Observations on the projection from the perihypoglossal nuclei to the cerebellar cortex and nuclei in the cat. A retrograde WGA-HRP and fluorescent tracer study.

The origin and distribution of cerebellar cortical and nuclear afferents from the perihypoglossal nuclei have been studied by means of retrograde transport after implants and injections of the wheat germ agglutinin-horseradish peroxidase complex in the cat. The projection reaches all the cerebellar nuclei as well as vermal, intermediate and lateral parts of the cerebellar cortex. It is bilateral with an ipsilateral predominance and originates from all the perihypoglossal nuclei. The majority of the projecting neurons are situated caudally in the nucleus prepositus, while smaller numbers of projecting neurons are located in the rostral part of this nucleus, in the rostral nucleus intercalatus and in the nucleus of Roller. Small and medium-sized spindle-shaped to round cells located throughout the nucleus prepositus and in the rostral nucleus intercalatus have widespread projections, reaching all parts of the cerebellar cortex and nuclei, whereas large multipolar cells located in the caudal ventromedial part of the nucleus prepositus and in the nucleus of Roller have projections only to the flocculus and nodulus and the lateral and intermediate cortices. Retrograde fluorescent double-labelling experiments were made to investigate possible axonal branching of the perihypoglosso-cerebellar fibres. In experiments with injections of rhodamine-B-isothiocyanate (RITC) in the left cerebellar hemisphere and implants of crystalline Fluoro-Gold in the right hemisphere, single- and double-labelled cells were found intermingled throughout the perihypoglossal nuclei. Experiments with cerebellar cortical injections of RITC and implants of crystalline Fluoro-Gold in the underlying nucleus, demonstrated single- and double-labelled cells in the nucleus prepositus and the rostral nucleus intercalatus, while only single-labelled RITC neurons were seen in the group of large neurons in the ventromedial part of the nucleus prepositus and the nucleus of Roller. After injections of RITC in the cerebellar cortex and implants of crystalline Fluoro-Gold in the abducent nucleus on the same side, double-labelled neurons were found only in the rostral nucleus prepositus.