Changes in axon birefringence associated with excitation: implications for the structure of the axon membrane

Changes in birefringence during nerve excitation were examined in squid giant axons which were flattened after removal of the major portion of the axoplasm by either mechanical or enzymatic means. Birefringence changes were observed at the center as well as near the edges of the flattened axons; this finding suggests that there are changes in the refractive index for linearly polarized light with its electric vector directed parallel to the long axis of the axon. Changes in birefringence during action potentials were found to be smaller in pronase treated axons than in those treated mechanically. This finding was interpreted as indicating involvement of a longitudinally oriented macromolecular structure at or near the membrane in nerve excitation. The results of a simple mathematical analysis is presented in an appendix, showing the variation in the lateral direction of the birefringence changes recorded from an intact (i.e., unflattened) axon.