Figure 14-10 Spontaneous ectopic impulses in "abnormal" nerves are blocked by very low concentrations of local anesthetics. When 10% to 20% of the Na⁺ channels close very slowly, as caused here by the addition of a peptide neurotoxin (ATX), but otherwise an intrinsic property of channels that are upregulated after nerve injury, spontaneous impulses appear (trace B). Occurring as bursts of action potentials separated by quiet periods, these impulses appear similar to the ectopic discharges in a peripheral nerve after injury. Very low concentrations of lidocaine, equal to plasma levels during intravenous infusions that can reverse neuropathic pain, strongly suppress (C) and eventually abolish (D) these spontaneous discharges, an effect that is reversed when lidocaine is removed from the nerve (E). In contrast, electrically stimulated action potentials are unaffected by such low lidocaine concentrations (e.g., see Fig 14-8B ). (From Persaud N, Strichartz G: Micromolar lidocaine selectively blocks propagating ectopic impulses at a distance from their site of origin. Pain 99:333–340, 2002.)