Both anatomical and physiological studies in cats substantiated the existence of a tha-lamus-cortex-thalamus neuronal circuit in which neurons in the centre median send fibers towide areas of the cerebral cortex and the cerebral cortex, the motor cortex in particular, maysend the fibers back to the parafascicular nucleus. The cortical neurons concerned tend todischarge spontaneously and constantly. These spontancous discharges can be readily inhib-ited by the corticopetal impulses from the centre median, which itself can be activated bygeneral afferent impulses. On the basis of this observation it is assumdd that constantly dis-charging corticothalamic neurons may exert a tonic excitatory influence on the pain-receivingcenter, 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 centremedian 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 thecentromedian nucleus may reinforce the inhibitory action of the centromedian neurons so as toinhibit the spotaneous discharges of the cortical neurous. A self-regulating mechanism is thusformed by which a stabilized state of brain excitability can be maintained. Both anatomical and physiological studies in cats substantiated the existence of a tha-lamus-cortex-thalamus neuronal circuit in which neurons in the center median send fibers towide areas of the cerebral cortex and the cerebral cortex, the motor cortex in particular, maysend the fibers back to the parafascicular nucleus. The cortical neurons tend tend todischarge spontaneously and constantly. These spontancous discharges can be readily inhib-ited by the corticopetal impulses from the center median, which itself can be activated by generous afferent impulses. On the basis of this observation it is assumdd that that dis-charging corticothalamic neurons may exert a tonic excitatory influence on the pain-receiving center, para ascicular nucleus, so as to keep it in a state of constant alertness to the tissue-damaging danger signals and that the inhibitory action of ascending fibers from centremedian would serve to suspend the excitatory action of the cortex on the parafascicul The corticofugal fibers in this forebrain circuit terminating in the centromedian nucleus may reinforce the inhibitory action of the centromedian neurons so as to tohibit the spotaneous discharges of the cortical neurous. A self-regulating mechanism is thus by a stabilized state of brain excitability can be maintained.