Intracellular oscillatory rhythms in pyramidal tract neurones in the cat

Abstract

Intracellular micro-electrode records from pyramidal tract neurones during spindle bursts in cerveau isolé pyramidal cat preparations, or with light barbiturate anaesthesia, has shown a close relationship between spindle waves and much larger oscillations in membrane potential of PT cells. Intracellular oscillations are composed of excitatory (depolarizing) and inhibitory (polarizing) components of 5–17 mV amplitude. Excitatory components correspond to the surface negative spindle wave or recruiting response, and are most prominent in stable cells with a high resting MP and low rate of spontaneous firing. Inhibitory phases of intracellular oscillation, most prominent in partially depolarized active cells are not so clearly reflected in the form of surface spindle waves. The inhibitory phase may arrest spontaneous firing and decrease the probability of intracellular invasion of antidromic impulses. Although intracellular oscillations and occasionally a burst of spindle waves may be evoked by antidromic volleys, such stimuli are far less effective than intralaminar thalamic stimulation in controlling spindle bursts. Axon collateral inhibition, though present and long lasting in PT neurones, does not seem to be the most important mechanism generating spontaneous spindle bursts in the cortex. Spindle bursts are most likely composed of synchronized alternating excitatory and inhibitory post-synaptic potentials distributed over the extent of the soma-apical dendritic membrane of pyramidal cells, the more superficially located synapses being largely responsible for the surface negative excitatory waves. Surface positive waves may be also excitatory and slow surface negative waves may be associated with long lasting polarizing waves in PT neurones. Multiple intracortical, thalamo-cortical, and cortico-thalamic reverberating rhythmic systems, normally loosely linked and capable of a degree of independent action, must be involved in the generation of the variety of waves and rhythms of the EEG. Pacemaker neuronal systems in the thalamus probably provide the timing for many cortical rhythms, even though intracortical axon collateral or other self re-exciting circuits may participate in generating rhythmically repeated discharge, and may become at times independent of thalamic control.