Painless somatosensory and nociceptive signals from dental primary afferents relay in trigeminal brainstem nuclei. This report analyzed brainstem fMRI signal activity in response to 5 different electric current intensities applied to the maxillary right canine
Method:
13 right handed male volunteers (aged 22-49 yrs; mean 33.6yrs) with no dental pain experience during the preceding year participated. Pathologies of test teeth were excluded by professional dental and x-ray examination. 5 different stimulus intensities (individual pain-detection-threshold “PDT” -40%, “PDT” -10%, “PDT” +20%, “PDT” +40% and “PDT” +60%) were applied to the right maxillary canine and volunteers were specifically trained to focus and rate distinctly their intensity perception during the fMRI session.
Result:
We found 6 right-sided and 3 left-sided activation clusters in the brainstem. Superior to the activity in the principal sensory nucleus, we observed two small clusters in a region probably involving the mesencephalic trigeminal nucleus (left: 4 voxels; right: 20 voxels). Main activity clusters were located to the principal sensory trigeminal nucleus (left: 154 voxels; right: 94 voxels). Further activation patterns were observed alongside the spinal trigeminal nucleus with a main left lateralized focus. More inferiorly, an activation cluster covered of the superior medulla bilaterally (left: 21 voxels; right 12 voxels). Focusing on intensity coding properties, none of these areas demonstrated a significant effect (all p > 0.3).
Conclusion:
Electric dental stimulation of current amplitudes below and above the pain threshold evoked robust fMRI signals covering all trigeminal brainstem nuclei. However, no significant intensity coding effect was observed. This result corroborates previous findings suggesting that somatosensory related intensity coding is primarily a “higher” cognitive function. An alternative explanation of this negative finding is that the 3 Tesla magnetic field strength was insufficiently strong. Therefore, future investigations using ultra-high MR (7T) might elucidate more detailed functions of trigeminal brainstem nuclei.