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Chapter 6
to fl ow directly across the junction through the connecting
channels from one neuron to the other. This depolarizes the
membrane of the second neuron to threshold, continuing the
propagation of the action potential. Communication between
cells via electrical synapses is extremely rapid. Although numer-
ous in cardiac and smooth muscles, electrical synapses are not
commonly found in the mammalian nervous system.
Figure 6–25
shows the basic structure of a typical chemi-
cal synapse. The axon of the presynaptic neuron ends in a slight
swelling, the axon terminal, which holds the
synaptic vesicles
that contain the neurotransmitter. The postsynaptic membrane
adjacent to the axon terminal has a high density of intrinsic and
extrinsic membrane proteins that make up a specialized area
called the
postsynaptic density.
Note that in actuality the
size and shape of the pre- and postsynaptic elements can vary
greatly (
Figure 6–26
). A 10- to 20-nm extracellular space, the
synaptic cleft,
separates the pre- and postsynaptic neurons and
prevents
direct
propagation of the current from the presynaptic
neuron to the postsynaptic cell. Instead, signals are transmitted
across the synaptic cleft by means of a chemical messenger—a
neurotransmitter—released from the presynaptic axon termi-
nal. Sometimes more than one neurotransmitter may be simul-
taneously released from an axon, in which case the additional
neurotransmitter is called a
cotransmitter.
These neurotrans-
mitters have different receptors on the postsynaptic cell.
In general, the neurotransmitter is stored on the presyn-
aptic side of the synaptic cleft, whereas receptors for the neu-
rotransmitters are on the postsynaptic side. Therefore, most
chemical synapses operate in only one direction. One-way
conduction across synapses causes action potentials to trans-
mit along a given multineuronal pathway in one direction.
Mechanisms of
Neurotransmitter Release
As indicated in
Figure 6–27
, neurotransmitter is stored in
small vesicles with lipid bilayer membranes. Prior to activa-
tion, many vesicles are docked on the presynaptic membrane
Direction of action
potential propagation
Terminal of
presynaptic
axon
Vesicle
docking
site
Synaptic
vesicle
Mitochondrion
Synaptic cleft
Postsynaptic
density
Postsynaptic
cell
Figure 6–25
Diagram of a chemical synapse. Some vesicles are docked at
the presynaptic membrane, ready for release. The postsynaptic
membrane is distinguished microscopically by the postsynaptic
density, which contains proteins associated with the receptors.
(a)
(b)
(d)
(c)
Figure 6–26
Synapses appear in many forms, as demonstrated here in views (a) to
(d). The presynaptic terminal contains synaptic vesicles.
Redrawn from Walmsley et al.
Convergence
Divergence
Figure 6–24
Convergence of neural input from many neurons onto a single neuron,
and divergence of output from a single neuron onto many others.
Arrows indicate the direction of transmission of neural activity.
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