Signal Propagation in Myelin Sheath as Dielectric Waveguide in the Mid-Infrared to Terahertz Spectral Range

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. We experimentally demonstrated the high contrast of mid-infrared reflectivity/refractivity between the myelin sheath and other structures in nervous tissue [1]. It is found that, with a certain mid-infrared to terahertz spectral range, the myelin sheath possesses $\mathrm{a}\approx 2$ - 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 $(\approx 2\mu \mathrm{m})$ 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 infrared and THz energy concentration in myelin mainly depends on the myelin thickness, the difference of dielectric constant between the myelin and axon fluid, rather than the absolute values of the inner radius and inner dielectric constant of axon. 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.