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FIGURE 9.7 In vivo imaging of the same multiply innervated neuromuscular junctions in a neonatal
mouse. Two transgenic mouse lines, each expressing fluorescent proteins in their motor neurons, were mated,
and the progeny examined. Images were obtained from the same sternomastoid muscle fiber over the course
of several days. In this example, one of the motor terminals (blue and insets) occupies a larger percentage
(70%) of the postsynaptic territory at P11. It gradually withdraws from the junction (arrows) over the next
four days. The withdrawing axon is marked by an asterisk at P14 and 15. Scale bars are 10 mm. (From Walsh
and Lichtman, 2003)
variety of species and neural structures, although there
are exceptions. There is significant remodeling of
retinal axons during topographic map formation in the
visual midbrain (Reh and Constantine-Paton, 1984;
Simon and O'Leary, 1992). In frogs, single retinal affer-
ents innervate successively posterior locations within
the tectum as development proceeds. In mammals,
retinal axons innervate topographically incorrect posi-
tions at birth, and these terminals are gradually with-
drawn over two weeks. Synapse elimination is also
found in invertebrates. In the cricket, single sensory
neurons are functionally connected with two different
interneurons during an early stage of development.
Gradually, the strength of one connection doubles
while the other connection is completely eliminated
(Chiba et al., 1988). As we will see below, excitatory
synaptic transmission has been implicated in the
rearrangement of connections. However, inhibitory
afferents can also become more refined during devel-
opment. Inhibitory projections from the MNTB to the
LSO (see Figure 8.28) undergo a striking refinement
during the first postnatal week, followed by a decrease
in the arbor size of single afferents by about 25% along
the tonotopic axis (Sanes and Siverls, 1991; Kim and
Kandler, 2003).
While the functional implications of terminal expan-
sion are not clear, some neural circuits probably
require broad connectivity with the sensory world.
Many systems may employ a mixed strategy, with
some axons expanding in territory while others retract.
In the chick auditory system, two sets of axons con-
verge on the nucleus laminaris , one innervating the
dorsal dendrites and the other the ventral dendrites.
Both sets of afferents spread out along the tonotopic
axis during the embryonic period analyzed, suggest-
ing that synapses are being added (Young and Rubel,
1986). However, the ventral terminal arbors form
directly above their parent axon at first, but nearly half
of them come to lay at a significant distance later in
development. Since it is unlikely that the entire axon
shifts its position, individual synaptic contacts are
probably eliminated from one region of the terminal
while new synapses are added at a different position.
The connections from sensory axons to motor neurons
appear to be maintained from the outset. Inappro-
priate monosynaptic connections from sensory axons
to spinal motor neurons are not found with intra-
cellular recordings, although there may be significant
immaturities within the polysynaptic pathways
(Seebach and Ziskind-Conhaim, 1994; Mears and
Frank, 1997).
SOME TERMINALS EXPAND
OR REMAIN STABLE
NEURAL ACTIVITY REGULATES
SYNAPTIC CONNECTIONS
There are a few afferent pathways where synapse
elimination is either scarce or absent . In contrast to the
axonal arbors described above (see Figures 9.5 and
9.6), there are retinal cells with large visual receptive
fields (Y-cells) whose arbors expand during develop-
ment (Sur et al., 1984; Florence and Casagrande, 1990).
We encounter use-dependent changes of our
nervous system on a regular basis. Sensory informa-
tion is processed by our nervous system, and stored as
memories (see BOX: Remaining Flexible). Our hands
gradually become more adept at working with a newly
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