The thalamo‐cortical projection of the nucleus submedius in the cat

The cortical projection of the nucleus submedius (Sm) was studied in the cat with the autoradiographic and horseradish peroxidase (HRP) methods. The results indicate that Sm projects topographically on to layer 3 of a distinct agranular cortical field that occupies the posterolateral gyrus proreus, the adjacent fundus of the rhinal sulcus, and the postero‐ventral portion of the medial wall of the presylvian sulcus. This cortical field is denoted the ventrolateral orbital cortex (VLO), consonant with previous nomenclature in the rat (Krettek and Price, '77a). The more ventral part (VLOβ) is cytoarchitectonically distinct from the dorsal part (VLOα); the former receives input from the anterior part of Sm (Sm_a), while the latter receives superficial layer 1 of VLO probably also arises from Sm, and there may be an input to layers 5 and 6. The corticothalamic projection from VLO to Sm reciprocates the ipsilateral thalamocortical projection and also has a moderate contralateral component. A dense, subpial layer 1 input to VLO arises from cells of the ventromedial nucleus (VM) subjacent to Sm. The present experiments also indicate that clusters of cells in VM probably provide input to layer 3 of the cortex in the fundus of the presylvian sulcus, as well as area 6aβ in the lateral wall of the presylvian sulcus and the ventral bank of the cruciate sulcus. Results from the HRP experiments additionally indicate that VLOβ and the anteroventral (Sm_v) portion of VLOα are reciprocally connected with the ventral agranular insular cortex and the cingulate cortex, ipsilaterally, while the posterodorsal (Sm_d) portion of VLOα is instead connected with specific portions of the somatosensory cortical areas bilaterally. All portions of VLOα appear to project to the ventrolateral periaqueductal gray. In light of the recent suggestion that (Sm_d) is involved with nociception (Craig and Burton, '81), the present results suggest that the related portion of VLOα may serve as a cortical representation for noxious stimuli.