[Method of visually classifying synapses from a multiplicity of attributes]

Seven electron microscopic signs of synapses were recognized and classified visually. Altogether 2250 synapses of the stratum radiatum of the hippocamp of 15 white rats were examined on a statistical basis. Five rats (750 synapses) showed a significant decrease in the magnitudes of two signs of the synapses (the active zone length and the mean sectional area of the terminal) after the elaboration of the conditioned reflex. The concentration of synaptic vesicles adjucent to the active zone rose as compared to that seen in a passive control group (5 rats, 750 synapses). An active control group (5 rats) demonstrated a significant increase in the postsynaptic membrane thickness and sectional area of the terminal occupied by synaptic vesicles. The concentration of vesicles neighboring the active zone was noticeably lowered.