Propagation Characteristics of Myelinated and Un-myelinated Nerve Fibres in guiding 200 nm to 2000 nm EM Wave

Introduction: Infrared Nerve Stimulation (INS) is becoming popular because of its potential to provide targeted stimulation [1]. Recently it was claimed that myelin sheath can guide light (200nm – 1300nm) [2], however... [ view full abstract ]

Introduction: Infrared Nerve Stimulation (INS) is becoming popular because of its potential to provide targeted stimulation [1]. Recently it was claimed that myelin sheath can guide light (200nm – 1300nm) [2], however propagation characteristics were not reported for wavelengths l > 1500nm, common in INS [1]. We present them here for l up to 2000nm for both myelinated and unmyelinated nerve fibres (MNF and UNF).

Methods: Maxwell’s equations were solved by FDFD (Matlab) on the cross-section of the nerve fibre to obtain its modal characteristics. The effective index (n_eff) of the first three modes was evaluated and the single mode operating wavelength range was determined for both MNF and UNF, using a 4 um diameter axon. The MNF overall diameter was 6.66 um (StructureNerve). The refractive indices of the fibre cytoplasm, the myelin sheath, and the outside medium were set as 1.34, 1.44 and 1.38 respectively [3].

Results and Discussion: The optical power propagating through UNF is confined by the index of the fibre’s cytoplasm (1.38) being higher than its surrounding (1.34). The effective indices of the first three propagating modes were determined and plotted in (PropCharcUnmyelinated) for 200nm ≤ l ≤ 2000nm. The UNF is single-moded for l > 1700 nm. In the MNF, optical power is confined within the myelin sheath (1.44). The effective indices of the MNF as shown in (PropCharcMyelinated) indicate that it supports more modes than the UNF and the myelin sheath operates in a single-moded condition for wavelengths longer than 1980 nm. This article determines light propagation characteristics of nerve fibres for a range of wavelengths, making it very useful for future INS designs.   

[1] Mou et al. “A Simulation Study of the Combined Thermoelectric Extracellular Stimulation of the Sciatic Nerve of the Xenopus Laevis: the Localized Transient Heat Block”, IEEE TBME, 59.6 (2012).

[2] Kumar et al. "Possible existence of optical communication channels in the brain" Scientific reports 6 (2016). [3] Antonov et al. "Measurement of the radial distribution of the refractive index of the Schwann's sheath and the axon of a myelinated nerve fiber in vivo" J. Applied Spectroscopy 39.1 (1983).