Expansion of Afferent Vestibular Signals After the Section of One of the Vestibular Nerve Branches

The anterior branch of N. VIII was sectioned in adult frogs. Two months later the brain was isolated to record in vitro responses in the vestibular nuclei and from the abducens nerves following electric stimulation of the anterior branch of N. VIII or of the posterior canal nerve. Extra- and intracellularly recorded responses from the intact and operated side were compared with responses from controls. Major changes were detected on the operated side: the amplitudes of posterior canal nerve evoked field potentials were enlarged, the number of vestibular neurons with a monosynaptic input from the posterior canal nerve had increased, and posterior canal nerve stimulation recruited stronger abducens nerve responses on the intact side than vice versa. Changes in the convergence pattern of vestibular nerve afferent inputs on the operated side strongly suggest the expansion of posterior canal-related afferent inputs onto part of those vestibular neurons that were deprived of their afferent vestibular input. As a mechanism we suggest reactive synaptogenesis between intact posterior canal afferent fibers and vestibularly deprived second-order vestibular neurons.