Alpha-motoneuron excitability at high altitude

Summary

It has been hypothesized that chronic hypobaric hypoxia could lead to inhibition of theα-motoneuron pool, thus limiting the maximal activation of working skeletal muscles. To test this hypothesis six subjects [32 (SEM 2) years] were evaluated in resting conditions, at sea level and after acclimatization at 5,050 m. The recruitment curves of the Hofmann-reflex (H-) and the direct muscle-response (M-) of the right soleus muscle were obtained by stimulating the posterior tibeal nerve with different intensities while recording the electromyogram of the soleus muscle. From the recorded data the netα-motoneuron excitability (ratio of maximal H-reflex to M-response H_max : M_max ratio), the threshold and gain for both responses, obtained from linear regressions through the rising phase of the recruitment curves of both responses, as well as the latency times of both responses were determined. The latency times and the H_max :M_max ratio were unchanged at altitude. The thresholds of both responses and the gain of the M-response were unaltered. The gain of the H-response was significantly higher at altitude when compared to sea level. It is concluded that in the acclimatized subjects at rest the signal conduction velocity through the different parts of both pathways was unaltered and therefore nerve and muscle conduction velocity as well as synaptic and muscle end-plate transmission were unchanged, that the recruitment of the H-reflex was slightly facilitated after acclimatization to high altitude suggesting increased excitability of theα-motoneurons, through either postsynaptic facilitatory changes in the soma or a different descending drive, and that the unchanged H_max:M_max ratio indicated no change in the net excitatory and inhibitory influences on theα-motoneuron pool. The above hypothesis is thus not strengthened by the results that were, however, obtained in resting conditions.