Effect of vertical linear acceleration on H-reflex in decerebrate cat. II. Sinusoidal stimuli.

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Abstract
This experiment extends a previous investigation of the effects of vertical linear acceleration on spinal cord excitability. Seven normal and 5 bilaterally labyrinthectomized cats were subjected to controlled steady sinusoidal oscillation in the vertical direction at 0.5, 1.35 and 2.5 Hz. Spinal motoneuron pool excitability was tested during these periods of oscillation by means of the H-reflex technique. In normal cats, modulation of motoneuron pool excitability was seen at all 3 frequencies of oscillation, with maximal H-reflex amplitude occurring roughly at the time of maximum downward acceleration. The modulation was most pronounced at the highest oscillation frequency. No significant modulation was seen in 4 of the 5 bilaterally labyrinthectomized cats as a result of steady oscillation. The 5th, which had been adapting to the deficit for 26 mo., responded to the acceleration stimulus with an appropriate cyclical modulation of its H-reflex amplitude. Confirming the results obtained previously during step changes in vertical linear acceleration, these results show a significant reflex effect of the otolith organs on lumbosacral motoneuron pool excitability during sinusoidal oscillation in the vertical direction. That the otolith organs can respond to relatively high-frequency stimulation and cause rapidly changing reflex effects in the cord was confirmed. The otolith-spinal response seen may make a significant contribution to the part of locomotor activity concerned with the landing phase of stepping or running movements.