Diagonal band

The diagonal band of Broca (DBB) is part of the basal forebrain cholinergic complex and comprises vertical and horizontal limbs with associated neuronal populations.

Vertical limb of diagonal band of Broca

The nucleus of the vertical limb of the diagonal band contains the second-largest cholinergic cell group in the human brain and has major projections to the hippocampus. Its anatomical boundaries overlap with neighbouring basal forebrain nuclei, particularly the medial septal region, which can make precise delineation difficult.

Horizontal limb of diagonal band of Broca

The horizontal limb contains neurons distributed among the fibres of the horizontal DBB and beneath the substantia innominata; in primates, only a relatively small proportion of these neurons are cholinergic.

The diagonal band of Broca (DBB) also contains non-cholinergic neuronal populations, including GABAergic and glutamatergic neurons.

Functional and clinical correlates

Through its connections with the hippocampal and other limbic regions, particularly the prominent hippocampal projections of the vertical diagonal band, the DBB contributes to learning and memory, with the vertical diagonal band particularly implicated in memory retrieval. Neuronal loss within the vertical DBB has been reported in ageing and several neurological disorders, with cholinergic neuronal loss in Lewy body dementias being of particular clinical interest.