[Effect of 120 days head-down tilt (HDT) on characteristics of tendinous reflexes]

Time dynamics of variations in the spinal reflex mechanisms was evaluated during 120-d HDT (-6 degrees). Already within the first days in HDT the knee and Achilles reflexes were noted to sharply become easy to fulfil, the conclusion made from a significant drop of thresholds, an appreciable rise of amplitudes of reflex responses and an expansion of the range of stimulations. However, dynamics of these variations was diverse. The abrupt drop of reflex thresholds determined already on HDT day 2 was persistent throughout the experiment, whereas maximal rise of the tendon reflex amplitude was seen on the initial phase of HDT (days 15-30). Later on, a clear-cut downward trend of the electromyographic amplitude was observed. The experimental data suggest that hyperreflexia during HDT is one of the manifestations of partial deafferentation of the motoneuron pool due to diminution of the proprioceptive afflux. At the same time, hyperactivity of the reflex mechanisms is masked by the peripheral muscular effects of the same factor, i.e. HDT.