FREEZE-ETCHING STUDY OF THE AXOSOMATIC SYNAPSES IN THE RAT SENSORIMOTOR CORTEX

The intramembranous structure of axosomatic synapses in the rat sensor imotor 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 st ructure as the surrounding nonjunctional membrane on both fracture fac es. Probably this is the freeze-etching equivalent of symmetric axosom atic synapses. Asymmetric synapses were found on perikarya of nonpyram idal neurons. In freeze-etched replicas, the postsynaptic non-pyramida l neuronal membrane contains aggregates of particles on the extracellu lar half (E-face) as an intramembranous equivalent of asymmetric synap ses. The particle packing density in these aggregates was measured and compared with that at the E-face of dendritic shaft and dendritic spi ne membranes. Intramembranous characteristics of the axosomatic synaps es in the rat sensorimotor cortex are discussed in terms of plasticity of the synaptic contact zone.