Paramembranous densities of 'C' terminal-motoneuron synapses in the spinal cord of the rat

A category of large boutons forming synapses with the soma and proximal dendrites of spinal motoneurons was studied in glutaraldehyde-fixed, non-osmicated tissue stained with uranyl acetate and lead citrate. The identity of these boutons with 'C' boutons was indicated by their shape, frequency and distribution as well as by the ultrastructural characteristics of the boutons and the associated postsynaptic structures. In contrast to previous descriptions based on osmicated tissue, this study demonstrates that paramembranous densities are a feature of 'C' terminal-motoneuron synapses.