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1.8. Efferent Projection of BFC neurons

Although there is considerable species variation in the precise locations of cholinergic projection neurons in the basal forebrain, the efferent projections of these cells follow basic organizational principles in all vertebrate species studied. That is BFC neurons show a rough ventrolateral-dorsomedial and rostro-caudal topography towards their target areas, although some projections, possibly determined by developmental factors, do not fit into a simple topographical scheme. In tha rat, neurons within the medial septum and nucleus of the vertical limb of the diagonal band (MS/VDB; also termed Ch1/Ch2 according to the classification of Mesulam et al., 1983b) provide the major cholinergic innervation of the hippocampus; cholinergic cells within the horizontal limb of the diagonal band and magnocellular preoptic nucleus (HDB/MCP; Ch3) project to the olfactory bulb, piriform, and entorhinal cortices; cholinergic neurons located in the ventral pallidum, sublenticular substantia innominata (SI), globus pallidus, internal capsule, and nucleus ansa lenticularis, collectively termed the nucleus basalis (Ch4), project to the basolateral amygdala, and innervate the entire neocortex (Sofroniew et al., 1982; Armstrong et al., 1983; Mesulam et al., l983a,b; Lamour et al., l984b; Rye et al., l984; Woolf et al., l984; Amaral and Kurz, l985; Carlsen et al., l985; Wainer et al., l985; Woolf et al., l986; Z‡borszky et al., 1986a; Luiten et al., l987; Nyakas et al., l987; Sofroniew et al., l987; Fisher et al., l988; Gaykema et al., 1990).

In the primate the corticopetal cholinergic cells (Mesulam et al., 1983b;Mesulam et al., 1986a; see also Butcher and Semba, 1989; Heimer et al.,1991a) are subdivided according to the topography of their projections. Thus, the anteromedial sector (Ch4am) projects to medial cortical areas including the cingulate gyrus; the anterolateral compartment (Ch4al) to ventral orbital, frontal, and parietal opercular regions, as well as the amygdala; the intermediate sector (Ch4i) to lateral frontal, parietal, peristriate and temporal regions including the insula; and the posterior sector (Ch4p) to superior temporal and temporopolar areas (Mesulam et al., l983a, l986a; Mesulam and Geula, l988). * (Fig.1.8-1) and (Fig.1.8-2) demonstrate the main principles of this projection in primates and rodents, respectively.

Fig.1.8-1 Fig.1.8-2