Gradual and reversible central vestibular reorganization in frog after selective labyrinthine nerve branch lesions |
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Authors: | Goto Fumiyuki Straka Hans Dieringer Norbert |
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Institution: | (1) Department of Physiology, Pettenkoferstrasse 12, 80336 Munich, Germany, |
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Abstract: | Postlesional reorganization of vestibular afferent and commissural inputs onto second-order vestibular neurons was studied
in the isolated brain after unilateral section of the N.VIII, of the ramus anterior (RA) of N.VIII, of the utricular (UT)
or of the anterior vertical and horizontal canal nerves in combination. RA nerve section eliminated the inputs from utricular,
anterior vertical and horizontal canal organs. In the first set of experiments we recorded field potentials on the operated
side of the vestibular nuclei 2 months after RA nerve section. These responses were evoked by electrical stimulation of the
RA nerve or of the posterior vertical canal nerve on the operated or on the intact side. The amplitudes of afferent field
potentials evoked by stimulation of the spared posterior vertical canal nerve were increased. The amplitudes of afferent field
potentials evoked by stimulation of the axotomized RA nerve remained unaltered. After N.VIII section the commissural, but
not the afferent, field potentials increased significantly on the operated side following stimulation of N.VIII on the intact
and on the operated side, respectively. After UT nerve section no change in commissural but an increase in the amplitude of
afferent field potentials from each of the three intact canal nerves was observed on the operated side. In the context of
earlier results these findings imply that second-order vestibular neurons, disfacilitated due to afferent nerve section, became
receptive to additional, excitatory synaptic inputs, preferentially from intact vestibular nerve afferent fibers. The reduced
excitation via afferent nerve inputs was thereby replaced by other afferent nerve inputs from spatially inadequate vestibular
end-organs. The synaptic terminals of inactivated afferent nerve fibers were maintained and not repressed. The process of
central reorganization after vestibular nerve lesion was activity related, the expansion of signals restricted to inputs from
intact fibers, its extent graded and its onset delayed with respect to the onset of corresponding spinal changes and to the
onset of postural recovery after the same type of nerve lesion. After the section of RA nerve or of an individual nerve branch
the labyrinthine end-organs remained intact and were not removed as after unilateral labyrinthectomy (UL). Peripheral reinnervation
of the end-organs was thus excluded after UL, but expected after one of the former types of lesion. Functional reinnervation
of the utricular macula was mirrored behaviorally by the reappearance of severe postural deficits following a second RA nerve
section. These lesion-induced postural deficits began to reappear if the repeated RA nerve section was delayed with respect
to the first by about 3 months. We therefore studied postlesional reorganization in the brainstem 3 months after the first
RA nerve section. Reinnervation of the utricular macula was accompanied by a rapid decline of the increased amplitudes of
afferent and commissural vestibular field potentials towards control values, suggesting the reversibility of the lesion-induced
central reorganization.
Electronic Publication |
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Keywords: | Vestibular compensation Synaptic plasticity Postlesional reorganization Regeneration |
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