Hypoxia and monosynaptic reflexes in humans.
The recruitment curves of the monosynaptic Hoffmann (H) reflex and of the direct motor (M) excitation of alpha-motor fibers of the posterior popliteal nerve were studied in seven human subjects in normoxic and hypoxic conditions at sea level. The amplitude of the H and M responses were determined from the computerized full-wave rectified and integrated surface electromyographic (EMG) signal derived from bipolar surface electrodes placed over the soleus muscle. Hypoxic exposure [end-tidal O2 fraction (FETO2) = 0.066 +/- 0.003 and end-tidal CO2 fraction (FETCO2) = 0.0504 +/- 0.001 (SE)] did not affect the maximal M (Mmax) response but decreased significantly (7%) the maximal H (Hmax) response. The Hmax/Mmax ratio decreased from 0.60 to 0.53. Furthermore, by fitting the rising phase of the recruitment curves of the H and M responses vs. stimulus intensity with linear regressions, hypoxia was found to produce a significant decrease of similar magnitude (6%) in the threshold of both the H and M responses with no change in slope. Using a constant stimulus strength eliciting an H response of half the maximum (H50%) of the control conditions, hypoxia resulted in a 50% increase in the amplitude of the H response within 12 min. These results suggest that the effects of hypoxia on the nervous system consist of a direct depolarizing action on the peripheral alpha-fibers and 1A sensory fibers and of a central effect on supraspinal structures affecting the spinal alpha-motoneurons.
Willer
J C
JC
Laboratory of Clinical Neurophysiology, Faculté de Médecine Saint-Antoine, Paris, France.
Miserocchi
G
G
Gautier
H
H
D000328:Adult; D004558:Electric Stimulation / Q000379:methods; D004576:Electromyography; D006181:H-Reflex; D006801:Humans; D000860:Hypoxia / Q000503:physiopathology*; D008297:Male; D008875:Middle Aged; D009068:Movement; D011999:Recruitment, Neurophysiological; D012023:Reflex, Monosynaptic; D013979:Tibial Nerve / Q000502:physiology