Modeling DRG stimulation

Prior DRG modelling work has focused on building a model of afferent fibers to predict the probability of recruiting a distribution of fibers as a function of stimulus intensity. This model demonstrated that as stimulation amplitude increased, the number of fibers recruited increased exponentially. Furthermore, it also predicted that medium-diameter fibers may be recruited with greater probability than large-diameter fibers. While these results formed the basis for our in vivo experimentation with penetrating stimulation electrodes, the model does not account for cell bodies and assumes that the site of activation always occurs at the nodes of Ranvier in axons.

Results from our epineural stimulation experiments may contradict this assumption about the site of activation within PAs in the DRG. When the DRG is viewed in cross-section, the cell bodies of PAs are clustered around the circumference, while axons are more densely located near the center of the structure. This geometric arrangement means that epineural electrodes are much closer to PA cell bodies than to axons, and given that voltage drops exponentially with distance from an electrode and that epineural stimulation can achieve a high degree of selectivity, it is possible that the site of activation may not be always be the nodes of Ranvier. A key innovation of this proposal is the development of a computational model of the DRG that can replicate its unique geometric arrangement and may shed light on the mechanisms of action and locus of activation of epineural and penetrating stimulation.

A second key innovation of this project is the development of a platform that will allow for optimization of the design of epineural electrodes for a somatosensory neuroprosthesis. The design of an optimal electrode involves determining the ideal number, location, and size of stimulation electrode contacts, and designing stimulation parameters that maximize the dynamic range and selectivity of stimulation. Contemporary studies have used a similar model-based approach to design epineural electrodes that maximize the selectivity of stimulation of peripheral nerves for motor and somatosensory neuroprostheses. Their simulations investigated multiple electrode configurations to determine the optimal number and location of contacts for maximum selectivity. The advantage of such an approach is that it allows for rapid iteration on the design of electrodes to determine the effects of electrode size, shape, arrangement, and number on the selectivity of stimulation. While this approach has been used extensively in designing electrodes for stimulation of peripheral nerve, this study would mark the first application of model-based design for epineural electrodes at the DRG.