Figure 1.

Theta oscillations. (A) Example local field potential (LFP) trace during track running. (B) Simplified schematic of theta generation. The medial septum-diagonal band of Broca (MSDB) sends theta-rhythmic GABAergic (g), cholinergic (c) and glutamatergic projections to the hippocampus (glutamatergic neurons appear to entrain other septal neurons, but not hippocampal theta, and are not represented here). Rhythmic inhibition (red, GABA) of highly interconnected perisomatic interneurons (g) constrains the timing of pyramidal cell spikes. The entorhinal cortex (EC) also sends theta-paced outputs to the hippocampus through the perforant path. CA3 outputs targeting stratum radiatum constitute another likely intrahippocampal theta generator. In addition, the dendritic membrane of CA1 pyramidal cells is endowed with resonant properties at theta frequency. Finally, hippocampal long-range backpropagating interneurons target the MSDB, arguing for cyclical (non-unidirectional) mechanisms in the generation of theta. (C) Power spectral density of dorsal CA1 LFP as a function of running velocity (logarithmic scale; average over four rats). At low velocities, power is dominated by a single peak located between 6 and 7 Hz. As velocity increases, the peak quickly narrows, shifts toward 8 Hz, while additional peaks (harmonics) develop at frequencies n × 8 Hz (up to n = 6 for dorsal CA1 region, n = 2 for intermediate CA1). Reproduced from Sheremet et al. (2016).