The activity of thalamus and cerebral cortex neurons in rabbits during "slow wave-spindle" EEG complexes

"Slow wave-spindle" complexes were studied during slow wave sleep in rabbits at the thalamic (medial thalamus) and cortical (upper and lower layers of the sensorimotor cortex) levels. Slow wave complexes are biphasic positive-negative complexes or triphasic complexes with a predominantly negative component. Spindles have characteristics close to those of spontaneous sleep spindles. Complexes arise singly, as though inserted into the rhythm of spontaneous sleep spindles, or in series with periods similar to the spindle rhythm. Medial thalamus neurons and some cortical neurons had the same activity during waves as during spindles: if the neuron decreased (increased) its spike frequency in a spindle, then decreases (increases) in frequency were also seen in slow waves; if the neuron produced trains of discharges during spindles, then trains of activity were also seen from the slow-wave part of "slow wave-spindle" complexes. The membrane potential changed in a similar fashion: on a background of hyperpolarization which started at the slow wave, individual depolarization oscillations appeared in the EEG wave rhythm; these oscillations were not always accompanied by spike trains. The slow wave mechanism, the rhythms of isolated complexes and simultaneous complexes and spontaneous sleep spindles may share a common underlying mechanism: slow, cyclical variations in excitability in thalamocortical neuronal networks, which have previously been demonstrated for spindle-like activity. The possibility that there are common mechanisms for slow waves in complexes and other EEG slow waves, particularly delta activity, remains hypothetical.