The three frequencies: the alpha-motoneuron pool as transmitter of rhythmicities in cortico-spinal motor drive

In the study of corticospinal communication, the conceptual mechanism called communication through coherence (CTC, Fries, 2005) gets increasing support by experimental evidence. The concept comprises the assumption that neuronal groups communicate through coherent oscillatory activity in specific frequency bands roughly in the range between 10 - 80 Hz, divided in alpha-, the beta- and the gamma-band. Especially when corticospinal interaction gives support to the CTC principle, this is regarded as strong evidence (Van Elswijk, 2007). An interesting question for the corticospinal tract is how centrally evoked oscillatory phenomena are translated by the alpha-motoneuron pool into motoneuron activity. Thereby, the properties of the alpha-motoneuron, especially its long afterhyperpolarized (AHP) phase, are utmost relevant in signal transmission. A number of authors have already looked into such spinal transmission and motor neuron firing patterns (e.g. Farina et al., 2004; Meyers et al., 2007; Williams and Baker, 2009). Three different types frequencies or better frequency bands play a role in the discussion. First, there is a specific alpha-, beta-, or gamma-frequency band in which the corticospinal interaction apparently occurs (the 1st frequency). Since frequencies in these bands are in the order of the firing frequencies of motor neurons (the 2nd frequency), attempts are made to interpret the couplings in the 1st frequency directly from this 2nd one. We will elucidate that for a proper understanding these have to be separated conceptually. And then the frequency content of the involved electrophysiological signals (EEG, MEG, EMG; the 3rd frequency) plays a role. They lead for instance to a regular question whether and why the EMG signal should be rectified before a coherence analysis is done, a question hardly ever posed for the EEG signal. By using a model for corticospinal transmission (Kleine et al., 2001, Matthews. 1996), we investigated the effectiveness and the properties of oscillatory central drive transmission to the muscle, its relation with the firing patterns of the alpha-motoneurons, the mutual interaction of those firing patterns, and the consequences for especially the surface EMG signal. In this attempt, we clearly distinguish the three frequency bands involved from the beginning to avoid confusion.