Medullary on- and off-cell responses precede both segmental and thalamic responses to tail heating.

Medullary on- and off-cell responses to tail heating were studied in lightly anesthetized rats. In 10 animals the electromyographic (EMG) activation of tail muscles was recorded simultaneously with on- or off-cells. The on-cell burst or the off-cell pause always preceded segmental EMG activation by about 0.5 sec. In turn, EMG activation preceded visible tail flick by about 0.09 sec. In 13 other animals ventrobasal thalamic unitary responses were recorded simultaneously with on- and off-cell responses. On-cell bursts or off-cell pauses always preceded thalamic responses by about 0.4 sec. These results support the notion that on- and off-cells play a regulatory role in both segmental and ascending nociceptive transmission.