Specialized subregions in the cat motor cortex: anatomical demonstration of differential projections to rostral and caudal sectors.

Ipsilateral cortico-cortical and thalamo-cortical projections to the cat motor cortex were determined from the locations of retrogradely labeled neurons following single small intracortical injections of HRP in area 4 gamma. These projections were also examined by studying the distribution of anterogradely transported axonal label following multiple injections of HRP or of tritiated amino acids in areas 1-2 of SI and in area 2pri (SII). The number of retrogradely labeled cells in areas 1-2 and in area 2pri differed markedly between HRP injection sites located in the precruciate (anterior sigmoid gyrus) and postcruciate (posterior sigmoid gyrus) subregions of area 4 gamma. These associational projections from primary and secondary somatosensory cortices were dense to postcruciate subregions but weak to the precruciate subregions. The associational projections from areas 1-2 and from area 2pri to the postcruciate subregion of area 4 gamma were topographically organized, but no clear topographic organization could be demonstrated for the precruciate projection. Anterograde terminal labeling following injection of either HRP or tritiated amino acids into areas 1-2 and area 2pri confirmed the preferential projection of somatosensory cortex to the postcruciate subregion of motor cortex. The projection from somatosensory areas 1-2 was uniform over its terminal field, but that from area 2pri was more patchy and complex. HRP injections in area 4 gamma gave rise to lamellae of labeled neurons in the ventrolateral nucleus of thalamus (VL). A topographic relationship was found between the site of injection and the location of the lamella of labeled neurons. The percentage of retrogradely labeled neurons in the shell zone surrounding the border of the ventrolateral nucleus and the ventrobasal complex (VB) was greater following postcruciate than precruciate injections, whereas fewer retrogradely labeled neurons were found in central lateral nucleus (CL) after postcruciate injections than after precruciate injections. These observations support the hypothesis that differential cortical and thalamic projections to different subregions of area 4 gamma may give rise to the different physiological properties of neurons observed in these subregions (Vicario et al. 1983; Martin et al. 1981).