Introduction: Inhibitory connections between neurons of the sensory trigeminal nucleus and the
hypoglossal nucleus have been described in the cat (Tomioka et al., 1999). We investigated
if these inhibitory pontomedullary projections are also present in man by applying
a newly developed stimulation and recording technique. Methods: In a preliminary study we examined 14 healthy subjects (9 males, 5 females, age:
42±13 years) with a specially designed enoral stimulation and recording device. The
device allowed unilateral electrical stimulation of the trigeminal (V2) innervated
palatal mucosa and simultaneous recording of the electromyographic activity from both
halves of the tongue using surface electrodes. The stimulation intensity was at least
5-fold the perception threshold and was increased up to a maximum of 20 mA. During
stimulation the subject pressed his/her tongue against the recording device. Five
recordings were registered and the electromyographic activity was averaged and rectified.
Results: The examination was well tolerated by all subjects without adverse events. In all
subjects we observed one bilateral suppression period of tongue muscle activity following
unilateral electrical stimulation of the mucosal V2 afferents. The silent period started
at 41.1±4.7 ms and ended at 82.4±12.5 ms. The mean duration of the silent period was
41.4±10.2 ms. Conclusion: Analogously to animal studies, we demonstrated that inhibitory pontomedullary projections
between sensory neurons of the enoral endings of the 2nd trigeminal nerve and hypoglossal
neurons are also present in man. The silent period was bilaterally organized and showed,
in contrast to the exteroceptive suppression of masseter muscle activity, only one
suppression period. The trigemino-hypoglossal silent period is most probably a nociceptive
protection reflex and further investigations in patients with defined brainstem lesions
have to clarify its diagnostic relevance.
