| The prefrontal cortices and the thalamus have important roles in cognitive, emotional and executive functions. The anterior medial nucleus (AM) of the thalamus occupies a central position in pathways processing emotional and mnemonic information. The ventral anterior nucleus (VA) of the thalamus links the prefrontal cortex with the basal ganglia in a circuit that has a role in the initiation of movement. The goal of this study was to investigate the extent of bidirectional interactions of these two thalamic nuclei with prefrontal cortices using modern neuronal tracing procedures and probes, and quantitative analyses. Another goal was to determine the laminar organization of projection neurons in prefrontal cortices directed to these two thalamic nuclei, as well as the principal thalamic nucleus for the prefrontal cortex, the mediodorsal nucleus (MD).; The results showed that the AM nucleus receives preferential projections from orbitofrontal and medial prefrontal areas, associated with emotions and long-term memory, and from frontal polar area 10, associated with processing multiple events temporarily to accomplish specific tasks in behavior. In the temporal lobe, neurons projecting to the AM nucleus were found preferentially in the hippocampal formation, which is associated with long-term memory, and in the amygdala, which is associated with emotions. The VA nucleus receives preferential projections from dorsal and medial prefrontal cortices, which are associated with executive functions, and from the basal ganglia and the thalamic reticular nucleus. Unlike corticothalamic projections from sensory areas which originate predominantly from layer VI, a comparatively high proportion of projection neurons in prefrontal cortices directed to AM, MD, and VA (∼10–50%) originated from layer V and the rest originated from layer VI. The unique populations directed to VA, MD and AM from neurons in layer V in prefrontal areas may have a role in recruiting several cortical areas through widespread projections to layer I [for reviews see (Steriade et al., 1997; Rouiller and Welker, 2000)]. The prefrontal circuits through the AM, VA and MD nuclei of the thalamus may represent specialized channels in neuronal circuits that convey specific emotional and mnemonic information, selected by the prefrontal cortex in carrying out executive functions in behavior. |
