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the protection of the brain immediately after trauma, during surgical procedures
and in recovery of patients (Sessler, 2001; Varathan
et al
. 2001; McIntyre
et al
.
2003). We studied the effect of hypothermia on network dynamics, by observing
the apparent changes in the network activity during and following application of
cold (Rubinsky
et al
. 2007; Rubinsky
et al
., in press).
In our experiments, we exposed cultured neuronal networks to deep
hypothermia by pumping chilled water through channels in the walls of the life-
supporting chamber. This effectively reduced the temperature of the culture to
19°C. The cultures were maintained in this hypothermic state for 20 hours.
Following 20 hours of deep hypothermic conditions the cooling system was shut
down and the temperature was returned to 37°C while continuously recording for
additional 5 hours. This was repeated on 3 separate cultures.
The most pronounced and immediate observation was the drastic decrease in
the SBE rate (Fig. 12.8, top) seen within a few minutes after application of
hypothermia. This low level of activity remained constant throughout the whole
duration of hypothermia application. Moreover, as is shown in Rubinsky
et al.
(2007), during hypothermia there was a gradual decrease in burst width and burst
intensity.
Immediately following hypothermia the network exhibited a dramatic
increase in activity, with a temporary overshoot reaching 3 times the SBE rate
compared to the activity level prior to hypothermia. This overshoot lasted for
over an hour and was also accompanied by a slight elevation in the averaged
inter-neuron correlation, which afterwards gradually returned to initial levels.
This overshoot could be explained by an increase in the network excitability,
as compensation for the strong inhibition of the network activity during
hypothermia. Approximately two hours after termination of hypothermia
application the network activity gradually returned to its pre-hypothermia SBE
rate and activity level.
Homeostasis of the network activity before and after hypothermia was also
seen in the classification of the SBEs into motifs. In this example the measure of
similarity between SBEs was based on correlation analysis, as explained in Segev
et al.
(2004) and Rubinsky
et al.
(2007). In the bottom of Fig. 12.8 we see the
clustered SBE similarity matrix prior to hypothermic exposure in one of the
cultures. The resulting clustered matrices exhibit a clear block organization of
SBEs with high similarity within the blocks and low levels in between blocks,
where each block represents a separate motif of spatio-temporal pattern of
propagation. In the figure we detect three such motifs prior to hypothermia
application. These three motifs are quite evident when seen in the plots of the
