Representative examples of response to first noxious heat exposure among anatomically identified myelinated nociceptors across different postnatal ages. For each cell, somal APs (and first derivatives, below), response to noxious heat, and camera lucida reconstructions of central anatomy (20 serial sections each) are shown from left to right. A: P4 laminae I–V AMH. B: P5 laminae I/IIo AMH. C: P9 laminae I–V AM. D: P9 laminae I/IIo AM. Note that the AM in C failed to respond to noxious heat despite stimulation for >30 s above 53°C, whereas the AM in D failed to respond despite stimulation ≤60°C. CV, mechanical threshold, and heat threshold for AMHs were, respectively, 1 m/s, 0.7 mN, 52°C (A) and 0.7 m/s, 1.56 mN, 45°C (B); CV and mechanical threshold for AMs were 1.3 m/s, 5.7 mN (C) and 0.7 m/s, 0.7 mN (D). Calibration bars = 20 mV and 5 ms (spikes, A–D), 10 s (heat traces, A–D), and 100 μm (camera lucida reconstructions, A–D).