Partial restitution of lesion-induced deficits in the horizontal vestibulo-ocular reflex performance measured from the bilateral abducens motor output in frogs.

Abstract

The responses of the bilateral abducens nerves to small table velocity steps in the dark were measured in four groups of animals: One group was intact prior to recording (controls), one group was hemi-labyrinthectomized the day before the recordings (acute HL), the horizontal canal nerve was sectioned the day before the recordings (acute HCN) in another and the last group was hemi-labyrinthectomized between 60 and 90 days prior to recording (chronic HL). In controls (N = 6) the slopes of the change in discharge rate to increasingly larger velocity steps increased maximally with about 200 imp/s per 1 degree/s and decreased maximally with about -60 imp/s per 1 degree/s. This difference is explained by low resting rates and by recruitment of spontaneously inactive vestibular afferent, central vestibular and abducens neurons. Results obtained from acute HL (N = 4) and acute HCN (N = 4) animals were practically identical. In neither case was a spontaneous nystagmic activity pattern observed. Results differed from those obtained in controls due to an asymmetric reduction in responsiveness. Comparison of the slopes of the evoked increases and decreases in discharge rates of abducens nerves to increasingly larger velocity steps with those in controls show that normal abducens responses are predominantly controlled by crossed excitation and by uncrossed inhibition. Disinhibition and disfacilitation play minor roles. In chronic HL animals (N = 6) that had posturally recovered to a similar degree, responses evoked by steps towards the intact side at larger velocity steps were slightly reduced with respect to those in acute HL or HCN animals. Responses evoked by steps towards the lesioned side differed between individuals. They were either similar to those in controls (N = 1), to those in acute animals (N = 2) or lay between these two extremes (N = 3). The improvement in response to velocity steps towards the lesioned side in 4 of 6 animals is explained by an increase in activity released by disinhibition. This inhibition in turn is controlled by horizontal canal-dependent input from the intact side. Plugging of this canal abolished all direction-specific responses in this plane in the dark, suggesting that the partial restitution of function of horizontal reflex performance depends exclusively on signals derived from receptors of this canal.