Quantification of ultrastructural symmetry at molluscan chemical synapses.

Two criteria were developed to identify symmetrical synapses in the tentacle ganglion of the snail Achatina fulica. First, the concentration of electron lucent vesicles within 20 nm of one membrane had to be less than three times the concentration at the opposed membrane. Second, paramembranous densities had to be present in both cells, as unanimously judged by three independent observers. Nine synapses (27% of sample) satisfied both criteria. It is suggested that these synapses transmit chemically in two directions.