Projections from neurons innervating the masseter muscle to the subnucleus oralis of the spinal trigeminal nucleus and adjacent lateral reticular formation in the rat.

The projection from the masseter to the subnucleus oralis of the spinal trigeminal nucleus (Vo) and adjacent parvicellular reticular nucleus (PCRt) was studied with retrograde and transganglionic transport of horseradish peroxidase (HRP) combined with fluorescent double labeling method. The labeled axons and terminals within the medulla showed a specific distribution. The field of labeling formed a continuous band from rostro-laterally to caudo-medially. At the level of caudal Vo the labeling region became the most expanded in the medulla, which occupied dorso-medial Vo (Vodm) and adjacent PCRt about half by half. The ratio of masseter primary afferent neurons in the mesencephalic trigeminal nucleus (Vme) to that in the trigeminal ganglion (TG) was found to be 7.7-8.5:1 following injection of HRP into the masseter nerve. However, the proportion between the Vme and the TG neurons which build up the pathway from the masseter to the Vodm and the adjacent PCRt was 8.7-10:1, revealed by means of fluorescent double labeling when different tracers were applied at the peripheral and central targets.