Fine structure of primary afferent axon terminals of slowly adapting cutaneous receptors in the cat.

Three slowly adapting type I and two slowly adapting type II afferent fibres from the lumbosacral cord of the cat were intra-axonally labelled with horseradish peroxidase and processed for light and electron microscopy. Terminals from both types of afferent exhibited similar ultrastructural features in that both formed contacts with one to five post-synaptic profiles, including dendritic shafts and spine heads, some of which contained vesicles. The stained axons were themselves post-synaptic in axo-axonic synapses. Maximum diameters of slowly adapting boutons and the dendritic shafts on which they terminated were measured. The present results indicate that there is considerable overlap in the morphological characteristics studied for all large myelinated cutaneous afferent boutons. It is not possible therefore to distinguish between these on ultrastructural grounds alone.