Ultrastructural analysis of medial brain stem afferents to the superficial dorsal horn.

Axonal projections of medial brain stem areas rich in serotonin-containing neurons were identified in layers I and II of cat medullary dorsal horn using EM autoradiography. Following [3H]amino acid injections into the brain stem, labeled axonal endings were found throughout layers I and II but were most numerous in layer I. Three different morphological types of endings could be distinguished. Each type resembled serotonergic axonal endings identified in previous experiments. The labeled endings formed both symmetrical and asymmetrical synapses on dendritic shafts and spines and occasionally on a neuronal soma suggesting that the major site of action of the descending serotonergic afferents is on the neurons in layers I and II.