Crossed effects on central vestibular neurons in the horizontal canal system of the frog
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Neuronal discharges were recorded extracellularly in curarized frogs (Rana esculenta) with glassmicropipettes. Vestibular neurons, which were activated by ipsilateral horizontal angular acceleration and suppressed during deceleration (type 1) were found mainly in the medial part of the ventral vestibular nucleus.
In contrast to type 1 vestibular neuron of the cat, this type of neuron in the frog was not suppressed, but facilitated by electrical stimulation of the contralateral vestibular nerve, irrespective of stimulus intensity. In most cases the facilitation was subthreshold for producing full action potentials when single stimulation was applied.
Single shocks applied to the contralateral vestibular nerve evoked small negative field potentials (maximum peak amplitude, 300 μV) in the vestibular nucleus. The threshold for the potential was 2.5–3.0 times threshold for the field potential recorded in the vestibular nucleus on the stimulated side. The latency ranged from 5.0 to 8.0 msec.
Intracellular recordings were obtained from the neurons in the medial part of the ventral vestibular nucleus. These neurons exhibited monosynaptic EPSP in response to ipsilateral vestibular nerve stimulation. Following electrical stimulation of the contralateral vestibular nerve polysynaptic EPSPs were found with latencies between 5.0 and 10.0 msec and no IPSPs were detected.
The EPSPs induced by single supramaximal shocks to the contralateral vestibular nerve were in most cases not large enough to evoke full action potentials. In more than half of the cells recorded from, partial spikes were superimposed on the EPSPs.
In the vestibular neurons which send axons to the peripheral receptor organs, EPSPs were induced by stimulation of the contralateral vestibular nerve. There were no highly significant differences between the time courses of these EPSPs and those described in 4.
Key wordsFrog vestibular neurons Type 1 response Crossed vestibular facilitation
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