CHAPTER 7
Nervous Tissue
119
Figure 7-2A.
Types of stains for nervous tissue.
The various features of nervous tissue are more difF cult to
visualize than are the features of most other histological
tissues. Consequently, several
special stains
are commonly
used with nervous tissue.
Golgi preparations
stain all of the
processes of an individual neuron (dendrites, cell body, and
axon) but react with only a very small percentage of the total
number of neurons (Purkinje cell,
upper left
).
Nissl stains
react with rough endoplasmic reticulum and, therefore,
allow the shape and size of cell bodies to be visualized but do
not stain dendrites and axons (spinal motor neurons,
upper
right
).
Myelin stains
allow the visualization of myelinated
F bers but do not react with cell bodies or dendrites (spinal
cord,
lower left
). Myelinated F ber tracts are, therefore, dark,
and areas with high concentrations of neuron cell bodies are
light.
H&E stains
are often used in the diagnosis of patho-
logical conditions and sometimes used to stain
normal ner-
vous tissue (posterior root ganglion,
lower right
).
A
Golgi
Nissl
H&E
Myelin
Figure 7-2B.
Information transmission in the nervous
system.
The primary functions of the
nervous system
are to
transfer
information
(in the form of
action potentials
) from one
place to another and to
process that information
to gener-
ate sensory experience, perceptions, ideas, and motor activ-
ity. Information is carried in the form of action potentials
along
axons
(
red arrows 1 and 2
). At the ends of axons,
there are
axon terminals
, where the electrochemical action
potential causes the release of molecules called
neurotrans-
mitters.
These molecules act upon
receptor complexes
in the
dendrites and somas of the next neuron in a series (e.g.,
3
)
at regions of
synapses
(
red dashed circle
). The action of the
neurotransmitters may be either excitatory or inhibitory on
the
postsynaptic membrane
. When the excitatory infl
uences
on a neuron exceed the inhibitory infl
uence by a certain
threshold amount, that neuron generates an action poten-
tial that is then transmitted onto yet another neuron.
J. Lynch
1
2
3
Information transmission in the nervous system
B
Figure 7-2C.
Elements of the synapse.
A typical
chemical synapse
consists of a
terminal bouton
(a swelling at the end of an axon terminal) that includes
a
presynaptic membrane
, a specialized
postsynaptic mem-
brane
, and a space between the two (the
synaptic cleft
). The
terminal bouton contains many
synaptic vesicles
that con-
tain neurotransmitter molecules. When an action potential
arrives at the axon terminal, a complex chemical process
is initiated that culminates in the fusion of some vesicles
with the presynaptic membrane and the discharge of their
neurotransmitter molecules via
exocytosis
into the synaptic
cleft where they can act on
receptors
in the postsynaptic
membrane. The postsynaptic membrane is thickened in the
immediate vicinity of the synapse as a result of the dense
concentration of receptor protein complexes in that region
(±ig. 7-3A). Both the presynaptic and postsynaptic regions
contain numerous mitochondria, which supply the energy
needed by the synaptic transmission process.
J. Lynch
C
Axon terminal
(terminal bouton)
Mitochondrion
Presynaptic
membrane
Postsynaptic
membrane
Dendrite
Postsynaptic
density
Microtubule
Vesicle
Synaptic cleft
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