Electron microscopic observations of nodes of Ranvier in the external cuneate nucleus.

During the course of an investigation of the synaptic organization of the external cuneate nucleus (ECN) in the Sprague-Dawley albino rat, the ultrastructural morphology of nodes of Ranvier in the neuropil has been studied. The majority of nodes observed have the basic morphological features of conventional central nodes but there is individual variation with regard to length, surface area and cytoplasmic organelles. In addition, nodes with multiple myelinated branches are observed. Some nodes of Ranvier were observed to form specialized synaptic boutons. Two types of nodal synaptic boutons were present; a simple type and a complex type. Simple nodal boutons were observed more frequently. These nodes usually synapse upon a single dendritic element; the portion of the node opposite the presynaptic area has a morphology similar to conventional nodes. Complex nodal boutons are of greater dimension than simple nodal boutons and are usually in contact with several neuronal elements. They may be presynaptic to dendritic shafts or spines and are occasionally observed to be postsynaptic to small axonic profiles, a synaptic relationship which, until this report, has not been demonstrated in the central nervous system (CNS). The possible functional significance of these observations is discussed.