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The influence of bag₂ and chain intrafusal muscle fibres on secondary spindle afferents in the cat

R. Durbaba, A. Taylor, P. H. Ellaway and S. Rawlinson

Static -motor activity is strongly modulated by a particular phase relationship to the cyclic movements of locomotion, and this has a profound effect on the firing patterns of muscle spindle afferents. Whilst primary afferents are affected by both static and dynamic -motor output, secondary afferents are affected significantly only by the static system acting via the intrafusal bag₂ and chain fibres. It is therefore important to know how fluctuating patterns of static -motor activity affect secondary afferents and to relate this to the actions of bag₂ and chain fibres. We have studied the action of single static axons on secondary afferents in cat hindlimb muscles. Various physiological methods were explored to identify which of the intrafusal muscle fibres were being activated in each case, including the use of random stimulation and ramp frequency stimulation. The effects were also recorded of 1 Hz sinusoidally frequency-modulated -axon stimuli and the amplitude and phase of the resulting afferent modulation related to the involvement of the bag₂ and chain fibres. It was found that bag₂ fibres are effective in biasing the secondary discharge, but their modulating action is relatively weak and involves a marked phase lag. Chain fibres acting alone cause strong modulation with very little phase lag. Mixed bag₂ and chain-fibre action is most effective in modulating afferent discharge and causes intermediate values of phase lag. The results are discussed in relation to the control of natural movements and it is concluded that an important function of the static motor system is to provide a signal to sum algebraically with the length-related signal. The results do not suggest that it could also usefully control stretch sensitivity.

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