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.
second somesthetic cortex
fourth somesthetic cortex