A TRACT-TRACING STUDY OF THE CENTRAL PROJECTIONS OF THE MESENCEPHALICNUCLEUS OF THE TRIGEMINUS IN THE GUPPY (LEBISTES RETICULATUS, TELEOSTEI), WITH SOME OBSERVATIONS ON THE DESCENDING TRIGEMINAL TRACT

We studied the central projections of the mesencephalic nucleus of the trigeminal nerve (MesV) in the guppy (Lebistes reticulatus), after ap plication of horseradish peroxidase or fluorescein dextran amine into the eye orbit. A small number (1 to 13) of large mesencephalic trigemi nal neurons were solid labelled in the ipsilateral rostral mesencephal on. At the level of the trigeminal nerve entrance, the united process of each mesencephalic trigeminal cell bifurcates, giving rise to a per ipheral branch that exits in the trigeminal nerve and a descending bra nch that runs caudally in a medial bundle separated from the descendin g trigeminal tract. This bundle passes close to the visceromotor nucle i of the medulla oblongata. Descending processes give rise to short co llaterals to the descending nucleus of the trigeminus and the ventrola teral reticular area. Most MesV descending fibres terminate in this ve ntrolateral field at the transition of the medulla to the spinal cord, but one or two fibres could be followed to the C6 level, where they g ive rise to collaterals to the dorsal funicular nucleus. No collateral s directed to the trigeminal motor nucleus, the cerebellum, or the mes encephalic tegmentum were observed. These projections were also compar ed with those of the descending trigeminal tract. Copyright (C) 1997 E lsevier Science Inc.