Only the Fastest Corticospinal Fibers Contribute to β Corticomuscular Coherence

Area of Science:

• Neuroscience
• Motor Control
• Human Physiology

Background:

• Corticospinal transmission studies often use brain stimulation, which favors fast-conducting axons.
• This bias raises questions about the representativeness of findings for volitional activity in mild contractions.
• Investigating endogenously generated brain activity offers an alternative to study corticospinal transmission.

Purpose of the Study:

• To investigate the transmission speeds of cortical beta rhythms (∼20 Hz) to arm and leg muscles in humans during tonic mild contractions.
• To introduce and validate methodological improvements for estimating corticomuscular beta transmission delays.

Main Methods:

• Proposed using cumulant density (cross-covariance) over directed coherence for more accurate delay estimation in bidirectional systems.
• Utilized spiking motor unit activity instead of interference electromyography to eliminate motor unit action potential shape influence.
• Applied these improved methods to estimate corticomuscular transmission delays in 19 human participants.

Main Results:

• Descending corticomuscular beta transmission delays were found to be only 1-2 ms slower than expected from the fastest corticospinal pathways.
• Simulations using macaque histologic data on axon conduction velocities suggested two scenarios for fast transmission.
• These scenarios involve either selective transmission by the fastest corticospinal axons or transmission by the entire pool with natural cancellation.

Conclusions:

• The improved methodology accurately measures delays in cortical beta activity transmission to muscles.
• Observed transmission speeds are remarkably similar to those of the fastest corticospinal axons.
• Findings suggest efficient neural mechanisms underlying fast corticomuscular transmission, with implications for understanding neural interactions.