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Basic and Applied Aspects ofVestibular Function J.C. Hwang, N.G. Daunton and V.I. Wilson (Eds.)© Hong Kong University Press, Hong Kong, 1988 RESPONSES OF VESTIBULAR NUCLEAR NEURONS TO BIDIRECTIONAL OFF-VERTICAL AXIS ROTATIONS IN NORMAL AND HEMILABYRINTHECTOMIZED CATS Y.s. Chan, Y.M. Cheung and Ie. Hwang Department ofPhysiology, Faculty ofMedicine, University ofHong Kong, Sassoon Road, Hong Kong. Abstract The responses of tilt-sensitive vestibular nuclear units to slow constant velocity off-vertical axis rotations (0VAR) in 3600 clockwise (CW) and counterclockwise (CCW) directions werestudied in labyrinth-intactand acute unilaterally labyrinthectomized cats, with an aim to understand the contribution of otolith inputs from either side. Inintact animals, the dischargerate ofeach unitshowed aposition-dependent modulation for any unidirectional rotation. The spatial location of the CW discharge maximum differed from its CCW counterpart. The characterized directional axes (the averaged positions of CW/CCW discharge maxima) of neurons were distributed along theroll and pitch axes and the subtended angles. Units were classified into either phase lead or phase lag group based on the location of CW discharge maximum relative to the CCW counterpart along the rotatory locus. On increasing the velocity of rotation, there was a progressive shift of the discharge maximum in the direction ofrotation. At 1.75°/s, 51n7 units showed phase lead; while at 15°/s, 23{27 units showed phase lag. Increase in the amplitude of head tilt resulted in an increase in amplitude modulation. The response gains of the units however remained unaffected by both the direction and the velocity of rotation (1.75-15°/s) In acute unilateral labyrinthectomized animals, 29 of 53 units showed responses similarto those found inintactcats. Theremaining units, however, showed discharge modulation only to OVAR in either the CW or CCW direction but not to both. Some of these latter neurons became responsive to bidirectional OVAR, with comparable gains in either direction, when the head tilt exceeded 15° or the velocity of rotation was above 4.7°/s. Our results indicate that the otoliths on each side do exert influence on vestibular nuclearneurons during OVAR. Headpositional signals from otoliths on each sidecouldprovide a coordinate for motorcommands. In lesioned animals, the directional asymmetric responses of vestibular nuclear neurons may account for the motor asymmetries after acute hemilabyrinthectomy. Keywords: cat, vestibular nuclear neurons, otolith, directional sensitivity, hemilabyrinthectomy 64 Chan etal. Introduction Acute unilateral labyrinthectomycausescharacteristic posturalandoculomotorasymmetries in vertebrates (Schaeffer and Meyer, 1974). In cats, such lesion results in asymmetry in the spontaneous discharge rate and response sensitivity of bilateral vestibular nuclear neurons that were sensitive to eitherpitch tilt (Chan etal., 1983) orroll tilt (Lacouretal., 1985). Since some vestibular nuclear neurons give rise to vestibulo-ocular (Hwang and Poon, 1975) or vestibulospinal pathways (Wilson and Peterson, 1981), these asymmetric activities of vestibular nuclear neurons on the two sides of the brainstem may function to mediate an imbalance in the extraocular and spinal motoroutputs observed immediately after hemilabyrinthectomy . To understand how bilateral inputs from the otolith organs (Chan et al., 1977; Hoshino andPompeiano, 1977; Wilson etal., 1978) interactin processing spatial information derived from rotation of the head tilted at fixed angles from the horizontal plane, natural vestibular stimulations in the form of constant velocity off-vertical axis rotations (OVAR) (cf. Chan et al., 1985) or wobble rotations (cf. Schor et al., 1984) can be employed. Such modes of stimulation impose on the otolith organs a sequential change in gravitational vector during a complete rotation in the animal's horizontal plane. Various vestibular reflexes have been described resulting from these head positional changes in space: continuous nystagmus during OVAR (Young and Henn, 1975; Cohen et al., 1983) and vestibulospinal reflexes acting on the forelimb and shoulder muscles during wobble rotation (Wilson et al.. 1986). Hitherto, no attempt has been made to study the effect of acute hemilabyrinthectomy on the processing of otolith-related information in central vestibular neurons during OVAR. The . present study is directed towards evaluating the spatial and directional coding properties of static tilt-sensitive vestibular nuclear units in labyrinth-intact and hemilabyrinthectomized cats during bidirectional OVAR. Methods Experiments wereperformed on two groups ofdecerebrate cats: a labyrinth-intactgroup and a hemilabyrinthectomized group. Hemilabyrinthectomy was performed on the right side via a ventral approach. The animal was mounted on a stereotaxic frame with the head maintained in the earth's horizontal plane. The cervical vertebral column was oriented perpendicular to gravity (cf. Vidal et al., 1986) with the C2 vertebra clamped to the stereotaxic frame, while the body and the extended limbs of the animal were...

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