Changes in axon birefringence during the action potential.

1. Observations have been made on the changes in optical retardation accompanying the passage of impulses along crab leg nerves and squid giant axons.2. The nerves were mounted on the stage of a polarizing microscope, at 45 degrees to the planes of polarization and analysis, brightly illuminated with white light. During the nerve impulse the intensity of the light passing the analyser decreased temporarily by 1 part in 10(3)-10(6). Signal-averaging techniques were used to obtain an acceptable ratio of signal to noise.3. The changes in light intensity recorded under these conditions were shown to arise almost entirely from alterations in retardation, with little or no interference from scattering, absorption, linear dichroism or optical rotation effects; the occurrence of stimulus and coupling artifacts was also ruled out.4. In the squid giant axon, the retardation change was shown to be located in a thin cylinder immediately surrounding the axoplasm, and to have a radially oriented optic axis.5. The time course of the decrease in optical retardation was very similar to that of the action potential recorded with an intracellular electrode, suggesting that the retardation closely followed the electrical potential across the membrane.