162
Chapter 6
There are both electrical and concentration gradients
driving sodium into the cell, while for potassium, the elec-
trical gradient opposes the concentration gradient (review
Figure 6–12). Opening channels that are permeable to all
small positively charged ions therefore results in the simul-
taneous movement of a relatively small number of potassium
ions out of the cell and a larger number of sodium ions into
the cell. Thus, the
net
movement of positive ions is into the
postsynaptic cell, causing a slight depolarization. This poten-
tial change is called an
excitatory postsynaptic potential
(
EPSP,
Figure 6–28
).
The EPSP is a graded potential that spreads decremen-
tally away from the synapse by local current. Its only function
is to bring the membrane potential of the postsynaptic neuron
closer to threshold.
Inhibitory Chemical Synapses
At inhibitory synapses, the potential change in the postsynaptic
neuron is generally a hyperpolarizing graded potential called
an
inhibitory postsynaptic potential
(
IPSP,
Figure 6–29
).
Alternatively, there may be no IPSP but rather
stabilization
of the membrane potential at its existing value. In either case,
activation of an inhibitory synapse lessens the likelihood that
the postsynaptic cell will depolarize to threshold and generate
an action potential.
At an inhibitory synapse, the activated receptors on the
postsynaptic membrane open chloride or potassium channels;
sodium permeability is not affected. In those cells that actively
regulate intracellular chloride concentrations via active
transport out of the cell, the chloride equilibrium poten-
tial is more negative than the resting potential. Therefore,
as chloride channels open, chloride enters the cell, produc-
ing a hyperpolarization—that is, an IPSP. In cells that do not
actively transport chloride, the equilibrium potential for chlo-
ride is equal to the resting membrane potential. Therefore, a
rise in chloride ion permeability does not change the mem-
brane potential but is able to increase chloride’s infl uence on
the membrane potential. This makes it more diffi cult for other
ion types to change the potential and stabilizes the membrane
at the resting level without producing a hyperpolarization.
Increased potassium permeability, when it occurs in the
postsynaptic cell, also produces an IPSP. Earlier we noted that
if a cell membrane were permeable only to potassium ions,
the resting membrane potential would equal the potassium
equilibrium potential; that is, the resting membrane potential
would be about –90 mV instead of –70 mV. Thus, with an
increased potassium permeability, more potassium ions leave
the cell and the membrane moves closer to the potassium
equilibrium potential, causing a hyperpolarization.
Synaptic Integration
In most neurons, one excitatory synaptic event by itself is not
enough to reach threshold in the postsynaptic neuron. For
example, a single EPSP may be only 0.5 mV, whereas changes
of about 15 mV are necessary to depolarize the neuron’s mem-
brane to threshold. This being the case, an action potential
can be initiated only by the combined effects of many excit-
atory synapses.
Of the thousands of synapses on any one neuron, prob-
ably hundreds are active simultaneously or close enough in
time that the effects can add together. The membrane poten-
tial of the postsynaptic neuron at any moment is, there-
fore, the result of all the synaptic activity affecting it at that
moment. A depolarization of the membrane toward thresh-
old occurs when excitatory synaptic input predominates, and
either a hyperpolarization or stabilization occurs when inhib-
itory input predominates (
Figure 6–30
).
A simple experiment can demonstrate how EPSPs and
IPSPs interact, as shown in
Figure 6–31
. Assume there are
Figure 6–28
Excitatory postsynaptic potential (EPSP). Stimulation of the
presynaptic neuron is marked by the arrow. Drawn larger than
normal; typical EPSP = 0.5 mV.
Time (ms)
0
–70
10
20
Threshold
EPSP
Membrane potential (mV)
Figure 6–29
Inhibitory postsynaptic potential (IPSP). Stimulation of the
presynaptic neuron is marked by the arrow. (This hyperpolarization
is drawn larger than a typical IPSP.)
Time (ms)
0
–70
10
20
Threshold
IPSP
Membrane potential (mV)
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