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All cells have
>1 input
100
Far apart ( )
50
Close together ( )
0
All cells have
only 1 input
0
10
20
30
40
50
114
204
87
Days
FIGURE 9.23 Synapse elimination depends on synapse distance. Two motor axons were positioned on
the same muscle, either close to one another or at a distance. Intracellular recordings were made from muscle
fibers to monitor polyneuronal innervation, as shown in Figure 9.2. When the synapses were close together
(blue circles), synaptic elimination occurred within a few weeks. When the synapses were far apart (red
circles), synaptic elimination failed to occur. (Adapted from Kuffler et al., 1977)
functional AChR clusters (Westerfield et al., 1990). In
contrast, sensory deprivation studies suggest that the
disuse of a synapse leads to its weakening or elimina-
tion, especially if other synapses are active. Synaptic
terminals “compete” for the distinction of activating
the postsynaptic neuron, and “stable” connections
depend, in part, upon an active transmitter-receptor
system. Therefore, the presence of synaptic transmis-
sion is not an absolute requirement for the initial for-
mation of a synapse but influences its subsequent
development and stability.
The activity of a neighboring synapse can be
harmful to an inactive one. This mechanism was tested
at an unusual muscle in the rat foot, called the lum-
brical muscle, which receives its innervation via two
separate peripheral nerves (Figure 9.24). It is possible
to electrically stimulate one group of motor axons
during the normal period of synapse elimination, and
find out what happens to the synapses of unstimulated
axons. The strength of the nerve-muscle connection
was determined indirectly by measuring the size of
nerve-evoked muscle contractions, where a larger con-
traction signifies a stronger connection. When one
nerve is stimulated for about six days, the unstimulated
axons are much less effective at producing a muscle
contraction (Ridge and Betz, 1984). That is, the unstim-
ulated axons are either eliminated from the muscle in
disproportionate numbers, or their synapses become
weaker.
How long does it take for one synapse to extinguish
its neighbor? The onset of synapse elimination was
studied using a very simple culture system that con-
tained two presynaptic neurons and one postsynaptic
cell, a myocyte (Figure 9.25A). Whole cell recordings
were made from the myocyte, while the activity of one
or both presynaptic neurons was controlled with
stimulating electrodes. In the first experiment, the
synaptic currents elicited by each neuron were first
measured, and then one of the neurons was stimulated
for several seconds. Within moments of this brief pro-
cedure, the unstimulated neuron produces much
smaller synaptic currents, and the effect lasts for the
duration of the experiment (Lo and Poo, 1991). When
one synapse is able to modify the operation of a second
one, the interaction is referred to as heterosynaptic .
Two additional observations seem to fit neatly into
the puzzle. First, if the two synapses are separated
from one another by >50 mm, then activation of one
synapse is not able to suppress its neighbor. Two
synapses can apparently compete for the right to acti-
vate a postsynaptic cell, but the interaction occurs over
a very short distance, consistent with results from the
adult NMJ and Aplysia cultures (Kuffler et al., 1977;
Glanzman et al., 1991). Second, when both neurons are
stimulated at the same time, there is also no change in
the strength of either synapse. That is, synaptic activ-
ity is able to protect a synapse from the ill effects of an
active neighbor.
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