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 /spl gamma/-motoneurones were active. One (/spl gamma/sT) tensed the secondary ending by tonically activating intrafusal fibres at the same pole as the ending. A second /spl gamma/-motoneurone (/spl gamma/sC) 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 /spl gamma/sC activity. However, there was no significant peak in the coherence between group Ia or II afferents and /spl gamma/sT activity. These results suggest that coupling of group Is and II afferents was determined by the group II activity and controlled by the coupling /spl gamma/-motoneurone.