Neuronal Synchronization and Thalamocortical Rhythms in Sleep, Wake and Epilepsy

Excerpt

Neuronal synchronization can be defined as a correlated appearance in time of two or more events associated with various aspects of neuronal activity. Neuronal synchronization depends on chemical and electrical synaptic as well as ephaptic and non-specific interactions. We consider two distinct types of neuronal synchronization: local synchronization that is responsible for the generation of local field potentials; and long-range synchronization, detected with distantly located electrodes and mediated primarily via chemical synaptic interactions, which contributes to the EEG synchronization. Neocortical synchronization during sleep and wakefulness is often associated with rhythmic oscillations of neuronal activity: slow oscillation, delta, spindle, beta, gamma and ripples. Normal thalamocortical oscillations [sleep or wake oscillations] are generated as a result of both local and long-range synchronization. During paroxysmal [seizure] activity, the role of chemical synaptic interactions decreases because of alterations in ionic composition that impairs synaptic transmission. Synchronized activities in large population of neurons (such as neocortex) may occur as nearly simultaneous patterns across an entire population or as propagating waves. Neocortical synchronization is controlled by the activities in ascending systems: cholinergic, norepinephrinergic and serotoninergic. The presence of cortico-thalamo-cortical feedback loops contribute to the synchronization of cortical activities. We propose that all the types of neuronal interactions contribute to the generation of synchronous oscillatory activities, but the ratio of their contribution is different for different types of oscillations.