31.01.2012 17 c.t.
The physics of nerves, anesthesia and lipid membrane channels
by Prof. Dr. Thomas Heimburg
from Niels Bohr Institute, University of Copenhagen, Denmark
Ludwig Prandtl Hörsaal, Am Faßberg 11 AI-Gebäude
At physiological temperature, many biomembranes are found in a physical state slightly above the melting transition of the membrane lipids. We show that this condition gives rise to the possibility of the propagation of electromechanical pulses (solitons) in membrane cylinders that share many properties with the action potential in nerves. Among those properties are the reversible heat change measured in nerves under the influence of the action potential, a mechanical shortening and thickening of the nerve, but also the excitability by voltage, local cooling, and by mechanical stimulus. Depending on boundary conditions one can obtain voltage pulse trains with minimum pulse distance (refractory periods) and undershoot (hyperpolarization).
The underlying physics is that of the fluctuation-dissipation theorem that contains strict thermodynamic couplings between heat capacity, compressibility and lifetimes of membrane processes. It contains a role for all thermodynamic variables (not only voltage). We show that our electromechanical picture also contains a mechanism for anesthesia that lies in the melting point depression caused by these drugs, in agreement with the well-known Meyer-Overton correlation for anesthetics. The presence of anesthetics results in the reduction of the membrane excitability. Further, a direct consequence of the membrane fluctuations is the observation of spontaneous formation of ion-channel-like conduction events in the pure lipid membrane indistinguishable from those attributed to ion channel proteins. Since the theory is of macroscopic thermodynamic nature it does not make statements on processes on the molecular scale. However, it is mostly consistent with known pharmacology provided that the macroscopic conservation of heat is maintained.
Summarizing, we provide thermodynamic picture for many excitatory processes in biomembranes that let pulse propagation, ion conductance, and action of drugs all be part of one self-consistent physical description.
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