Synaptic organization of the vestibulo-collic pathways from six semicircular canals to motoneurons of different neck muscles.

The pattern of inputs from six semicircular canals to neck motoneurons was investigated by stimulating six ampullary nerves electrically and recording intracellular potentials from motoneurons of the rectus capitis dorsalis (RD), the complexus (COMP) and the obliquus capitis caudalis (OCA) muscles at the upper cervical cord of the cat. RD and COMP motoneurons received disynaptic excitation from bilateral anterior and contralateral horizontal ampullary nerves and disynaptic inhibition from bilateral posterior and ipsilateral horizontal ampullary nerves. OCA motoneurons received excitation from ipsilateral vertical and contralateral horizontal ampullary nerves and inhibition from contralateral vertical and ipsilateral horizontal ampullary nerves. Ipsilateral disynaptic inhibitory postsynaptic potentials and contralateral disynaptic excitatory postsynaptic potentials to these motoneurons were mediated by the medial longitudinal fasciculus (MLF) and the other postsynaptic potentials by the extra-MLF pathways. The results indicated that motoneurons of a neck muscle have its own characteristic pattern of inputs from six semicircular canals.