The sensory surface of the face of the rat is topographically represented in the brainstem trigeminal complex (Nord, '67), and in parallel with this the underlying facial musculature is also represented in a topographic fashion in the facial nucleus (Papez, '27; Martin and Lodge, '77; Watson and Sakai, '78). It has been recently reported that in the young rat three distinct representations of the vibrissae are present in the sensory portion of the brainstem trigeminal complex (Belford and Killackey, '79). Within this perspective, the specific connectivity between the brainstem trigeminal complex and the facial nucleus was investigated in adult rats by the Fink-Heimer technique.
The two major sensory nuclei of the brainstem trigeminal complex, the spinal trigeminal nucleus and the principal sensory nucleus, differ in their projection patterns to the facial nucleus. While the principal sensory nucleus sends sparse projections to the ipsilateral lateral and dorsal subdivisions of the facial nucleus, the spinal trigeminal nucleus sends differential projections to various subdivisions of the facial nucleus depending on their origin with respect to three cytoarchitectonically different subnuclei that compose the spinal trigeminal nucleus. It is concluded that the magnocellular portion of subnucleus caudalis projects rather heavily to the ipsilateral lateral subdivision of the facial nucleus, while the projections from the subnucleus interpolaris are sparser and distributed more widely to parts of the lateral, dorsal and intermediate subdivisions of the facial nucleus ipsilaterally. In contrast to ipsilateral facial projections from the rest of the brainstem trigeminal complex, the projections from the subnucleus oralis of the spinal trigeminal nucleus are bilateral and confined to the intermediate subdivision of the facial nucleus. However, ipsilateral projections of the subnucleus oralis are denser than the very sparse contralateral projections.
In addition to the facial projections from the brainstem trigeminal complex, projections from the upper portions of the cervical cord to the medial subdivision of the facial nucleus were observed. These projections are bilateral, and those fibers destined for the contralateral medial subdivision cross over below the level of the pyramidal decussation.