An EM analysis of the synaptic connections of horseradish peroxidase‐filled stalked cells and islet cells in the substantia gelatinosa of adult cat spinal cord

Abstract Two major interneurons of the outer part of Rexed's layer II (IIa) were impaled with microelectrodes, had their primary inputs characterized, and were subsequently filled with horseradish peroxidase. Their fine structural characteristics and synaptic connections were then analyzed electron microscopically. Two islet cells, whose rostrocaudally oriented dendrites were largely confined within layer IIa, received primary input form small myelinated axons. A stalked cell, whose cell body was situated on I/II border had a cone‐shaped dendritic arbor which traversed layer IIa as well as the inner part of Rexed's layer II (IIb) and rostrocaudal dendritic branches which ran for part of their course along the I/IIa border. It received primary input from small myelinated as well as form unmyelinated axons. Both cell types received asymmetrical axodendritic synapses from primary endings in layer IIa and IIb glomeruli and widely separated symmetrical axodendritic synapses from small nonprimary endings outside of glomerli. The presence of aggregates of synaptic vesicles in the dendrites of the layer IIa islet cells but not in the staked cell dendrites constitutes the major fine structural difference between these interneurons. Islet cell dendtrites from symmetrical synapses on several different kinds of neural processes. They usually send either a single type 2 spine (spines which contain synaptic vesicles) or dendtritic shaft into layer IIa and IIb glomeruli, where they form dendrodendritic synapses on adjacent type 1 spines (spines without synaptic vesicles) and on other small dendritic shafts. Some islet cell type 2 spines also form dendroaxonic synapses on the primary endings. Outside of the glomeruli, islet cell dendrites also form dendrodendritic synapses on type 1 spines and different sized dendritic shafts. They often approach other dendritic shafts. They often approach other dendritic shafts forming small bundles of dendrites in which they are reciprocally linked by dendrodendritic synapses to other synaptic vesicle‐containing dendrites. At bead‐like enlargements of their dendritic shafts and along some of the shafts of unmyelinated axons. The unmyelinated axon of the islet cell forms symmetrical synapses on layer II dendritic shafts and spines outside of glomeruli. The role of the layer IIa islet cell as an inhibitory interneuron is discussed.