Fig. 4. Rise times of synaptic potentials. (A) Four excitatory postsynaptic potentials (EPSPs) with different rise times recorded from the same neurone. The dotted line passes through the peak of the fastest EPSP (a). The EPSPs shown in b and c had similar rise times but different amplitudes, suggesting either that they had been produced in the same site with different quantal contents or that the two EPSPs had been produced on different sides of the gap junction. The EPSP shown in d was smaller and slower, suggesting that it was produced electrotonically at a point more more distant from the soma being recorded. (B) Characteristic rise time distribution of one Retzius neurone recorded in normal saline solution. The distribution had two Gaussian components marked by the arrows. (C) The EPSP rise time distribution of a Retzius neurone recorded in the presence of 1 mmol l^-1 Mg²⁺ in the external fluid. Again, two Gaussian peaks, which are marked by the arrows, were present. In this case, the peak at the long rise time was clearer. (D) Theoretical model showing two Retzius neurones with dendrites coupled by a resistance (r) and sharing a common presynaptic input. The lettering of the inputs corresponds to those in B and C and has as reference the recording electrode.