Ultrastructural observations in the frog spinal cord in relation to the generation of primary afferent depolarization.

The spatial distribution of different types of synaptic boutons over the motoneuron cell body and dendrites was studied both in the normal and in the chronically deafferented frog spinal cord. In both preparations, endings with S-type (spherical) vesicles were more numerous than those with F (flat) vesicles, the S F synapse ratio being approximately constant when measured at the soma, the proximal dendrites or the fine dendrites. However, the proportion of S boutons, as well as the proportion of degenerated boutons of afferent fibers in close apposition with other boutons, was higher on the fine dendrites in the lateral neuropile than on the motoneuron cell body and proximal dendrites, and in all cases significantly higher than the proportion of F boutons showing close appositions with other boutons. Degenerated boutons from descending fibers were found to synapse over the motoneuron cell body and proximal dendrites, but these boutons were rarely seen to be closely apposed to other boutons. The possible relation of these findings to the mechanisms generating primary afferent depolarization is discussed.