Journal of Applied Physiology, Vol 38, Issue 1 86-90, Copyright © 1975 by American Physiological Society
Effects of increased ambient pressure and nitrogen on man's monosynaptic reflexes
B. Bishop, H. Hoffman, I. Wallis and D. Shindell
Neurological signs during dives may result from altered excitability of
central neurons. The present study assesses the effect of an increase in
pressure from 1 to 3 ATA on the excitability of muscle spindles and alpha
motoneurons by comparing the EMG amplitudes of the mechanically and
electrically elicited monosynaptic reflexes of the gastrocnemius-soleus
muscle in 10 normal adults breathing a normoxic oxygen-nitrogen gas
mixture. At the surface the amplitude of the electrically elicited H
response was matched to that of the mechanically elicited Achilles tendon
reflex (ATR), but at depth these amplitudes became significantly different.
In every subject the amplitude of the ATR, which depends upon the
excitability of both muscle spindles and the alpha motoneurons, was reduced
on an average of 38% (with a range of 12-75%). The H response bypasses the
muscle spindles and hence, depends primarily upon alpha motoneuron
excitability. Its amplitude was unaltered in four, reduced in three, and
increased in three subjects. Since the ATR was always depressed despite the
direction of change in the H response, we have concluded that an increase
in ambient pressure (i.e., pressure per se, or nitrogen, or both) must have
decreased the responsiveness of muscle spindles to the tendon tap via a
reduction in fusimotor activity.
