The fine structure of neurones belonging to the
Postsynaptic Oorsal Column (PSDC) system and the primary
afferent fibre types known to project onto them in the cat
was studied using intracellular and retrograde labelling
with horseradish peroxidase.
Both intracellularly and retrogradely labelled neurones
were selected for ultrastructural analysis. Four types of
bouton were found in contact with P3QC somata and bendrites.
(1) Boutons containing spherical, agranular vesicles forming
asymmetrical synapses. These could also be observed
terminating on other unlabelled profiles in the surrounding
(2) Boutons containing oval or pleomorphic vesicles
(3) Boutons containing round agranular vesicles which formed
the central terminal of glomerular complexes in laminae IIIII of the dorsal horn.
(4) Large electron lucent terminals containing spherical
agranular vesicles and forming multiple asymmetrical
contacts onto P30C dendrites.
Other complex synapses in which PSDC dendrites
participated included axoaxonic and triadic arrangements.
The P 3D C system is therefore more complex at the
ultrastructural level than another ascending system (the
spinocervical tract) which originates in the dorsal horn,
and the functional implications of this complexity are
Terminals from Group la, slowly adapting Type I (3AI) and
rapidly adapting pad (RA) afferent units were also studied.
Boutons formed by all three afferent types contained
spherical agranular vesicles, however the Group la boutons
were larger and generally formed fewer synapses with
postsynaptic structures than the cutaneous axons. SAI and RA
axons formed asymmetrical synapses onto dendritic shafts and
spines and participated in both simple axodendritic and more
complex arrangements involving other unlabelled dendrites
and presynaptic axonal profiles. They differed in the
average number of postsynaptic targets but the differences
were not considered sufficiently great to allow
identification of unlabelled primary afferent profiles on
ultrastructural grounds alone.