Full-field interferometric imaging of propagating action potentials.

Currently, cellular action potentials are detected using either electrical recordings or exogenous fluorescent probes that sense the calcium concentration or transmembrane voltage. Ca imaging has a low temporal resolution, while voltage indicators are vulnerable to phototoxicity, photobleaching, and heating. Here, we report full-field interferometric imaging of individual action potentials by detecting movement across the entire cell membrane. Using spike-triggered averaging of movies synchronized with electrical recordings, we demonstrate deformations up to 3 nm (0.9 mrad) during the action potential in spiking HEK-293 cells, with a rise time of 4 ms. The time course of the optically recorded spikes matches the electrical waveforms. Since the shot noise limit of the camera (~2 mrad/pix) precludes detection of the action potential in a single frame, for all-optical spike detection, images are acquired at 50 kHz, and 50 frames are binned into 1 ms steps to achieve a sensitivity of 0.3 mrad in a single pixel. Using a self-reinforcing sensitivity enhancement algorithm based on iteratively expanding the region of interest for spatial averaging, individual spikes can be detected by matching the previously extracted template of the action potential with the optical recording. This allows all-optical full-field imaging of the propagating action potentials without exogeneous labels or electrodes. A promising non-invasive technique for measuring electrical activity in neurons and other cells works by observing how cells deform in response to changes in their electric potential. Existing methods for analyzing cells’ electrical activity are invasive and may affect natural cell behavior. Now, Tong Ling and co-workers from Daniel Palanker’s lab at Stanford University in the US demonstrated a method that monitors changes in cell shapes during electrical spikes – or ‘action potentials’ - using quantitative phase microscopy. Action potential occurs when the negatively-charged cell interior depolarizes due to a stimulus, triggering a rapid voltage change across the cell membrane. Ling’s team demonstrated that cells deform in response to these relatively large (0.1 V) voltage shifts, revealing exactly when action potentials occur. Their technique may facilitate non-invasive analysis of neuronal networks and other cellular activities.