Morphology and distribution of the synapses to the spinal motoneuron of the frog.

The quantitative distribution of the different structural elements on the surface of the motoneurons (MN) in the spinal cord of the frog was studied in thin sections and freeze-fracture replicas. In particular, the different synaptic types and their distribution on the MN soma and dendrites are described. In thin sections three types of synapses were discerned: the S-type with spherical vesicles, the F-type with flattened vesicles, and finally the C-type synapse with spherical vesicles and a subsynaptic cistern. The synaptic covering at the MN surface as determined by thin sections is about 36%. In freeze-fracture replicas were observed boutons with and without gap junctions, and C-type boutons. When using the latter technique the synaptic covering was found to be 43%. With both techniques special attention was paied to the morphology of the C-type bouton and both the subsynaptic and extrasynaptic cisterns. The soma membrane over the sub- and extrasynaptic cisterns reveals characteristic and very similar morphological features with respect to both the distribution and size of the membrane particles. the possible functions of the two cisternal types are discussed.