The origin of the initial heat associated with a single impulse in mammalian non-myelinated nerve fibres.

1. A study has been made of the temperature changes associated with the passage of a single impulse in rabbit desheated vagus nerves.2. The initial changes consist of an evolution of positive heat followed by a reabsorption of most of it; i.e. there is a phase of positive and a phase of negative heat production.3. The size of the positive heat, its time of onset, and the time of onset of the negative heat have been measured by an analogue method of analysis. In addition, these parameters, together with the size of the negative heat and the duration of both phases of initial heat, have been studied with the aid of a computer, and also by conventional heat block analysis.4. At about 5 degrees C the measured positive heat is 7.2 mucal/g. impulse. It starts as soon as the compound action potential reaches the thermopile and lasts for about 107 msec.5. This positive heat decreases with increasing temperature, the ratio of heat at 4 degrees C to that at 14 degrees C being 1.86.6. The measured negative heat at about 5 degrees C is 4.9 mucal/g. impulse. It starts 102 msec after the onset of positive heat, and lasts for about 240 msec.7. When the sodium of Locke solution is replaced by lithium the positive heat is reduced by 19%, but the negative heat is increased by 22%.8. In potassium-free solutions the positive heat is hardly affected (increase of 5%), but the negative heat is more than doubled. As a result the nerve may become briefly colder than its initial temperature by about 2 mu degrees C.9. The effect of sodium-deficient solutions on the positive heat is somewhat variable, but the negative heat is consistently diminished.10. Replacement of the chloride of Locke solution by sulphate or nitrate has little effect on the positive heat. The negative heat is reduced in size by 26% and in duration by 22%.11. Replacement of most of the sodium of Locke solution by barium reduces or abolishes the negative heat, and increases the measured size of the positive heat nearly threefold.12. Veratrine (10(-5) g/ml.) produces a nearly tenfold increase in the net positive heat.13. Ouabain (1 mM) and antimycin A (1 mug/ml.) applied for 30-60 min have little effect on the initial heat production.14. Over the temperature range 5-15 degrees C, and for the ionic solution changes described above, there is close agreement in timing between the positive heat and the rising phase of the action potential, and between the negative heat and the falling phase.15. Because of the inevitable temporal dispersion of the action potential over the face of the thermopile, the observed temperature changes are smaller than those which occur at a single point in the nerve close to a stimulating electrode. The corrected value of the positive heat at 5 degrees C is 24.5 mucal/g. impulse, while that of the negative heat is 22.2 mucal/g. impulse.16. The heats of mixing of the ions in solution that interchange during the action potential are much too small to account for the observed initial heats, but an exchange of ions associated with fixed charges might make a significant contribution to the heats.17. The condenser theory, according to which the positive heat represents the dissipation of electrical energy stored in the membrane capacity, while the negative heat results from the recharging of the capacity, appears unable to account for more than half of the observed temperature changes.18. It seems probable that the greater part of the initial heat results from changes in the entropy of the nerve membrane when it is depolarized and repolarized.