Central and peripheral causes of hyperreflexia in humans breathing 5% trimix at 650 m

To clarify the mechanism of increased stretch reflex responsiveness in deep divers (hyperbaric hyperreflexia), comparative studies of stretch (T) and Hoffmann (H) reflexes were done on three men breathing 5% N2-0.5 bar O2-balance He at pressures up to 650 m of seawater (msw) (Atlantis IV simulated dive, F.G. Hall Laboratory, Duke Medical Center). Electromyography revealed increases at depth of up to 160% in the T reflex recruitment ratio (T reflex/Mmax) compared with surface controls. The H reflex recruitment ratio (Hmax/Mmax) did not change significantly. It is concluded that hyperbaric hyperreflexia is mainly due to increased muscle spindle sensitivity, presumably arising as a central effect on gamma-motoneuron activity. However, a purely peripheral effect of pressure on the spindle end-organ itself is not ruled out. Increases of 100-200% in muscle twitch peak force are reported and provide evidence that pressure can act directly on peripheral physiology. Postreflex clonic potentials (rebounds) during voluntary plantar flexion were significantly increased both in amplitude and number, leading to sustained clonus in one subject. In this respect, 5% N2 was less effective than 10% N2 of Atlantis III in controlling underdamping of the reflex loop. Conversely, the twitch contraction rate and clonic frequency in this study were only half as slowed compared with results from the earlier dive.