Relationship between electroacupuncture analgesia and descending pain inhibitory mechanism of nucleus raphe magnus.

Raphe-spinal (R-S) neurons were identified in the nucleus raphe magnus (NRM). The conduction velocity of their axons was calculated to be about 15-60 m/sec. The great majority of R-S neurons did not respond clearly to non-noxious stimuli, such as brushing hair or lightly pressing the skin, but they did respond to noxious stimuli (pricking or clamping), with increases or decreases in their firing rates. The receptive fields of the R-S neurons responding to noxious stimuli were very wide covering almost all of the body. The responses of R-S neurons to noxious stimulation were excitatory (increasing firing rates), inhibitory (decreasing) and unresponsive in type, and we unexpectedly found 3 reversibly excitatory-inhibitory type R-S neurons, the firing rate of inhibitory R-S neurons being higher than that of excitatory R-S neurons. The responses of the excitatory-inhibitory reversible neurons to noxious stimulation were also related to the background firing rates. The effects of electroacupuncture (EA) on the excitatory R-S neurons were mainly to increase their firing rates, and to inhibit their nociceptive responses. After the transection of DLF, the R-S neurons could still be activated by EA, but the post-inhibitory effects of EA on their nociceptive responses were obviously reduced. It is suggested that the EA can activate NRM, a supraspinal area mediating a negative feedback circuit modulating pain, thus inducing analgesia via descending inhibition.