An electron microscopic study of primary afferent terminals from slowly adapting type I receptors in the cat

Primary afferent fibers transmitting impulses from slowly adapting (SA) Type I receptors in the glabrous skin of the hind paw of the cat were injected intraaxonally in the spinal cord with horseradish peroxidase (HRP). At the light microscopic level, terminal arborizations were observed in the medial dorsal horn extending up to 6 mm rostrocaudally in and near the seventh lumbar segment. Boutonlike swellings labelled with HRP were distributed in clusters in Rexed's laminae III‐VI. There was a tendency for the most dorsal clusters from an individual fiber to be located rostrally and for the most ventral clusters to be located caudally. At the electron microscopic level, a combination of morphometric analysis and serial reconstruction revealed the following: (1) All the boutons labelled with HRP contained predominantly clear, round synaptic vesicles, 40–50 nm in diameter. (2) Labelled boutons (n = 75) had cross‐sectional longest dimensions of 1.72 ± 0.53 μm (Mean ± S.D.), perimeters of 4.95 ± 1.52 μn, and areas of 1.18 ± 0.59 μm². Their shapes in section varied from rounded to elongated forms. (3) The sizes of labelled boutons decreased significantly and linearly with depth from lamina IV to VI. The shapes of the bouton cross sections also became rounder with depth in the dorsal horn. (4) About 72% of synaptic contacts associated with HRP‐filled boutons were with dendritic spines and shafts; most of these synapses were of the asymmetric type. (5) The remainder (28%) of the appositions were synapselike contacts between labelled boutons and unlabelled structures containing flattened or pleomorphic vesicles, and occasional dense‐cored vesicles. (6) We observed no unequivocal axosomatic contacts made by labelled boutons. (7) The lengths of synaptic appositions with dendritic spines (0.46 ± 0.20 μm) or with dendritic shafts (0.51 ± 0.18 μm) were significantly greater than the synapselike contacts with vesicle‐containing unlabelled structures (0.29 ± 0.09 μm). (8) Complex neuropilar organization was occasionally seen with labelled boutons as central elements, although simpler organizations were much more common. In summary, HRP‐labelled fibers ended predominantly in boutons containing clear, round vesicles forming axospinous and axodendritic synapses. This is consistent with physiological observations of postsynaptic excitation of dorsal horn cells by the primary afferent fibers from SA Type I receptors in the glabrous skin.