The representation of experimental tooth pain from upper and lower jaws in the human trigeminal pathway.

FMRI was used to study the differences of cerebral processing of nociceptive input from the 2nd and the 3rd branches of the trigeminal nerve by electrical stimulation of the tooth pulps of the upper and lower canines. The focus of the study was an investigation of the different levels of the trigeminal system in brainstem, thalamus and in cortical regions which are known to be involved in pain processing. Increased blood oxygen level dependency (BOLD) signals were found ipsilaterally in the trigeminal ganglion and the spinal nucleus (SpV) of the trigeminal nerve. SpV-related activations showed some somatotopic organization. Bilateral activation was found in the structures of the antinociceptive system in the midbrain. Contralateral activations were encountered at the level of the pons. In the thalamus ipsilateral activations were found in the ventral parts. Bilateral activation occurred in the medial dorsal nuclei. At the cortical level BOLD activations were encountered bilaterally in the primary somatosensory cortex (S1, lateral pain system), the cingulate and insular cortex (medial pain system). In the cortex a small difference in the representation of the two trigeminal branches was detected only in S1 on both hemispheres. These findings demonstrate that trigeminal pain markedly activates the lateral and medial pain projection systems and the majority of the affected brain regions showed no difference regarding the input from lower or upper tooth. This lack of discrimination may explain why sometimes it is difficult for patients to locate the exact source of the intraoral clinical pain conditions.