Modulation of temporalis muscle exteroceptive suppression by limb stimuli in normal man

The effects of noxious and non-noxious limb stimulations on the second exteroceptive suppression of voluntary temporalis muscle activity (ES2) were studied in healthy human volunteers. Duration of temporalis ES2 was measured on averaged rectified responses obtained after stimulating the labial commissure at an intensity of 25 mA. Single peripheral electrical stimuli applied over nerve trunks or over the skin before the labial stimulus decreased ES2 duration. This effect was most pronounced after cutaneous stimuli, especially of the index finger, and it was not observed when the conditioning stimulus was a 10 second, high frequency train. For stimulation at the index finger, temporalis ES2 inhibition progressively increased with intensity from 10 mA to 40 mA; it was maximal for an interstimulus interval between 50 and 140 ms. After naloxone (0.4 mg or 4 mg, i.v.) there was a partial reversal of the index-induced ES2 depression, but this effect was not significant. Immersion of one hand in water heated at 47 degrees C produced a short-lasting ES2 reduction. These results are comparable, though not similar, to the inhibition of the digastric reflex (or jaw opening reflex) observed in animals after limb stimuli and to the depression of the spinal flexion reflex reported in man after heterotopic peripheral stimuli. Although peripheral stimuli were able by themselves to suppress temporalis EMG activity in some subjects, it is likely that they reduce labial-induced ES2 via activation of brainstem structures, such as periaqueductal gray matter or raphe magnus nucleus, which are thought to inhibit the medullary inhibitory interneurons mediating ES2.