Thalamic posterior ventral neurons with bifurcating axons to the first and second somatosensory areas in the cat, demonstrated by the fluorescent retrograde double labeling technique.

The thalamic posterior ventral neurons with bifurcating axons to both the first and second somatosensory cortical areas (SI and SII) in the cat were examined by the fluorescent retrograde double labeling technique. After injection of Evans blue (EB) into the SI, and of 4',6-diamidino-2-phenylindol.2HCl (DAPI) into the SII of the same hemisphere, EB- and DAPI-labeled cells were observed predominantly in both the posterolateral ventral and the posteromedial ventral nuclei of the thalamus. Although EB single-labeled and DAPI single-labeled cells tended to occupy separate regions within the posterior ventral nuclei, a small number of cells double-labeled with both EB and DAPI were detected in the border zone between two single-labeled cell groups. These observations indicate that some cells in the posteromedial and posterolateral ventral nuclei project both to the SI and SII by bifurcating axons.</p

An experimental study on the thalamic afferents to the second and fourth somesthetic cortices in the cat

The present study was undertaken to trace the thalamic projections to the second and fourth somesthetic areas in the anterior ectosylvian gyrus of the cat using the technique of retrograde axonal transport of horseradish peroxidase. The projections of the posterolateral and posteromedial ventral nuclei (VPL, VPM) to the second somesthetic area (SⅡ) are organized somatotopically. The posterior portion of SⅡ (hindlimb area) receives fibers largely from the dorsolateral part of VPL, the middle portion of SⅡ (forelimb area) from the ventromedial part of VPL, and the anterior portion of SⅡ(face area) from VPM. These topical projections are more loosely organized and less densely arranged than those to the first somesthetic area (SI). The SⅡ receives a few fibers from the medial geniculate nucleus, particularly its magnocellular and dorsal principal parts, and from the suprageniculate nucleus. The posterior part of SⅡ near the secondary auditory area receives many fibers from the medial geniculate and suprageniculate nuclei, and only a few fibers from the lateral central and paracentral nuclei. The fourth somesthetic area (SIV), located in the dorsal bank of the anterior ectosylvian sulcus, receives fibers mainly from the dorsal principal and magnocellular parts of the medial geniculate nucleus, and from the suprageniculate nucleus. The SIV receives a few fibers from VPL and VPM in a somatotopical manner. The posterior portion of SIV receives fibers mainly from the dorsolateral part of VPL, the middle portion of SIV from the ventromedial part of VPL, and the anterior portion from VPM. In addition, SIV receives a few fibers from the lateral central, paracentral, ventral lateral and ventral medial nuclei. The SIV, together with the most posterior part of SⅡ, forms an auditory area, receiving many fibers from the medial geniculate and suprageniculate nuclei