Projection of Cortical Beta Band Oscillations to a Motor Neuron Pool across the Full Range of Recruitment.

Cortical beta band oscillations (13-30 Hz) are associated with sensorimotor control, but their precise role remains unclear. Evidence suggests that for low-threshold motor neurons (MNs), these oscillations are conveyed to muscles via the fastest corticospinal fibers. However, their transmission to MNs of different sizes may vary due to differences in the relative strength of corticospinal and reticulospinal projections across the MN pool. Consequently, it remains uncertain whether corticospinal beta transmission follows similar pathways and maintains consistent strength across the entire MN pool. To investigate this, we examined beta activity in MNs innervating the tibialis anterior muscle across the full range of recruitment thresholds in a study involving 12 participants of both sexes. We characterized beta activity at both the cortical and motor unit (MU) levels, while participants performed contractions from mild to submaximal levels. Corticomuscular coherence remained unchanged across contraction forces after normalizing for the net MU spike rate, suggesting that beta oscillations are transmitted with similar strength to MNs, regardless of size. To further explore beta transmission, we estimated corticospinal delays using the cumulant density function, identifying peak correlations between cortical and muscular activity. Once compensated for variable peripheral axonal propagation delay across MNs, the corticospinal delay remained stable, and its value (∼14 ms) indicated projections through the fastest corticospinal fibers for all MNs. These findings demonstrate that corticospinal beta band transmission is determined by the fastest pathway connecting in the corticospinal tract, projecting similarly across the entire MN pool.