To characterize the neural network in an increasingly intact system, we sliced hCSs into 250-μm sections and performed acute whole-cell recordings (Fig. 5f). We found that 80% of neurons (n = 15 cells from hCSs differentiated from two hiPSC clones) fire action potentials in response to depolarizing current steps from a holding potential of −65 mV (Supplementary Fig. 8a). The large majority of neurons (86%, n = 15; cells from hCSs differentiated from two hiPSC clones) showed spontaneous synaptic activity (Fig. 5g; Supplementary Fig. 8b) that was reduced by kynurenic acid, a glutamate receptor blocker (Fig. 5g; Supplementary Fig. 8c; n = 6 cells; P = 0.0008). Following patch-clamp recordings, neurons were labeled with biocytin to reveal their morphology (Fig. 5h). Importantly, in response to extracellular electrical stimulation, we observed large-amplitude excitatory postsynaptic potentials (EPSPs; >20 pA), demonstrating that cortical neurons in hCSs participate in network activity (Fig. 5i; Supplementary Fig. 8d,e). To determine whether these synaptic responses are capable of driving spike firing in hCS neurons, we performed current-clamp recordings in which no holding current was applied while administering
