Freeze-etching study of the axosomatic synapses in the rat sensorimotor cortex.

The intramembranous structure of axosomatic synapses in the rat sensorimotor cortex was studied by means of thin sections and freeze-etched replicas. In thin sections, symmetric synaptic junctions were located on perikarya of pyramidal and non-pyramidal neurons. In freeze-etched preparations, the postsynaptic neuronal membrane exhibited the same structure as the surrounding nonjunctional membrane on both fracture faces. Probably this is the freeze-etching equivalent of symmetric axosomatic synapses. Asymmetric synapses were found on perikarya of non-pyramidal neurons. In freeze-etched replicas, the postsynaptic non-pyramidal neuronal membrane contains aggregates of particles on the extracellular half (E-face) as an intramembranous equivalent of asymmetric synapses. The particle packing density in these aggregates was measured and compared with that at the E-face of dendritic shaft and dendritic spine membranes. Intramembranous characteristics of the axosomatic synapses in the rat sensorimotor cortex are discussed in terms of plasticity of the synaptic contact zone.