Temporal summation in human peripheral axons when stimulated transcutaneously with a 10-kHz waveform

Area of Science:

• Neuroscience
• Biomedical Engineering
• Rehabilitation Medicine

Background:

• Transcutaneous spinal cord stimulation (TSCS) uses 10-kHz waveforms for motor function rehabilitation post-spinal cord injury.
• Kilohertz-frequency waveforms may enhance subthreshold depolarization summation, but optimal burst parameters remain unstudied.

Purpose of the Study:

• To systematically investigate the optimal number of pulses within a burst for transcutaneous nerve stimulation.
• To determine the impact of varying pulse counts on nerve stimulation efficacy and charge.

Main Methods:

• Eleven adults underwent transcutaneous ulnar nerve stimulation with 10-kHz waveforms containing 1, 2, 4, 6, 8, or 10 pulses.
• Compound muscle action potentials (CMAPs) and sensory nerve action potentials (SNAPs) were recorded.
• Stimulus-response curves were analyzed for threshold current, total charge, and efficacy (CMAP amplitude/total charge).

Main Results:

• Increasing pulse count shifted stimulus-response curves leftward for current and rightward for total charge.
• Higher pulse counts decreased threshold current but increased total charge at thresholds and maximal responses.
• Stimulation efficacy decreased with increased pulse count, and onset latencies were delayed with six or more pulses.

Conclusions:

• The study provides evidence for subthreshold depolarization summation in human sensory and motor axons via TSCS.
• Optimal pulse count for summation is unclear due to trade-offs between current and charge.
• Waveforms with more than six pulses are suboptimal, showing no further current reduction but increased total charge.