Midbrain projections to the trigeminal, facial and hypoglossal nuclei in the opossum. A study using axonal transport techniques.

It has been proposed (see Berntson and Micco for review) that circuits intrinsic to the midbrain play an important role in the elaboration and control of behaviors involving the motor nuclei of the trigeminal, facial and hypoglossal nerves (e.g. defense, threat, attack); but because of technical problems, it has been difficult to analyze their organization. Using the horseradish peroxidase technique we have localized those midbrain neurons which project to each of the above nuclei and by using the autoradiographic method we have plotted the intranuclear distribution of their axons. Using both techniques, we have seen that mesencephalic projections to oral-facial motor nuclei strongly favor the nucleus of the facial nerve. Cells ventral to the cerebral aqueduct, including the ventral periaqueductal gray, the interstitial nucleus of Cajal, the nucleus of Darkshchewitsch and the rostral oculomotor nucleus provide major midbrain-facial projections in the opossum. Their axons terminate densely and bilaterally within areas innervating auricular muscles and to a lesser extent, the platysma sheet. The projection to the caudal auricular area of the facial complex is particularly dense. Neurons within and dorsal to the red nucleus project to regions of the contralateral facial nucleus reported to supply buccolabial, zygomatic and cervical musculature. There is also a minor tectal projection to the facial nucleus. Direct projections to the hypoglossal nuclei also arise within the periaqueductal gray and interstitial nucleus, but if such regions influence the motor trigeminal nucleus, it is mainly by way of dendrites that extend outside the nucleus or by at least one synaptic delay. The mesencephalic nucleus of the trigeminal nerve, however, projects strongly to the motor trigeminal nucleus. These data are discussed in light of their possible functional significance.