Effect of vertical linear acceleration on H-reflex in decerebrate cat. I. Transient stimuli.

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Abstract
The effects of natural vertical linear accelerative stimulation of the vestibular otolith organs on lumbosacral motoneuron pool excitability were examined. Five normal and 4 bilaterally labyrinthectomized decerebrate cats were subjected to controlled step changes in acceleration of 686 cm/s2 in the downward direction. Spinal motoneuron pool excitability was measured during these maintained accelerations by H-reflex testing. The earliest response seen was a marked suppression of excitability of rapid onset, which gradually declined back to zero by about 250 m. Superimposed on this curve, and having a slightly delayed onset, was a significant period of reduced suppression in the interval from 50-100 ms. This disinhibition or facilitation was absent after labyrinthectomy and the degree of overall suppression was greatly reduced. A significant reflex effect of the otolith organs on lumbosacral motoneuron pools associated with foot antigravity muscles was demonstrated. Since the response quickly adapts to steady acceleration, the system as studied would not be expected to respond to gravity, except during a rapid tilting of the gravity vector. There may be an otolith-spinal contribution to locomotor control. The pathways involved in this otolith-spinal reflex appear to be complex, and indirect evidence suggests cerebellar participation.