Chemically distinct compartments of the thalamic VPM nucleus in monkeys relay principal and spinal trigeminal pathways to different layers of the somatosensory cortex.

The ventral posteromedial nucleus (VPM) of the monkey thalamus was investigated with combined immunocytochemical, histochemical, and connection-tracing techniques. Injections of anterogradely transported tracers were placed selectively in the caudal nucleus of the spinal trigeminal nuclear complex, and retrogradely transported horseradish peroxidase (HRP) or fluorescent dyes were placed on the surface or into the depths of defined parts of the trigeminal representation in the first somatic sensory area (SI) of the cerebral cortex. The results are correlated with those of the preceding paper (Rausell and Jones, 1991), which demonstrated the presence of 2 domains in the nucleus on the basis of different patterns of cytochrome oxidase (CO) staining and calcium-binding protein immunoreactivity. The cells of the CO-defined rod and matrix domains receive inputs from different components of the trigeminal afferent system and project to different layers of SI. The large- and medium-sized relay cells of the CO-rich rods, which are immunoreactive for parvalbumin, all project to middle layers of SI. The small relay cells of the weakly-stained CO-matrix, surrounding and intervening between the rods, are immunoreactive for 28-kDa calbindin and project to superficial layers (I and II) of SI. Anterograde tracing studies reveal that the rod domain in VPM is innervated by fibers arising in the contra- and ipsilateral principal trigeminal nucleus, while the matrix domain (and calbindin-positive domains in adjacent nuclei) are innervated by fibers arising in the caudal nucleus of the spinal trigeminal complex. These results demonstrate the modularity and parallel streaming of the functional components of the trigeminal part of the somatic sensory system and suggest that lemniscal and nonlemniscal elements of the system gain access by separate routes to different layers of the SI cortex.