Figure 5.

Suprathreshold and subthreshold responses of GFP+ neurons. A, Spike trains in response to current steps of increasing amplitudes. Aa–Ac, Ad–Af, and Ag–Ai are responses to high, medium, and low current levels, respectively; the three current steps for each neuron are shown superimposed in Aj–Al. The asterisks in Ad denote interruptions in firing characteristic of stuttering X94 neurons; some GIN neurons also stuttered, but X98 neurons never did. The insets in Ag–Ai are the first action potential from the corresponding trace, shown at half the vertical scale and at a 100-fold expanded horizontal scale; note the pronounced difference in spike widths between lines. The arrowheads in Ah and Ai point to the characteristic triphasic AHP in X98 and GIN neurons. Calibration: 40 mV (Aa–Ai), 1000 pA (Aj–Al), 200 ms. B, Superimposed voltage responses (Ba–Bc) to the hyperpolarizing and small depolarizing current steps shown in Bd–Bf, in three other neurons. Note the very low input resistance of the X94 neuron compared with that of the X98 and GIN neurons (much larger current steps required to elicit similar voltage changes). The asterisks in Ba–Bc indicate the sag attributable to the hyperpolarization-activated cationic current I_h. The open arrowheads in Ba and Bc denote a depolarizing rebound, also attributable to I_h. The filled arrowheads in Bb point to bursts of action potentials riding on low-threshold Ca²⁺ spikes. Calibration: 20 mV (Ba–Bc), 300 pA (Bd–Bf), 200 ms.