Abstract

We have investigated the intrinsic optical properties of cultured Aplysia neurons and the changes in these properties associated with an action potential. By using conventional dark-field optics to collect light scattered at angles >4.5°, we recorded single action potentials in real time. The relative change in the level of scattered light during an action potential was ~3 × 10^-4, and the signal-to-noise ratio was typically 10. The optical signal followed the same time course as the electrical signal. Furthermore, optical techniques allowed signals to be measured from fine neuronal processes. The angular dependence of light scattered from a resting neuron was measured and compared with calculated scattering from a nonabsorbing dielectric cylinder with an inhomogeneous index distribution. The data were consistent with a cylinder in which the index distribution was significantly peaked within the cell—not necessarily at the center. The average value of the cell index with respect to water was 1.02. Comparison with the changes in the scattered light observed in the dark-field measurements implies that the change in scattering reflects a change in the index of refraction of the cell. Based on these estimates, we have built a phase-sensitive microscope that can directly measure this phase shift. This should increase both the signal-to-noise ratio and the spatial resolution.

© 1990 Optical Society of America