ABSTRACT
The mammalian central nervous system displays several electroencephalographic (EEG) rhythms that differ in their frequencies, behavioral correlates and in the neuronal mechanisms that are responsible for their generation. In this chapter I concentrate on the role of intrinsic properties and synaptic inputs underlying the rich repertoire of electrical behaviors exhibited by neurons in thalamocortical and cortico-hippocampal circuits. It will be shown that although the basic delta oscillation of thalamocortical neurons is generated by the interplay of two intrinsic inward currents, the activity patterns observed in these cells either in wakefulness or in sleep, in physiological states as well as in pathological conditions, are more than the simple expression of the intrinsic membrane conductances of neurons. Indeed, synaptic potentials (either rhythmic or randomly occurring) and different transmitters modulate, abolish, and sometimes are fully responsible for the occurrence of specific electrical activity patterns. In contrast to thalamocortical cells, pharmacologically or physically isolated hippocampal neurons are not capable of exhibiting membrane potential oscillations. In hippocampal principal cells it is the synaptic inputs that play the major role in the generation of electrical rhythms such as the hippocampal theta and 40-Hz oscillations. However, intrinsic currents can modulate the synaptically driven hippocampal theta oscillations. Finally, I discuss some of the available evidence and recent hypotheses for the functional roles of neuronal oscillations, and for the roles of rhythmic as well as random synaptic events which bombard neuronal membranes at particular frequencies.
