410
UNIT 3
Regulation and Integration of the Body
11
For each nerve impulse reaching the presynaptic termi-
nal, many vesicles (perhaps 300) empty into the synaptic
clef. Te higher the impulse ±requency (that is, the more
intense the stimulus), the greater the number o± synaptic
vesicles that ±use and spill their contents, and the greater the
effect on the postsynaptic cell.
4
Neurotransmitter diffuses across the synaptic cleft and
binds to specific receptors on the postsynaptic membrane.
 
5
Binding of neurotransmitter opens ion channels, creating
graded potentials.
When neurotransmitter binds to the recep-
tor protein, this receptor changes its three-dimensional shape.
Tis change in turn opens ion channels and creates graded po-
tentials. Postsynaptic membranes ofen contain receptor pro-
teins and ion channels packaged together as chemically gated
ion channels. Depending on the receptor protein to which the
neurotransmitter binds and the type o± channel the receptor
controls, the postsynaptic neuron may be either excited or
inhibited.
6
Neurotransmitter effects are terminated.
Te binding o±
a neurotransmitter to its receptor is reversible. As long as
it is bound to a postsynaptic receptor, a neurotransmitter
continues to affect membrane permeability and block recep-
tion o± additional signals ±rom presynaptic neurons. For this
reason, some means o± “wiping the postsynaptic slate clean”
is necessary. Te effects o± neurotransmitters generally last
a ±ew milliseconds be±ore being terminated in one o± three
ways, depending on the particular neurotransmitter:
Reuptake
by astrocytes or the presynaptic terminal,
where the neurotransmitter is stored or destroyed by en-
zymes, as with norepinephrine
Degradation
by enzymes associated with the post-
synaptic membrane or present in the synapse, as with
acetylcholine
Diffusion
away ±rom the synapse
Synaptic Delay
An impulse may travel at speeds o± up to 150 m/s (300 mi/h) down
an axon, but neural transmission across a chemical synapse is com-
paratively slow. It reflects the time required ±or neurotransmitter to
be released, diffuse across the synaptic clef, and bind to receptors.
²ypically, this
synaptic delay
lasts 0.3–5.0 ms, making transmission
across the chemical synapse the
rate-limiting
(slowest) step o± neural
transmission. Synaptic delay helps explain why transmission along
neural pathways involving only two or three neurons occurs rapidly,
but transmission along multisynaptic pathways typical o± higher
mental ±unctioning occurs much more slowly. However, in practical
terms these differences are not noticeable.
Check Your Understanding
15.
What structure joins two neurons at an electrical synapse?
16.
Events at a chemical synapse usually involve opening both voltage-
gated ion channels and chemically gated ion channels. Where are
these ion channels located and what causes each to open?
For answers, see Appendix H.
Postsynaptic Potentials
and Synaptic Integration
Distinguish between excitatory and inhibitory postsynaptic
potentials.
Describe how synaptic events are integrated and modified.
Many receptors on postsynaptic membranes at chemical
synapses are specialized to open ion channels, in this way
converting chemical signals to electrical signals. Unlike the
voltage-gated ion channels responsible ±or APs, however,
these chemically gated channels are relatively insensitive to
changes in membrane potential. Consequently, channel open-
ing at postsynaptic membranes cannot possibly become sel±-
ampli±ying or sel±-generating. Instead, neurotransmitter re-
ceptors mediate graded potentials—local changes in mem-
brane potential that are
graded
(vary in strength) according
to the amount o± neurotransmitter released and the time it
remains in the area.
Table 11.2
compares graded potentials
and action potentials.
Chemical synapses are either excitatory or inhibitory, de-
pending on how they affect the membrane potential o± the post-
synaptic neuron.
An EPSP is a local
depolarization of the
postsynaptic membrane
that brings the neuron
closer to AP threshold.
Neurotransmitter binding
opens chemically gated
ion channels, allowing
Na
+
and K
+
to pass
through simultaneously.
An IPSP is a local
hyperpolarization of the
postsynaptic membrane
that drives the neuron
away from AP threshold.
Neurotransmitter binding
opens K
+
or Cl
channels.
10
20
30
+30
–55
–70
0
Time (ms)
(a) Excitatory postsynaptic potential (EPSP)
Membrane potential (mV)
Threshold
Stimulus
10
20
30
+30
–55
–70
0
Time (ms)
(b) Inhibitory postsynaptic potential (IPSP)
Membrane potential (mV)
Threshold
Stimulus
Figure 11.18
Postsynaptic potentials can be excitatory
or inhibitory.
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