Soleus H reflex studies in humans at 40-70 bar in helium-oxygen-nitrogen (trimix)

Excitability of the Hoffmann (H) reflex was studied in 6 subjects at pressures of 40 to 70 bar during two trimix exposures (10% N2, 0.5 bar O2, the remainder He; Atlantis II and III simulated dives, Duke University Medical Center). In Atlantis II (12 h to 47 bar, 1 + 3 days to 66 bar), the H reflex recruitment ratio (Hmax/Mmax) was increased in 2 out of 3 subjects and reduced in the 3rd, while the usual facilitatory effect on the H reflex of a hand-grip reinforcement maneuver was reduced in all subjects. These results are similar to those previously seen in He-O2 dives and suggest a slight hyperreflexia mediated by an increase in alpha motoneuron excitability. In Atlantis III (1.5 days to 47 bar, 7 days to 66 bar) H recruitment ratios were normal in 2 subjects, and reduced in 1 subject (P less than 0.001). Early (III) and late (V) inhibitory phases of the H reflex recovery cycle were not significantly altered, while a 50-ms delay in the recovery cycle rebound (phase IV) in 5 out of 6 subjects at 66 bar (P less than 0.01) is attributed to the slowed muscle contraction. However, the increased amplitude of the rebound and the presence of clonic electromyographic potentials in Atlantis III are evidence of stretch receptor hyperexcitability that apparently persisted when H reflex excitability was normal. It is concluded that increased reflex responsiveness at depth is not promoted exclusively by either enhanced alpha motoneuron excitability or stretch receptor sensitivity but is mediated via both systems. Slower compression and the presence of N2 appear to preserve normal spinal inhibitory feedback and alpha motoneuron excitability but do not prevent increases in stretch receptor sensitivity.