Synaptic organization of the pedunculopontine tegmental nucleus of the cat

The synaptic organization of the feline pedunculopontine tegmental nucleus (PPN) was studied electron microscopically. The bouton covering ratios were calculated in various sizes of PPN neurons, and the ratios of large neurons (56%) were found to be much higher than those of small neurons (16%). The PPN neuron dendrites usually showed some varicosities, and spines were observed on both somatic and dendritic profiles. Among a total of 1021 synapses sampled at random, axosomatic, axodendritic and axospinous synapses comprised 21.7, 61.2 and 14.1%, respectively. On the basis of the postsynaptic junction, these synapses were classified into the symmetric (66.3%) and the asymmetric (33.7%) types. The percentage of symmetric synapses was much higher on the soma (91.0%), and the large (69.4%) and medium-sized (63.2%) dendrite, while that of asymmetric synapses showed a higher value on the small dendrite (55.5%) and the dendritic spine (50.8%). Axoaxonic, dendrodendritic and dendroaxonic synapses, although not so frequent, were, in part, involved in the serial synapse or the synaptic triad. It is concluded that some PPN neurons are spiny, and that axosomatic, axodendritic and axospinous synapses are the main synaptic constituents and besides those synapses a more complex synaptic organization exists in this nucleus.