Dual pathways for thermal afferents from the cat's tongue

In the lightly anesthetized cat, pathways transmitting cold afferents from the tongue to the first somatosensory cortex (SI) and to the preoptic/anterior hypothalamic (PO/AH) area were analyzed electrophysiologically. Thalamic cold-sensitive neurons that are responsive to cold stimulation of the tongue were found in the dorsomedial and caudal part of the medial ventroposterior nucleus (VPM). These thalamic neurons received synaptic inputs from the neurons in the subnucleus caudalis of the spinal trigeminal nuclei. Many of the neurons in the SI activated by stimulation of the cold-sensitive area in the VPM responded to cold stimulation of the tongue. Blocking the neural activity around the thalamic cold-sensitive area by injecting a solution of xylocaine resulted in a lack of responsiveness to cold stimulation of the tongue in the cortical cold-sensitive neurons. There were neurons in the PO/AH area responsive to such cold stimulation. These neurons were activated by electrical stimulation of the cold-sensitive area in the subnucleus caudalis, but not by stimulation of the cold-sensitive area in the VPM. These results suggested the possible existence of dual pathways for cold afferents from the tongue. One is the pathway to the SI through the VPM and the other to the PO/AH area. The signal transmission speed of the former was markedly faster than that of the latter.