Corticothalamic resonance, states of vigilance and mentation

During various states of vigilance, brain oscillations are grouped together through reciprocal connections between the neocortex and thalamus. The coh erent activity in corticothalamic networks, under the control of brainstem and forebrain modulatory systems, requires investigations in intact-brain a nimals. During behavioral states associated with brain disconnection from t he external world, the large-scale synchronization of low-frequency oscilla tions is accompanied by the inhibition of synaptic transmission through tha lamocortical neurons. Despite the coherent oscillatory activity, on the fun ctional side there is dissociation between the thalamus and neocortex durin g slow-wave sleep. While dorsal thalamic neurons undergo inhibitory process es due to the prolonged spike-bursts of thalamic reticular neurons, the cor tex displays, periodically, a rich spontaneous activity and preserves the c apacity to process internally generated signals that dominate the state of sleep. In vivo experiments using simultaneous intracellular recordings from thalamic and cortical neurons show that short-term plasticity processes oc cur after prolonged and rhythmic spike-bursts fired by thalamic and cortica l neurons during slow-wave sleep oscillations. This may serve to support re sonant phenomena and reorganize corticothalamic circuitry, determine which synaptic modifications, formed during the waking state, are to be consolida ted and generate a peculiar kind of dreaming mentation. in contrast to the long-range coherent oscillations that occur at low frequencies during slow- wave sleep, the sustained fast oscillations that characterize alert slates are synchronized over restricted territories and are associated with discre te and differentiated patterns of conscious events. (C) 2000 IBRO. Publishe d by Elsevier Science Ltd. All rights reserved.