CONNECTIONS BETWEEN OTOLITH RECEPTORS AND NECK MOTONEURONS

Intracellular recordings from ipsilateral and contralateral neck exten sor and flexor motoneurons were studied to determine the connections b etween the utricular nerve and neck motoneurons and between saccular n erves and neck motoneurons in cats. Stimulating electrodes were placed within the left utricular nerve or saccular nerve. Electrical stimula tion of the utricular nerve evoked excitatory postsynaptic potentials (EPSPs) with disynaptic latency in almost all ipsilateral extensor mot oneurons and disynaptic EPSPs in all ipsilateral flexor motoneurons. I n contralateral extensor and flexor motoneurons, the initial response to utricular stimulation was usually inhibitory postsynaptic potential s (IPSPs) with trisynaptic linkage. Thus, the utriculo-neck connectivi ties essentially differentiated between the left and right side. In re sponse to stimulation of the saccular nerve, the initial response in b ilateral extensor motoneurons was EPSPs. Ipsilateral projections were always disynaptic; contralateral projections could be di- or trisynapt ic. IPSPs were evoked in almost all bilateral flexor motoneurons with di-or trisynaptic latencies. The sacculo-neck motoneuron relationship thus has a qualitatively, bilaterally symmetric organization with rega rd to flexors and extensors.