Myelin Sheath as a Dielectric Waveguide for Signal Propagation in the Mid‐Infrared to Terahertz Spectral Range

TLDR

The myelin sheath dramatically speeds nerve signal propagation. The authors experimentally and theoretically examined myelinated nerves with synchrotron‑based FTIR microspectroscopy. The study shows that myelin sheath, with a ~2‑fold higher refractive index and ~2 µm thickness, acts as a dielectric waveguide confining infrared energy and enabling millimeter‑scale propagation with minimal loss, and that nodes of Ranvier amplify the signal, providing a model for infrared/terahertz neurotransmission and applications in label‑free detection and brain‑machine interfaces.

Abstract

Abstract The myelin sheath enables dramatic speed enhancement for signal propagation in nerves. In this work, myelinated nerve structure is experimentally and theoretically studied using synchrotron‐radiation‐based Fourier‐transform infrared microspectroscopy. It is found that, with a certain mid‐infrared to terahertz spectral range, the myelin sheath possesses a ≈2‐fold higher refraction index compared to the outer medium or the inner axon, suggesting that myelin can serve as an infrared dielectric waveguide. By calculating the correlation between the material characteristics of myelin and the radical energy distribution in myelinated nerves, it is demonstrated that the sheath, with a normal thickness (≈2 µm) and dielectric constant in nature, can confine the infrared field energy within the sheath and enable the propagation of an infrared signal at the millimeter scale without dramatic energy loss. The energy of signal propagation is supplied and amplified when crossing the nodes of Ranvier via periodic relay. These findings provide the first model for explaining the mechanism of infrared and terahertz neurotransmission through myelinated nerves, which may promote the development of biological‐tissue label‐free detection, biomaterial‐based sensors, neural information, and noninvasive brain–machine interfaces.