Piecing together the Puzzle of Basal Forebrain Anatomy

Abstract

The basal forebrain contains a seemingly heterogeneous collection of structures including nucleus accumbens, olfactory tubercle, septum, diagonal band nuclei, bed nucleus of stria terminalis, substantia innominata, olfactory cortex, hippocampus formation and amygdaloid body. It is also traversed by a number of large fiber tracts, e.g. fornix, stria terminalis, diagonal band of Broca, medial forebrain bundle, inferior thalamic peduncle, and ventral amygdalofugal pathway, to which the various basal forebrain structures contribute axons in order to establish connections between themselves and with other parts of the brain. Hypothalamus, the main diencephalic component of the basal forebrain, is one such region closely related to many of the telencephalic basal forebrain structures and fiber tracts. These intimate relations to the hypothalamus provided much of the anatomical rationale to bring the above-mentioned basal forebrain structures together as integral parts of the “limbic system”. This has contributed to the popular view of forebrain organization in which the neocortex is related to the basal ganglia or the “extrapyramidal motor system” through the well-known cortico-striato-pallidal pathways, while so-called limbic structures, e.g. septum, nucleus accumbens, amygdaloid body and allocortical areas like hippocampus and olfactory cortex are characterized foremost by their relation to the hypothalamus, a major regulator of autonomic and endocrine functions.