Connections between otolith receptors and neck motoneurons

Intracellular recordings from ipsilateral and contralateral neck extensor and flexor motoneurons were studied to determine the connections between the utricular nerve and neck motoneurons and between saccular nerves and neck motoneurons in cats. Stimulating electrodes were placed within the left utricular nerve or saccular nerve. Electrical stimulation of the utricular nerve evoked excitatory postsynaptic potentials (EPSPs) with disynaptic latency in almost all ipsilateral extensor motoneurons and disynaptic EPSPs in all ipsilateral flexor motoneurons. In contralateral extensor and flexor motoneurons, the initial response to utricular stimulation was usually inhibitory postsynaptic potentials (IPSPs) with trisynaptic linkage. Thus, the utriculo-neck connectivities essentially differentiated between the left and right side. In response to stimulation of the saccular nerve, the initial response in bilateral extensor motoneurons ws EPSPs. Ipsilateral projections were always disynaptic; contralateral projections could be di- or trisynaptic. 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 regard to flexors and extensors.