Fig. 5.

Distribution of primary afferent depolarization (PAD) in the spinal cord. A: extracellular recording (EC; black) of the group Ia afferent volley (field) in lamina I near the dorsal columns (DC) in response to S4 dorsal root (DR) stimulation (1.2 × threshold (T), for volley), expanded 10× in gray. Intracellular (IC) spike (red) in response to the same DR stimulation after penetrating a nearby proprioceptive group Ia afferent (S4), demonstrating that extracellular fields are negative when there is a nearby positive intracellular event. Afferent resting at −72 mV. B: same cell as in A, but on longer time scale and stimulation set lower (1.1 × T), subthreshold to direct orthodromic spike in the same afferent. Intracellular recording (red) shows a PAD in response to the DR stimulation. Extracellular recording (black) shows the afferent volley field, followed by a second field corresponding to the synaptic excitation of local interneurons in the dorsal horn, and finally a slow long-lasting negative field corresponding to PAD (expanded in gray, 7×; truncated at vertical line). Subtraction of the EC from the IC recordings gives the actual transmembrane potential (green). C: representative intracellular (red) and extracellular (black) recordings from different laminae in the spinal cord during dorsal root stimulation (~2.0 × T), with peak PAD observed in the deep dorsal horn (positive red, IC and negative black, EC), including both an early, fast phasic PAD and later tonic PAD (n = 22). In the ventral horn, only a transient 20 ms long PAD was seen in the ventral afferent terminals recorded intracellularly (Lamina IX, red, n = 4), and extracellularly there was no negative PAD field (n = 20/20). Scale 0.5 mV for dorsal EC fields, 0.1 mV for ventral EC fields and 2 mV for all IC recordings.