Trisynaptic Inhibition from the Contralateral Vertical Semicircular Canal Nerves to Neck Motoneurons Mediated by Spinal Commissural Neurons. Sugiuchi, Yuriko, Yoshiko Izawa, and Yoshikazu Shinoda. Department of Otolaryngology, and Department of Physiology, School of Medicine, Tokyo Medical and Dental University, 1-5-45, Yushima, Bunkyo-ku, Tokyo 113, Japan.
APStracts 2:0004N, 1995.
SUMMARY AND CONCLUSIONS
1. Neck motoneurons usually receive disynaptic excitation and inhibition from individual semicircular canal nerves. However, in motoneurons of some neck muscles, trisynaptic inhibition is evoked by stimulation of the contralateral vertical canal nerves. The present study was performed to analyze this pathway and the location and properties of the last-order interneurons responsible for mediating this trisynaptic inhibition from the contralateral vertical canal nerves to neck motoneurons in anesthetized cats. 2. Bipolar stimulating electrodes were implanted on the contralateral anterior (ACN), lateral (LCN) and posterior canal nerve (PCN), and postsynaptic potentials (PSPs) evoked by electrical stimulation of individual canal nerves were intracellularly recorded from motoneurons of the obliquus capitis inferior (OCI), longus capitis (LC), and rectus capitis posterior (RCP) muscles. Stimulation of the contralateral ACN evoked trisynaptic IPSPs in OCI and LC motoneurons and disynaptic EPSPs in RCP motoneurons. Stimulation of the contralateral PCN evoked di- and trisynaptic IPSPs in OCI and RCP motoneurons and disynaptic EPSPs in LC motoneurons. Stimulation of the contralateral LCN evoked disynaptic EPSPs in all of the motoneurons examined. 3. To determine the pathway which mediates these trisynaptic IPSPs from the vertical canal nerves to neck motoneurons, a lesion was made in the lower medulla, and the patterns of PSPs evoked by stimulation of the three contralateral canal nerves were compared before and after the lesion. Interruption of the ipsilateral medial longitudinal fascicle (MLF) abolished all disynaptic EPSPs and IPSPs from the three contralateral canal nerves in OCI, LC and RCP motoneurons. In contrast, trisynaptic IPSPs evoked by stimulation of the contralateral ACN or PCN remained unaffected by sectioning the MLFs bilaterally. Sectioning of the contralateral LVST eliminated the trisynaptic IPSPs in OCI and LC motoneurons evoked by contralateral ACN stimulation and trisynaptic IPSPs in OCI and RCP motoneurons evoked by contralateral PCN stimulation, but did not affect disynaptic EPSPs and IPSPs. 4. Stimulation of the contralateral LVST in the lower medulla after sectioning the bilateral MLFs evoked disynaptic IPSPs in OCI, LC and RCP motoneurons. Since the LVST only projects ipsilaterally, this finding indicates that the last-order interneurons which mediate the trisynaptic inhibition through the LVST are most likely commissural neurons located in the spinal cord. 5. To determine the locations of last-order commissural neurons terminating on OCI motoneurons, WGA-HRP was injected into the OCI muscle nerve at C1. In addition to retrogradely-labelled motoneurons, many neurons were transneuronally labelled mainly in lamina VIII and sparsely in laminae VII and IX of the contralateral ventral horn. 6. Spinal commissural neurons receiving disynaptic excitation from the ipsilateral vertical canal nerves through the LVST were penetrated and injected with HRP intraaxonally. These neurons were located in lamina VIII. Stem axons ran dorsomedially and, after crossing the midline, bifurcated into ascending and descending branches. These branches gave rise to multiple axon collaterals which projected to ipsilateral neck motor nuclei. In the motor nuclei, axon terminals of these collaterals appeared to make contact with cell bodies and proximal dendrites of labelled neck motoneurons. 7. Microstimulation of contralateral lamina VIII evoked predominantly monosynaptic IPSPs in OCI, LC and RCP motoneurons. These monosynaptic IPSPs were facilitated by conditioning stimulation of the contralateral ACN in OCI and LC motoneurons and of the contralateral PCN in OCI and RCP motoneurons. These results show that there are last-order inhibitory commissural neurons in lamina VIII. The present study has elucidated that trisynaptic inhibition from the contralateral vertical canal nerves is mediated through the contralateral LVST and inhibitory commissural neurons in lamina VIII of the upper cervical cord to OCI, LC and RCP motoneurons. The trisynaptic connections between the vestibular afferents and neck motoneurons, in addition to the disynaptic connections, may play an important role in the vestibulocollic reflex. 

Received 19 September 1994; accepted in final form 9 January 1995.
APS Manuscript Number J587-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on  3 April 1995.