The controversial issue of the synaptic structure underlying pain modulation in the superficial dorsal horn of the spinal cord

At present，the occurrence of first modulation of pain transmission in the superficial dorsal horn， especially in the substantia gelatinosa (SG: lamina II)，is firmly established. Nociceptive primary，afferent central terminals (C-terminal) arising from nonmyelinated C-fibers and myelinated Aδ- fibers terminate in the SG and make synaptic glomeruli，in which they are centrally situated and surrounded by a number of dendrites and a few axonal terminals. One of the structural characteristics of C-terminals is indented，sinuous，or scalloped electron-dense axoplasm full of round synaptic vesicles (CI-terminal)， while an other is larger roundish， electron-lucent axoplasm with many large dense core vesicles and mitochondria (CII-terminal). They are FRAP-and SP-or CGRP-positive and show intense capsaicinsensitivity. lnhibitory interneurons containing GABA，glycine or opioids are exclusively present in the SG. They contact with C-terminals，forming synaptic glomeruli. However，the synaptic relationship between C-terminals and inhibitory interneurons is controversial，i.e.，the postsynaptic contacts of inhibitory interneurons to C-terminals is not accepted. However，from previous reports concerning these synaptic structures，the presynaptic relationship of C-fibers to the inhibitory interneurons seem to be more enhanced，although species differences might be present.