The influence of the gamma system on cross-correlated activity of Ia muscle spindles and its relation to information transmission.

In the decerebrate cat during static or slowly varying (less than 0.3 Hz) muscle stretches, the activity of muscle spindle (MS) pairs was poorly correlated and such correlation was not changed after ventral root sectioning. With slightly faster sinusoidal stretches (0.3-20 Hz) activity in pairs of MS was also poorly correlated. However, sectioning of the ventral roots produced preferential firing frequencies in the same muscle spindle pairs and increased their degree of correlation. The increase in correlation between MS activity detected after suppressing the gamma bias appeared to arise from extrinsic 60 Hz power line vibration in the range of micrometers. Nevertheless, activation of the gamma system could suppress such phase locking. When frequencies above 20 Hz were used, the gamma system could not decorrelate the MS pair activity since the two units became locked to their common input signal. It is suggested that decorrelation of MS activity by gamma influence may improve the fidelity of the information transmitted by the Ia MS ensemble by filtering distortion harmonics, as well as damping tremor oscillations in the stretch reflex loop.