PhD defense by Tian Wang – Niels Bohr Institute - University of Copenhagen

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PhD defense by Tian Wang

Title: Studies on the action potential from a thermodynamic perspective

Nerve impulse, also called action potential, has mostly been considered as a pure electrical phenomenon. However, changes in dimensions, e.g. thickness and length, and in temperature along with action potentials  have been observed, which indicates that the nerve is a thermodynamic system.   
The work presented in this thesis focuses on the study of the following features of nerve impulses, and interpretations from a thermodynamic view are provided. (1) Two impulses propagating toward each other are found to penetrate through each other upon collision. The penetration is found in both bundles of axons and nerves with ganglia. (2) Attempts have been made to measure the temperature change associated with an action potential as well as an oscillation reaction (Briggs-Rauscher reaction) that shares the adiabatic feature. It turns out that some practical issues need to be solved for the temperature measurement of the nerve impulses, while the measured temperature change during the oscillation reaction suggests that there are a reversible adiabatic process and a dissipative process. (3) Local anesthetic effect on nerves is studied. Local anesthetic lidocaine causes a significant stimulus threshold shift of the action potential, and a slight decrease in the conduction velocity. (4) The conduction velocity of nerve impulses as a function of the diameter of the nerve is investigated with stretched ventral cords from earthworms. The velocity is found to be constant with a decrease of the diameter, indicating that the conduction velocity is independent of the diameter of the nerve. All the above results can be explained by a thermodynamic theory for nerve impulses, i.e. the Soliton theory, which considers the nerve impulses as electromechanical solitons traveling without dissipation.  
Finally, the magnetic field generated by a nerve impulse is measured with a sensitive atomic magnetometer developed by our collaborators from the Quantum Optics (QUANTOP) group in our institute. The magnetometer can be operated at room or body temperatures, and magnetic field from nerve impulses can be measured several millimeters away. This provides a promising technique for medical applications.