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J Neurophysiol (April 12, 2006). doi:10.1152/jn.00765.2005
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Submitted on July 19, 2005
Accepted on April 4, 2006

Contribution of M-waves and H-reflexes to contractions evoked by tetanic nerve stimulation in humans

Piotr M Klakowicz1, Evan RL Baldwin1, and David F Collins1*

1 Centre for Neuroscience, University of Alberta, Edmonton, Canada

* To whom correspondence should be addressed. E-mail: dave.collins{at}ualberta.ca.

Tetanic neuromuscular stimulation evokes contractions by depolarizing motor axons beneath the stimulating electrodes. However, we have shown that extra torque can develop due to the discharge of spinal neurons recruited by the evoked sensory volley. The present experiments investigated whether extra torque in the ankle plantar and dorsiflexors was associated with enhanced H- reflexes. The tibial and common peroneal nerves were stimulated using 7s trains (20Hz for 2s, 100Hz for 2s, 20Hz for 3s). Extra torque was defined as significantly more torque during 20Hz stimulation after the 100Hz burst (time2) than before it (time1). In 9 of 11 subjects, extra plantarflexion torque developed during stimulation just above motor threshold. In these 9 subjects, torque increased from 8-13{per thousand} MVC (time1 to time2, the soleus H-reflex increased from 1319{per thousand}Mmax and the M-wave of {bsim}2{per thousand}Mmax did not change significantly. To evoke extra dorsiflexion torque, greater stimulation intensities were required. In 6 of 13 subjects, extra torque developed at intensities that evoked an M-wave of 5-20{per thousand}Mmax at time1. In these 6 subjects, torque doubled from 2-4{per thousand} MVC (time1 to time2) while tibialis anterior (TA) H-reflexes and M-waves did not change significantly (H-reflex from 0.8-2{per thousand} Mmax; M-wave from 12-14{per thousand} Mmax). In 7 of 13 subjects, extra torque developed at higher stimulation intensities (35-65{per thousand} Mmax). In these 7 subjects, torque increased from 13-20{per thousand} MVC, while TA H-reflexes and M-waves were not significantly different (H-reflex from 0.7-1{per thousand} Mmax; M-wave from 49-54{per thousand} Mmax). Thus, enhanced H-reflexes contributed to extra plantarflexion, however, other factors generated extra dorsiflexion.




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