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recorded from (Figure 8.8A). The tight seal between
the large tip of a whole-cell recording electrode and the
muscle membrane (see Box: Biophysics: The Nuts and
Bolts of Functional Maturation) permits the recordings
to continue while a small round muscle cell, called a
myoball , is detached from the substrate and reposi-
tioned in the culture dish. By using this technique, it
is possible to observe spontaneous synaptic events
within seconds of contact (Figure 8.8B), and they con-
tinue to increase in both rate and amplitude over the
first 10 to 20 minutes (Xie and Poo, 1986). Nerve-
evoked synaptic transmission that is great enough to
elicit an action potential can be found within 15 s of
nerve-muscle contact. However, in most cases, evoked
synaptic responses continue to increase during the first
15 minutes of contact (Figure 8.8C; Sun and Poo, 1987;
Evers et al., 1989). Certain adult-like characteristics of
synaptic transmission, such as depression and facilita-
tion, are also present immediately after contact in the
neuromuscular system. Clearly, functional maturation
proceeds briskly at the NMJ in vitro. However, most
analyses of the mammalian CNS, both in vitro and in
vivo, indicate that synaptic properties take days or
weeks to reach maturity (see below).
In comparing the development of synaptic structure
and function, it is interesting that the maturation of
transmission seems to evolve far more rapidly. In
the chick ciliary ganglion, synaptic potentials can be
recorded before synapses are detected with an electron
microscope (Landmesser and Pilar, 1972). Similarly,
when a muscle is manipulated into contact with a
growth cone in a Xenopus culture, the recorded synap-
tic currents can be quite large at contacts that show no
appreciable differentiation (Buchanan et al., 1989).
Therefore, a rapid phase of functional maturation
occurs over minutes, and is due primarily to develop-
mental events that have preceded contact: the expres-
sion of a neurotransmitter release mechanism by the
growth cone and neurotransmitter receptors by the
postsynaptic cell.
Spontaneous PSCs
Evoked PSCs
Immediately after contact
5 minutes after contact
FIGURE 8.8 Muscle cell contact enhances spontaneous and
evoked transmission. A. Cultures of Xenopus spinal neurons were
grown in culture, and whole cell pipets were used to record from
round muscle cells and to manipulate them into contact with the
neuron. B. A continuous recording from a muscle cell shows spon-
tanneous transmission (downward deflections) during the first 20
minutes after contact. C. Nerve evoked postsynaptic currents
(downward deflections) increased in amplitude from the moment of
contact to 5 minutes later. (Adapted from Evers et al., 1989; Xie and
Poo, 1986)
motor axons grow out of the Xenopus spinal cord and
form functional synapses on the developing myotubes
over a period of hours (Kullberg et al., 1977). In the
fruit fly, it takes only eight hours for neuromuscular
transmission to reach a mature level of function
(Broadie and Bate, 1993a). However, it is nearly impos-
sible to record from a cell at the exact moment that it
is first contacted by a growth cone in vivo.
Fortunately, the appearance of synaptic transmis-
sion can be explored with great accuracy in dissociated
cultures. When intracellular recordings were obtained
from isolated Xenopus muscle cells, and the formation
of a neurite contact was visually monitored on a micro-
scope, it was found that synaptic potentials could
be elicited within minutes of lamellopodial contact
(Kidokoro and Yeh, 1982). To provide even better tem-
poral resolution, muscle cells were manipulated into
contact with growing neurites while they were being
Growth cones usually slow down when they enter
their target, and this may involve a signal to halt
growth cone motility and encourage synapse forma-
tion (see Chapter 6). Evidence for a target stop signal
was found in a system where newborn mouse basilar
pontine nuclei were co-cultured with their target
neurons, the granule cells of the cerebellum (Baird et
al., 1992). Pontine neurites grow rapidly on cerebellar
glial cells (greater than 100 mm/hr), indicating that glia
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