技術論文とプレゼンテーション

Biophotons are ultraweak photons emitted by living cells as a byproduct of metabolic activity [1]. Elevated biophoton production in neurons has led to speculation that photons may play a role in intercellular communication [2-3]. For such signaling to occur, photons must be able to propagate through neural structures and reach neighboring cells. Axons have been proposed as potential optical waveguides due to their geometry and refractive index contrast with the surrounding medium [4]. To investigate the light-guiding properties of axons, green fluorescent proteins (GFP) expressed in neuronal nuclei were excited using two-photon microscopy. A tapered optical fiber probe was used to measure optical signals near axons and detect signatures consistent with evanescent coupling (Fig. 1a-b). Measurements were performed along the length of the axon and compared with surrogate waveguide experiments to identify expected coupling behavior. The emitted fluorescence was detected using a HAMAMATSU photomultiplier tube while the probe was positioned near the axon. A decrease or oscillatory variation in measured intensity as the probe is translated along the axon would be consistent with evanescent coupling and guided propagation of light. However, initial experiments showed that the measured intensity increased as the probe moved farther from the cell body (Fig. 1c), suggesting that the excited neuron was not the only optical source contributing to the detected signal. These results indicate that scattering from surrounding cells may significantly influence measurements and obscure signatures of guided propagation. To better understand the experimental observations, two-dimensional electromagnetic simulations were performed using the Electromagnetic Waves, Frequency Domain module in COMSOL Multiphysics. Fluorescent proteins within the nucleus were modeled as line-current point sources, and the neuron, axon, and tapered fiber probe were included to replicate experimental conditions. Additional neighboring cells were incorporated into the model to investigate scattering effects observed during measurements. Perfectly matched layer (PML) boundary conditions were used to absorb outgoing radiation, while a perfect magnetic conductor (PMC) boundary condition was applied on the side containing the optical source. The simulations are being used to replicate the experimental environment and evaluate how scattering from neighboring cells couples into the tapered fiber probe. By providing precise control over the optical environment, COMSOL enables investigation of conditions that are difficult to realize experimentally, e.g. scattering of a nucleus without an axon present (Fig. 2a-b). These results will help distinguish between scattered and guided light and guide future experiments aimed at demonstrating optical propagation within axons.