Fig. 1.

Current heterogeneity between neurons. Whole-cell voltage clamp shows the presence of at least two voltage-activated outward K currents and a voltage-activated inward Na current (I_Na) in developmentally mature neurons (19 hr AEL and older). K currents include a fast, inactivating A-type (I_A) current and a slower, inactivating, delayed rectifier (I_K)-like current. These neurons also express inward Ca currents, but these are masked by the outward K currents under the conditions used for these recordings (but see Fig.4). A, The majority of neurons examined exhibit prominent I_A andI_K currents. The remainder of neurons predominantly express either I_A(B) or I_K(C). I_Na is present in most but not all neurons (e.g., absent in B; see Fig.6). The presence or absence of I_Na appears unrelated to neuronal K current characteristics. D, Currents were evoked from voltage steps (15 mV increments; range, −60 to +45 mV; 50 msec) applied from a conditioning prepulse of −90 mV. Recordings were obtained in normal whole-cell saline (see Materials and Methods).