Chapter 11
Fundamentals of the Nervous System and Nervous Tissue
413
11
 
GRADED POTENTIAL (GP)
ACTION POTENTIAL (AP)
 
POSTSYNAPTIC POTENTIAL (A TYPE OF GP)
 
 
EXCITATORY (EPSP)
INHIBITORY (IPSP)
 
Function
Short-distance
signaling;
depolarization
that spreads to
axon hillock; moves
membrane potential
toward
threshold for
generating an AP
Short-distance signaling;
hyperpolarization that
spreads to axon hillock;
moves membrane
potential
away
from
threshold for generating
an AP
Long-distance signaling; constitutes the nerve impulse
Initial effect
of stimulus
Opens chemically
gated channels that
allow simultaneous
Na
1
and K
1
fluxes
Opens chemically gated
K
1
or Cl
2
channels
Opens voltage-gated channels; first opens Na
1
channels, then K
1
channels
Peak
membrane
potential
Depolarizes; moves
toward 0 mV
Hyperpolarizes; moves
toward
2
90 mV
1
30 to
1
50 mV
Table 11.2
(continued)
and the site of action potential generation (the axon hillock).
Accordingly, inhibitory synapses occur most oFen on the cell
body and excitatory synapses occur most oFen on the dendrites
(±igure 11.19d).
Synaptic Potentiation
Repeated or continuous use of a syn-
apse (even for short periods) enhances the presynaptic neuron’s
ability to excite the postsynaptic neuron, producing larger-than-
expected EPSPs. Tis phenomenon is called
synaptic poten-
tiation
. Te presynaptic terminals at such synapses contain
relatively high Ca
2
1
concentrations, a condition that triggers
the release of more neurotransmitter, which in turn produces
larger EPSPs.
Synaptic potentiation also brings about Ca
2
1
influx via den-
dritic spines into the postsynaptic neuron. Brief high-frequency
stimulation partially depolarizes the postsynaptic membrane.
Tis partial depolarization causes certain chemically gated
channels called
NMDA
(
N-methyl-D-aspartate
)
receptors
to al-
low Ca
2
1
to enter, something that only happens when the mem-
brane is depolarized. As Ca
2
1
floods into the cell, it activates
of biochemical and electrical characteristics) with different
types of target neurons. How is all this conflicting informa-
tion sorted out?
Each neuron’s axon hillock keeps a running account of all the
signals it receives. Not only do EPSPs summate and IPSPs sum-
mate, but also EPSPs summate with IPSPs. If the stimulatory
effects of EPSPs dominate the membrane potential enough to
reach threshold, the neuron will fire. If summation yields only
subthreshold depolarization or hyperpolarization, the neuron
fails to generate an AP (±igure 11.19d).
However, partially depolarized neurons are
facilitated
—that
is, more easily excited by successive depolarization events—
because they are already near threshold. Tus, axon hillock
membranes function as
neural integrators
, and their potential
at any time reflects the sum of all incoming neural information.
Because EPSPs and IPSPs are graded potentials that decay
the farther they spread, the most effective synapses are those
closest to the axon hillock. Specifically, inhibitory synapses are
most effective when located between the site of excitatory inputs
Na
+
K
+
Cl
K
+
Na
+
K
+
mV
Time
–70
0
mV
Time
–70
0
mV
Time
–70
+50
0
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