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Neuroscience. 2012 Sep 6;219:214-33. doi: 10.1016/j.neuroscience.2012.05.065. Epub 2012 Jun 7.

Somatotopic direct projections from orofacial areas of secondary somatosensory cortex to trigeminal sensory nuclear complex in rats.

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Department of Oral Anatomy and Neurobiology, Graduate School of Dentistry, Osaka University, Suita, Osaka 565-0871, Japan.


Little is known about the projections from the orofacial areas of the secondary somatosensory cortex (S2) to the pons and medulla including the second-order somatosensory neuron pools. To address this in rats, we first examined the distribution of S2 neurons projecting to the trigeminal principal nucleus (Vp) or oral subnucleus (Vo) of the trigeminal sensory nuclear complex (TSNC) after injections of a retrograde tracer, Fluorogold (FG), into five regions in the Vp/Vo which were responsive to stimulation of trigeminal nerves innervating the orofacial tissues. A large number of FG-labeled neurons were found with a somatotopic arrangement in the dorsal areas of S2 (orofacial S2 area). This somatotopic arrangement in the orofacial S2 area was shown to closely match that of the orofacial afferent inputs by recording cortical surface potentials evoked by stimulation of the trigeminal nerves. We then examined the morphology of descending projections from these electrophysiologically defined areas of the orofacial S2 to the pons and medulla after injections of an anterograde tracer, biotinylated dextranamine (BDA), into the areas. A large number of BDA-labeled axon fibers and terminals were seen only in some of the second-order somatosensory neuron pools, most notably in the contralateral TSNC, although the labeled terminals were not seen in certain rostrocaudal levels of the contralateral TSNC including the rostrocaudal middle level of the trigeminal interpolar subnucleus. The projections to the TSNC showed somatotopic arrangements in dorsoventral, superficial-deep and rostrocaudal directions. The somatotopic arrangements in the Vp/Vo closely matched those of the electrophysiologically defined central projection sites of the orofacial trigeminal afferents in the TSNC. The present results suggest that the orofacial S2 projects selectively to certain rostrocaudal levels of the contralateral TSNC, and the projections may allow the orofacial S2 to accurately modulate orofacial somatosensory transmission to higher brain centers including the orofacial S2 itself.

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