
Lab 5: Nervous System
I. Peripheral Nerves and the Myelin Sheath
A. Introduction
In this laboratory we will examine nerves, neuronal cell bodies, and ganglia. Let us start
with a few important definitions:
Nerve fiber = multicellular, containing both an axon and surrounding myelin sheath. The axon
comes from a single neuron, but the myelin sheath is made by a train of many myelinating
Schwann cells. In the case of unmyelinated axons, the unmyelinated fiber shares each Schwann
cell with several other unmyelinated axons.
Nerve = a bundle or bundles of nerve fibers.
Ganglion = clusters of neuron cell bodies in the peripheral and autonomic nervous systems, as
well as associated glial cells and axons. Therefore, ganglia can be distinguished from peripheral
nerves by the presence of neuron cell bodies.
Axons are neuronal processes specialized for electrical impulse conduction. EMs show a
cytoskeleton rich in microtubules (neurotubules) and intermediate filaments (neurofilaments).
The organelles associated with protein synthesis are rare in axons, but abundant in neuronal cell
bodies, where the membrane channels, ion pumps and synaptic machinery needed in axons and
dendrites are synthesized. Axons may be long (spinal cord to foot) or short (many interneurons of
CNS). Myelin is formed by Schwann cells in the PNS and produces segmental insulation
interrupted between successive Schwann cells at the nodes of Ranvier, where ion channels and
ion pumps are localized and support saltatory impulse conduction.
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2011
B. Slide Descriptions
Webslides 0020_O and 0021_O: Sciatic nerve, monkey, cross-section and longitudinal
section, TB-AF
The sciatic nerve is a mixed nerve, containing sensory axons from neuron cell bodies in
dorsal root ganglia and motor axons from neurons in spinal cord gray matter. Like all larger
peripheral nerves bundles, it is also mixed in the sense of containing both somatic and
autonomic nerve fibers.
Scan the slide at low power to see the fascicular organization, identifying epineurium
(dense connective tissue surrounding the nerve and filling in between nerve bundles or fascicles),
perineurium (thin layers of flattened cells immediately surrounding and defining each fascicle),
and endoneurium (fine connective tissue within each fascicle between nerve fibers--see your
text for orientation). Around smaller fascicles, the epineurium becomes very thin, leaving the
bundle surface visible as the circumferential wrapping of perineurial cells. Note that the
perineurium gives nerve bundles sharply defined boundaries that makes them easy to
distinguish from vessels, ducts, CT, and muscle fibers. The nuclei visible within each nerve
bundle include crescent-oval Schwann cell nuclei snugly tangent to nerve fibers, and smaller,
denser nuclei belonging to endoneurial fibroblasts or the endothelium of capillaries running
through the endoneurium, usually parallel to nerve fibers.
At higher magnification you can see the unstained rings representing individual myelin
sheaths surrounding axons. Unmyelinated nerve fibers (which typically occupy at least 5% of
a nerve's cross-section) can be visualized as axons without surrounding myelin sheaths.
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Webslide 0014_O: Peripheral nerve, mammal, cross and longitudinal sections
Here you see a nerve lying parallel to 1-2 medium-small blood vessels, cut from a block
that presents both l.s. and c.s. views. Here myelin remains only as a pink network of residual
protein. Although the endoneurium is very hard to spot, the epineurial CT and the perineurial
wrapping are easily seen. Note in c.s., the axons in the center of many nerve fibers.
Unmyelinated fibers are impossible to distinguish here, but you should be able to observe the
nuclei of Schwann cells, endothelial cells, and endoneurial fibroblasts.
Webslide 0012_O: Monkey ear
Locate the peripheral nerve in association with other tissues in the bottom right quadrant
and the upper right quadrant of the slide. What criteria are useful in identifying a peripheral
nerve? Look for both myelinated and non-myelinated fibers. The nervous system, like the
vascular system, is very pervasive. You will see it a lot in future slides.
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2011
II. Basic Cell Types of the Central Nervous System and Autonomic
Nervous system
A. Introduction
In these slides, cell types can be best distinguished in specially stained slides, where a
small number of cells and their processes are densely stained. However, the "empty spaces" in
this material are deceptive--they contain unstained cell processes whose cell-cell relationships are
poorly illustrated. Weblide #19 of this lab, on the other hand, contains practically no "empty
space." It enables you to see the region of synaptic contacts (neuropil), but the appreciation of
basic cellular shape and orientation is considerably more difficult. It is necessary to combine
information from both kinds of preparations to get a feeling for the cellular configurations.
B. Slide Descriptions
Webslide 0302_O (cat): Spinal cord
This section contains a cross section of the spinal cord. The tiny central canal recalls the
origin of the nervous system as an infolding which closes off to form the neural tube. Locate the
central canal in the middle of your section and examine the lining layer of ependymal cells.
Compare the ependymal nuclei with the neuronal nuclei you observed in previous slides.
Ependymal cells also line the ventricles of the brain that are continuous with the central canal of
the spinal cord.
During development, newly formed neurons proliferate adjacent to the canal to form a
mantle layer, which becomes the gray matter in the central region of the mature spinal cord.
This gray matter occupies a butterfly-shaped cross-section in this mature spinal cord. In the gray
matter, examine the large motor neuron cell bodies carefully. Depending on how the cells are
cut, you may see the nucleus, nucleolus, and axon hillock, as well as the Nissl bodies (rough
ER) filling the cytoplasm. Note also the smaller nuclei of neuroglial cells. Neuropil refers to
the regions in gray matter that lie between cell bodies, devoid of nuclei but complexly crowded
with neuronal cell processes and synapses.
The neurons of the mantle layer produce processes that grow outward to form the fiber
tracts of white matter, which carry parallel myelinated axons for long-range interneuronal
contacts in CNS. The white matter fills most of this cross-section external to the central
butterfly-shaped region of gray matter. This marginal zone is white primarily because of the
abundance of myelinated axons, which you can observe in your sections.
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Webslide 0019A_O: Spinal cord, mammalian, silver stain
Quickly repeat the observation of the spinal cord as described above, noting the
differences that can be readily observed due to the silver staining of the neuropil. Also note the
dorsal root ganglia on either side of the spinal cord that contain large neuronal cell bodies similar
to those that you will observe in the next slide.
Webslide 0018_O: Gasserian Ganglion, mammal, TB-AF
The Gasserian ganglion (the sensory ganglion of the fifth cranial nerve) is a collection of
pseudo-unipolar cells (pseudo because the single cell process arising from the soma splits into
two functionally polarized processes). In response to tactile stimuli on the face a nerve impulse
travels along the axon, passes through the ganglion containing the cell bodies, and finally reaches
a synapse in the sensory nucleus of cranial nerve V in the brain stem. This is the first neuron
(primary neuron) in a pathway whose activity will eventually result in your awareness of being
touched.
In this slide, you will see the large round cell bodies of these primary neurons, also called
ganglion cells. The nuclei are large and stain faintly. The single nucleolus, if it happens to be in
the plane of section, stains darkly, giving the nucleus a bull's eye appearance typical of many
neurons. Around each neuronal soma are smaller flattened cells, whose darkly stained nuclei
form a ring around the ganglion cells. These are specialized astroglial cells called satellite cells
(not to be confused with satellite cells of skeletal muscle).
In the same plane of section you will also see collections of light blue disks surrounded
by unstained structures that look like white donuts. These are the axons and myelin sheaths of
the ganglion cells passing through the plane of section. Look among the myelinated fibers for the
wedge-shaped, densely staining nuclei of the Schwann cells, whose membranes form the myelin
sheaths.
Among the cells and fibers in the Gasserian ganglion you can observe capillaries, many
still containing easily identified red blood cells. Note the connective tissue coverings of the
ganglion.
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Webslide 0032_O: Ileum, monkey, H & E
In this section of the ileum, you will see autonomic ganglia where axons (whose cell
bodies are in the spinal cord) synapse with intrinsic neurons (whose cell bodies are located here
in the GI tract). These ganglia (called the Auerbach's or myenteric plexus) are found all along the
GI tract between the circular and longitudinal layers of the outer smooth muscle. Like other parts
of the peripheral nervous system, these ganglia are covered by a thin connective tissue layer,
essentially a perineurium. Like CNS, but unlike other (non-enteric) autonomic ganglia, these
enteric ganglia exclude connective tissue, and contain only neurons and glial (supporting) cells.
Look for the typical neuronal bull's eye nuclei. Neurons are smaller and glial supporting cells
fewer in autonomic ganglia than in sensory ganglia, so they rarely appear as obvious rings of
satellite cells like those seen in Webslide #18. A good example of a neuron in an Auerbach’s
plexus can be found using overlay D. (as a review, other identifications: A- simple columnar
epithelium with microvilli, B- mast cell, and C- simple squamous epithelium)
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