Pallidal neurons in the rat.

The globus pallidus has been examined in rat brains with Golgi methods. Most of the impregnated cells, the typical pallidal neurons, have relatively large cell bodies and thick, infrequently branched dendrites that are several hundred microns long. Most dendrites have one or two spines, some of them are moderately spiny, and a few are quite spiny. Although the dendrites generally end by simply becoming thinner and beaded, they occasionally form special dendritic ramifications, which are similar to the complicated dendritic endings reported in primate brains. The variability in the size of the somata and in the structure of the dendrites is not sufficiently consistent to permit dividing the neurons into distinctive subsets. However, two forms of dendritic trees can be defined. The neurons in the center of the pallidum have radiate dendritic trees, whereas the cells along the borders have compressed dendritic trees. Two axonal patterns have been seen: ones with and ones without collaterals. All of the axons are beaded. Two other cell types were found. The special border cells along the external medullary lamina in caudal pallidum have dendrites that extend for some distance into the caudate-putamen. They otherwise resemble typical pallidal neurons. Small neurons that were infrequently impregnated may be interneurons, but their axons were not visualized. Their dendrites are short, varicose, and have a few spines. The spherical dendritic trees have a radius of 150-170 micron. Two sorts of axons that are probably afferent fibers were observed. The more common ones are nonbeaded, thin axons that have several boutons en passant and collaterals spaced along their length. In comparison, the other afferent fiber has numerous swellings, boutons en passant, and collaterals that are crowded together. They appear to invest the dendrites closely.