Regenerative Peripheral Nerve Interface Signal Validation via Conscious Response to Tactile Stimuli
Daniel C. Ursu, PhD, MMSc, Theodore A. Kung, MD, Melanie G. Urbanchek, PhD, R. Brent Gillespie, PhD, Paul S. Cederna, MD, Stephen W.P. Kemp, PhD.
University of Michigan, Ann Arbor, MI, USA.
Purpose: Regenerative Peripheral Nerve Interfaces (RPNIs) are neurotized autologous free muscle grafts equipped with electrodes to record myoelectric signals for prosthetic control. Validation of RPNI signal patterning in conscious rats is critical for RPNI acceptance as a prosthesis control modality. This work quantifies RPNI activity in response to randomly timed tactile stimuli. Electromyography (EMG) signals are recorded and synchronized to Center of Pressure (COP) displacement measurements while the rat is stationary on a force plate. Balance-perturbation reflexes are initiated by monofilament stimulation. The purpose of this study is to characterize signal transduction from an RPNI during baseline and voluntary responses initiated by aperiodic tactile stimuli.
Methods: Two rats received two separate RPNI constructs each, consisting of free left EDL and TA muscle transfers to the left thigh, respectively. The EDL was re-innervated with the proximal end of a transected tibial nerve fascicle, while the TA was re-innervated with the fully transected peroneal nerve. Bipolar, epimysial, wire electrodes were positioned on each RPNI construct; electrode wires were subcutaneously tunneled under the skin of the dorsum and attached to a head-cap mounted to the rat skull using dental cement and fine bone screws. Rats were conditioned to stand stationary on a force plate, retract the hind limb in response to a monofilament stimulus, and were evaluated at 6 months post-surgery. A metallic monofilament attached to a sensing trigger was used to both provide a tactile stimulus and record time of skin contact. A 3 kHz data acquisition system was used to synchronize force plate readouts and EMG signals with the timing of the tactile stimulus. An evaluation protocol consisting of 20 stimuli was fit to a 2x2 pseudorandom Latin Squares design. The varied design parameters were hind limb being stimulated (ipsilateral or contralateral) and stimulus location (paw or upper thigh).
Results: Analysis of the balance perturbation data revealed that EMG signaling in the RPNI constructs was minimal at pre-stimulus baseline, becoming active only when COP displacement occurred in response to the monofilament tactile stimulus (Fig 1). All timing delays between RPNI firing and COP displacements were matched and correlated with stimulus location and stimulated limb. RPNI activation occurred only in response to the monofilament stimulus; furthermore, the two RPNI constructs displayed EMG activity independent of one another, with minimal signal cross-talk.
Conclusions: RPNI signal transduction showed baseline and voluntary myoelectric RPNI signal activity consistent with COP displacement measurements initiated by a tactile stimulus. These data validate that RPNI signaling is governed by volitional and reflexive processes appropriate to prosthetic control.
Fig 1: Illustration depicting the synchronized EMG signal (top-left) and force plate outputs (bottom) of a rat undergoing left paw retraction in response to a tactile stimulus.
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