Observations on the knee-jerk reflex in oxygen-helium at 31 and 43 bars.

Sensitivity of the knee-jerk reflex was measured in three men during two experimental saturation dives, a fast compression dive to 300 msw and a slow compression dive to 420 msw. In all cases, the force produced at the ankle by the reflex (isometric) contraction was augmented after compression. The most marked increases were observed during the shallower but faster compression dive. During decompression, reflex sensitivity remained enhanced during the 300-m dive but returned to control levels during the 420-m dive. Furthermore, the usual facilitatory effects of Jendrassik's maneuver were abolished during the 300-m dive. Peripheral nerve conduction velocity and synaptic delay apparently remained unaffected. The conclusion that the observed hyper-reflexivity results from supra-spinal influence on alpha-motor-neuron excitability and possibly on gamma-efferent activity is discussed in relation to other reflex studies in hyperbaric O2-He and O2-N2 environments.