It is widely believed that the theta rhythm in the hippocampus is caused by the rhythmic input from the medial septum-diagonal band of Broca (MSDB). The main MSDB output is formed by GABAergic projection neurons which are divided into two subpopulations and fire at different phases of the hippocampal theta rhythm. The MSDB also contains projection cholinergic, glutamatergic, and non-projection GABAergic neurons. These cell populations innervate each other and also GABAergic projection neurons and participate in the formation of the synchronous rhythmic output to the hippocampus. The purpose of this study is to work out a model of interactions between all neural populations of the MSDB that underlie the formation of the synchronous septal theta signal. The model is built from biologically plausible neurons of the Hodgkin-Huxley type and its architecture reflects modern data on the morphology of neural connections in the MSDB. The model satisfies the following requirements: (1) a large portion of neurons is fast-spiking; (2) the subpopulations of GABAergic projection neurons contain endogenous pacemaker neurons; (3) the phase shift of activity between subpopulations of GABAergic projection neurons is equal to about 150°; and (4) the strengths of bidirectional connections between the subpopulations of GABAergic projection cells are different. It is shown that the theta rhythm generation can be performed by a system of glutamatergic and GABAergic non-projection neurons. We also show that bursting pacemaker neurons in the subpopulation of projection GABAergic neurons play a significant role in the formation of stable antiphase outputs from the MSDB to the hippocampus.

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Brain rhythms are essential for information processing in neuronal networks. Oscillations recorded in different brain regions can be synchronized and have a constant phase difference, that is, they can be coherent. Coherence between local field potential (LFP) signals from different brain regions may be correlated with the performance of cognitive tasks, indicating that these regions of the brain are jointly involved in the information processing. Why does coherence occur and how is it related to the information transfer between different regions of the hippocampal formation? In this article, we discuss possible mechanisms of theta and gamma coherence and its role in the hippocampus‐dependent attention and memory processes, since theta and gamma rhythms are most pronounced in these processes. We review in vivo studies of interactions between different regions of the hippocampal formation in theta and gamma frequency bands. The key propositions of the review are as follows: (1) coherence emerges from synchronous postsynaptic currents in principal neurons as a result of synchronization of neuronal spike activity; (2) the synchronization of neuronal spike patterns in two regions of the hippocampal formation can be realized through induction or resonance; (3) coherence at a specific time point reflects the transfer of information between the regions of the hippocampal formation; (4) the physiological roles of theta and gamma coherence are different due to their different functions and mechanisms of generation. All hippocampal neurons are involved in theta activity, and theta coherence arranges the firing order of principal neurons throughout the hippocampal formation. In contrast, gamma coherence reflects the coupling of active neuronal ensembles. Overall, the coherence of LFPs between different areas of the brain is an important physiological process based on the synchronized neuronal firing, and it is essential for cooperative information processing.

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We propose a model of the main rhythms in the hippocampal CA1 field: theta rhythm; slow, middle, and fast gamma rhythms; and ripple oscillations. We have based this on data obtained from animals behaving freely. We have considered the modes of neuronal discharges and the occurrence of local field potential oscillations in the theta and non-theta states at different inputs from the CA3 field, the medial entorhinal cortex, and the medial septum. In our work, we tried to reproduce the main experimental phenomena about rhythms in the CA1 field: the coupling of neurons to the phase of rhythms, cross-rhythm phase–phase coupling, and phase–amplitude coupling. Using computational experiments, we have proved the hypothesis that the descending phase of the theta rhythm in the CA1 field is formed by the input from the CA3 field via the Shaffer collaterals, and the ascending phase of the theta rhythm is formed by the IPSPs from CCK basket cells. The slow gamma rhythm is coupled to the descending phase of the theta rhythm, since it also depends on the arrival of the signal via the Shaffer collaterals. The middle gamma rhythm is formed by the EPSPs of the principal neurons of the third layer of the entorhinal cortex, corresponds to experimental data. We were able to unite in a single mathematical model several theoretical ideas about the mechanisms of rhythmic processes in the CA1 field of the hippocampus.