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Chapter 10. Ocular Reflexes > Vestibular Compensation

10-5. Vestibular Compensation

It has long been known that a unilateral labyrinthectomy causes the development of pronounced nystagmus (Vidal et al., 1998). After the lesion, the static deficits generally disappear in a few days, whereas restoration of the dynamic, vestibular-related synergies is much slower and only partial. When the flocculus was lesioned unilaterally 40–70 days earlier, recovery from the effects of the contralateral labyrinthectomy was severely delayed. When such lesions were made after recovery for 16–60 months, they produced only a transient asymmetry of the vestibuloocular responses. These observations suggest that the flocculus is required for initiating (but not maintaining) the compensatory process following peripheral lesions of the vestibular system (cat; Courjon et al., 1982). The behavioral recovery from unilateral labyrinthectomy was accompanied by an asymmetric expression of isoforms of PKC in flocculus Purkinje cells. There was also a regionally selective increase in the number of PKC-immunoreactive Purkinje cells contralateral to the lesion (rat; Goto et al., 1997). In another study, the compensation was retarded after application of PKC inhibitors through the cerebral ventricle (rat; Balaban et al., 1999). It thus appears that conjunctive LTD, which requires PKC, has a role in vestibular compensation.

Certain changes related to vestibular compensation have been found to occur also in the brainstem. Unilateral labyrinthectomy caused an increase in the number of GABAergic neurons in vestibular nuclei (cat; Tighilet and Lacour, 2001), an increase in mRNA coding BDNF in medial vestibular nucleus (mouse; Li et al., 2001), and an increase in glycinergic quantal current amplitudes and frequency of glycinergic quantal events in medial vestibular nuclear neurons (mouse; Lim et al., 2009). A neuronal network simulation also suggested a change in commissural inhibitory connections after this lesion (Graham and Dutia, 2001). Together with cerebellar adaptation by the flocculus, considerable reorganization of the vestibular neuronal network appears to underlie vestibular compensation.


  

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