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Horseradish peroxidase was injected into the medial and lateral columns of the facial nucleus of the rat. Following medial injections, cells were labelled by retrograde transport in the ipsilateral spinal trigeminal nucleus (caudalis) both medial vestibular nuclei, contralateral midbrain reticular formation and nucleus of the lateral lemniscus. The periaqueductal grey, interstitial nucleus and nucleus of Darkschewitch were also labelled ipsilaterally. Injections into the lateral column of the facial nucleus labelled the spinal trigeminal nucleus (oralis) and parabrachial nuclei ipsilaterally and the Darkschewitch and red nuclei contralaterally.

The representation of facial muscle groups in the facial nucleus of rat was examined by retrograde transport of HRP. Motoneurons supplying muscle groups are arranged in longitudinal columns. Those supplying nasolabial muscles are located in the lateral and ventral intermediate segments, posterior auricular muscles in a medial column, platysma in a n intermediate column; the lower lip and ocular muscles are in the ventral and dorsal segments respectively of the intermediate column. The posterior belly of the digastric muscle is supplied by motoneurons extending from the dorsal aspect of the facial nucleus to the caudal pole of the trigeminal motor nucleus.Several techniques have been employed to determine the central organization of the facial nucleus in a variety of species. These include chromatolysis (Marinesco, 1899; Van Gehuchten, 1898;Yagita, 1910;Papez, 1927;Vraa-Jensen, 1942), the Gudden method (Courville, 1966) loss of cholinesterase in the nucleus (Dom et al., 19731, and application of peroxidase to nerve stumps (Provis, 1977). All of these methods require section of peripheral nerves, although more than one peripheral facial nerve branch has been shown to supply some muscles (Provis, 1977). In addition, recent studies have cut adjacent nerves to avoid their uptake of protein tracer (Kume et al., 19781, or prepared only some facial nerve branches to avoid current spread from stimulation Martin and Lodge, 1977). These methods may have imposed an artificial organization on the nucleus. Similarly, the placement of lesions in parts of the facial nucleus with subsequent examination of individual muscles for evidence of degenerating motor end plates (Szentagothai, 1948) imposes restrictions upon the information that has been obtained. It is therefore not surprising that 3-6 anatomical subdivisions of the nucleus described from coronal Nissl-stained sections (Marinesco, 1899; Van Gehuchten, 1898;Yagita, 1910;Papez, 1927;Hogg, 1928;Nishi, 1965; Courville, 1966) have been related to peripheral branches of the facial nerve rather than to individual muscles or muscle groups.This study was undertaken to map the representation of muscle groups in the facial nucleus of the rat by retrograde transport of horseradish peroxidase (HRP) after its injection into individual facial muscle groups. It was a n essential prerequisite to tracing of central connections of the facia1 nucleus because afferents have been shown to terminate preferentially within the facial nucleus from the red nucleus (Courville, 1966;Martin and Dom, 1970;Mizuno and Nakamura, 1971;Yu et al., 1972;Edwards, 1972), the trigeminal nerve Stewart and King, 1963; Erzulumulu and Killackey, 19791, the hypoglossal nerve (Tanaka et al., 19711, and from respiratory centers (Sherrey and Megirian, 1977). Conflicting maps of the representation of muscles within the facial nucleus render the significance of these inputs unclear. MATERIALS AND METHODSThirty 250-g female hooded rats (Wistar strain) were given injections of 1-5 p1 of a 50% solution of HRP ...