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Both anatomical and physiological studies in cats substantiated the existence of a thalamus-cortex-thalamus neuronal circuit in which neurons in the centre median send fibers to wide areas of the cerebral cortex and the cerebral cortex, the motor cortex in particular, may send the fibers back to the parafascicular nucleus. The cortical neurons concerned tend to discharge spontaneously and constantly. These spontaneous discharges can be readily inhibited by the corticopetal impulses from the centre median, which itself can be activated by general afferent impulses. On the basis of this observation it is assumed that constantly discharging corticothalamic neurons may exert a tonic excitatory influence on the pain-receiving center, parafascicular nucleus, so as to keep it in a state of constant alertness to the tissue-damaging danger signals and that the inhibitory action of the ascending fibers from centre median would serve to suspend the excitatory action of the cortex on the parafascicular nucleus, resulting in relief of pain. The corticofugal fibers in this forebrain circuit terminating in the centromedian nucleus may reinforce the inhibitory action of the centromedian neurons so as to inhibit the spontaneous discharges of the cortical neurons . A self-regulating mechanism is thus formed by which a stabilized state of brain excitability can be maintained.

On radiant heat stimulation of the sole of the hind foot of rat, the neurons in the parafascicular nucleus of thalamus would not respond until the skin temperature was raised to 45 degrees C. If there had been existing discharges in these neurons prior to the test, these discharges may be inhibited by the innocuous heat stimulus at temperatures below 45 degrees C. The warmth, or the heat below the innocuous level was unable not only to produce but also to inhibit the nociceptive neuronal discharges incurred by previous nocuous stimulation. This phenomenon is regarded as a manifestation of one of the general principles on which the interpretation of acupuncture analgesia is based.