Characterization of blink reflex interneurons by activation of diffuse noxious inhibitory controls in man

The blink reflex consists of an early, pontine R1-component and a late, medullary R2-component. R1 and R2 can be evoked by innocuous stimuli, but only the R2 also by painful heat, suggesting that the R2 is mediated by wide dynamic range neurons (WDR) of the spinal trigeminal nucleus. Remote noxious stimuli suppress the activity in WDR neurons via activation of diffuse noxious inhibitory controls (DNIC), whereas low-threshold mechanoreceptive neurons (LTM) are unaffected. In order to characterize the trigeminal interneurons of R1 and R2 we investigated the modulation of the blink reflex by remote painful heat. The blink reflex was elicited in 11 healthy subjects by innocuous electrical pulses applied to the left supraorbital nerve. The remote, painful heat stimuli were applied by a Peltier type thermode to the left volar forearm. Remote painful heat of 44 to 46 degreesC significantly suppressed the R2 by 15% (p<0.01), while the R1 remained unchanged. These results provide further evidence that the R2 is mediated by medullary WDR neurons and the R1 by pontine LTM neurons.