The ventroposterior inferior nucleus in the thalamus of cats: a relay nucleus in the Pacinian pathway to somatosensory cortex.

The ventroposterior region of the thalamus of mongrel cats was searched to locate zones activated by somatic stimuli. By using stimuli that selectively excited Pacinian corpuscles, areas activated by this class of afferent fibers were differentiated from regions activated by other classes of cutaneous mechanoreceptors. The results showed that Pacinian inputs excite neurons in the ventroposterior inferior nucleus (VPI) of the thalamus, whereas other more dorsal zones within the ventroposterior thalamus receive inputs from other mechanoreceptor classes. This definition of the VPI tended to be larger and to extend further lateral than some published descriptions. Horseradish peroxidase (HRP) was injected into ventroposterior zones shown by electrophysiological recordings to receive inputs from Pacinian afferents. Subsequently, labeled cell bodies were observed in the caudal poles of the dorsal column nuclei, a region previously shown to be activated by Pacinian afferents. Very few labeled cells were found in the central region of these nuclei, a region previously shown to be activated by other classes of cutaneous mechanoreceptors. Electrophysiological recordings were used to locate a small portion of the second somatosensory cortex driven by Pacinian stimuli. When HRP was injected into this region cell bodies in the VPI and the lateral part of the posterior group were labeled, but few or no labeled cells were found in ventroposterior lateral nucleus. We hypothesize that the VPI receives Pacinian information from a cytoarchitecturally distinct region in the caudal poles of the dorsal column nuclei. Further, we suggest that a major cortical target for the VPI is a subdivision of the second somatosensory cortex. These studies do not exclude the possibility that Pacinian inputs have other thalamic and cortical targets.