Periterminal synaptology of dorsal root glomerular terminals in the substantia gelatinosa of the spinal cord in the rhesus monkey.

The synaptic glomerular complexes surrounding dorsal root terminals in the substantia gelatinosa were reconstructed from six sets of 50--140 gapless ultrathin serial sections prepared in the transverse plane of the spinal cord in adult rhesus monkeys. All three types of glomerular terminals described in the preceding paper (Knyihar-Csillik et al., '82) were identified: (1) DSA (endings of superficial collaterals); (2) LDCV (endings of marginal collaterals); and (3) RSV (endings of deep collaterals). Each type of terminal forms a glomerular complex which invariably includes presynaptic dendrites which are intercalated between primary terminals and the postsynaptic (conventional) dendrite. Since the latter also receives direct input from the primary sensory terminal the synaptic organization assumes triadic arrangements, suggesting that primary afferent impulses may be subjected to a postsynaptic modulation through inhibitory action of presynaptic dendrites. In glomeruli with DSA as the central element, several triadic systems are usually interrelated, possibly as a structural basis for prolonged retardation of impulses. Adjacent glomeruli, containing DSA and LDCV terminals, are coupled together by a series of triadic systems fed by DSA terminals enabling association between superficial and marginal collaterals. RSV terminals are presynaptic to somata and dendrites of substantia gelatinosa cells that presumably exert inhibition upon terminals of all three kinds of primary sensory collaterals. In addition RSV terminals are postsynaptic to numerous F boutons which presumably derive mainly from axons of substantia gelatinosa cells; similar F boutons impinge upon presynaptic and other dendrites surrounding DSA terminals. The complicated but orderly synaptic architecture of these types of primary afferents may be regarded as a structural basis for first-order analysis and modulation of the nociceptive information within the primate central nervous system.