Current of injury

Du Bois-Reymond (1852) wrote of a "negative variation" of the muscular current during contraction. He preferred it to "decrease" because he could not yet tell whether the current only weakened or reversed its direction.¹ He also discussed priority. He had first written that Matteucci stated the frog current disappears during tetanus, and had thought Matteucci observed the negative variation before him.² He later judged that this wording did not do justice to his own work.² A committee report quoted in the abstract called the negative variation a fundamental fact that directly explains Matteucci's induced contraction.²

Bose (1902) described the same effect in nerve. Stimulation reduces an existing difference of potential, so the current of injury diminishes and the nerve shows a negative variation.³ He put this as a current of action that opposes the current of injury, running from the more excited to the less excited region.³ In 1926 he used the effect as a method in plants. Injuring the tissue at one electrode abolished its excitability, so the response at the other electrode appeared as a negative variation of the current of injury.⁷ He noted a complication. The injured tissue may recover, which reduces or removes the response, so a fresh injury has to be made.⁷

Bayliss (1915) tied excitation to increased permeability of the cell membrane. If the membrane at rest is impermeable to only one ion of an electrolyte within the cell, it is polarised, and the current of rest, injury current or demarcation current is accounted for.⁴ If that semipermeability is lost in excitation, the negative variation follows.⁴ Lillie (1923) reported that Hober's salt experiments on muscle supported Overton's view that resting muscle is impermeable to sodium salts. Potassium, rubidium and ammonium salts produced an injury current or local negativity.⁵ Lillie gave 0.05 volt as the usual maximum variation in contraction, similar to the demarcation current. Some observers found the action current could reach 0.08 volt. He said the exact physico-chemical meaning could not yet be stated.⁶

Tyler (2017) reviewed work on voltage changes at wounds. Wounding human skin produces an ionic flux, the injury current, typically 1 to 10 μA/cm2.⁸ Amputated newt limbs showed 10 to 100 μA/cm2 in the remaining epidermis. Newly amputated human fingertips, which can regenerate, more usually in children, gave 22 μA/cm2.⁸ The review reports the suggestion that regenerating and non-regenerating systems differ in their bioelectric characteristics.⁸ O'Hara-Wright and colleagues (2022) describe the injury potential, or demarcation current, as a potential difference between intact epithelium and the wound. Current leaks from the wound edge, the path of least resistance, and creates an electrical field.⁹