Three-dimensional morphology and distribution of pallidal axons projecting to both the lateral region of the thalamus and the central complex in primates

This study presents a three-dimensional analysis of pallido-thalamic axons and axonal endings in the monkey (Macaca mulatta and M. irus). Injections of the anterograde tracer biocytin were made in the dorsal, associative region of the medial pallidum. Numerous axonal endings were observed within the pallidal territory of the lateral region of the thalamus and the central complex. Individual axons were reconstructed from serial sections and traced in three dimensions. Two axons made a collateral branch in the ventral part of the lateral region and ended in the central complex. In the pallidal territory of the lateral region, axons divided several times before ending in different parts of the territory in a 'bunch', a characteristic dense terminal arborization. Axonal endings in the central complex were differently organized. Our data show that associative medial pallidal information is distributed throughout the pallidal territory of the lateral region and the pars media of the central complex by means of individual axons with numerous branches and axonal endings specific to each of the two targets.