Forward Dendritic Spikes - Advancing Artificial Intelligence Through Biological Process Applications - page 50

Parametric Studies: Diving Deep into
Dendrites

port such functioning. The particular concept of
aborted forward propagation of distally initiated
spikes did not represent any challenge since it was
well known from early theoretical studies in axons
that flaring cables produce the blockade of spike
propagation (Goldstein and Rall, 1974). A similar
conclusion was reached in some theoretical stud-
ies applied to active dendrites (Tóth and Crunelli,
1998). However, opposing to this widespread
notion, we found that the moderate flare of the
apical shaft in pyramidal cells actually works as
a loudspeaker amplifying inward currents and
fostering the forward propagation of distally
initiated spikes (López-Aguado et al., 2002). The
mystery is not other than the optimum balance
between capacitive load and electroregenerative
capability of a given segment of dendrite. Even
with the moderate excitability attributed to this
cell element, the small flare cannot counterbal-
ance the increasing current density produced by
forward spikes (Figure 1).
In other studies, we have used a realistic py-
ramidal cell model calibrated with in vivo and
in vitro results to explore how some well known
factors govern the synaptic initiation of dendritic
action potentials and we have determined their
ability to influence cell output. Among others, we
have cross-analyzed the influence of the synaptic
loci and strength, the role of different dendritic
channels, the relative excitability of the axon and
dendrites, and inhibition. When considering only
the axon vs. the main dendrite, we found several
types of spiking, namely, aborted dendritic spikes,
full forward and pseudosaltatory propagation,
and backpropagation. The number and variety
of partial conclusions on dendritic functioning
is very large (Ibarz et al., 2006). Some follow,
just to name a few. Clustered dendritic inputs
are more effective in producing output decision
in dendrites than spatially dispersed inputs. The
higher ratio of the axon-to-dendrite excitability
was correlated with spike initiation at the axon
for inputs in the middle part of the dendritic tree,
but this effect was ill-defined for distal inputs.

More beneficial are the models aiming to explore
the impact of a given parameter on the electrogen-
esis of the different neuron domains, regardless
of whether they reproduce or not specific experi-
mental data. Some have been dedicated to study
the effects of specific dendritic architectures, the
fine geometry of dendrites, or the inhomogeneous
distribution of channels on spike propagation.
The exhaustive analysis of the parametric space
is necessary since different combinations of so-
matodendritic channel types and distributions can
reproduce the physiological events of interest at a
single locus (e.g., Cook and Johnston, 1999), with-
out this implying that the events in other neuron
sites are even closer to reality. That is, one may
reproduce faithfully the electrical behavior of a
small neuron domain at the expenses of getting
out of range everywhere else.
In addition, when some parameters are fixed,
the number of possible solutions for the model
is limited enormously. While it may be advan-
tageous when the choice is correct, it may be
misleading in other cases. For instance, using a
fixed channel assortment in different prototype
model neurons, Vetter et al. (2001) showed that
dendritic architectures may define the mode of
spike initiation and propagation. When this par-
ticular channel assortment was implemented in
pyramidal-like cells, dendritically initiated spikes
do not propagate forwardly. However, there are
already numerous experimental observations
showing this is not true (Larkum et al., 2001;
Williams, 2004). Obviously, different cells have
dissimilar channel assortment, surely because
it is the function and not the structure what has
been preserved by evolution.
One important issue is the computational role
of the main apical dendrite. Since the initial ex-
perimental observations favored the idea of back
over forward propagation of spikes along this cell
element, the model studies also appeared to sup-