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In Vivo Pressure-Response Recordings of Tactile Sensation in the Rat
Kristoffer B. Sugg, MD; Melanie G. Urbanchek, PhD; Elmer K. Kim, MS; Gregory J. Gerling, PhD; Paul S. Cederna, MD; Nicholas B. Langhals, PhD
University of Michigan, Ann Arbor, MI, USA
Introduction: Newer generations of neuroprostheses display improved motor function, but continue to lack meaningful, graded somatosensory feedback. Amputees must rely on visual cues to execute simple tasks vital to daily living. A potential solution is to interface force sensors affixed to prosthetic fingertips with afferent peripheral nerve fibers. The purpose of this study is to characterize the afferent response in the rat whole sural nerve to mechanical stimulation at different levels of pressure. Our hypothesis is increasing pressure increases both the amplitude and firing rate of sensory nerve action potential (SNAPs).
Methods: A depilated neurocutaneous flap containing sural nerve mechanoreceptors was elevated off the rat's dorsolateral foot (n = 4). The flap was rotated into a skin-flap chamber and secured to a silicone elastomer base. A custom mechanical indenter fitted with a 3-mm diameter probe tip and 2.5 N load cell provided ramp-and-hold stimuli for 5 seconds duration on the flap's epidermal surface at depths of 1, 2, and 3 mm. In vivo multi-unit action potentials were recorded from the whole sural nerve proximal to the flap.
Results: Increased displacement of the indenter tip resulted in increased force (pressure) detected by the load cell (Peak Force = 2.86*Displacement – 1.50; R2 = 0.9923). Accordingly, the SNAP firing rate increased with 5, 50, and 100 spikes/sec recorded at depths 1, 2, and 3 mm, respectively (Fig. 1). Peak-to-peak SNAP waveform amplitude also increased with 0.05±0.01, 0.08±0.02, and 0.11±0.03 mV at depths 1, 2, and 3 mm, respectively. The largest SNAP waveform amplitudes were demonstrated in the early-moving phase of displacement and the smallest amplitudes in the late-hold phase (Fig. 2)
 | Figure 1. Increased displacement of the indenter tip increased peak force (measured at the indenter tip) and SNAP firing rate |
 | Figure 2. Larger SNAP waveforms are elicited in the early-moving phase of displacement than in the late-hold phase. |
Conclusions: Sensory neural responses were recorded from orthotopic rat whole sural nerves in response to localized pressure on the skin. Varying the depth of skin indentation produced unique, repeatable SNAP amplitudes and firing rates. Findings were used to program a simple sensory algorithm relating pressure to firing rate.
Acknowledgements: This work was sponsored by the Defense Advanced Research Projects Agency (DARPA) MTO under the auspices of Dr. Jack Judy through the Space and Naval Warfare Systems Center, Pacific Grant/Contract No. N66001-11-C-4190, and the Plastic Surgery Foundation's Research Fellowship Grant (Kristoffer B. Sugg, MD).
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