Effects of stretch on dynamic fusimotor after-effects in cat muscle spindles

Conditioning stimulation of dynamic fusimotor axons leaves persistent after-effects which increase the responses of primary endings to test dynamic stimuli. Such after-effects are abolished by muscle stretch. Destruction of these after-effects depends on the following. (a) Amplitude of stretch: with symmetrical triangular stretches of moderate velocity, an extension of soleus by 4-5 mm totally abolishes the after-effects. Lesser stretches cause a graded reduction. (b) Velocity of relaxation: for a given amplitude of stretch there is greater destruction of after-effects when it is followed by a slow rate of relaxation than after rapid relaxation. (c) After-effects tested late in ramp stretch are more resistant to destruction by stretch than those increasing test dynamic responses early in ramp stretch. Stretch itself produces after-effects which enhance test responses to dynamic but not to static fusimotor stimulation. Interactions between conditioning dynamic stimulation and stretch suggest that both these effects occur in the same intrafusal elements, the bag fibres.