Characterization of the functional connections during development. (A) Number of detected functional connections in Helix neuronal cultures treated with KCl (black line) and 5-HT (red line). (B) Trend of the cross-correlogram peak values during development (KCl black, 5-HT red). (C) Estimation of the synaptic latency: in the case of 5-HT treatment, the delays are always higher than the corresponding with KCl. (D-E) Schematic representations of APs intracellularly recorded from pre and postsynaptic cells linked by a chemical (A) or electrical (B) connection. The first derivate of signals are shown in the bottom panels. (F) Bar graph showing the mean synaptic delays measured in relation to the type of connection (chemical and electrical) compared with the type of treatment (5-HT and KCl).