Fig. 3.

Evoked action potential (AP) discharge in ascending projection neurons during the early postnatal period. Direct current injection through the patch electrode at increasing intensities (bottom to top) revealed 4 firing patterns in developing spino-PB and spino-PAG lamina I neurons. A: tonic neurons fired APs throughout the 800-ms depolarizations. All traces in panel originate from the same lamina I projection neuron. B: phasic neurons exhibited APs at the beginning of the current step but did not discharge spikes throughout the prolonged depolarization, as irregular gaps in their firing were evident at many stimulus intensities (arrows). C: delayed neurons were distinguished by a long latency to the first spike that varied with stimulus intensity. D: bursting neurons were identified by their slow plateau potentials with superimposed bursts of high-frequency AP discharge. Inset, example of spike afterdepolarization (see boxed region). E and F: when APs were evoked by intracellular current injection from the resting membrane potential, the majority of both spino-PB (E) and spino-PAG (F) neurons exhibited tonic firing during the first 3 postnatal weeks, while a more even distribution of firing patterns was evident by P30–32. Sample sizes were the same as described in Fig. 2.