During the action potential changes are observed in the birefringence (BR) of the membrane of the nerve fiber, associated with changes in the electric field in the membrane. Calculations show that at the peak of the action potential, the membrane BR is increased by 1.5 • 10 "4, and that the recordable optical effect is approximately a hyperbolic function of the axon diameter. For comparison, behaviour of BR of artificial phospholipid membranes (APMs) was investigated under identical conditions. The membrane was held at 45 ~ to the direction of the incident light. No changes in BR were found in a pure APM, which is practically impermeable to ions. Hence it is concluded that the effect observed on the axon is not the result of direct action of the electric field on the lipids {Kerr effect), but is connected with processes in the complex structure of the membrane. Experiments were carried out with APMs containing the iodide carrier diborenyl mercury, both forms of which are charged. The volt-ampere characteristic curve of the APM in this case is N-shaped. Square pulses of amplitude (150-200 my) sufficient to bring the APM to the lower part of the decreasing segment of the characteristic curve were applied to the APM. Under these conditions optical effects similar in magnitude to those on the axon, and of the same sign, are observed. If changes in BR in the APM were of the same magnitude as in the axon, according to the calculation a change in the light flux of 6 x 10 -6 would be observed. The experimental results give 0.7-5 • 10 -6 . About 30% of the magnitude of the effect is due to amplitude changes in light passing through the APM, which was not observed on the axon.

Optical effects on such APMs can be explained by changes in the APM structure during displacement of charged carriers under the influence of the electric field to one edge of the membrane. A similar phenomenon perhaps takes place in the axonal membrane also.

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