Many techniques may modulate peripheral nerve activity. Infrared light (IR) can excite or inhibit nerves. Compound action potentials (CAPs) are often measured as an endpoint, focusing on complete block, or overall amplitude reduction. To our knowledge, no standard techniques determine whether CAP sub-components have been modulated. Treatments may alter timing of CAP components as well as blocking them. How can these be distinguished?
We developed a numerical simulation in which extracellularly recorded action potentials were summed, assuming a Gaussian distribution for their onset time. Onset time for sub-populations was delayed (shifting), or amplitudes were reduced to zero (blocking). We demonstrated that area under the rectified curve, divided by the entire duration of the CAP, provided a more stable measure of change than other options (e.g., power). Regions must be selected such that the CAP’s individual components do not shift out of the analysis window. The largest reductions in area under the curve due to shifts were ~55% due to destructive interference, which is likely to be much larger than typically observed experimentally. In contrast, blocking components could reduce the area under the curve to zero.
The analysis was applied to sequential nerve stimulations. At every point, variance of the normalized area was computed. Choosing regions of lowest variance across stimulations defined an objective criterion for boundaries between CAP subcomponents. Analysis was applied to IR effects on CAPs recorded in the pleural-abdominal connective of Aplysia californica and musk shrew vagus. Slower conducting CAP subcomponents were selectively blocked before faster subcomponents.
Junqi Zhuo, Emilie H. Lothet, E. Duco Jansen, Charles C. Horn, Hillel J. Chiel, and Michael W. Jenkins, "Analysis of components of compound action potentials in response to infrared laser light (Conference Presentation)," Proc. SPIE 10052, Optogenetics and Optical Manipulation, 100520N (Presented at SPIE BiOS: January 29, 2017; Published: 19 April 2017); https://doi.org/10.1117/12.2253275.5371339437001.
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