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FIGURE 8.2 Development of synapse morphology. A. The contact of an auditory nerve endbulb (EB) onto
its postsynaptic target, the spherical bushy cell (SBC), is shown at several ages. The end bulb initially forms
a small contact on the spherical bushy cell in newborn mice. In adults, the endbulb forms an extremely large
ending on the spherical busy cell. B. When contacts from newborn animals are examined at the ultrastruc-
tural level, there is little evidence of synapse formation. (Right) In adults, these contacts display many signs
of mature synapses, including presynaptic vesicles and a postsynaptic membrane density. (Limb and Ryugo,
in the initial stages of synaptogenesis, as discussed
below. There are also many examples of presynaptic
protrusions being engulfed by muscle membrane,
termed a coated pits . These observations show an intense
interaction at the initial site of contact. The first sign of
differentiation is found below the postsynaptic mem-
brane, where the Golgi apparatus accumulates and
coated vesicles proliferate, both of which probably con-
tribute to the construction of the postsynaptic density.
As for the presynaptic terminal, it is only after about 24
hours of contact that vesicles begin to accumulate at the
site of contact. Thus, the structure of a synapse appears
to mature over a relatively long period in these in vitro
studies, and the postsynaptic cell is the first to display
any sign of differentiation.
The location of synapse formation is extremely
important to the operation of the nervous system.
Even at the nerve-muscle junction, motor synapses
form at distinct central locations on the myofibers. On
the typical central neuron there are many more
options. Synapses that form near the soma are thought
to have a greater voice in deciding whether the neuron
will fire an action potential. Thus, inhibitory synapses
are often found nestled up around the cell body so that
they can halt activity efficiently. In contrast, many
synapses form on dendritic spines where their activity
provides tiny potentials that must be summed together
to produce a significant change in the neuron's mem-
brane potential. When many different types of affer-
ents synapse on a postsynaptic neuron, each with a
distinct functional role, then the problem becomes
quite difficult indeed. Does synaptogenesis proceed in
any particular sequence, and how does each synapse
know where to form on the postsynaptic cell?
Early observations from Golgi stained spinal cord
material showed axonal growth cones and dendritic
growth cones seemingly reaching for one another, sug-
gesting that axodendritic synapses result from an early
trophic interaction. The electron microscopist has been
able to show where synapses are added because both
the presynaptic terminal and the postsynaptic location
(e.g., soma, dendrite, spine) are identifiable at high
magnification. In many systems, including the NMJ,
the spinal cord, the hippocampus, and the cortex, con-
tacts seem to form initially on postsynaptic processes.
For example, muscle cells extend tiny processes, called
myopodia , just before motor terminals arrive, and these
postsynaptic processes are the prefered site of contact
(Ritzenthaler et al., 2000). In the embryonic mouse
spinal cord, nearly 75% of axodendritic synapses are
found on dendritic growth cones (Vaughn et al., 1974).
Even in the cortex and hippocampus, axodendritic
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