Mechanisms of compensation for vestibular deficits in the frog. I. Modification of the excitatory commissural system.

In hemilabyrinthectomized frogs excitatory responses of central vestibular neurons to electrical stimulation of the remaining vestibular nerve were recorded extra- and intracellulary at different stages (0, 3, and 60 days) after the operation. The output pattern of ipsilateral vestibular neurons sending an axon across the midline via the vestibular commissure to the deafferented nucleus did not change postoperatively. The synaptic efficacy of these commissural axons ending on partially deafferented vestibular neurons on the lesioned side increased with time. This enhanced synaptic potency was associated with a shortening in time to peak and duration and an increase in amplitude of the evoked EPSPs. As a result most vestibular neurons were readily excited by single shock stimulation of the contralateral vestibular nerve, a finding which was rarely observed in control animals. These plastic changes are explained by the assumption of reactive synaptogenesis. The consequences of this modification for the readjustment of static and dynamic vestibular reflexes are discussed.