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The organization of projections from the cholinergic neurons of the basal forebrain to neocortex and associated structures was investigated in the rhesus monkey with the help of horseradish peroxidase transport, acetylcholinesterase histochemistry, and choline acetyltransferase immunohistochemistry. Four groups of neurons contained cholinergic perikarya and were designated as Chl-Ch4. The C h l group corresponds to the medial septa1 nucleus; about 105% of its neurons are cholinergic, and it provides a substantial projection to the hippocampus. The Ch2 group corresponds to the vertical nucleus of the diagonal band; a t least 70%' of its neurons are cholinergic, and it is the major source of innervation that the hippocampus and hypothalamus receive from the Chl-Ch4 complex. The Ch3 group most closely corresponds to the horizontal nucleus of the diagonal band; only 1 % of its neurons can definitely be shown to be cholinergic, and it is the major source of Chl-Ch4 projections to the olfactory bulb. The Ch4 group most closely corresponds to the nucleus basalis of Meynert; a t least 90% of its neurons are cholinergic, and it has projections to widespread areas of cortex and to the amygdala. In fact, the Ch4 group provides the single major source of cholinergic innervation for the entire cortical surface. In this respect, it is analogous to the raphe nuclei and to the nucleus locus coeruleus, which constitute the major sources of widespread cortical serotonergic and noradrenergic innervation, respectively.The extensive Ch4 group can be divided into several subdivisions. Each subdivision has a preferential set of targets for its projections even though the connection patterns contain considerable overlap. The anteromedial subdivision of Ch4 is the major source of cholinergic projections to areas on the medial aspect of the cerebral hemispheres: the anterolateral Ch4 subdivision is the major source of cholinergic projections to frontoparietal opercular areas and to the amygdala; the intermediate Ch4 subdivision provides the major cholinergic input for a variety of dorsal prefrontal, insular, posterior parietal, inferotemporal, and peristriate areas; and the posterior subdivision of Ch4 provides the major cholinergic innervation of superior temporal and immediately adjacent areas.

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The pedunculopontine tegmental nucleus (PPTn) was originally defined on cytoarchitectonic grounds in humans. We have employed cytoarchitectonic, cytochemical, and connectional criteria to define a homologous cell group in the rat. A detailed cytoarchitectonic delineation of the mesopontine tegmentum, including the PPTn, was performed employing tissue stained for Nissl substance. Choline acetyltransferase (ChAT) immunostained tissue was then analyzed in order to investigate the relationship of cholinergic perikarya, dendritic arborizations, and axonal trajectories within this cytoarchitectonic scheme. To confirm some of our cytoarchitectonic delineations, the relationships between neuronal elements staining for ChAT and tyrosine hydroxylase were investigated on tissue stained immunohistochemically for the simultaneous demonstration of these two enzymes. The PPTn consists of large, multipolar neurons, all of which stain immunohistochemically for ChAT. It is present within cross-sections that also include the A-6 through A-9 catecholamine cell groups and is traversed by catecholaminergic axons within the dorsal tegmental bundle and central tegmental tract. The dendrites of PPTn neurons respect several nuclear boundaries and are oriented perpendicularly to several well-defined fiber tracts. Cholinergic axons ascend from the mesopontine tegmentum through the dorsal tegmental bundle and a more lateral dorsal ascending pathway. A portion of the latter terminates within the lateral geniculate nucleus. It has been widely believed that the PPTn is reciprocally connected with several extrapyramidal structures, including the globus pallidus and substantia nigra pars reticulata. Therefore, the relationships of pallidotegmental and nigrotegmental pathways to the PPTn were investigated employing the anterograde autoradiographic methodology. The reciprocity of tegmental connections with the substantia nigra and entopeduncular nucleus was investigated employing combined WGA-HRP injections and ChAT immunohistochemistry. The pallido- and nigrotegmental terminal fields did not coincide with the PPTn, but, rather, were located just medial and dorsomedial to it (the midbrain extrapyramidal area). The midbrain extrapyramidal area, but not the PPTn, was reciprocally connected with the substantia nigra and entopeduncular nucleus. We discuss these results in light of other cytoarchitectonic, cytochemical, connectional, and physiologic studies of the functional anatomy of the mesopontine tegmentum.