Afferent and efferent connections of cerebellar lobe C1 of the mormyrid fish Gnathonemus petersi: an HRP study.

The corpus of the highly developed cerebellum of the weakly electric fish Gnathonemus petersi is differentiated into four lobes, numbered C1 to C4. The present paper deals with the extrinsic connections of the rostralmost lobe C1. Relevant nuclei were studied in normal histological material and HRP injections were placed in lobe C1, the neighbouring pedunculus valvulae, and the brainstem. The largest number of afferents to lobe C1 originates from the nucleus lateralis valvulae, a large nucleus of tightly packed small cells in the dorsal midbrain tegmentum. In Gnathonemus this nucleus encompasses nine subdivisions, of which the rostral, caudal, and exterolateral parts project in particular to lobe C1. Larger neurons in the dorsal midbrain tegmentum and presumed mesencephalic trigeminal neurons project to C1 as well. In the rhombencephalon, afferents to lobe C1 arise from the first funicular nucleus, the lateral reticular nucleus, and the inferior olive. Efferents of lobe C1 have been found to arise from a peculiar cell type in the Purkinje cell layer (so-called eurydendroid neurons) and to project predominantly to the nucleus of the fasciculus longitudinalis medialis, the nucleus reticularis superius and medius, and the trigeminal motor nucleus. Additional small projections terminate in the tectum mesencephali and in the nucleus reticularis inferior. Compared with other parts of the mormyrid cerebellum as well as with the cerebellum of other teleosts, the connections of lobe C1 appear to be quite restricted and specialized. In this respect the connections with the trigeminal nerve via the first funicular nucleus, the mesencephalic trigeminal nucleus, and the trigeminal motor nucleus are of particular interest. The absence of central cerebellar nuclei intercalated in the efferent cerebellar connections, in combination with the presence of a precerebellar nucleus (lateralis valvulae) involved in the afferent cerebellar connections, represents a remarkable difference between teleosts and other vertebrate classes.