Ipsilateral utricular and semicircular canal interactions from electrical stimulation of individual vestibular nerve branches recorded in the descending medial longitudinal fasciculus.

Interactions from electrical stimulation of individual vestibular end organ nerves recorded from second order vestibular neurons projecting caudally in the medial longitudinal fasciculus (MLF) have been investigated in unanesthetized, decerebrate decerebellate cats. Canal-canal and utricle-canal interactions were recorded using various condition-test parameters. The brain stem was sectioned to eliminate influence of the contralateral vestibular nuclei, and the condition-test experiments were repeated. The type of interaction observed (inhibition or facilitation) and the degree to which it occurred were dependent on the MLF recording site. Also, in the case of utricular interactions, these were dependent on the site of utricular stimulation. These dependencies indicate a topological arrangement from receptor cell through the second order neuronal projection pathways. Sectioning of the brain stem produced changes in the interaction patterns, indicating masking and/or balancing of the ipsilateral interactions. In interactions between the anterior and posterior canals, instances of no apparent interaction occurring before sectioning converted to inhibition after sectioning. This indicates that some facilitative interaction occurs involving feedback from the contralateral vestibular nuclei, and this facilitation masks or balances ipsilateral inhibition. In interactions between the anterior and horizontal canals, contralateral inhibitory influence masks ipsilateral facilitatory interactions. Contralateral facilitatory influence again balances ipsilateral inhibition in interactions between the horizontal and posterior canals. The degree of inhibition and facilitation observed indicates that considerably more polysynaptic ipsilateral convergence on second order vestibular neurons occurs than has been demonstrated in intracellular studies. Only inhibition was observed when the anterior canal was conditioned by stimulation of the utricular nerve after sectioning. In all other pairings, mixed effects were observed. It is suggested that this is related to known functional connections of semicircular canal second order neurons with neck motoneurons. Not only is canal output affected by utricular activity, but each canal affects utricular output in a different manner.