SOMATOTOPIC REPRESENTATION WITHIN THE FACIAL MOTOR NUCLEUS OF THE CHICKEN STUDIED BY MEANS OF HORSERADISH-PEROXIDASE

The distribution in the chicken of motoneurons innervating the hyoling ual muscles, i.e. the M. branchiomandibularis (BM), M. ceratoglossus ( CG), M. interceratobranchialis (CB), M. serpihyoideus (PH), M. stylohy oideus (YH) and one of the mandibular muscles, M. depressor mandibulae (DM), was examined by retrograde transport of horseradish peroxidase conjugated with wheatgerm agglutinin. Labelled motoneurons are found i n the three subnuclei of the facial (VII) nucleus, as well as hypoglos sal (XII) nucleus. The distribution of motoneurons projecting to the D M are observed in the three subnuclei, those of the BM, PH and YH in t he intermediate and ventral subnuclei and those of the CB and CG in th e intermediate subnucleus. Motoneurons projecting to the PH, YH, CB an d CC are also distributed in the XII nucleus. The ratio of labelled mo toneurons of the VII to XII nuclei decreases the PH, YH, CB and CG in that order, and the ratio of labelled ones of the ventral to intermedi ate subnuclei decreases the BM, PH and YH in that order. The topograph ical and functional aspects of the subdivision of the motor nucleus ar e discussed.