The injection of anterograde tracers into the ventral division of the medial geniculate body (MGV) of both rats and rabbits labels terminal axons in layer I of auditory cortex as well as the more conventional terminal arbors in layers III/IV. Whether these layer I projections represent a separate lemniscal pathway to the molecular layer or arise as collaterals of axons terminating in III/IV has not been addressed. Focal injections of the anterograde tracers biocytin or biotinylated dextran amine were made into the MGV of young rabbits. Serial section reconstruction of single MGV axons in auditory cortex revealed that layer I axons were collaterals of thalamocortical afferents that formed multiple divergent patches within III/IV. MGV collaterals to layer I often coursed tangentially for several millimeters before terminating. In some cases, the layer I collaterals descended to arborize within a thalamocortical patch in layers III/IV. These results suggest considerable radial and tangential divergence in the auditory thalamocortical pathway and argue for an expanded role for layer I in the processing of specific sensory stimuli.
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