Ultrastructure of Pacinian corpuscle primary afferent terminals in the cat spinal cord

The glabrous skin of the hindlimb of the cat contains 3 types of low-threshold mechanoreceptors: Pacinian corpuscles (PC), and slowly and rapidly adapting receptors. In the present study, 12 primary afferent fibers transmitting impulses from PC were injected intra-axonally with horseradish peroxidase (HRP) in the spinal cord to examine the morphology of their terminals in the dorsal horn. At the light microscopic level, terminal arborizations were observed in laminae II–VI of the dorsal horn, extending up to 7 mm rostrocaudally in and near the seventh lumbar segment. Bouton-like swellings, predominantly (67%) of the en passant type, were distributed in two discrete clusters, one concentrated rostrally in Rexed's laminae III–IV, and the other concentrated caudally in lamina V. At the electron microscopic level, a combination of morphometric and serial reconstructive analyses with 3 fibers revealed the following. (1) Boutons labelled with HRP invariably contained clear round vesicles, approximately 40 nm in diameter. Labelled bouton sections had longest dimensions of1.84 ± 0.63 μm. Their shapes varied from rounded to elongated forms with occasional scalloped appearances. (2) A majority (73%) of the contacts associated with HRP-filled boutons were made with dendritic spines and shafts. Thick postsynaptic densities were usually associated with these synapses, although thinner densities were also observed. (3) 24% of the contacts made by labelled boutons were synapse-like contacts with unlabelled vesicle-containing structures. The vesicles in the unlabelled structures were usually pleomorphic, but sometimes round. These contacts were identified as ‘synapse-like’ because labelling obscured possible landmarks necessary for definitive identification of synapses. However, in most of these contacts, there was an accumulation of vesicles near the cleft on the unlabelled side, suggesting that the labelled boutons were postsynaptic. (4) Only 3% of the contacts made by labelled boutons were axosomatic. (5) The lengths of contacts with dendritic spines(0.49 ± 0.23 μm) or with dendrites proper(0.45 ± 0.20 μm) were significantly longer than those with vesicle-containing unlabelled structures(0.31 ± 0.18 μm). (6) The portions of cross-sectional bouton contours devoted to synaptic or synapse-like contacts accounted for 9–13% of the perimeters. The larger the bouton, the longer the summed lengths of contacts. (7) Serial reconstruction of selected labelled boutons revealed both simple and quite complex synaptic organizations, including glomeruli with labelled boutons as the central component. (8) We observed no systematic and significant laminar differences in the morphology and synaptic arrangement of labelled boutons. The presence of clear round vesicles in HRP-labelled boutons, as well as the predominance of axospinous and axodendritic synapses, is morphologically consistent with postsynaptic excitation of dorsal horn cells by the primary afferent fibers from PC in the glabrous skin. Compared with afferent terminals from slowly adapting receptors, PC afferent terminals are somewhat larger, synapse on smaller dendrites, and display greater diversity in morphology and synaptology.