Figure 2.

Net increase in hyperpolarizing current in the central region of an axon in response to an action potential as the axon’s temperature is gradually increased from 6.3 °C to 25 °C around the center of the axon. (A). Schematic of experiment. The left-hand side of the model axon (0.5 mm diameter) was held at 6.3 °C, and then, from 40 to 60 mm from its left-hand side, its temperature was gradually increased to 25 °C (see methods, simulating temperature changes in the model, equations (2) and (3)). An action potential was initiated using a current pulse of 2000 nA (1 ms duration) injected into the left-hand side of the axon, and the sodium, potassium and net currents were measured at the center of the axon. Sodium current, potassium current, net current and charge transfer were measured at three positions: position 1—region of the axon at the control temperature (25% along its length). Position 2—center of axon, region within which temperature is changing (50% along its length). Position 3—region of the axon at the new, higher temperature (75% along its length). (B) Temperature profile along the axon, ranging from 6.3 °C to 25 °C from 40 mm to 60 mm. (C)–(F) Results for the Hodgkin/Huxley model. (C) Inward sodium current. (D) Outward potassium current. (E) Net current. (F) Inward versus outward charge transfer. (G)–(J) Results for the modified Hodgkin/Huxley model. (G) Inward sodium current. (H) Outward potassium current. (I) Net current. (J) Inward versus outward charge transfer. Both models show a net increase in hyperpolarizing (outward) current in the central region as temperature increases in response to an action potential; in the modified model, the net outward current is also larger in the region of elevated temperature, although the currents are much smaller.