Integrative Action of Thalamus in the Process of Acupuncture for Analgesia

Abstract

It was intended in this study to test the validity of the hypothesis that acupuncture analgesia is essentially a function of the central nervous system resulting from the inhibitive interaction between the afferent impulses arising from the point of acupuncture and those from the site of pain in the brain especially in the thalamus. Experiments performed in albino rats and rabbits showed that certain neurons in nucleus parafascicularis (Pf) and also nucleus centralis lateralis (CL) of thalamus could give rise to characteristic unit discharges in response to nocuous stimuli, and these discharges could be abolished be morphine. Evidences were presented to show this kind of characteristic discharges were concerned with pain. Pain responses of the Pf and CL neurons could be inhibited by electrical needling of certain acupuncture points, squeezing the Achilles tendon or weak electrical stimulation of a sensory nerve. Too strong stimulation, however, tended to exaggerate the response to pain. One kind of spontaneous rhythmic discharges of Pf neurons were demonstrated to be driven by the incessant inflow of pain impulses from the surgical wound of the experimental animals. Spontaneous discharges of this kind, like the evoked pain discharges, could be enhanced by nocuous stimuli and inhibited by an innocuous stimulus, the mechanical squeeze of the Achilles tendon or the needling of acupuncture points. The duration of inhibition of the spontaneous pain discharges of Pf neruons incurred by an innocuous stimulus varied with the frequency of the progressing discharges at the moment of stimulation, or in other words, with the excitability level of the neuron, suggesting that the efficacy of acupuncture for analgesia may be determined mainly by the state of brain excitability.