FIG. 2.

Characteristics of layer 2/3 neocortical GIN cells. A: cumulative plots of the lowest current value (pA) that elicited action potentials for each cell type (i.e., firing threshold). B: average firing frequencies of GIN, regular-spiking (RS), and fast-spiking (FS) cells as a function of stimulus current, measured during the last 200 ms of 600-ms constant-current steps. Currents are normalized to the threshold (i.e., the lowest current amplitude that yielded action potentials for each cell). C: GIN cell responses to current steps at −100 and 40 pA. An example of the afterdepolarization (ADP) characteristic of GIN cells is enlarged in the inset (from area above horizontal line in C). With injection of stronger currents (e.g., 300 pA in bottom trace in C), GIN cells display spike frequency adaptation. In addition, characteristic action potential height and afterhyperpolarization (AHP) profiles of GIN cells can be observed in this trace. Note that the peak of the first action potential is the most positive in the trace and the depth of the first AHP is the most negative. D: FS cell responses to current steps at −100 and 180 pA (top) and 500 pA (bottom). The lack of an ADP is shown in the inset of the top panel (enlarged from area above horizontal line in D). Note also the delay in firing in the top panel in D, which is characteristic of FS neurons with injections of just-suprathreshold currents. E: GIN cells have a more positive resting membrane potential (fine dashed line) and a more negative action potential threshold (coarse dashed line) than RS (F) and FS (G) cells. Arrows indicate resting membrane potentials for each cell type. H: average resting membrane potentials and action potential thresholds.