Midbrain neuronal responses to local and spinal cord temperatures.

Water-perfused thermodes were implanted over the lumbothoracic spinal cord and unilaterally in the midbrain of urethan-anesthetized rabbits. Single-unit activities were recorded with steel microelectrodes from the thermosensitive neurons in the midbrain reticular formation (MRF), and the effects of heating and cooling of the spinal cord were studied. Of 38 cold-sensitive MRF neurons studied, 7 units decreased their firing rate upon elevation of spinal cord temperature (Tsc) and 3 units showed the opposite type of response to Tsc. The remaining 28 cold units were not affected by the changes in Tsc between 30 and 43 degrees C. Of 17 warm units, 3 units increased and one unit decreased the firing rate during spinal cord heating. These results suggest that the temperature signal arising from thermosensitive structures in the spinal cord may be transmitted to some of the locally thermosensitive neurons in the MRF.