Neuroscience Letters, 50 (1984) 209-215 209 Elsevier Scientific Publishers Ireland Ltd. NSL 02927 CAN FUSIMOTOR ACTIVITY POTENTIATE THE RESPONSES OF MUSCLE SPINDLES TO A TENDON TAP? Di. MORGAN, A. PROCHAZKA* and U. PROSKE** Departments of Electrical Engineering and Physiology, Monash University, Clayton, Victoria 3168 (A ustralia) (Received April 16th, 1984; Revised version received June 20th, 1984; Accepted July 3rd, 1984) Key words: muscle spindle - tendon jerk - fusimotor reflex - afferents - motoneuron 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 fusirnotor stimulation increased the number of impulses to 6.3 and the instantaneous rate to 557 impulses/s. Combined stimula- tion 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. When a muscle with central nervous connections intact is rapidly stretched it responds with a brief, synchronous reflex contraction, the tendon jerk. Because of the importance of the tendon jerk as a diagnostic tool in clinical medicine, interest has continued to focus on this reflex and on the underlying neural mechanisms. Un- til recently it was widely believed that the fusimotor system was able to influence the size of the afferent volley in response to a tendon tap and could therefore deter- mine the size of the reflex contraction (for review see ref. 11). In man the evidence came from comparison of the tendon jerk with the reflex evoked by electrical stimulation of afferent fibres (H-reflex), before and after reinforcement (Jendrassik manoeuvre). Reinforcement was seen to potentiate the tendon jerk but not the H- reflex and this difference was attributed to the action of the fusimotor system [1,15,17]. However, in recent years evidence has accumulated which suggests that * On leave from Sherrington School of Physiology, St. Thomas's Hospital Medical School, London, SE1 7EH, U.K. ** Author for correspondence. 0304-3940/84/$ 03.00 © 1984 Elsevier Scientific Publishers Ireland Ltd.