High-speed interferometric imaging reveals dynamics of neuronal deformation during the action potential

Abstract

Neurons undergo nanometer-scale deformations during action potentials, and the underlying mechanism has been actively debated for decades. Previous observations were limited to a single spot or the cell boundary, while movement across the entire neuron during the action potential remained unclear. Here we report full-field imaging of cellular deformations accompanying the action potential in mammalian neuron somas (−1.8 to 1.4 nm) and neurites (−0.7 to 0.9 nm), using high-speed quantitative phase imaging with a temporal resolution of 0.1 ms and an optical path length sensitivity of <4 pm per pixel. The spike-triggered average, synchronized to electrical recording, demonstrates that the time course of the optical phase changes closely matches the dynamics of the electrical signal. Utilizing the spatial and temporal correlations of the phase signals across the cell, we enhance the detection and segmentation of spiking cells compared to the shot-noise–limited performance of single pixels. Using three-dimensional (3D) cellular morphology extracted via confocal microscopy, we demonstrate that the voltage-dependent changes in the membrane tension induced by ionic repulsion can explain the magnitude, time course, and spatial features of the phase imaging. Our full-field observations of the spike-induced deformations shed light upon the electromechanical coupling mechanism in electrogenic cells and open the door to noninvasive label-free imaging of neural signaling.

On the Coupling between Mechanical Properties and Electrostatics in Biological Membranes

“…Full-field imaging of primary neuronal cultures isolated from rat cortical tissues during an action potential was performed using high-speed quantitative phase imaging by Ling. et al [ 158 ]. This technique enables 0.1 ms temporal resolution and an axial path length sensitivity of 4 pm per pixel.…”

The optoretinogram reveals the primary steps of phototransduction in the living human eye

“…In general, the physiological processes associated with neural activity affect the cell's refractive index (9) and shape (10,11), which together alter light propagation, including changes in light scatter ing (12,13), polarization (14), and optical path length (OPL) (5). Similarly, cellular deformations accompanying the transmembrane voltage change during action potentials have been documented in crustacean nerves (15), squid giant axons (16), and mammalian neurons (11,17). Interferometric imaging of these changes, in general, and cellular deformations associated with variations in the cell potential, in particular, offer a noninvasive labelfree optical approach to monitor the electrical activity in neurons (10).…”

“…Using atomic force microscopy, Zhang et al (21) demonstrated that human embryonic kidney (HEK) 293 cells deformed proportionally to the membrane potential change (1 nm per 100 mV). Using quan titative phase microscopy, Ling et al (10) demonstrated cellular deformations of up to 3 nm during the action potential in spiking HEK293 cells and about 1 nm in mammalian neurons (11). Detect ing the millisecondfast and nanometerscale cellular deformations in transmission is very challenging because of the OPL difference between the cells and the surrounding medium scales with the refractive index difference, which is only ≈0.02.…”

Nanosecond-Resolution Photothermal Dynamic Imaging via MHz Digitization and Match Filtering

“…Beyond the mid-IR photothermal process, PDI can fertilize research related to imaging of short lifetime events that includes but not limits to visible, near-infrared, and other transient physical perturbations 46,47 . Lock-in free PDI offers nanoseconds temporal resolution that is ultimately limited by the photon detector response.…”