Large dorsal horn neurons which receive inputs from numerous substance P-like immunoreactive axon terminals in the laminae I and II of the chicken spinal cord

Large neurons outlined with numerous substance P (SP)-like immunoreactive (LI) boutons were detected immunocytochemically in the dorsal horn of the chicken spinal cord at the light microscopic level. The cervical enlargement was mainly used for observations. By electron microscopy, asymmetrical synapses were observed between the SP-LI axon terminals and the soma and dendrites of the large neurons. Cell bodies of the large neurons were mostly localized in the lamina I and the region lateral to the lamina I. Some of the cell bodies were also located in the lamina II. Their dendrites extended in the lamina I, in the region lateral to the lamina I, and deeply in the lamina II. In the lamina II, dendrites of these neurons formed synapses with SP-containing central terminals in synaptic glomeruli known to originate from primary afferents. The findings suggest that these large neurons receive nociceptive information directly from primary afferents. In the light of previous investigations, these neurons are considered to be pain-transmitting long ascending tract neurons. (C) 1997 Elsevier Science Ireland Ltd.