Computational model of optogenetic stimulation in a peripheral nerve

Stimulation has been a key technique for studying underlying mechanisms of the nervous system. Electrical stimulation has been the predominant method for eliciting desired muscle responses for decades, yet methodologies remain invasive and low in selectivity of tissue stimulated. Current injection affects all local tissue types and can lead to damaging immune responses that threaten both nerves and equipment alike. Optogenetics provides a solution for such stimulation difficulties by increasing specificity and decreasing risk to tissue. Via genetic modifications, opsins (light-sensitive proteins) are added to neurons, and can be activated by light to cause neuron excitation. Through preliminary in vivo testing in transgenic mice expressing channelrhodopsin (ChR2) we validate that multiple beams of light have an additive effect and increase the response from muscles innervated by the target nerve. Measuring hindlimb flexion increases with increase in number of light sources present. To further characterize this additive effect, a Monte Carlo computer model was generated to simulate a random-walk of photons passing through nerve tissue. The model shows that light beams can aggregate within the nerve, although are limited. When using collimated light, multiple beams converging on the interior region of the nerve cannot result in a higher intensity than outermost layer of tissue nearest a single light source. This model serves as a tool to aid future animal studies by determining light emission parameters, specifically prescribing the need for optically-focused light, when attempting to selectively stimulate regions deep in the interior of a given nerve. Such capability will allow for high spatial resolution of stimulation in peripheral nerves giving finer control of excitation in downstream tissue.