Inhibition determines membrane potential dynamics and controls
action potential generation in awake and sleeping cat cortex.
Michelle Rudolph, Martin Pospischil, Igor Timofeev and Alain
Destexhe.
Journal of Neuroscience 27: 5280-5290, 2007.
Abstract
Intracellular recordings of cortical neurons in awake cat and monkey
show a depolarized state, sustained firing and intense subthreshold
synaptic activity. It is not known what conductance dynamics
underlies such activity, and how neurons process information in such
highly stochastic states. Here, we combine intracellular recordings
in awake and naturally sleeping cats with computational models to
investigate subthreshold dynamics of conductances and how conductance
dynamics determine spiking activity. We show that during both
wakefulness and the "up-states" of natural slow-wave sleep, membrane
potential activity stems from a diversity of combinations of
excitatory and inhibitory synaptic conductances, with dominant
inhibition in most of the cases. Inhibition also provides the
largest contribution to membrane potential fluctuations.
Computational models predict that in such inhibition-dominant states,
spikes are preferentially evoked by a drop of inhibitory conductance,
and that its signature is a transient drop of membrane conductance
prior to the spike. This pattern of conductance change is indeed
observed in estimates of spike-triggered averages of synaptic
conductances during wakefulness and slow-wave sleep up-states. These
results show that activated states are defined by diverse
combinations of excitatory and inhibitory conductances with
pronounced inhibition, and that the dynamics of inhibition is
particularly effective on spiking, suggesting an important role for
inhibitory processes in both conscious and unconscious cortical
states.
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