Can fusimotor activity potentiate the responses of muscle spindles to a tendon tap?

Experiments on the cat soleus muscle have determined the effect of selective activation of the fusimotor system on the responses of muscle receptors to a simulated tendon tap. Primary endings of spindles responded in the passive muscle with an average 4.3 impulses at a mean instantaneous rate of 502 impulses/s. Static fusimotor stimulation at 100 pulses/s increased the number of impulses during the tap to 4.8 but dropped the mean instantaneous rate to 400 impulses/s. Dynamic fusimotor stimulation increased the number of impulses to 6.3 and the instantaneous rate to 557 impulses/s. Combined stimulation of the two axons gave intermediate values. We consider these effects as rather feeble. The tendon jerk in man shows a large increase in reflex amplitude following a reinforcement manoeuvre (Jendrassik manoeuvre). Based on our animal experiments we conclude that such increases cannot be accounted for simply in terms of selective engagement of the fusimotor system.