|5. Neural Oscillations at the Intrinsic Resonance Frequency||MT|
Neural oscillations are rhythmic or repetitive neural activity in the central nervous system. Figure 1 shows two examples for a single neuron, whose membrane potential changes periodically either below or above the threshold. The subthreshold oscillation does not generate action potentials whereas stronger stimulation may produce a train of action potentials (spikes) at certain frequency.
Most neurons in the central nervous system respond selectively to inputs at a preferred frequency. This feature is known as "resonance", which arises from both passive and active membrane properties. The passive property always filters out high frequencies, while the active property may filter out low frequencies, thereby resulting in a preferred frequency (Hutcheon and Yarom, 2000; PDF; Figure 2). The active property is determined by ion channels, particularly the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel, which regulates the alpha rhythm generated by the lateral geniculate nucleus (LGN) of the thalamus (Kanyshkova et al., 2009; Hughes et al., 2011), and the theta rhythm originating from the medial septum (Varga et al., 2008). The delta rhythm could be produced by coupling between the HCN channel and a low threshold calcium channel (Lüthi and McCormick, 1998). The gamma band (30 - 80 Hz) is regulated by Kv1 (Sciamanna and Wilson, 2011). Another potassium channel, Kv3, is important for faster spiking (Erisir et al., 1999). Hence, a group of neurons can oscillate at the same frequency, as long as they have the same intrinsic property.
The oscillation frequency may also depend on network activity. For instance, the sleep spindles, characterized by intermittent oscillations at the frequency 8-14 Hz, are generated in the thalamus through an interaction between the GABAergic neurons of the thalamic reticular nucleus and the excitatory thalamic relay cells (Lüthi and McCormick, 1998).
An oscillation is characterized not only by the frequency, but also by the phase. For a group of neurons to oscillate at the same frequency and phase (i.e., synchronize), they must interact with each other, or with the same oscillating source. Three forms of interactions have been well established: (1) via gap junctions (Chapter 6), (2) via GABAergic interneurons (Chapter 7) and (3) via ephaptic coupling (Chapter 8).
Author: Frank Lee