Septal nuclei

The septal nuclei are a group of basal forebrain nuclei situated beneath and contiguous with the septum pellucidum. In the cholinergic basal forebrain classification, the septal region corresponds mainly to the Ch1 and Ch2 cell groups. It includes the ventrolateral, dorsolateral, intermediolateral and septofimbrial nuclei, with the vertical limb of the diagonal band of Broca situated medially.

Ch1 group of septal nuclei

The Ch1 group corresponds to the medial septal nucleus and is composed predominantly of small- to medium-sized neurons. Butler et al., suggested approximately 10% being cholinergic.

Ch2 group of septal nuclei

The Ch2 group corresponds to the nucleus of the vertical limb of the diagonal band of Broca and contains larger neurons, with approximately 70% being cholinergic.

Butler et al. use the Ch1–Ch2 region collectively when discussing the septal basal forebrain. For teaching purposes, Ch1 = medial septal nucleus, while Ch2 = nucleus of the vertical limb of the diagonal band of Broca. And together they constitute major magnocellular/cholinergic components of the septal basal forebrain region. This also explains why the vertical DBB can appear under both “diagonal band” and “septal” descriptions. Anatomically and cytoarchitecturally these basal forebrain cell groups are closely continuous.

Functional correlates

The septal nuclei are strongly interconnected with the hippocampal formation through the fimbria and fornix. This septo-hippocampal system plays an important role in generating and regulating hippocampal theta rhythm, which is closely associated with learning and memory.