The somatotopic organization of nociceptive-specific (NS) and wide dynamic range (WDR) units in the nucleus ventralis posterolateralis (VPL) of the ventrobasal (VB) complex was studied. Experiments were carried out on adult cats anesthetized with urethane-chloralose. The recording sites of nociceptive units were marked by the electrophoretic deposition of pontamine sky blue from the recording microelectrode, and subsequently identified histologically in cresyl-violet stained sections. 2. It was found that NS units were located in the dorsal and ventral shell regions of the caudal VPL, whereas WDR units were located in a narrow zone of the shell region, just rostral to the NS zone. 3. NS units in the dorsal shell region had receptive fields on the contralateral dorsal surface of the body, whereas NS units in the ventral shell region had their receptive fields on the ventral aspect of the contralateral integument. 4. In the dorsal shell region, the contralateral body surface was represented in an orderly sequence. Units responding to noxious stimulation of the upper-most cervical dermatome were found in the most medial part of the shell region of the VPL, whereas those responding to stimulation of successively more caudal dermatomes were located more laterally, and in serial order. Units responding to noxious stimulation of the sacral dermatomes were thus found in the most lateral part. 5. In the ventral NS zone, the pattern was distorted by disproportionately large areas devoted to the fore- and hindpaw pads. In this region, therefore, NS units with receptive fields on the fore- and hindpaw pads were intermingled with other NS units that could have receptive fields located anywhere on the ventral body surface. It should be noted, however, that the remainder of the body surface was still represented in an orderly sequence; cervical dermatomes being represented medially, with successively more caudal dermatomes being represented progressively more laterally. 6. A similar somatotopic pattern was recognized in the distribution of the low-threshold centers of the receptive fields of WDR units.
