The nucleus corporis pontobulbaris of the North American opossum.

The nucleus of the pontobulbar body (PBu) in the North American opossum is located, for the most part, adjacent to the motor root of the trigeminal nerve. Material prepared by degeneration and autoradiographic methods shows that the PBu receives projections from the facial motor-sensory cortex, red nucleus, spinal cord and cerebellum. The latter fibers probably take origin within the fastigial nucleus. Each of the afferent connections ends in a restricuted part of the PBu, but there is considerable overlap. Use of the horseradish peroxidase technique reveals that the PBu projects to the spinal cerebellum (anterior lobe, pyramis and paramedian lobules), to visual-auditory areas of the vermis and to the lobus simplex as well as to crus I and II of the hemispheres. Although there is some topography to such projections, it is not sharply defined and many regions of the PBu contain labelled neurons after injections of horseradish peroxidase into widely separate areas of the cerebellar cortex. Because of its embryogenesis and position, the PBu is often considered part of the dorsolateral basilar pons. It appears from our material, however, that the organization of PBu afferent and efferent connections is different from that of the adjacent basilar pons, and arguments for considering the PBu a separate precerebellar nucleus are presented.