Tetanus toxin reduces local and descending regulation of the H-reflex.

INTRODUCTION: Skeletal muscles that are under the influence of tetanus toxin show an exaggerated reflex response to stretch. We examined which changes in the stretch reflex may underlie the exaggerated response.

METHODS: H-reflexes were obtained from the tibialis anterior (TA) and flexor digitorum brevis (FDB) muscles in rats 7 days after intramuscular injection of tetanus toxin into the TA.

RESULTS: We found effects of the toxin on the threshold, amplitude, and duration of H-waves from the TA. The toxin inhibited rate-dependent depression in the FDB between the stimulation frequencies of 0.5–50 HZ and when a conditioning magnetic stimulus applied to the brain preceded a test electrical stimulus delivered to the plantar nerve.

CONCLUSIONS: Tetanus toxin increased the amplitude of the Hwave and reduced the normal depression of H-wave amplitude that is associated with closely timed stimuli, two phenomena that could contribute to hyperactivity of the stretch reflex.