PROCESS DIAGRAM
Synapses Help Neurons Communicate
with Other Cells
As you have just learned, a stimulus evokes an action poten-
tial in a neuron. The neuron conducts the action potential
down its axon to the
axon terminal
(see Figure
7.3a), and to another cell (neuron, effector) at
a junction called the
synapse
. The neuron that
sends the message is called the
presynaptic neuron,
and the one that receives the message is called
the
postsynaptic neuron.
Some synapses are con-
sidered to be
electrical synapses.
These junctions
are generally found between two neurons that
lie very close to one another. The plasma mem-
branes of the two cells are connected by gap
junctions, and the ionic signal passes through
these gaps from one cell to the other.
Most synapses, however, are
ch em ica l syn -
apses.
The electrical message (action poten-
tial) in the presynaptic neuron gets converted
to a chemical message with the release of a
neurotransmitter
. This chemical messenger
drifts across the synapse, binds to receptors
on the postsynaptic cell, and transmits information to the
postsynaptic cell (Figure 7.6).
There are several different neurotransmitters in the human
body, including
acetylcholine (ACh)
, dopamine, serotonin,
and norepinephrine. They are stored in structures
in synaptic bulbs called synaptic vesicles. Some neu-
rotransmitters cause depolarizations in the postsyn-
aptic cell, while others cause hyperpolarizations in
the postsynaptic cell. A single neuron produces only
one type of neurotransmitter.
Generally, synaptic transmission goes only one
way, from a presynaptic cell to a postsynaptic cell.
However, one neuron may receive input from several
others. For example, a typical CNS neuron receives
input from 1,000 to 10,000 synapses. Some synapses
may be excitatory and cause depolarizations, while
others may be inhibitory and cause hyperpolariza-
tions. Many toxins and drugs affect synaptic transmis-
sion either by blocking it altogether or by modulating
the amount of neurotransmitter released.
Neurons can play different roles in neural path-
ways. They can send, receive, or process informa-
tion. Sensory neurons help relay information from
synapse
(SYN-aps)
The functional junction
between two neurons
or between a neuron
and an effector, such
as a muscle or gland.
A synapse may be
electrical or chemical.
neurotransmitter
One of a variety of
molecules within axon
terminals that are
released into the syn-
aptic cleft in response
to a nerve impulse and
that change the mem-
brane potential of the
postsynaptic cell.
S ynaptic transm ission • Figure 7.6
NNER
-N a+
e o
e o.-
Synaptic
cleft
i+
Na-
Action potentials in the presynaptic neuron set off a series of
ionic events that cause the release of a neurotransmitter
such as acetylcholine. The neurotransmitter binds to the
postsynaptic neuron and
elicits an action potential
in this cell.
.........
t
Presynaptic
neuron
Postsynaptic
neuron
Direction of impulse
Axon terminal of presynaptic neuron
Synaptic vesicle
Action potential
arrives.
Neurotransmitter
Synaptic bulb
Ca2+
Action
potential
Na+ channels open and
depolarization causes
Ca2+ channels to open.
Calcium causes synaptic
vesicles to fuse with neuron
membrane, dumping
neurotransmitter into
synapse.
Postsynaptic
neuron
Neurotransmitter binds
receptor, receptor opens
Na+ enters postsynaptic
neuron and depolarizes cell,
causing action potential.
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