The alpha-motoneuron pool as transmitter of rhythmicities in cortical motor drive.

Abstract

OBJECTIVE: Investigate the effectiveness and frequency dependence of central drive transmission via the alpha-motoneuron pool to the muscle. METHODS: We describe a model for the simulation of alpha-motoneuron firing and the EMG signal as response to central drive input. The transfer in the frequency domain is investigated. Coherence between stochastical central input and EMG is also evaluated. RESULTS: The transmission of central rhythmicities to the EMG signal relates to the spectral content of the latter. Coherence between central input to the alpha-motoneuron pool and the EMG signal is significant whereby the coupling strength hardly depends on the frequency in a range from 1 to 100 Hz. Common central input to pairs of alpha-motoneurons strongly increases the coherence levels. The often-used rectification of the EMG signal introduces a clear frequency dependence. CONCLUSIONS: Oscillatory phenomena are strongly transmitted via the alpha-motoneuron pool. The motoneuron firing frequencies do play a role in the transmission gain, but do not influence the coherence levels. Rectification of the EMG signal enhances the transmission gain, but lowers coherence and introduces a strong frequency dependency. We think that it should be avoided. SIGNIFICANCE: Our findings show that rhythmicities are translated into alpha-motoneuron activity without strong non-linearities.