Illustrations that might account for the case in which the fast signal in type IIIa contains the presynaptic and postsynaptic action potential components. In A, both the fast and slow signals were reduced in the presence of APV together with CNQX. The enlargements of the signals are shown on theright. This expanded time base reveals that the presynaptic and postsynaptic action potentials are overlapped. In such a case, the slow signals were often relatively large. InB, the fast signal is not altered in the presence of APV and CNQX. In this case, the slow signal was often small. Thin traces are control in normal Ringer’s solution; thick traces are recordings made in APV (190 μm)- and CNQX (5 μm)-containing solution.