Noxious thermal input from the rat tail: Modulation by descending inhibitory influences

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Abstract
In anesthetized rats, single fibers were dissected from the tail nerves. Fibers were found which became excited when the temperature of water surrounding the tail was raised above 40.degree. C. Firing rate increased with stepwise increases in temperature, showing first a transient outburst followed by adaptation to a static level. Corresponding neurons were also found in the dorsal horn at the entry zone of the roots coming from the tail. The cord neurons had a higher threshold temperature of 42.5-45.degree. C. When the spinal cord was reversibly blocked by cooling in the thoracic region, the threshold of the dorsal horn neurons was reduced to that of the afferent fibers. At suprathreshold temperatures dorsal horn activity was greater during cord blockade. Apparently dorsal horn neurons responding to noxious heating are subject to a tonic descending inhibitory control.