NERVOUS TISSUE
The nervous system consists of all nervous tissue in the body. It is divided
anatomically into the
central nervous system and
the
peripheral nervous system.
Central Nervous System (CNS)
The CNS consists of the brain (encephalon), which is enclosed in the skull,
and the spinal cord, which is contained within the vertebral canal. Nervous
tissue of the CNS does not contain connective tissue other than that in the
three meninges (dura mater, arachnoid membrane and pia mater) and in the walls
of large blood vessels. Collagenous fibers or fibrocytes/blasts are
consequently not observed, which is quite unlike other tissues. Because of the
absence of connective tissue, fresh CNS tissue has a very soft, somewhat
jelly-like consistency. The two major classes of cells that make up the nervous
tissue are nerve cells,
neurones, and supporting
cells,
glia.
Neurons
The vast majority of neurones is generated before birth. Persisting stem
cells give rise to a
small number of new neurones
throughout the lifetime of mammals, including humans. The permanent addition of
neurones may be important for the maintenance and plasticity of some parts of
the CNS, but it is insufficient to replace neurones that die because of disease
or acute damage to the CNS. Neurones should last a lifetime. Mature neurones
are not mitotically active, i.e. they do not divide.
Neurones are generally large cells. Neural activity and its control require
the expression of many genes, which is reflected in the large and light nuclei
of most neurones. The keys to the understanding of the function of a neurone
lies in (1) the shape of the neurone and, in particular, its processes, (2) the
chemicals the neurone uses to communicate with other neurones (
neurotransmitters) and (3) the ways in which the
neurone may react to the neurotransmitters released by other neurones.
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The shape of the neurone and its processes
Neurones have long processes, which extend from the part of the cell body
around the nucleus, the perikaryon or soma. The processes can be divided into two
functionally and morphologically different groups, dendrites
and axons.
Dendrites are part of the receptive surface of
the neurone. As a rule, neurones have one to several primary
dendrites, which emerge from the perikaryon.
Primary dendrites may divide into secondary, tertiary
etc. dendrites. Dendrites can be smooth, or they can be studded with
small, mushroom-shaped appendages, which are called spines.
Each neurone has as a rule one axon,
and never more than one axon which emerges from the perikaryon or
close to the trunks of one of the primary dendrites. The point of origin of
the axon from the perikaryon is the axon hillock.
The axon may, like the dendrites, branch as it travels through the nervous
tissue to its destination(s). The axon is the "transmitting"
process of the neurone.
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The axon forms small, bulb-shaped swellings called boutons at the ends (terminal boutons) or along the
course (boutons en passant) of its branches. Synapses
are morphologically specialised contacts between a bouton formed by one
neurone, the presynaptic neurone, and the cell
surface of another neurone, the postsynaptic neurone.
Synaptic vesicles contain the neurotransmitters.
Synaptic vesicles typically accumulate close to the site of contact between
the bouton and the postsynaptic neurone. The release of the neurotransmitter
from the synaptic vesicles into the synaptic cleft,
i.e. the space between the bouton and the postsynaptic neurone, mediates the
transfer of information from the pre- to the postsynaptic neurone. Both the
release of the synaptic vesiscles and the mediation of the response to the
transmitter require membrane-associated specialisations - the pre- and postsynaptic densities.
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The shape and orientation of the dendritic tree of the neurone determines
the amount and type of information that may reach the neurone. The course of
its axon determines to which neurones this information may be passed on. The
location of the neurone within the CNS determines to which major system the
neurone belongs.
There are several hundred functionally different areas, i.e.
groups of neurones, in the CNS. Based on their location, the shape of their
dendritic tree and the course of their axon, several thousand types of neurons
can be distinguished in the CNS.
Transmitters
Neurotransmitters either excite or inhibit the postsynaptic neurone. The
most prominent excitatory transmitter in the CNS is L
-glutamate.
The most prominent inhibitory transmitter in the CNS is
GABA
(gamma-amino butyric acid). Other "main" neurotransmitters are e.g.
dopamine, serotonin, acetylcholine, noradrenaline and glycine. Each neurone
uses only one of the main transmitters, and this transmitter is used at all
synaptic boutons that originate from the neurone.
One or more of the "minor" transmitters (there are several dozens of
them - such as cholecystokinin, endogenous opioids, somatostatin, substance P)
may be used together with a main transmitter.
The molecular machinery that is needed to mediate the events occurring at
excitatory synapses differs from that at inhibitory synapses. Differences in
the morphological appearances of the synapses accompany the functional
differences. The pre-and postsynaptic densities are typically of equal width,
or
symmetric, at inhibitory
synapses. The postsynaptic density is thicker than the
presynaptic density at
asymmetric synapses, which
are typically excitatory.
Receptors
Usually there exists a multitude of receptors which are all sensitive to one
particular neurotransmitter. Different receptors have different response
properties, i.e. they allow the flux of different ions over the plasma membrane
of the neurone or they may address different second messenger systems in the
postsynaptic neurones. The precise reaction of the neurone to the various
neurotransmitters released onto its plasma membrane at the synapses is
determined by the types of receptors expressed by the neurone.
sections of spinal
cord - H&E, luxol
fast blue/cresyl violet (LFB/CV), toluidine blue, Giemsa
Thoracic Spinal Cord, sheep - LFB/CV
Most neurones have a light, large nucleus with a distinct nucleolus. The
cytoplasm of many neurones contains fairly large amounts of rough endoplasmatic
reticulum, which may aggregate within the cytoplasm of the neurone to form
Nissl-bodies. Nissl-bodies are prominent in motor
neurones located in the ventral horn of the grey matter of the spinal cord. The
neurites are difficult to identify in most types of stained sections. Only the
most proximal segments of the primary dendrites are seen clearly. The size of
the perikaryon depends on the level of activity of the neurone and the length
of the processes which the neurone has to support. An usable range for the size
of the perikaryon would be 15 - 50 µm, although much smaller and much larger
neuronal perikarya exist.
Draw the spinal cord at low magnification and indicate the
distribution of grey matter and white matter. Find a nice group of neurones in
the grey matter and draw them at a high magnification. Finally, have a look at
the white matter and identify the nuclei of glial cells. You will find similar
nuclei also in the grey matter.
Thoracic spinal Cord - H&E, silver stain
These slides show the same major features as the LFB/CV stained sections. Try
to identify neurones (primary dendrites, Nissl-bodies) and glial cell nuclei in
the H&E stained section. Differentiate between grey and white matter. The
LFB stain showed the myelin sheath nicely. In the H&E stained section we
instead can see large, cross-sectioned axons in the white matter. The feltwork
of nerve fibres, neuronal and glial cell processes is also called
neuropil.
Part of the cytoskeleton of neurones is (like the reticular connective tissue
fibers) argyrophilic, i.e. they "love" silver and can be stained by
silver stains. Aside from the neurones and their processes, fine fibrils are
visible in the neuropil. Many of the fibrils represent axons travelling in the
grey and white matter of the spinal cord.
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Title:
Central Nervous System (CNS)
by:
om
at
2013-02-15T03:31:00+07:00
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Central Nervous System (CNS)