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

AIM: Corticospinal interaction gives support to the conceptual mechanism called communication through coherence, which is the assumption that neuronal groups communicate through coherent oscillatory activity. In this respect it is interesting how centrally evoked oscillatory phenomena are translated into EMG activity. We investigated the transfer properties of central drive transmission via the alpha-motoneuron pool to the muscle. METHODS: A model based on that of P. Matthews (J Physiol. 1996;492:597-628) was constructed (Figure 1). This model was used for the simulation of alpha-motoneuron firing patterns and the EMG signal as response to central drive input. Short-term synchrony was introduced by assuming a common input to each pair of alpha-motoneurons. The cortical input was modulated to investigate the transfer through the alpha-motoneuron pool in the frequency domain. Coherence between stochastical central input and EMG signal is also evaluated. Furthermore, the effect due to the often-used EMG rectification is investigated. RESULTS: Modulated cortical input is transferred with only a limited level of non-linearity. The alpha-motoneuron firing frequencies do play a role in the frequency distribution of the amplitude spectrum. However, no preference over proportionality in the region of the firing frequencies was found. Coherence analysis between the summed central input to the alpha-motoneuron pool and the EMG signal is large whereby the coupling strength does not depend on frequency in a range from 1 to 100 Hz. Common central input to pairs of alpha-motoneurons strongly increases the coherence levels as well as the amplitudes in the frequency spectrum. Rectification of the EMG signal introduces a clear frequency dependence. Especially the motoneuron firing frequency range is emphasized. CONCLUSION: Centrally evoked 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 the coherence and introduces a strong frequency dependency. Because of its non-linearity, we think that rectification should better be avoided.