Plane-specific brainstem commissural inhibition in frog second-order semicircular canal neurons

Commissural inputs of identified second-order semicircular canal neurons we re studied by separate stimulation of each of the three canal nerves on eit her side in in vitro frog brains. The spatial pattern of these inputs was f urther investigated in those second-order canal neurons that received a mon osynaptic input from only one ipsilateral canal nerve (91%). Since similar results were obtained in the presence as in the absence of the cerebellum, commissural inputs must have been relayed via fibers crossing in the brains tem. Following stimulation of individual semicircular canal nerves, commiss ural inputs were either inhibitory or excitatory. A commissural inhibition was evoked in the majority of the recorded neurons (79%) by stimulation of the coplanar semicircular canal nerve on the contralateral side. In the rem aining neurons, a commissural excitatory input was evoked. A commissural ex citation, originating from the two noncoplanar semicircular canals, predomi nated in most (68%) of the recorded neurons and was independent of the type of second-order canal neuron. The onset latency of the canal plane-specifi c commissural inhibitory potentials was di- or trisynaptic. Stimulation of the contralateral VIIIth nerve evoked excitatory commissural responses. The canal plane-specific commissural inhibition therefore might have been mask ed by commissural excitatory responses as in earlier studies. The similar o rganization of the canal plane-specific commissural inhibition in frog and cat corroborates the notion of a phylogenetically conservative, basic vesti bular organization. The presence of a canal plane-unspecific commissural ex citation, however, appears to be a feature that is specific to frogs. The f unctional implications of these similarities and differences are discussed.