N. Dieringer, VESTIBULAR COMPENSATION - NEURAL PLASTICITY AND ITS RELATIONS TO FUNCTIONAL RECOVERY AFTER LABYRINTHINE LESIONS IN FROGS AND OTHER VERTEBRATES, Progress in neurobiology, 46(2-3), 1995, pp. 97
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.