Synaptic organization of the external cuneate nucleus in the rat

The normal synaptic organization of the rat external cuneate nucleus (ECN) has been investigated. The characteristic feature of the ECN neuropil is complex synaptic arrangements termed synaptic glomeruli. These involve both axo-dendritic and axo-axonic synapses and usually are isolated from the surrounding neuropil by lamellar glial sheaths. Glial isolation of synaptic glomeruli is conspicuous near capillaries. Central glomerular bouton profiles vary in size (2-9 micron diam) and configuration. They form asymmetrical synapses onto dendritic shafts and spines and occasionally receive symmetrical synapses from en passant bouton profiles. Other forms of axo-dendritic synapses not associated with synaptic glomeruli are observed. The type frequently observed is formed by one or more boutons (0.5 4.0 micron diam) synapsing onto a dendrite or dendritic spine. "Isolated" axo-dendritic synapses are surrounded by glial lamellae. Elongated "giant" boutons make multiple synaptic contacts along a dendritic shaft. Serial axo-axonic synapses are found mainly in caudal regions of the nucleus. Axo-somatic synapses are formed by small boutons (0.3.2.0 micron diam) contacting medium (16-24 micron) and small (9-14 micron) neurons. The least frequently observed bouton type in the rat ECN contains numerous granular vesicles, 80-100 nm in diameter. These boutons may contact dendrites, neuronal somas or other boutons.