Effects of remote deep somatic noxious stimuli on a jaw reflex in man

The effects on an inhibitory jaw reflex of activating deep somatic afferent nerves in a remote part of the body (the arm) were studied in 13 humans. Electromyographic recordings were made from the active masseter of the long-latency (mean 42.0 ± 1.1 ms) inhibitory reflex evoked by electrical stimulation of the upper lip. Immediately after a 1-min conditioning period during which the participants compressed a hand-held spring once a second while ischaemia was produced in the arm with an inflated pneumatic cuff, the magnitude of the inhibitory reflex decreased significantly (by 43%). The reflex recovered within 5 min to a magnitude that was not significantly different from its pre-conditioning value. The arm exercise or the ischaemia alone produced no significant changes in the reflex. Furthermore, neither of these last two conditions was reported to be painful, whereas the ischaemic exercise produced pain in all but one participant. It is concluded that activation of remote nociceptive, but not non-nociceptive, deep somatic nerves can modulate jaw reflexes in man.