The after‐effects of impulses in the giant nerve fibres of Loligo

The giant nerve fibres of Loligo and Sepia differ from most excitable tissues in that the spike of an isolated axon is followed by a brief period of hyperpolarization, which is often called the positive phase (Curtis & Cole, 1942; Hodgkin & Huxley, 1939; Weidmann, 1951). There is evidence that the positive phase occurs because the permeability of the membrane to potassium increases during the second half of the spike, and does not at once return to its resting value (Hodgkin & Huxley, 1952d). Since the resting potential of an isolated axon may be 15-30 mV less than the equilibrium potential for potassium ions, the persistence of the state of increased potassium permeability during the refractory period raises the membrane potential above its resting value and generates a pQsitive phase. This suggestion is supported by the observation that the membrane potential during the positive phase is markedly affected by small changes in the concentration of potassium outside the fibre (Hodgkin & Katz, 1949a, Hodgkin & Keynes, 1955). Thus, increasing the external potassium concentration from 0 to 20 mm, which decreases the resting potential by only about 10 mV, reduces the membrane potential during the positive phase by 30 mV. The sensitivity of the positive phase to changes in potassium concentration forms the basis for interpreting the observations described in this paper; the starting-point was provided by an experiment in which the effects of calciumdeficient solutions were being examined. Like many previous observers we found that reducing the concentrations of calcium and magnesium made the giant axon of Loligo spontaneously active. When fibres were near the point at which they fired continuously they often gave long trains of impulses in response to a single shock. In these trains we noticed that the positive phases of the spikes were not of equal size, but declined during the initial stages of the