Cytoarchitecture and somatic sensory connectivity of thalamic nuclei other than the ventrobasal complex in the cat

This study is a re‐examination, using autoradiographic and axonal degeneration methods, of the distribution of spinal, dorsal columnlemniscal and cortico‐thalamic fibers within the thalamus of the cat. The emphasis was placed upon making an exact cytoarchitectonic delineation of regions outside the ventrobasal complex which receive spino‐thalamic fibers and corticofugal fibers arising in the sensory‐motor regions of the cerebral cortex. It is concluded, in agreement with Boivie ('70, '71a,b) that spino‐thalamic fibers arising below the level of the lateral cervical nucleus terminate, not in the ventrobasal complex, but in a recognizable portion of the ventrolateral complex adjacent to the ventrobasal complex. This region appears to be also the thalamic relay for Group I muscle afferents. Combined experiments in which, in the same animal, the dorsal column‐lemniscal path was labeled autoradiographically and the spino‐thalamic path by the Nauta method, indicate virtually no overlap of the two systems in the ventral nuclear complex. Spinal fibers also end in the medial division of the posterior group (Pom), which extends posteriorly as a small‐celled zone along the ventromedial aspect of the magnocellular medial geniculate nucleus. Reports of spinal terminations in the magnocellular nucleus proper may result from a failure to recognize the extent of Pom. A third part of the thalamus receiving spinal fibers consists of a posteriorly situated group of large, deeply staining cells belonging to the central lateral nucleus which lie mainly posterior to the internal medullary lamina and which, as Mehler ('69) has mentioned, have previously been confused with the centre médian and parafascicular nuclei. Corticofugal fibers arising in the somatic sensory cortex terminate in both the ventrobasal complex and the spinal part of the ventrolateral complex as well as in the central lateral nucleus and Pom and this confirms the work of Rinvik ('68a). Those arising in the motor cortex terminate only in (a different part of) the ventrolateral complex and in the centre médian nucleus.