Figure 1. Glomerular activation drives asynchronous subthreshold and suprathreshold excitation of GCs. A, Circuitry of the MOB. Excitatory cell classes are shown in black. Inhibitory cell classes targeting excitatory cells are shown in red. Inhibitory cell classes targeting other inhibitory cells are shown in blue. GL, Glomerular layer; IPL, internal plexiform layer; ETC, external tufted cell; PGC, periglomerular cell; sSAC, superficial short-axon cell; EPL-IN, EPL-interneuron; GL-dSAC, GL-projecting dSAC; EPL-dSAC, EPL-projecting dSAC; GCL-dSAC, GCL-projecting dSAC; IBP, intrabulbar projection; LOT, lateral olfactory tract. B, Experimental design. GC activity was monitored using whole-cell patch-clamp recordings while single glomeruli were activated by low-level stimulation of OSN axons. C, Bright-field image (top; scale bar, 100 μm) and morphological reconstruction (bottom) of a representative sGC. Gray lines in this and all subsequent reconstructions correspond to the cell layers labeled in A. M, Medial; P, posterior. D, Representative voltage trace (top) and raster plot of spike times across multiple trials (middle) after a single OSN stimulation pulse (bottom) for the sGC shown in C. The highlighted trial in this and all subsequent raster plots corresponds to the representative trace shown. Red arrow marks a spontaneous spikelet. E, F, Same as C and D for another sGC showing a barrage of asynchronous EPSPs and unreliable, long-latency firing. G, H, Same as C and D for a representative dGC showing subthreshold excitation and spikelets (inset: suprathreshold response to step current injections; calibration: 0.5 s, 20 mV/100 pA). I, Histogram of mean spike probabilities across all trials. Note that several GCs exhibit only subthreshold responses. J, K, Histograms of mean number of spikes evoked (J) and mean first-spike latencies (K) per suprathreshold trial.