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Unit 5: Nerve Tissue
Neurons, fibers, fascicles, & ganglia
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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.
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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Learning the histological components of a sensory neuron
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
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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
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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
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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.
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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
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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
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Spinal Ganglion- Staining: Hematoxylin-Eosin-Methylene Blue
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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