Branching projections from mesopontine nuclei to the nucleus reticularis and related thalamic nuclei: a double...

Branching projections from pedunculopontine and laterodorsal tegmental nuclei to different thalamic targets were studied by means of a double retrograde tracing technique. The results show a topographic distribution of mesopontine neurons projecting to different thalamic targets. In addition, the present data demonstrate that a small percentage (< or = 5%) of mesopontine neurons projecting to the intralaminar nuclei or to the rostral pole of the reticular nucleus innervate both these areas by means of branching axons. By contrast, a large number of mesopontine neurons projecting to the sensorimotor thalamic nuclei send axon collaterals to the caudal part of the reticular nucleus. The present findings support the hypothesis of an inhomogeneity of different sectors of the thalamic reticular nucleus. Thus, this nucleus can be differentiated into two functional areas, in accordance with their connections with functionally different cortical fields and thalamic districts. The possibility that these two areas of the thalamic reticular nucleus subserve different mechanisms during sleep phenomena is discussed.