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Synchronization and information processing in thalamus and cortex during slow sleep oscillation

Igor Timofeev
Dept of Anatomy and Physiology, Laval University and The Centre de Recherche Université Laval Robert-Giffard (CRULRG), Canada

During quiet wakefulness cortical neurons are spontaneously active. Slow-wave sleep is characterized by spontaneous alternations of active (depolarized) and silent (hyperpolarized) states. Active (not UP) states are also recorded in thalamocortical neurons during cortical active states. During silent states the neuronal firing is absent. It is not clear how the active states are generated and how the alteration of active and silent states affect information transfer in thalamocortical system. I will provide data showing that active states are triggered by summation of subthreshold spike-independent synaptic events (minis). The active states start when the minis depolarize postsynaptic membrane to the firing threshold. Once started, the active states propagate to different directions. In neocortex of cats the active states start at any location but most often from a border of area 5 and 7. The end of active states occurs almost simultaneously in all involved neurons suggesting synaptic and not intrinsic mechanism responsible for active state termination. Multisite intracellular and field potential recordings show that deeply laying neurons lead the onset of active states. Generation of activity in cortical and thalamocortical neurons leads to a decrease in extracellular concentration of Ca 2+. As consequence, in neocortex the efficiency of synaptic transmission is highest during silent network states and rapidly decreases upon development of active states. Together with high level of active inhibition, this results in overall low efficiency of cortical network in information transfer during active phases of sleep. In thalamus too, generation of low-threshold Ca 2+ spikes by postsynaptically located T-channels leads to brief extracellular Ca 2+ depletion and increased failure of synaptic responses. I propose that both, active and silent network states of sleep are associated with a reduction of information transfer in thalamocortical network. Although the neurons fire during active network states the timing of action potentials is unreliable that likely causes bizarre conscious experience.

Supported by CIHR and NSERC