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Prelab Exercise 3 – NERVE
Prelab Exercise 3
NERVE TISSUE
Nerve tissue, particularly that comprising the central nervous system (CNS, i.e., brain
and spinal cord) is amazingly complex in organization as well as function. Therefore,
only a few selected regions will be examined in this lab (they will be investigated in more
detail later, in the neuroscience course). In this laboratory session you will also have
time to study examples of nerve tissue from the peripheral nervous system (PNS).
Next to the blood vascular system, nerve tissue is the most widespread in its distribution
throughout the body and is the most highly specialized for electrical excitability,
conductivity and synaptic transmission. The cell of major interest in this tissue is the
neuron (or nerve cell), though it is not the most numerous component of the tissue.
Supporting cells, neuroglia, outnumber it at least ten to one in the CNS and many times
that along peripheral nerve fibers. The neuron consists of a cell body (soma or
perikaryon) and all of its processes (one axon and usually many dendrites). It is the most
important structural and functional unit of the nervous system.
STAINS
Various types of stains emphasize different neuronal structures. The most commonly
used stains for neural tissue are Nissl stains: hematoxylin, thionin, cresyl violet,
methylene blue, gallocyanin. These basic dyes react preferentially with acidic
components in tissue sections such as DNA, RNA, chromatin, ribonucleoprotein and
phospholipids. In neural tissue this results in staining of chromatin in the cell nuclei, the
nucleolus and the Nissl bodies (ribosomes). Processes of both nerve cells (axons and
dendrites) and glial cells are almost totally unstained by these Nissl stains. In the CNS,
there is a light-staining meshwork of these neural processes along with the processes of
glial cells. Together this is called the neuropil.
The staining of neuronal processes may be accomplished by the staining of
neurofibrillar components through silver or other metallic stains, or by
immunohistochemical means. Axonal processes (particularly myelinated ones) may be
visualized by staining of myelin by, for instance, sudan or osmic acid. The Bodian
method stains neuronal processes, however it may not be optimal for observing the
configuration of cell processes because it stains most of the nerve cell processes in a
section, making it difficult to see the tree for the forest (or to follow the processes of one
neuron). There are, however, certain modified metallic stains impregnate only 1-2% of
the cells in a section along with their processes, thus allowing visualization of the
configuration of nerve cell processes of a single neuorn. These are the Golgi stains
which impregnate tissue by reduced metallic salts such as silver, mercury, chromium, or
tungsten. These methods are notoriously capricious and time-consuming and their mode
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Prelab Exercise 3 – NERVE
of action is still not understood. They also result in some random deposition of salts,
which can complicate visualization of the stained neurons and their processes.
CENTRAL NERVOUS SYSTEM
Spinal Cord
The spinal cord is roughly cylindrical in shape in cross section and is composed of an
inner butterfly or H-shaped core of gray matter surrounded by an outer cortex of white
matter. In general, gray matter consists mainly of nerve cell bodies and the proximal
portions of their processes as well as neuroglia. The white matter consists mainly of
myelinated nerve fibers also with supporting neuroglia.
Transverse section of spinal cord at mid-lumbar level
(Weigert stain for myelin)
Gray matter. Note the anterior (ventral) and posterior (dorsal) horns of gray matter,
united by a thin gray commissure containing the small ependymal-lined central canal.
Observe the large motor neurons (nerve cells) in the anterior horn. Many of these cells
contain darkly stained neurofibrils. The neurons are also partially outlined by darkly
stained nerve cell processes. Aside from the neurons and their processes the remainder of
the gray matter substance consists of supporting neuroglia cells and numerous small
blood vessels (both poorly stained). Keep in mind that the amount of gray matter at any
level of the spinal cord is related to the amount of the body that is innervated at that level.
Thus, the amount is greatest within the cervical and lumbosacral enlargements of the
cord, which innervate the upper and lower limbs respectively.
White matter. Note the separation of spinal cord white matter into posterior (dorsal),
lateral and anterior (ventral) columns. As in other regions of the CNS, the white matter
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Prelab Exercise 3 – NERVE
consists here of a mixture of nerve fibers (myelinated and unmyelinated), neuroglial cells,
and blood vessels. Nerve cell bodies are usually absent from white matter. Many of the
nerve fibers (axons) are myelinated which accounts for the white color of white matter (in
fresh preparations) and the intense dark staining in stains that stain myelin or the axons
themselves.
Cerebral cortex
As opposed to the gray matter of the spinal cord that is on the inside, The gray matter of
the cerebral and cerebellar cortex is on the surface. The purpose of examining the
following slides is to introduce you to the highly complicated organization and
microscopic structure of the higher brain centers. You are not required to learn these
features in detail at this time, though you should be able to distinguish cerebrum from
cerebellum and be able to distinguish the 3 main layers of the cerebellar cortex.
Additional detail will come later in your neuroscience course.
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Prelab Exercise 3 – NERVE
The pia mater and the next meningeal layer, the arachnoid have the same embryonic
derivation and provide the boundaries of the subarachnoid space (which is normally filled with
cerebrospinal fluid). These are often considered together as the pia-arachnoid
(leptomeningies). [See diagram above]. These two meningeal layers are joined by cobweb like
trabecular processes. (Hence, name arachnoid, spider). CSF circulates in the spaces between
the trabeculae (subarachnoid space). The inner surface of the pia forms a firm interface with a
special layer of brain glial cells to form a pial-glial (external limiting) membrane.
The large blood vessels of the brain (arteries and veins) are located between the pia and
arachnoid, lying within the subarachnoid spaces. The pia and arachnoid themselves, despite
being intimately related with the blood vessels, are avasular. As blood vessels penetrate the
brain they carry with them pia-glia tissues, which form a cuff around the vessel. At the capillary
level, only processes of glial cells (astrocytes and oligodendroglia) surround the basement
membrane of the capillary endothelium. The capillaries of the CNS are distinctive structurally
and functionally. Review the concept of the blood-brain barrier.
Now study the gray matter, first with the 4x, then with 10x and 40x objectives. You
will see many basophilic nuclei and occasional capillaries in a field of granular
eosinophilic material. Although it cannot be appreciated in this preparation the
eosinophilic material consists of an amazing network of the many processes of nerve and
glial cells. It is sometimes called the neuropil. Review the ultrastructure of neural tissue
in the “EM of Neural Tissue” module on the virtual histology site. The largest nuclei,
usually with a prominent nucleolus belong to nerve cells (neurons). Except for the
largest neurons, giant pyramidal cells, not much of the surrounding cytoplasm
(perikaryon, cell bodies, soma) will be seen. The more numerous smaller nuclei mostly
belong to glial and capillary endothelial cells. (Recall that glial cells outnumber nerve
cells by about 10 to 1 in the CNS).
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Prelab Exercise 3 – NERVE
The neurons of the mammalian cerebral cortex are not randomly distributed. Although it
may not be readily apparent to you from looking at this section, it has been shown that
they are arranged in six layers or laminae parallel to the brain surface (see the figure
below). It is not necessary at this time that you know the names and features of these
layers. Do appreciate, however, that the stratification you see in the sections does indeed
correspond to different population of cells and their processes.
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Prelab Exercise 3 – NERVE
ORGANIZATION OF THE HUMAN CEREBRAL CORTEX (NEOCORTEX)
The centrally located white matter contains no neuronal cell bodies. It consists mostly
of myelinated nerve fibers (going to or from the gray matter neurons), supportive glial
cells (astrocytes and oligodendrocytes), and blood vessels.
Cerebellar Cortex
The cerebellum (“little brain”) lies on the brain stem, largely hidden by the massive
cerebral hemispheres. It is mainly concerned with the integration and coordination of
complex motor activities and procedural learning (such as learning to ride a bicycle).
Like the cerebral cortex, the cerebellar cortex has a complex organization related to its
functional activities.
The gray matter can divided into three identifiable layers (see diagrams below). The
superficial, outermost layer is the molecular layer. It contains mainly dendrites and
axons from cells in the deeper layers but only a few nerve cell bodies. The middle layer
is dominated by the cell bodies of peculiar giant nerve cells, Purkinje cells, arranged in a
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Prelab Exercise 3 – NERVE
single layer of cells and is called, logically enough, the Purkinje cell layer. The deepest
layer, next to the white matter, is called the granular layer because it is packed with the
cell bodies of tiny neurons (granule cells), the most numerous cells in the nervous
system. The Purkinje cells are the most interesting cells. Not only are they huge
(estimate the diameter of the cell body) but they have fantastic candelabra like dendritic
trees that ramify in single plane in the molecular layer. These processes can not be seen
by this staining method, indicating why the study of neural tissues require sophisticated
methods that focus on the items that you need to visualize.
Ultrastructure of the CNS
Examine the ultrastructure of the CNS in figs. 1-8 of the “EM of neural tissue” module
of the virtual histology site.
PERIPHERAL NERVOUS SYSTEM
The peripheral nervous system is composed of all the neural tissue outside the brain and
spinal cord. It serves as the mediator of neural impulses traveling in both directions
between the CNS and body tissues. The entire system consists of ganglia, peripheral
nerves and their terminations. A peripheral nerve is defined as a group of nerve cell
processes, their supporting elements (neuroglia), and their terminations, all of which are
bound together by connective tissue and are located outside the CNS. A ganglion
consists of a collection of nerve cell bodies and their associated neuroglial tissue that is
located outside the CNS.
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Prelab Exercise 3 – NERVE
There are two types of motor (efferent) axons in peripheral nerve. Somatic motor axons
(to skeletal muscle) end as motor end plates. Visceral (autonomic) motor axons end as
terminals on myoepithelial cells, gland cells, or smooth muscle cells. These visceral
motor axons often form plexi within or near organs. Afferent (sensory) nerve fibers
have sense organs at their distal ends that transduce stimuli. These sense organs may
consist of specialized receptor cells or may be specializations of the nerve fiber itself.
This transduction converts various forms of energy into action potentials. The forms of
energy converted by receptors include: mechanical (touch and pressure); thermal
(degrees of warmth); electromagnetic (light); and chemical energy (odor, taste and
CO2 and O2 content of blood). The receptors in each of the sense organs are adapted to
respond to one particular form of energy at a much lower threshold than other receptors
respond to this form of energy.
Representation of a somatic nerve and a reflex arc. (note connective tissue
investments)
NERVE CELLS (NEURONS)
Multipolar neurons – Most neurons are multipolar. These cells typically have a coarsely
granular cytoplasm (due to Nissl substance), and a centrally-placed, pale staining
vesicular nucleus with a prominent nucleolus. Under the electron microscope, Nissl
substance corresponds with areas of rough endoplasmic reticulum and polyribosomes.
The Nissl substance of these cells extends into the base of the dendritic processes. Nissl
substance is lacking in the region of the cells that gives origin to the single axon process,
a region known as the axon hillock. Generally, the larger the neuron, the larger the
processes (and longer the axon).
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Prelab Exercise 3 – NERVE
Ganglion Cells - Ganglia in the peripheral nerve tissue contain neurons of two types.
Sensory ganglia have pseudounipolar neurons, while autonomic ganglia have multipolar
neurons.
Dorsal (sensory) root ganglion cells are pseudounipolar cells. They are globular in
shape and vary tremendously in size. The small cells have unmyelinated processes and
the large cells myelinated processes. A thin cellular capsule, comprised of satellite or
capsule cells, surrounds the cell bodies. These cells are continuous with the
neurolemmal sheaths of the cell processes. The stem process (primary neurite) bifurcates
in a T or Y fashion into two secondary processes: a peripheral process (“dendrite”)
which extends to some sensory receptor and a central process (“axon”) which enters the
spinal cord to synapse with neurons in the gray matter.
Autonomic Ganglia
The autonomic nervous system is a motor system that is involved in innervating smooth
muscle, heart muscle and glands. With the exceptions of the adrenal medulla, autonomic
pathways consist of a two-neuron chain (preganglionic and postganglionic neurons)
extending from the CNS to the peripheral structure. It is classified according to the cells
of origin in the CNS as the sympathetic (thoracolumbar) or parasympathetic
(craniosacral) division. Because it is a two-neuron pathway, there must be a synapse,
which occurs in a ganglion, which is also the location of the postganglionic cell body.
Sympathetic ganglia are located adjacent to the vertebrae (paravertebral ganglia;
sympathetic chain of ganglia) or along the aorta (prevertebral ganglia). These can be
seen in gross anatomy. Cells in these ganglia, which are surrounded by satellite cells,
send long postganglionic processes to the organ supplied. The small parasympathetic
ganglia are either very near or actually inside the organ supplied. These include the
ciliary, otic, sphenopalatine, or submaxillary ganglia in the head or ganglia in the wall of
organs such as the myenteric plexus (Auerbach) or submucosal plexus (Meissner) plexi
of the GI tract. Hence, parasympathetic neurons have short postganglionic processes.
PERIPHERAL NERVE TRUNKS
In looking at sections of mixed peripheral nerves remember that you are looking at the
peripheral processes of nerve cells (axons and dendrites), neuroglial supporting elements
(Schwann cells), connective tissue and associated small blood vessels.
Most of the fibers that you can see in tissues are myelinated. However, there are many
more unmyelinated fibers that can’t be seen without special staining. The different sizes
of axons reflect different speeds of conduction and different functions. Peripheral nervous
system myelin consists of the wrapping of a sequential chain of individual Schwann cells
around the axon, with a tiny gap (the node of Ranvier) in between. This is where the
axon is exposed to the interstitial environment and is a site of action potential generation.
Examine the structure of myelin on the “EM of neural tissue” section of the virtual
histology site.
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Prelab Exercise 3 – NERVE
CHECK LIST
Be able to identify the cellular components of neurons, including: (both light and
electron microscope level)
-cell body (perikaryon)
-axon hillock
-dendrites and axons
-myelin sheath
-nucleus and nucleolus
-Nissl bodies
-synapse
Be able to identify the four neuroglial cells of the CNS and the two neuroglial cells of
the PNS:
-microglia
-astrocytes
-Schwann
-ependymal
-oligodendrocytes
-satellite (capsule)
Identify these components of the CNS:
-white matter
-motor neurons
-meninges
-dura mater
-arachnoid
-pia mater
-cerebellar cortex
-molecular layer
-Purkinje cells
-granular layer
-gray matter
-spinal cord
-blood-brain barrier
-cerebral cortex
-giant pyramidal cells
Identify these components of the PNS:
-ganglion, both sympathetic and parasympathetic
-connective tissue sheaths of peripheral nerves
-epineurium -perineurium -endoneurium
-myelin sheath
-nodes of Ranvier and internodal segments
Review:
-neuromuscular junctions
-proprioceptive muscle spindles
-Nissl stains
-Silver stains
-Golgi stains
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