Thalamic projections to the feline motor cortex studied with horseradish peroxidase.

Cells in the thalamus projecting to the distal forelimb regions of the motor cortex, area 3a, and the sensory cortex were identified with horseradish peroxidase (HRP). These cortical areas were defined by mapping evoked potentials from superficial and deep radial nerve stimulation. Following injections into the motor cortex, the labelled cells were distributed throughout a large region (greater than 2 mm wide) in the ventral portion of the ventral lateral nucleus (VL). The border area between VL and the ventral posterolateral nucleus (VPL) also contained labelled cells. The region containing labelled cells following injections into area 3a was in the rostral, dorsal VPL, and overlapped with the region containing cells labelled by injections into the motor cortex. Cells labelled by injections into the sensory cortex were near the center of VPL distinctly separated from those labelled after injections into either area 3a or the motor cortex.