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Straka, Hans ORCID: 0000-0003-2874-0441; Dieringer, Norbert (1995): Spinal plasticity after hemilabyrinthectomy and its relation to postural recovery in the frog. In: Journal of Neurophysiology, Vol. 73, No. 4: pp. 1617-1631
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1. Brachial dorsal root-evoked ventral root responses were studied in the isolated brain/spinal cord preparation of frogs, One group of frogs (n = 20) had survived a hemilabyrinthectomy (HL) between 7 and 70 days. In another group of frogs (n = 30), a nerve branch to an individual labyrinthine organ was sectioned uni- or bilaterally 15 days before the recording session. In a third group of frogs (n = 5), a weight had been mounted eccentrically on the head for 15 days. A fourth group of intact frogs (n = 8) served as a control. 2. In chronic HL frogs (greater than or equal to 60 days postoperatively) the amplitudes of short- and long-latency ventral root potentials recorded on the operated side were consistently increased with respect to control values in response to all converging inputs tested. On the intact side most of these potentials were consistently increased as well, except for crossed long-latency responses after stimulation of the dorsal root on the operated side. 3. Practically identical responses were recorded in these preparations before and after the disconnection of the spinal cord from the brain stem at the level of the obex. Before this disconnection, ventral root potentials were recorded in response to electric stimulation of either one of the VIIIth nerves on the intact or on the operated side. Ventral root potentials recorded on the operated but not on the intact side were slightly increased in chronic HL frogs. 4. The time course of these changes was studied at intervals between 7 and 70 days after the lesion. The amplitudes of short-latency dorsal root-evoked ventral root potentials were increased relatively early (7-15 days) or relatively late (greater than or equal to 30 days) after HL. Ventral root potentials evoked by stimulation of either one of the N.VIII were significantly reduced in amplitude seven days after HL but normalized again or increased above control values after longer survival periods. These differences in the time courses suggest the presence of multiple, not singular mechanisms for intra-spinal changes. 5. Changes in dorsal root-evoked ventral root potentials similar to those after HL were seen 15 days after a selective unilateral section of the utricular, but not after a unilateral section of the horizontal canal or saccular nerve branch. Therefore these changes were initiated either by asymmetric utricular afferent inputs or by asymmetric proprioceptive inputs resulting from lesion-induced postural deficits. 6. These two possibilities were investigated in two different sets of experiments, In the first set, corresponding nerve branches on either side were lesioned. Two weeks after a bilateral utricular, but not after a horizontal canal nerve section, the dorsal root-evoked ventral root potentials recorded on either side of the isolated brachial spinal cord were significantly increased. 7. In the second set of experiments, frogs were equipped with a weight that was mounted eccentrically on the head. A normal head posture was actively maintained against this torque, and the forelimbs assumed a compensatory posture. In spite of the presence of asymmetric proprioceptive inputs under this condition, the dorsal root-evoked ventral root responses recorded 2 wk later were not different from those recorded in controls. 8. These results demonstrate that spinal plasticity after HL is directly related to the inactivation of utricular afferent inputs and is not a consequence of the lesion-induced postural deficits or of associated asymmetric reafferent proprioceptive inputs. 9. The early changes in propriospinal circuits after HL, as described here, parallel closely the known time course of postural recovery that proceeds exponentially in frogs and reaches similar to 50% 2-3 wk after HL. Known changes in the efficacy of inputs terminating in the vestibular nuclei on the operated side are delayed in their onset by similar to 4 wk and cannot account for the initial period of postural recovery. We therefore conclude that the normalization of postural tone after HL is an emergent property of adaptive processes that include changes in propriospinal circuits as a consequence of utricular afferent inactivation.