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. 2021 Jul 8;15:701080. doi: 10.3389/fncir.2021.701080

FIGURE 1.

FIGURE 1

Schematic diagram of the septal connections discussed in this article. The three major neuronal populations maintain a delicate intraseptal network. The medial septal region receives a variety of afferents from the lateral septum, PFC, and forms bidirectional connection with amygdala, EC and a broad range of other neocortical areas, hypothalamus, and brainstem. Unidirectional, overwhelmingly cholinergic efferent fibers are innervating the thalamus and the lateral and medial habenula. For simplicity, specific neurotransmitters and the intraseptal origins or targets of the pathways are only marked in the septohippocampal connections. The medial septum is highly interconnected with the hippocampus as well. These pathways have significant roles in generation or regulation of different hippocampal oscillations. Colored arrows indicate important connections contributing to oscillations in the septo-hippocampal axis. Magenta arrows show glutamatergic, cyan cholinergic, green (blunt) GABAergic, and orange serotonergic innervations. Glutamatergic neurons in the medial septum more likely regulate other theta generating medial septal neuronal populations; however, roles of their projections to the hippocampus in oscillations are largely unknown. The GABAergic neurons have roles in theta generation by disinhibiting the hippocampal pyramidal neurons. The cholinergic connections are not only important in the theta generation, but they also suppress SPW-R generation although whether this suppression acts on the hippocampal pyramidal or interneurons remains elusive. ACC, anterior cingulate cortex; dlTN, dorsolateral tegmental nucleus; EC, entorhinal cortex; IN, interneuron; LC, locus coeruleus; LH, lateral habenula; LHyp, lateral hypothalamus; MH, medial habenula; PCm, medial precentral cortex; NI, nucleus incertus; NTS, nucleus tractus solitarius; OB, olfactory bulb; OFC, orbitofrontal cortex; PCC, posterios cingulate region; PC, piriform cortex; pHyp, posterior hypothalamus; PFC, prefrontal cortex; PMB, posterior mammillary bodies; PY, pyramidal neuron; nPO, nucleus pontis oralis; RS, retrosplenial cortex; SI, substantia innominate; SuM, supramammillary nucleus; TMN, tuberomammillary nucleus; VLM, ventrolateral medulla; VTA, ventral tegmental area.