The basal forebrain is critically involved in functions representing the highest levels
of integration. Only recently has a relatively clear anatomical picture of this important
area begun to emerge. The territory that has generally been referred to as the "substantia
innominata" appears to be composed of portions of three recognizable forebrain structures:
the ventral striatopallidal system, the extended amygdala and the magnocellular corticopetal
system. (1) Rostrally, the striatopallidal system reaches ventrally to the base of
the brain. (2) Caudal to the ventral extension of the striatopallidal system elements
of the centromedial amygdala and bed nucleus of the stria terminalis are merged so
that these two areas together with this subpallidal corridor form a large forebrain
unit that might be described as an "extended amygdala". (3) Large cholinergic and
non-cholinergic corticopetal neurons form a more or less continuous aggregate that
is interwoven with the striatopallidal and extended amygdala systems in basal forebrain.
Consideration of morphological and connectional characteristics of basal forebrain
suggests that the corticopetal cell groups, together with magnocellular elements of
the striatum, serve similar functional roles for the striatopallidal system, the extended
amygdala, and the septal-diagonal band complex. Specifically, the output of medium
spiny neurons in striatum, extended amygdala, and lateral septum are directed toward
somewhat larger sparsely or moderately spiny neurons with radiating dendrites which
in turn project to diencephalon and brainstem or provide either local feedback (e.g.
in striatum) or distal feedback to cortex. The functional implications of this parallel
processing of descending forebrain afferents are discussed.