Cellular Bases of Rhythmic, Recurrent Activity in the Cerebral Cortex

Introduction

The brain is constantly active, even during deep sleep. In the cerebral cortex, this spontaneous activity (activity that is not obviously driven by a sensory input or a motor command) often occurs as periodic, rhythmic discharges. One form of rhythmic cortical activity that recently has been investigated extensively in vivo is the so-called "SLOW OSCILLATION" (see Steriade and colleagues, references here). The slow oscillation is characterized by periods of sustained depolarization interweaved with periods of hyperpolarization and silence at a rate of between once every 10 seconds to approximately once per every 2 seconds. The depolarized state is associated with low frequency neuronal firing and is termed the UP state, while the hyperpolarized state is referred to as, you guessed it, the DOWN state. The frequent transitions between the UP and DOWN state can make the membrane potential of the cortical neuron appear as a single channel recording, even though the slow oscillation is generated by the interaction of thousands of cells!
Slow oscillation in vivo and its disruption by stimulation of the brainstem. The top trace is an intracellular recording from a cortical pyramidal cell during the slow oscillation. The bottom trace is the electrocorticogram (EcoG). Repetitive stimulation of the brainstem results in activation of the EEG and suppresses the "down" state of the slow oscillation. From Steriade, M Amzica, F, Nunez A. 1993. Cholinergic and noradrenergic modulation of the slow (approximately 0.3 Hz) oscillation in neocortical cells. J. Neurophysiol. 70: 1385-1400. 

The slow oscillation is generated in the Cerebral Cortex, since removal of the thalamus does not block the slow oscillation and isolation of slabs of cortex retain this activity (reference). However, previous investigators have not demonstrated the slow oscillation in vitro in cortical slices. Here, we demonstrate the slow oscillation in cortical slices maintained in vitro, and we use this preparation to demonstrate how this activity is generated. Finally, we speculate on the functional significance of the ability of the cerebral cortex to generate rhythmic, recurrent activity and the possible functional significance of spontaneous activity in general.