The spinal trigeminal nucleus - considerations on the structure of the nucleus caudalis

The caudal part (nucleus caudalis) of the spinal trigeminal nucleus is considered to be the site of the second order neurons of the nociceptive pathways of the face. Recent studies have supported the co-participation in these circuits of the oral part of the same nucleus (nucleus oralis). The aims of the present study are: 1) to determine the morphology of the nucleus caudalis in human preparates; 2) to consider whether there is any structural basis for the pathways of signal transmission observed in animal experiments; 3) to provide evidence-based support for further consideration on the orofacial pathways. The studies were made using the Bielschowsky silver staining technique (on blocks) applied to drawn pieces of brainstems from human cadavers. On the sections the outer laminae of the nucleus are distinguishable, while the inner part hardly exposes any laminar configuration on transverse cuts. A marginal plexus with small polygonal or rounded small cells appears configured in 3 parts, namely dorsal, intermediate and ventral. Outer to the marginal plexus a clear band marks it off from the interstitial plexus, which appears more delicate. Within the marginal plexus is substantia gelatinosa with rare randomly distributed small or medium-sized cells. The inner magnocellular layers consist of clusters of small cells specifically allocated to fibre bundles, isolated small cells and large cells, pear-shaped or fusiform, appearing either bipolar or multipolar. The marginal and interstitial plexuses can represent the framework for modulation and vertical signal transmission within the spinal trigeminal nucleus, while the magnocellular layers seem to be mainly responsible for contralateral projection. It seems that the outer laminae of the spinal trigeminal nucleus may represent the receiver and the inner laminae the transmitter of the signal on the trigeminal pathway at brainstem level.