Ultrastructure and synaptic architecture of spinal motoneurons in the frog (Rana catesbeiana)

An electron-microscopic analysis of spinal motoneurons and their synapses was carried out in a frog (Rana catesbeiana). Six different types of boutons (S, F, M, P, C and GS) have been identified. Their distribution on spinal motoneuron somata and proximal dendrites is described. The mean linear percentage of the surface area covered by boutons is 26.1 +/- 1.9%. S-type boutons are preferentially concentrated on the soma and proximal dendrites. The relative number of S-type boutons (58.7%) was greater (p less than 0.01) than that of F-type boutons (41.3%). This is in contrast to mammalian spinal motoneurons where F-type boutons are much more numerous on the soma than S-type boutons. F-type boutons are randomly distributed and the average ratio of S:F-type boutons is 20:14 (S:F ratio = 1.4). In contrast, M-type boutons synapse exclusively on the distal part of the dorsal dendrites and are restricted to the intermediate zone or to the dorsal horn. P-type boutons form synapses upon the large M-type boutons. The polarity of these axoaxonic synapses is always from P to M. Similarities and differences between the synaptology of frog and mammalian spinal motoneurons are discussed.