AbstractThe ultrastructural organization of association nuclei in the primate thalamus is largely unexplored. In the present study we have combined electron microscopy with immunocytochemistry for the acetylcholine synthesizing enzyme choline acetyltransferase (ChAT) to assess the cholinergic synaptic organization of the mediodorsal (MD) nucleus in macaque monkeys. The cholinergic innervation of the MD nucleus showed striking regional variations with the greatest density of immunoreactive axons and varicosities found within the parvicellular division. Electron microscopic examination revealed that these ChAT immunoreactive (ChAT‐IR) axons were primarily small and unmyelinated. The majority of immunoreactive synaptic profiles were found within the extraglomerular neuropil (80.5%), with the remainder present in glomerular regions. Within the glomerular and extra‐glomerular neuropil ChAT‐IR profiles made contact with both conventional, presumably relay cell dendrites (CD), as well as with synaptic vesicle containing dendrites (SVCD) of local circuit neurons. In the glomeruli the frequency of synapses was approximately equal for CDs and SVCDs while in the extraglomerular areas 75% of the synaptic contacts were with CDs. ChAT‐IR synaptic profiles had a diversity of junctional complex morphologies. Within glomeruli they made symmetric synapses with CDs and predominantly asymmetric with SVCDs. The majority of extraglomerular contacts (60%) were classified as asymmetric and these as well as the smaller number of symmetric synapses contacted both CDs and SVCDs.In accord with results of physiological studies, these anatomical data indicate that cholinergic input to thalamic nuclei influences relay cell activity both directly and indirectly via local circuit neurons. © 1993 Wiley