Estimation of coupling frequency between afferents of primary and secondary endings of single muscle spindles during natural fusimotor activity

Fusimotor-generated coupling between group Ia and II afferents from single muscle spindles of tenuissimus muscles of cats was assessed by estimating the coherence with a fast Fourier transform (FFT) after applying a cosine waveform. The cosine waveform was formed from each interspike interval in the spike train. The coherence between afferents was investigated when two static γ-motoneurones were active. One (γsT) tensed the secondary ending by tonically activating intrafusal fibres at the same pole as the ending. A second γ-motoneurone (γsC) caused contractions at the opposite pole to the secondary ending and coupled the sensory output. Estimated coherence between group Ia and II afferents showed a peak at 9.2 Hz. The peak corresponded with a peak of the coherence between group II afferent and γsC activity. However, there was no significant peak in the coherence between group Ia or II afferents and γsT activity. These results suggest that coupling of group Is and II afferents was determined by the group II activity and controlled by the coupling γ-motoneurone.