Organization of the facial nucleus in the rat.

The topographic organization of the facial nucleus motoneurons in the rat was investigated with the horseradish peroxidase retrograde tracing method using tetramethyl benzidine as a chromogen. Facial muscles were 'represented' on the facial nucleus in an orderly fashion. For example, the motoneurons supplying the rostral (nasolabial) muscles are laterally placed, whereas those supplying the caudal (auricular) muscles are medially placed. Notable findings in this study are: (i) the relatively unremarkable size of the nasolabial (vibrissal) muscle representation; there does not seem to be an increase in size of this part of the nucleus, commensurate with the specialized sensory functions of the vibrissal area of the face and (ii) the neurons supplying the posterior belly of the digastric are found in the suprafacial nucleus which lies about 1 mm dorsal to the main facial nucleus.