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feed back more current into the parent dendrite.
The successive current injections contribute to
uphold the depolarizing envelope in the apical
shaft, which may be critical to set the pseudosalta-
tory forward AP conduction. Even lateral firing
alone without an apical shaft spike may still be
considered pseudosaltatory forward conduction
when it determines the immediate cell output.
arrival of inputs (Stevens and Zador, 1998; Softky
and Koch, 1993). An irregular pattern of synchro-
nous inputs can thus be translated in the occasional
firing of local spikes in a dendritic branch/domain,
collected by an excitable main dendrite that con-
veys all of them up to the soma/axon. The final
temporal structure of the output that can be read
in the axon is the summation of spikes generated
in all dendritic substations (Figure 4).
A generic conclusion that can be drawn is that
dendritic integration of local spikes is extraordi-
narily varied and subtle. The vast parametric space
within which the dendritic trees appear to work is
probably on the basis of the apparent unpredictabil-
ity of cell output. Whether this is true in real cells
awaits future studies. Recent work emphasizes
the importance of homeostatic mechanisms that
maintain firing regimes of a particular cell after
severe changes of some influential parameters,
as if neurons self-adapt the weight of intrinsic
electrogenic machinery in response to adverse or
unexpected conditions (Turrigiano, 2007). Being
that so, the range of some parameters, as found
in experiments, is much narrower than currently
thought, and large variability only results from
the variable perturbation of highly disturbing
recording techniques.
Alternatively, the unpredictability of the output
may reflect the subtle processing of inputs before
an output decision is taken. Most input patterns
would thus be insufficient to directly set an output.
Still, we speculate that the apical tree as a whole
is “tuned” to make immediate output decisions
for specific sequences or combinations of inputs
activated within a narrow temporal window. In
essence, that would be the main computational
difference as compared to the traditional view of
synaptic integration based on spatiotemporal sum-
mation of slow synaptic potentials. A few spikes
initiated in lateral branches will have an output
efficiency equivalent to hundredths of synaptic
potentials in absence of local spikes. Besides,
integration based on local spikes has the chance
OUTPUT DECISION IN DENDRITES:
WHAT FOR?
So far, the dendritic electrogenesis described in
the previous section remains in the category of the
theoretical possibilities, as it has not been shown
in actual experiments. Unfortunately, these have
not attained enough technical resolution to record
from intact dendritic branches without strongly
modifying their electrical behavior. Some prelimi-
nary studies (Gasparini et al., 2004) apparently
support the gross features.
We may ask what advantages can be obtained
from multisite dendritic decisions contributing to
cell output? One possibility is that local dendritic
spikes can be used as an internal language be-
tween different cell subregions so that cell output
becomes an elaborated response arising from the
cross-talk between somato-axonal and dendritic
spike generators. Such chattering between two
separated action potential trigger zones (axon and
apical dendrite) has already been shown in corti-
cal pyramidal cells to delineate the final output
pattern trough the axon (Larkum et al., 2001).
In the beginning of this chapter, we mentioned
that many neuron types fire very poorly and ir-
regularly. In general, these correspond to neurons
dealing with inputs of varied nature, i.e., neurons
ranking high in the canonical arbor of complex
associations. Their irregular firing is compatible
with output decision made in dendrites. Some
theoreticians have indicated that irregular firing
is not consistent with random input, as one might
think. Instead, they postulated the synchronous
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