AN ULTRASTRUCTURAL-STUDY OF CUNEOCEREBELLAR NEURONS AND PRIMARY AFFERENT TERMINALS IN THE EXTERNAL CUNEATE NUCLEUS OF GERBILS AS REVEALED BY RETROGRADE AND TRANSGANGLIONIC TRANSPORT OF HORSERADISH-PEROXIDASE

The present study examined the synaptic organization of cuneocerebella r neurons and their relationships with the primary afferents in the ge rbil external cuneate nucleus following multiple injections of horsera dish peroxidase over a widespread area in the cerebellum in conjunctio n with a simultaneous injection of horseradish peroxidase into the cer vical or brachial nerve plexus. The external cuneate nucleus is topogr aphically organized: the rostral portion receiving the primary afferen ts from the cervical plexus and the caudal portion primary afferents f rom the brachial plexus. This study attempted to correlate the synapto logy with the topography and different cytoarchitecture in these two s pecific regions in the external cuneate nucleus. Ultrastructurally, th e profiles of horseradish peroxidase-labelled cuneocerebellar neurons could be divided into three types, namely, small, medium and large on the basis of their cross-sectional areas. Axon terminals which formed axosomatic synapses could be classified into: round (Rs type; 22.2%), pleomorphic (Ps type; 55.6%) and flattened (Fs type; 22.2%) vesicle bo utons. The horseradish peroxidase-labelled dendritic elements of the c uneocerebellar neurons were postsynaptic to a greater number of axon t erminals which were also classified into lid (77.5%), Pd (18.8%) and F d (3.7%) type boutons. Some of the Rd boutons making direct synaptic c ontacts with the cuneocerebellar neurons originated from primary affer ents since they were simultaneously labelled by transganglionic transp ort of horseradish peroxidase. In the rostral external cuneate nucleus , synapses on cuneocerebellar neurons were more frequent on their prim ary dendrites as compared with those on the primary dendrites of the c audal cuneocerebellar neurons. The latter, on the other hand, showed m ore synapses on their distal dendrites. This may have functional impli cations with regard to the afferent inputs to cuneocerebellar neurons in the rostral and caudal external cuneate nucleus.