Infrared light can be used to modulate the activity of neuronal cells with broad generality and without any need for exogenous materials. The action potential response has been shown to be associated with heating due to the absorption of light by water in and around the illuminated tissues, which gives rise to at least two distinct processes: namely, the temperature pulses cause depolarizing capacitive currents due to an intramembrane thermo-mechanical effect, and in addition, temperature-sensitive TRPV ion channels (and likely, voltage-gated channels) drive additional membrane depolarization. However, substantial differences between the activation threshold of primary auditory neurons (<20 mJ/cm^2) and other neuronal types (>300 mJ/cm^2) in vivo have generated some controversy in the field. A temperature-dependent Hodgkin-Huxley type model, which combines capacitive currents and the experimentally-derived characteristics of voltage-gated potassium and sodium ion channels in primary auditory neurons, was used to accurately explain the in vitro response to 1870 nm infrared illumination. TRPV channels do not make a significant contribution in this case, suggesting that the detailed mechanism of the neuronal response to infrared light is dependent on the specific cell type. Furthermore, based on this detailed understanding of the cell behaviour, it is shown that action potentials cannot be generated at safe laser power levels. This suggests that the previously reported response of the auditory system to infrared stimulation in vivo might arise from a different mechanism, and calls into question the potential usefulness of the effect for auditory prostheses.
William G. A. Brown, Karina Needham, Alexander C. Thompson, Bryony Nayagam, and Paul R. Stoddart, "The role of membrane capacitance and ion channels in the response of primary auditory neurons to infrared light (Conference Presentation)," Proc. SPIE 10482, Optogenetics and Optical Manipulation 2018, 104820Z (Presented at SPIE BiOS: January 28, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2289779.5751488534001.
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