Neuronal Signaling and the Structure of the Nervous System
163
three synaptic inputs to the postsynaptic cell: The synapses
from axons A and B are excitatory, and the synapse from axon
C is inhibitory. There are laboratory stimulators on axons A, B,
and C so that each can be activated individually. An electrode
is placed in the cell body of the postsynaptic neuron that will
record the membrane potential. In part 1 of the experiment,
we will test the interaction of two EPSPs by stimulating axon
A and then, after a short time, stimulating it again. Part 1
of Figure 6–31 shows that no interaction occurs between
the two EPSPs. The reason is that the change in membrane
potential associated with an EPSP is fairly short-lived. Within
a few milliseconds (by the time we stimulate axon A for the
second time), the postsynaptic cell has returned to its resting
condition.
In part 2, we stimulate axon A for the second time before
the fi rst EPSP has died away; the second synaptic potential adds
to the previous one and creates a greater depolarization than
from one input alone. This is called
temporal summation
because the input signals arrive from the same presynaptic cell
at different
times.
The potentials summate because there are a
greater number of open ion channels and, therefore, a greater
fl ow of positive ions into the cell. In part 3 of Figure 6–31, axon
B is stimulated alone to determine its response, and then axons
A and B are stimulated simultaneously. The two EPSPs that
result also summate in the postsynaptic neuron; this is called
spatial summation
because the two inputs occurred at dif-
ferent
locations
on the same cell. The interaction of multiple
EPSPs through spatial and temporal summation can increase
the inward fl ow of positive ions and bring the postsynaptic
membrane to threshold so that action potentials are initiated
(see part 4 of Figure 6–31).
So far we have tested only the patterns of interaction of
excitatory synapses. Because EPSPs and IPSPs are due to oppo-
sitely directed local currents, they tend to cancel each other, and
there is little or no net change in membrane potential when both
A and C are stimulated (see Figure 6–31, part 5). Inhibitory
potentials can also show spatial and temporal summation.
Depending on the postsynaptic membrane’s resistance
and on the amount of charge moving through the ligand-gated
channels, the synaptic potential will spread to a greater or lesser
degree across the plasma membrane of the cell. The membrane
of a large area of the cell becomes slightly depolarized during
activation of an excitatory synapse and slightly hyperpolar-
ized or stabilized during activation of an inhibitory synapse,
although these graded potentials will decrease with distance
from the synaptic junction (
Figure 6–32
). Inputs from more
than one synapse can result in summation of the synaptic
potentials, which may then trigger an action potential.
Membrane potential (mV)
AA
A
A
B
B
B
A=
excitatory
B=
inhibitory
0
–70
Time
Threshold
Figure 6–30
Intracellular recording from a postsynaptic cell during times of (A)
excitatory synaptic activity when the cell is depolarized, and (B)
inhibitory synaptic activity when the membrane hyperpolarizes.
Time
–70
+30
1234
A
A
A
B
C
AA
B
A + B
A A B B
C
A + C
5
Membrane potential (mV)
Temporal
summation
Recording
microelectrode
Inhibitory
synapse
Excitatory
synapses
Spatial
summation
Threshold
Axon
Figure 6–31
Interaction of EPSPs and IPSPs at the postsynaptic neuron. Presynaptic neurons (A–C) were stimulated at times indicated by the arrows, and
the resulting membrane potential was recorded in the postsynaptic cell by a recording microelectrode.
Figure 6–31
physiological
inquiry
If this postsynaptic neuron had no active chloride pumps and the synapse from neuron C opened chloride channels, how would the traces in
panel 5 be different from what is shown?
Answer can be found at end of chapter.
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