Modulation of primary afferent discharge by dynamic and static gamma motor axons in cat muscle spindles in relation to the intrafusal fibre types activated

摘要

class="enumerated" style="list-style-type:decimal">Recordings were made from muscle spindle primary afferents from medial gastrocnemius and soleus muscles of the cat to study the modulating effects of varying γ-motor stimulation frequency at constant muscle length. Stimulus trains had a mean frequency of 50 Hz and were sinusoidally frequency modulated at 1 Hz, with an amplitude of modulation of ± 5 to ± 30 Hz.When dynamic γ-axons (γd) were selected for their pure effect on bag1 fibres, they were found to have very little modulating effect on afferent firing.Static γ-axons (γs) were tested with a random stimulus and correlation method to determine whether they acted purely on bag2 fibres, purely on chain fibres or on both together. Pure bag2γs-axons had weak modulating effects with large values of phase lag. Pure chain connections were effective in modulating with very little phase lag, but their mean gain was low. Mixed bag2 and chain axons were most effective and showed phase shifts proportional to gain.The effects of muscle length changes recorded previously from locomotor movements were also tested, with and without accompanying stimulation of mixed γs-axons with pulse trains recorded from γs-axons. This γs stimulation had a powerful effect in increasing afferent discharge during muscle shortening. The difference in afferent firing between the stimulated and non-stimulated conditions accurately predicted the profile of the γs stimulation.The results are discussed in relation to the ways in which the γ-motor system may be used in natural movements.