Physiological activity within peripheral nerves influences neural output in response to electrical stimulation: an <i>in vivo</i> study

<i>Objective.</i>Neuromodulation therapies are often applied to peripheral nerves. These nerves can have physiological activity that interacts with the activity evoked by electrical stimulation, potentially influencing targeted neural output and clinical outcomes. Our goal was to quantify changes in sensory neural unit activity in response to variations in electrical stimulation frequency and amplitude.<i>Approach.</i>In a feline model, we applied cutaneous brushing to evoke pudendal nerve afferent activity with and without electrical stimulation via a pudendal nerve cuff electrode. We recorded neural output with microelectrode arrays implanted in ipsilateral sacral dorsal root ganglia (DRG).<i>Main results.</i>Combined inter-spike interval distributions for all DRG units showed ranges of flattening, increases, and shifts in response to electrical stimulation. These distributions and changes within them due to electrical stimulation were largely driven by a select few units. Mixed-effects models revealed that quicker firing units generally decreased in firing rate in response to electrical stimulation and, conversely, slower firing units increased in firing rate. A unit's underlying firing rate also drove the magnitude of change in mean output firing rate in response to stimulation. Further, the models reported a small, negative correlation between the output mean unit firing rate and the applied electrical stimulation frequency.<i>Significance.</i>These results demonstrate the potential impact of electrical stimulation on underlying neural firing activity and output. Peripheral neuromodulation may normalize abnormal firing patterns in nerves contributing to pathological disorders or alter unrelated physiological activity in off-target neurons. These factors should be considered when selecting neuromodulation settings in animal subjects and human patients.