Organization of the first and the second somesthetic regions (SI and SII) in the rat thalamus

Organization of the rat somesthetic thalamus was studied by a combination of electrophysiologic, stereotaxic, and histologic methods. A monopolar recording electrode was inserted numerous times in any one preparation to explore systemically those thalamic regions which receive somesthetic input. The tip of the electrode was small enough to detect unit activity as well as slow potentials. The recording sites were spaced at 250 and 300 μ distances by using sensitive microdrives in all stereotaxic dimensions. For each recording sire maps were made of the peripheral projection fields which, upon tactile stimulation, evoked strong, medium strong, or weak responses. Punctures made by the recording electrode were reconstructed from sections cut by the freezing method and the geometric arrangement of the recording sites within the thalamus was reconstituted. Two thalamic regions for somesthetic projection (SI and SII) were found in the rat thalamus. They are similar to the first and the second somesthetic areas previously described with the rat cerebral cortex. The thalamic regions encompass the ventrobasal nuclear complex. In the relatively large region of SI there is contralateral projection of the body only. The snout is extensively represented througout this projction region whereas the hindquarters project on a smaller amount of thalamic tissue found in the extreme anterolateral position of SI. Region SII is relatively small. It is situated posterolaterally to SI. The animal's body is represented anew but this time bilaterally; snout dorsomedially, hindquarters ventrolaterally.