Abstract
Previous immunohistochemical studies have provided evidence that the external segment of the globus pallidus extends ventrally beneath the transverse limb of the anterior commissure into the area of the substantia innominata. Enkephalin-positive staining in the form of "woolly fibers" has been used as a marker for the globus pallidus and its ventral extension. Acetylcholinesterase staining of both fibers and cell bodies, frequently used as a marker for the basal nucleus of Meynert, is also found in the area of the substantia innominata. This study describes the differential distribution of enkephalin-positive woolly fibers and acetylcholinesterase staining on adjacent sections in both the monkey and human basal forebrain area in an attempt to define the relationship between the basal ganglia and the basal nucleus of Meynert. The results show that while both occupy large regions of the basal forebrain, they overlap very little. In both species investigated, dense concentrations of acetylcholinesterase-positive neurons lie, for the most part, outside the boundaries of the pallidal fibers. However, some scattered acetylcholinesterase cells do lie within the confines of the dorsal pallidum, and a more prominent group is found in the subcommissural ventral pallidum. These cells may constitute a group separate from the more densely packed acetylcholinesterase-positive cells in woolly fiber-free regions in that they may receive direct striatal input.