Arquitectura y conexiones talamocorticales del núcleo pulvinar medial en el mono tití común (Callithrix jacchus)

Abstract

The medial division of the pulvinar complex of the thalamus (MPul) is a massive structure in higher primates, particularly in humans. Unlike the rest of the pulvinar, MPul is not directly involved in visual processing and, as reported, it is connected to a variety of high order frontal, parietal, cingulate, insular and temporal cortical areas. The wiring of these multiple connections and their organization within MPul has not been investigated in detail. We used microiontophoretic injections of biotinylated dextran amine (BDA) in MPul to anterogradly label the axonal arborizations of thalamocortical neurons. Then, we mapped the whole tangential distribution in a reference flat cortical map and the laminar distribution of axons arising from small MPul neuronal clusters along its anteroposterior axis. Also, we analyzed MPul axon trajectories throughout the cerebral white matter. Subsequently, by means of optical fractionator we stereologically estimated the varicosity number within transects in selected cortical areas. Besides, we measured the maximal projection area of the axonal varicositites using camera lucida. To achieve more precision in the delimitation of the hodological sectors of MPul we made retrograde tracer deposits to map the global topography of the pulvinocortical populations that innervated several locations in frontal ventral, parietal posterior and temporal inferior cortices. We show that, within MPul, three partially overlapping sectors can be distinguished according to their axonal targets in the cerebral cortex. First, the rostrolateral sector is formed by densely packed but segregated neuronal populations which axons arborized to several terminal foci located in frontal, parietal, cingulate, or temporal areas. Second, the medial sector is formed by medial frontal and temporopolar projecting neurons. Third, the caudal sector has more prevalence of temporoinferior projecting somas. In all studied deposits, we saw that the laminar distribution pattern is mainly in layers 3b and 4 and layer 1. The varicosities´ maximal projection area is around 1 μm2, with small differences between areas and laminae. These features are consistent with the notion that PulM is a key neural hub in the neural networks that allows complex multisensory-guided social behaviors.