An ultrastructural study of serially sectioned Renshaw cells. III. Quantitative distribution of synaptic boutons.

The quantitative distribution of synaptic boutons on 17 presumed Renshaw cells has been studied ultrastructurally. All 17 neurons were postsynaptic to axon collateral boutons of intracellularly HRP-stained triceps surae alpha-motoneurons and were located in lamina VII, ventromedially to the main motor nuclei. In each of the presumed Renshaw cells, the values for mean length and mean area of apposition, percentage synaptic covering, and packing density of S-type, F-type, and S + F-type boutons were estimated on the cell body and in two dendritic compartments. The F/S percentage synaptic covering ratio was also calculated. The previously demonstrated differences within the present group of neurons, with respect to the site of axonal origin, were not accompanied by any corresponding differences in the quantitative distribution of synaptic boutons. However, it is suggested that the presumed Renshaw cells may possibly fall into two categories with respect to the F/S percentage synaptic covering ratio. The results are discussed in relation to previous studies on the neuronal architecture and synaptic types on the same presumed Renshaw cells, as well as in relation to earlier observations on the quantitative distribution of boutons on central neurons, particularly spinal alpha-motoneurons.