ABSTRACT
We report a procedure to detect electrical activity in cultured neurons by changes in their intrinsic optical properties. Using dark-field microscopy to detect scattered light, we observe an optical signal that is linearly proportional to the change in the membrane potential. Action potentials can be recorded without signal averaging. We use the dark-field method to show that there are substantial time delays between activity in the soma and in fine distal processes of identified Aplysia neurons. The biophysical basis for the change in optical properties of the neuron was deduced from measurements of the angular distribution of scattered laser light. An analysis of the data indicates that the radial component of the index of refraction of the membrane increases and the tangential components decrease concomitant with an increase in membrane potential. This is suggestive of a rapid reorientation of dipoles in the membrane during an action potential.