Studies of the motoneurons following the injection of horseradish peroxidase into the peripheral branches of the facial nerve in rats

With the method of retrograde transport of horseradish peroxidase, the present study demonstrated a topographical representation of the peripheral branches of the facial nerve within the rat facial nucleus. The labelled neurons relating to the different peripheral branches were characterized morphometrically with the aid of an image analyser system. While the results were in general agreement with the work of other investigators, a striking difference was observed in this study with respect to the spatial distribution of the labelled neurons of the cervical branch. In the latter, the cells of origin were derived from the dorsal parts of the lateral and intermediate division of the facial nucleus. The present study further demonstrated that the motoneurons giving rise to the respective peripheral branches of the facial nerve varied considerably in cell size. Thus, the neurons of the posterior auricular and the mental branches were significantly larger than those of the nasolabial and zygomatico-orbital branches. Another significant finding in this study was that the neurons supplying the mental branch were predominantly spindle-shaped. It is concluded that the heterogeneity in size and shape of the motoneurons in the facial nucleus is related to the topographical representation of the peripheral branches of the facial nerve.