The structural organization of the inferior and lateral subdivisions of the Macaca monkey pulvinar.

Previous light microscopic studies of Macaca pulvinar have demonstrated that both the inferior and adjacent portion of the lateral pulvinar subdivisions are reciprocally connected to the entire occipital lobe, including striate cortex. They differ in that inferior but not lateral pulvinar receives a projection from the superficial layers of the superior colliculus. In this study, the internal organization of these two subdivisions in compared by relating light microscopic Golgi morphology to the synaptic organization observed by electron microscopy. The Golgi impregnated neurons in inferior and lateral pulvinar are typical of other thalamic nuclei and are not qualitatively different in the two subdivisions. Projections neurons (PN) vary in cell body (15--40 micrometers) and dendritic tree (150--600 micrometers) diameters but bear the same varieties of dendritic appendages; spine-like, hair-like, and knot-like. Local circuit neurons (LCN) have smaller cell body diameters (10--20 micrometers) but can have very large dendritic field diameters (150--600 micrometers). They are best distinguished from PNs by their elaborate dendritic appendages, which have been identified as pre-synaptic dendrites in the EM. LCN axons are infrequently seen. In the EM both subdivisions contain four types of synaptic terminals. RS and RL terminal both contain round symaptic vesicles and make asymmetric synaptic contacts, but are subdivided on the basis of small (RS = 0.09 micrometers) versus large (RL = 2.2 micrometers) cross sectional diameters and organelle content. RLs contact larger caliber dendrites and frequently form synaptic complexes with presynaptic dendrites of LCNs, while RSs contact fine caliber dendrites and rarely take part in synaptic complexes. F terminal and P boutons both contain flat and pleomorphis vesicles and make symmetric synaptic contacts. They are characterized by vesicle number and cytoplasmic density. Fs are infrequently observed in pulvinar compared to P boutons and are of uncertain origin. P boutons can be equated with LCN dendritic appendages and have been identified as pre-synaptic dendrites. The quantitative distribution of each type is very similar in both subdivisions, avveraging RS 85%, RL 5%, F 0.3%, P 8% and unidentified 2%.