a single action potential, but some fired multiple spikes (Figure 1C). In total, the electrophysiological data indicate that the cells studied here display voltage-gated channels similar to those in central neurons. In addition to these signatures of electrical excitability, we also recorded spontaneous synaptic currents in some neurons (Figure 1D–F). In three control neurons (of 20 tested), the fast rise (0.5–0.7 ms) and decay (1.8–2.4 ms) of these currents and their reversal near 0 mV strongly suggested that they were AMPA-receptor EPSCs (Figure 1D–F). In three other neurons (2 of 17 from patients, 1 additional control) we saw occasional synaptic currents that were larger (40 to 325 pA, at −70 mV) and which decayed with time constants of 7 to 10 ms. Although the percentage of neurons showing synaptic currents was low, the results clearly demonstrate that neurons in the organoids form functional synaptic connections.
