The Influence of Unilateral Horizontal Semicircular Canal Plugs on the Horizontal Vestibulo-Ocular Reflex of the Rabbit

Abstract

Vestibulo-ocular reflexes provide a sensitive and convenient means of assaying the normal functions of the peripheral vestibular apparatus and for assaying damage to the central nervous system. The horizontal vestibulo-ocular reflex (HVOR) is ordinarily evoked by reciprocally modulated signals originating from the ampullae of the horizontal semicircular canals. The activity originating from each ampulla is increased by ipsilateral angular acceleration of the head and decreased by contralateral angular acceleration. This stimulus-modulated activity directly excites the ipsilateral medial and superior vestibular nuclei and indirectly inhibits contralateral medial and superior vestibular nuclei via a GABA-ergic commissural pathway (Kasahara and Uchino 1971; Precht et al. 1973) (Fig. 1). The vestibular nuclei send both excitatory and inhibitory projections to the oculomotor (III) and abducens (VI) nuclei (for review see Barmack and Hess 1980). This reciprocal organization is maintained from the peripheral vestibular apparatus to the peripheral oculomotor apparatus and, as a consequence, conjugate compensatory eye movements are evoked by head movements over a wide range of frequencies. This reciprocal organization can be drastically altered by unilateral labyrinthectomy or unilateral vestibular neurectomy. Such operations cause a nystagmus in which the slow phase is directed towards the damaged side (Dow 1938; Precht et al. 1966; McCabe et al. 1973; Schaefer and Meyer 1973; Baarsma and Collewijn 1975; Azzena et al. 1976). The spontaneous nystagmus which is caused by unilateral damage to the vestibular system can be attributed to a tonic imbalance of primary afferent activity.