120
UNIT 2
Basic Tissues
Figure 7-3A.
Structure of the synapse.
EM, scale line = 0.5
μ
m;
3
104,000
A high magnif cation electron micrograph oF an
axon terminal
and adjacent
postsynaptic membrane
is shown. Many
synaptic
vesicles
and three
mitochondria
are visible within the axon terminal. When an action potential reaches the axon terminal, some syn-
aptic vesicles Fuse with the
presynaptic membrane
and empty their neurotransmitter molecules into the
synaptic cleft
. The transmit-
ter molecules bind with receptor complexes in the postsynaptic membrane leading to either
depolarization
(excitatory infl
uence) or
hyperpolarization
(inhibitory infl uence) oF the postsynaptic membrane. The sum oF the excitatory and inhibitory infl
uences upon the
postsynaptic neuron determines whether it will f re an action potential or not. The large diFFerence in the thickness oF the presynaptic
and postsynaptic membranes makes this contact an
asymmetric synapse
. DiFFerences in regions oF
postsynaptic density
in diFFerent
synapses are probably a refl ection oF diFFerent types oF receptors in diFFerent postsynaptic membranes.
0.5
μ
m
0.5
μ
m
Presynaptic
Presynaptic
membrane
membrane
Postsynaptic
Postsynaptic
membrane
membrane
Synaptic vesicles
Mitochondrion
Membrane of
axon terminal
Presynaptic
membrane
Synaptic
cleft
Postsynaptic
density
0.5
μ
m
Postsynaptic
membrane
A
1.0
μ
m
1.0
μ
m
Dendrite
Microtubule
Axon terminal
Synaptic zone
Round
synaptic vesicle
Symmetric
synapse
Flattened
synaptic vesicle
Axon terminal
Mitochondrion
1.0
μ
m
B
Figure 7-3B.
S
tructure of the synapse.
EM, scale line =
1.0
μ
m;
3
35,000
Two
axon terminals
(
At
) Form synaptic contacts with a
den-
drite
(
Den
). The dendrite contains many
microtubules
, which
are more concentrated in dendrites than in axon terminals.
The smaller terminal contains predominantly
round vesicles
,
which are generally associated with excitatory neurotransmit-
ters, whereas the larger terminal contains many
fl attened ves-
icles
. Such vesicles are usually associated with inhibitory neu-
rotransmitters. A mixture oF round and fl attened vesicles is
termed a
pleomorphic
distribution. The larger axon terminal
Forms a synapse in which the postsynaptic membrane is about
the same thickness as the presynaptic membrane (
symmetric
synapse
). This type oF synapse is thought to indicate an
inhibi-
tory synapse
, whereas a synapse in which the postsynaptic
membrane is signif cantly thicker than the presynaptic mem-
brane (see, e.g., ±ig. 7-3A) is an
asymmetric synapse
and is
thought to be
excitatory
in its action. Numerous
mitochondria
are present in both the dendrite and the axon terminals. Other
types oF synapses not illustrated here include axon terminals
that contact neuron cell bodies or the initial segment oF axons,
axon terminals that contact other axon terminals, and recip-
rocal synapses at which two adjacent dendrites Form synap-
tic contacts with each other. In addition, terminal bundles oF
axons sometimes make multiple contacts through
boutons en
passage
rather than terminal boutons (see, e.g., ±ig. 6-10A).
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