Structural and electrophysiological effects of local anesthetics and of low temperature on myelinated nerves: Implication of the lipid chains in nerve excitability

X-ray scattering and electrophysiological experiments performed on toad sci atic nerves as a function of the exposure to either low temperature or tetr acaine yielded the following results: (i) the main structural effect is to thicken the individual membranes, thus to stiffen the acyl chains and incre ase the repeat distance of the one-dimensional lattice, phenomena that are typical of lipid-containing systems with disordered chains; (ii) the electr ophysiological effect is to decrease the amplitude and velocity of the comp ound action potential; (iii) the structural and physiological effects of th e two agents are practically identical. Since the structural and the electr ophysiological parameters have different origins in the nerves (the structu re regards the myelin sheath, the electrical signals originate at the nodes of Ranvier) it is inferred that tetracaine and low temperature exert simil ar effects on the membranes of both the myelin sheath and the nodes of Ranv ier. Also, since local anesthetics act by inhibiting the Na+ channels, thes e observations suggest that the acyl chain conformation modulates the chann el function and thus the generation of action potential. (C) 1999 Academic Press.