Signal processing in the tectothalamic pathway.

The pulvinar is the largest nucleus of the human dorsal thalamus and is affected in a variety of brain disorders, such as schizophrenia. The experiments described in this dissertation elucidate key features of tecto-pulvino-cortical pathways as a first step toward understanding their role in coding visual stimuli and coordinating appropriate responsive actions. The tree shrew is used as the animal model because the visual structures of the tree shrew brain display many of the features of the primate brain, and the tectopulvinar pathways are particularly enhanced in this species. The connections formed between the tectorecipient pulvinar nucleus and cortex were explored using tract tracing, immunohistochemistry, light, confocal, and electron microscopy. It was found that the pulvinar nucleus is reciprocally connected to two regions of the temporal cortex. Pulvinocortical terminals were found to contact dendritic spines of pyramidal cells, potentially influencing corticocortical projections to the striate cortex. Corticopulvinar synapses were found to be formed distal to tectopulvinar synapses on the dendrites of pulvinar neurons, suggesting that pulvinar neurons integrate inputs from the SC and temporal cortex. The membrane properties of neurons in the tectorecipient pulvinar were compared to those of neurons of the tree shrew dorsal lateral geniculate nucleus (dLGN), using whole cell recordings in slices maintained in vitro, western blotting, stereology, and neuron modeling techniques. These studies revealed that, compared to the dLGN, pulvinar neurons express a higher density of low threshold transient (T-type) calcium channels resulting in a greater propensity to fire with bursts of action potentials. These bursts may serve to increase the influence of pulvinocortical connections and/or synchronize the activity patterns of the multiple targets of the pulvinar nucleus. Finally, the properties of tectopulvinar synapses were explored using in vitro whole cell recordings in brain slices, immunohistochemistry and confocal microscopy. The results of these experiments suggest that the tectopulvinar terminals form convergent connections on pulvinar neurons and contain the vesicle-tethering proteins synapsin I and synapsin II. We suggest that these features allow pulvinar neurons to relay a dynamic range of visual signals from the SC in order to initiate and guide the appropriate responsive actions