Thalamocortical projection from the ventral posteromedial nucleus sends its collaterals to layer I of the primary somatosensory cortex in rat.

Here we examined quantitatively axonal projections originating from the ventral posteromedial thalamic nucleus (VPM) to layer I of the primary somatosensory cortex (SI) by extracellular and intracellular injections of biocytin as an anterograde tracer. Following the extracellular injections, two types of VPM afferents with different arborization patterns in SI were observed. The type I extended vertically, forming dense plexus in layers IV and VI, and projected collaterals to layer I. The type II rarely branched in SI, converged in the plexus formed by the type I, and projected no collaterals to the supragranular layers. The labeled fibers in layer I derived from the first type ran parallel to the brain surface, and their mean length was 339.7 +/- 87.5 microm. Intracellular injection into VPM neurons bearing both types of afferent demonstrated the full axonal arborization in both the reticular thalamic nucleus (Rt) and SI. The total length of the axon of a neuron bearing the type I was 86,968.8 microm, and the length of axonal collaterals in layer I of SI was 433.1 microm. The total axonal length of a neuron bearing the type II was very small. The present study is the first to demonstrate substantial projections from VPM to layer I of SI, and provide quantitative data on the entire extent of the axonal arborization of thalamocortical projections from single VPM neurons.