Freeze fracture study on three types of synapses in the Xenopus spinal cord

Three types of synapses (S-, F- and C-types) were identified in the thin-sectioned Xenopus spinal cord and their structure was analyzed with the freeze-fracturing technique. All three types of synapses showed similar specializations of the presynaptic membrane. This finding suggests that the three types of synapses may release their transmitters by a similar mechanism. By contrast, the three types of synapses revealed different specializations of the postsynaptic membrane. The E-face of the S-type postsynaptic membrane was characterized by a dense aggregate of large intramembrane particles, 12 to 15 nm in diameter. An aggregate of small particles, 8 to 9 nm, was evident on the E-face of the postsynaptic membrane of the F-type synapse. In the C-type synapse, there was a striking aggregate of intramembrane particles, 10 to 14 nm in diameter, on the P-face of the postsynaptic membrane. These characteristic features in the distribution of particles in the three types of postsynaptic membranes may reflect differences in the type of transmitter released or transmitter action on the postsynaptic neuron. The overall size of the area of aggregated particles on the P-face of the C-type postsynaptic membrane was coextensive with the underlying subsurface cistern (SSC) which showed partial occlusion of the lumen. This fact supports the view that the SSC is closely related to the C-type synaptic action which might be distinct from the other synaptic types.