Representation of whisker follicle intrinsic musculature in the facial motor nucleus of the rat.

Retrograde transport of wheatgerm-agglutinin horseradish peroxidase (WGA-HRP) and fluorescent tracers (true blue-TB, nuclear yellow-NY, and diamidino yellow-DY) from isolated whisker follicles was used to define the somatotopic organization of the facial (VII) motoneurons which innervate the intrinsic follicle muscles. Motoneurons supplying these muscles were restricted almost completely to the lateral (Martin and Lodge, '77) facial subnucleus and the motoneurons which innervated a given follicle were distributed over the entire length of this subnucleus. Cells projecting to dorsal (A-row) follicles were located in the most lateral part of the lateral subnucleus, while those supplying ventral (E-row) follicles were restricted to the medial part of the subnucleus. Injections of different tracers into rostral and caudal follicles within a given row revealed no somatotopic representation of the rostrocaudal axis of the whiskerpad. Additional control experiments demonstrated that some of the labelling obtained with WGA-HRP resulted from spread of this tracer to extrinsic muscles. This was not the case with the fluorescent tracers. The results of the control experiments suggested further that a significant percentage of the motoneurons in the lateral facial subnucleus innervate only intrinsic follicle muscles.