Enhanced peripheral motor responses mediated by spatiotemporal patterning of optogenetic nerve stimulation

Optogenetic stimulation of peripheral motor nerves offers several attractive properties for stimulation of muscle activity. One drawback of this approach is that the relatively slow channel kinetics of standard opsins such as channel-rhodopsin2 (ChR2) limit the frequency of stimulation trains that may be used compared to traditional electrical stimulation approaches. However, propagation of light-initiated action potentials along nerve axons relies on voltage-gated ion channels rather than light-sensitive opsins. In this study, we sought to leverage this property in order to increase the effective frequencies of optical pulse trains that may be used to stimulate opsin-labeled motor nerves in a transgenic Thy1-ChR2 mouse. Using a digital mirror device (DMD) connected to a 463 nm laser, we projected spatiotemporal optical stimulation patterns through a surgical macroscope onto an exposed nerve while recording corresponding muscle activity. By alternating illuminated nerve segments, we were able to increase the effective frequency of stimulation pulses that elicited downstream muscle contractions. While a single large area of illumination elicited similar or larger responses than alternating smaller segments of equal combined area, alternating illuminated segments demonstrated a slower decay in response amplitude and were more efficient in terms of energy usage. These results represent a first in vivo demonstration of this approach to overcoming limitations of typical channel kinetics but may still be used to complement development of opsins with faster kinetics.