Periterminal synaptic organization of primary afferents in laminae I and IIo of the rat spinal cord, as shown after anterograde HRP labelling.

The fine structure and periterminal synaptology of the primary afferent terminations in laminae I and IIo are examined in the rat, following anterograde labelling with horseradish peroxidase applied to the right C5-dorsal root. Labelled varicosities observed along the terminal arbors in parasagittal thick sections were relocated in ultrathin sections by electron microscopy. The labelled terminal profiles generated by the three primary afferent plexuses which can be identified by light microscopy in laminae I-IIo had similar fine structural features, except that axo-axonal contacts, although rare, were more frequent in the medial network plexus. Primary boutons were packed with agranular spherical vesicles and some large granular vesicles, and were mostly presynaptic to profiles of dendritic trunks of marginal cells. Unlabelled axonal profiles, either light with some flattened vesicles, or dense with round vesicles, were also presynaptic at symmetrical or asymmetrical contacts, respectively, to those dendritic profiles. It is suggested that such knobs of intrinsic origin are responsible for postsynaptic modulation of the primary noxious input. Although the 20 microns wide lamina IIo belongs cytoarchitectonically to lamina II and can be distinguished from lamina I by a decreased amount of myelinated fibres and large dendritic profiles, the periterminal synaptology was here found to be the same as in lamina I.