Fine structure of synapses associated with characterized postsynaptic dorsal column neurons in the cat

Fourteen dorsal horn neurons with axons projecting through the dorsal columns were identified either by electrophysiological methods (and subsequently injected with horseradish peroxidase) or by retrograde labelling with horseradish peroxidase in cats. All neurons were contacted by small (less than 2 micron) boutons containing spherical or elongated agranular vesicles. One neuron with its soma located in lamina III received additional contacts from central elements of glomerular complexes. Neurons with somata located more ventrally (deep lamina IV and V) were also postsynaptic to large (greater than 2 microns) electron lucent profiles which formed multiple synapses with the labelled cells. Some boutons presynaptic to postsynaptic dorsal column neurons were themselves postsynaptic to profiles containing pleiomorphic agranular vesicles at axoaxonic synapses. They also occasionally participated in triadic complexes. It is concluded that the synaptic arrangements formed by boutons in association with postsynaptic dorsal column neurons differ significantly from those associated with spinocervical neurons. Such differences might provide the anatomical substrate for the observed receptive field characteristics of these neurons.