Synaptic architecture of glomeruli in superficial dorsal horn of rat spinal cord, as shown in serial reconstructions.

Using serial section EM analysis, synaptic organization of glomeruli in lamina II of the dorsal horn of the rat has been examined. Four C1-terminals (small, dark and sinuous), four CIIa (large, light and regular, without neurofilaments) and four CIIb (with neurofilaments) at the centres of synaptic glomeruli of types I, IIa and IIb, respectively, were serially sectioned and reconstructed. Asymmetrical synapses between the central terminal (C) and dendritic profiles without synaptic vesicles (D) prevailed in all types of glomeruli. Symmetrical dendroaxonic contacts with presynaptic dendrites (V1----C) occurred practically only in type I glomeruli in which there were also more asymmetrical C----V1 contacts than in type II glomeruli. Symmetrical axoaxonic synapses V2----C were more abundant in type IIa and IIb glomeruli. Type IIa glomeruli had a significantly larger number of C----D synapses and of all synapses per unit area of C surface, than type IIb glomeruli. Triadic systems with C and D postsynaptic to V2 were nearly as numerous as those involving V1 in type I glomeruli. Triads with V2 were however largely preponderant in type IIa and virtually exclusive in type IIb. It thus seems that each of the three types of glomerulus has its own pattern of synaptic interactions which might reflect specific complexes of feed-forward and feed-back mechanisms. In type I glomeruli, excitation of second-order neurons by nociceptive C1 terminals may be controlled in similar proportions by presynaptic dendrites excited within the glomerulus by the C terminal itself, or by peripheral axons excited from outside the glomerulus. This kind of control is likely to prevail in type IIa glomeruli and to be the only efficient modulatory mechanism in type IIb glomeruli.