A prosencephalic neuronal circuit possibly involved in modulation of pain.

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.