Figure 5.

Five patterns of discharge recorded in response to current ramp stimulation in postnatal lumbar motoneurons (n = 38). The four classical types (A–D) as described in adult motoneurons (Bennett et al., 2001) are present and a fifth type called transient, since no discharge firing, or only a few spikes (<5), could be evoked during the descending phase (E). (A) type 1: linear F-I relationship where the firing frequency curves overlapped on the ascending and descending phases. (B) type 2: clockwise hysteresis pattern where the instantaneous frequency is lower in the descending phase for the same current intensity. (C) type 3: Linear F-I relationship with sustained firing in the descending phase. (D) type 4: Counter clockwise hysteresis where the frequency is higher during the descending phase; (E) type 5: transient discharge during the ascending phase with usually no discharge or only a few spikes in the descending phase. In this case which is frequent before P5 (40%), it was not possible to plot an F-I curve during the descending ramp. (F) distribution of the five discharge patterns according to the postnatal ages. Note that types 1 and 2 are the predominant types before P8 (n = 15/26) whereas the types 3 and 4 are most frequent at P8–P9 (n = 6/12) but rare before P8 (n = 3/26).