Abstract
Classic anatomical atlases depict a contralateral hemispheral representation of each side of the face. Recently, however, a bilateral projection of each hemiface was hypothesized, based on animal studies that showed the coexistence of an additional trigeminothalamic tract sprouting from the trigeminal principal sensory nucleus that ascends ipsilaterally. This study aims to provide an anatomical substrate for the hypothesized bilateral projection. Three post-mortem human brainstems were scanned for anatomical and diffusion magnetic resonance imaging at 11.7T. The trigeminal tracts were delineated in each brainstem using track density imaging (TDI) and tractography. To evaluate the reconstructed tracts, the same brainstems were sectioned for polarized light imaging (PLI). Anatomical 11.7T MRI shows a dispersion of the trigeminal tract (tt) into a ventral and dorsal portion. This bifurcation was also seen on the TDI maps, tractography results and PLI images of all three specimens. Referring to a similar anatomic feature in primate brains, the dorsal and ventral tracts were named the dorsal and ventral trigeminothalamic tract (dtt and vtt), respectively. This study shows that both the dtt and vtt are present in humans, indicating that each hemiface has a bilateral projection, although the functional relevance of these tracts cannot be determined by the present anatomical study. If both tracts convey noxious stimuli, this could open up new insights into and treatments for orofacial pain in patients.
Highlights
Orofacial nociceptive afferents, running through the trigeminal nerve and the trigeminal tract (TT), are believed to synapse in the ipsilateral trigeminal sensory nucleus complex (Sessle 2000)
We investigated the neuroanatomy of the dtt in human brainstems using, multiple, advanced neuroimaging techniques, including post-mortem, 11.7 anatomical magnetic resonance imaging (MRI) and diffusion-weighted MRI and several post-processing techniques (e.g., tract density imaging (TDI) and tractography)
The trigeminal nerve can be clearly observed entering the ventrolateral part of the pons, after which the hypo-intense signal of the tt can be seen traversing the brainstem
Summary
Orofacial nociceptive afferents, running through the trigeminal nerve and the trigeminal tract (TT), are believed to synapse in the ipsilateral trigeminal sensory nucleus complex (Sessle 2000). Second-order neurons cross and ascend as the trigeminothalamic tract to the contralateral ventro-posterior medial nucleus of the thalamus. Projections ascend to the primary and secondary somatosensory cortices and other cortical regions, such as the insula (Ralston 2005). The exact anatomy of the central portion of the trigeminal pathways in humans remains a point of discussion. Knowledge of the exact neuroanatomy of the trigeminal tracts will contribute to the understanding of orofacial pain and its treatment. Preclinical experiments provide evidence for a bilateral projection of the orofacial region causing activation of both thalami and both primary sensory cortices in healthy subjects, as
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