CORRELATED TEMPERATURE RESPONSES SUPPORT ROBUST ACTION POTENTIAL TIMING
Temperature-robust timing of action potentials is essential for neuronal communication and a prerequisite for learning and behavior. The timing of action potentials relies primarily on the coordination of action potential initiation between neurons and the maintenance of that timing along the length of their axons. Ion channels, imperative to both of these processes are affected by changes in the animal's internal and external temperature. Additionally, ion channel numbers and responses to temperature may differ between neurons. Yet, many animal behaviors are robust to temperature fluctuations, suggesting that the sites of action potential initiation as well as axons have mechanisms to maintain coordinated timing. This is particularly important when axons differ in their diameters, as axon diameter determines ion channel number. Mechanisms that support precisely timed action potential initiation between different neurons have been identified, but no such mechanisms have been identified for axons despite their essential role in maintaining timing. To determine if axons have mechanisms to maintain timing we utilized the crustacean pyloric circuit - a system where action potential timing at the initiation sites is temperature-robust. It is not known, however, if timing is maintained along the several centimeter long axons, which project into the periphery to activate the muscles involved in the associated behaviors. Our results indicate that despite the velocity along each pyloric axon being different at any given temperature, velocity increases by similar relative amounts in all axons as temperature increases. This suggests that the pyloric axons have similar ion channel properties but differ in their diameter. Surprisingly though, we find that within a limited range of physiological temperature range, the pyloric axons do maintain action potential timing. To predict the mechanisms by which axons of different diameters, including pyloric axons, can maintain action potential timing when temperature changes, we developed a computational model that allowed us to independently vary ion channel properties and compare timing between different axons. Our model results show that the temperature influence on velocity strongly depends on how much temperature affects the activation gate of the Sodium channel. Specifically, the ratio of temperature influences on the activation gates between two different diameter axons bests predicts how those two axons will maintain action potential timing: Action potential timing is best maintained over the largest temperature range if the smaller axon is more strongly affected by temperature than the larger one. Consequently, temperature-robust action potential timing requires coordination between axonal intrinsic properties.
DeMaegd, Margaret, "CORRELATED TEMPERATURE RESPONSES SUPPORT ROBUST ACTION POTENTIAL TIMING" (2019). University Research Symposium. 312.