Chapter 11
Fundamentals of the Nervous System and Nervous Tissue
411
11
with distance, they can and ofen do spread all the way to the
axon hillock. IF currents reaching the hillock are strong enough
to depolarize the axon to threshold, axonal voltage-gated chan-
nels open and an AP is generated.
Inhibitory Synapses and IPSPs
Binding oF neurotransmitters at inhibitory synapses
reduces
a
postsynaptic neuron’s ability to generate an AP. Most inhibitory
neurotransmitters hyperpolarize the postsynaptic membrane
by making the membrane more permeable to K
1
or Cl
2
. So-
dium ion permeability is not affected.
IF K
1
channels open, K
1
moves out oF the cell. IF Cl
2
chan-
nels open, Cl
2
moves in. In either case, the charge on the inner
Face oF the membrane becomes more negative. As the mem-
brane potential increases and is driven Farther From the axon’s
threshold, the postsynaptic neuron becomes
less and less likely
to “fire,” and larger depolarizing currents are required to induce
an AP. Hyperpolarizing changes in potential are called
inhibi-
tory postsynaptic potentials (IPSPs)
(±igure 11.18b).
Integration and Modification of Synaptic Events
Summation by the Postsynaptic Neuron
A single EPSP can-
not induce an AP in the postsynaptic neuron
(Figure 11.19a)
.
But iF thousands oF excitatory axon terminals fire on the same
postsynaptic membrane, or iF a small number oF terminals de-
liver impulses rapidly, the probability oF reaching threshold
soars. EPSPs can add together, or
summate
, to influence the
Excitatory Synapses and EPSPs
At excitatory synapses, neurotransmitter binding depolarizes
the postsynaptic membrane. However, in contrast to what hap-
pens on axon membranes,
chemically gated
ion channels open
on postsynaptic membranes (those oF dendrites and neuronal
cell bodies). Each channel allows Na
1
and K
1
to diffuse
simulta-
neously
through the membrane but in opposite directions.
Although this two-way cation flow may appear to be selF-
deFeating when depolarization is the goal, remember that the
electrochemical gradient For sodium is much steeper than that
For potassium. As a result, Na
1
influx is greater than K
1
efflux,
and
net
depolarization occurs.
IF enough neurotransmitter binds, depolarization oF the
postsynaptic membrane can reach 0 mV, which is well above an
axon’s threshold (about
2
50 mV) For “firing off” an AP. How-
ever, unlike axons which have voltage-gated channels that make
an AP possible,
postsynaptic membranes generally do not gener-
ate APs
. Te dramatic polarity reversal seen in axons never oc-
curs in membranes containing
only
chemically gated channels
because the opposite movements oF K
1
and Na
1
prevent exces-
sive positive charge From accumulating inside the cell. ±or this
reason, instead oF APs, local graded depolarization events called
excitatory postsynaptic potentials (EPSPs)
occur at excitatory
postsynaptic membranes
(Figure 11.18a)
.
Each EPSP lasts a Few milliseconds and then the membrane
returns to its resting potential. Te only Function oF EPSPs is to
help trigger an AP distally at the axon hillock oF the postsynaptic
neuron. Although currents created by individual EPSPs decline
Threshold of axon of
postsynaptic neuron
Threshold of axon
postsynaptic neur
n
of
ro
n
Excitatory synapse 1 (E
1
)
Excitatory synapse 2 (E
2
)
Inhibitory synapse (I
1
)
Resting potential
E
1
E
1
E
1
E
1
E
1
+ E
2
I
1
E
1
+ I
1
(a) No summation:
2 stimuli separated in time
cause EPSPs that do not
add together.
(d) Spatial summation of
EPSPs and IPSPs:
Changes in membane potential
can cancel each other out.
Membrane potential (mV)
0
–55
–70
(b)
Temporal summation:
2 excitatory stimuli close
in time cause EPSPs
that add together.
(c)
Spatial summation:
2 simultaneous stimuli at
different locations cause
EPSPs that add together.
Time
Time
Time
Time
E
1
E
1
E
1
E
2
I
1
E
1
Figure 11.19
Neural integration of EPSPs and IPSPs.
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