VESTIBULAR COMPENSATION - NEURAL PLASTICITY AND ITS RELATIONS TO FUNCTIONAL RECOVERY AFTER LABYRINTHINE LESIONS IN FROGS AND OTHER VERTEBRATES

Removal of the labyrinthine organs on one side is followed by a number of severe postural and dynamic reflex deficits. Some of these deficit s, in particular the posture of head and body, are normalized again ov er a period that varies strongly between species. Other, more persiste nt motor deficits are substituted, e.g. by the saccadic system. This p artial normalization of the function is accompanied by changes in resp onse properties of the central vestibular neurons on the operated side . Available evidence suggests the occurrence of reactive synaptogenesi s in cat and frog. In the latter species the synaptic efficacy of comm issural vestibular connections increases and the metabolic activity of central vestibular neurons on the operated side recovers post-operati vely. The onset of both changes, however, is delayed by about 30 days, which is too late to be causally related with the initial, rapid peri od of postural recovery in frog and cat. In frogs additional, early (7 -15 days p.o.) and late (45-60 p.o.) synaptic changes were detected in the brachial spinal cord. These multiple changes survive the isolatio n of the spinal cord and must be propriospinal in origin. Selective le sions of individual vestibular nerve branches indicate that inactivati on of utricular inputs is a sufficient and necessary condition to prov oke postural deficits and early spinal changes similar to those after hemilabyrinthectomy. Therefore, a close correlation between spinal pla sticity and postural recovery is indicated. In essence, the eliminatio n of vestibular afferent inputs results in a series of behavioral dist ortions that are partially normalized by a multitude of synaptic mecha nisms at distributed anatomical sites over different periods of time.