The accessory motor nuclei of the trigeminal, facial, and abducens nerves in the rat.

The cobalt labeling technique was applied to the trigeminal, facial, and abducens roots in order to investigate the location and morphology of motoneurons of the main and accessory motor nuclei of these cranial nerves in the rat. On the basis of soma-dendritic morphology, axonal trajectory, and tegmental location, three different accessory nuclei were distinguished in the brainstem. Neurons of the accessory nuclei of V and VII form a common club-shaped nucleus extending between the main motor nuclei of V and VII. The dendritic tree of these neurons is oriented in the dorsoventral direction and the axon invariably follows a dorsolaterally directed bent course. The accessory VI nucleus is located ventral and lateral to the main VI nucleus in the lateral tegmentum. The neurons have elongated perikarya, strong ventral and weak dorsal dendritic arbors, and the bent axonal course is directed dorsomedially. Neurons of the main VI nucleus have rounded perikarya and radial dendritic arborizations. Muscle groups innervated by the accessory and main motor nuclei have distinctly different functions.