Cervical effects on abducens motoneurons and their interaction with vestibulo-ocular reflex.

Abstract

The effect of neck afferents on abducens motoneurons and their interaction with the vestibulo-abducens reflex were examined in chloraloseanesthetized or unanesthetized, decerebrate cats. The test reflex elicited in the abducens nerve by stimulation of the contralateral vestibular nerve was inhibited by contralateral and facilitated by ipsilateral cervical dorsal root or neck joint stimulation. These reciprocal effects were obtained by stimulation at the level of C2 and C3, but not from C5 or lower. Contralateral and ipsilateral cervical stimulation induced IPSPs and EPSPs, respectively, in abducens motoneurons. The latencies were 2.8–6.0 msec for the IPSP and 2.8–5.3 msec for the EPSP after stimulation of the dorsal root. The labyrinthine-induced disynaptic IPSP or EPSP was facilitated by conditioning stimulation of the contralateral and ipsilateral cervical dorsal root, respectively. It is thus postulated that the cervico-abducens and vestibuloabducens reflex pathways converge upon common inhibitory or excitatory interneurons in the vestibular nuclie. Labyrinthine- and cervical-induced responses of the presumed interneurons in the vestibular nuclei or those of their axons recorded in the abducens nuclie were consistent with the above view. Lesion experiments in the brain stem indicated that afferent volleys from the neck joint ascend ipsilaterally in the spinal cord, cross to the contralateral side in the brain stem, and eventually project to the vestibular nuclei, thus interacting with the vestibulo-ocular reflex activity. A possible functional role of the cervical effects on the ocular motoneuron was briefly discussed.