Branching pattern and properties of vertical- and horizontal-related excitatory vestibuloocular neurons in the cat

1. The axonal trajectories of excitatory vestibuloocular neurons and their synaptic contacts with extraocular motoneurons were studied by means of spike-triggered signal averaging and microstimulation techniques. A majority of the excitatory neurons related to the vertical semicircular canals were located in the border of the descending and medial nuclei and the rostral half of the descending nucleus. 2. Individual vestibuloocular neurons activated by stimulation of the ampullary nerve of the anterior semicircular canal excited motoneurons within both the contralateral inferior oblique and contralateral superior rectus motoneuron pools. 3. Individual vestibuloocular neurons receiving input from the ampullary nerve of the posterior semicircular canal excited motoneurons in both the contralateral trochlear nucleus and contralateral inferior rectus motoneuron pools. The branching pattern of single vestibuloocular neurons activated by the anterior and posterior canals probably underlies conjugate eye movement during vertical head rotation. 4. Time to peak and shape indices of unitary excitatory postsynaptic potentials (EPSPs) suggested that the location of the synaptic contact of vestibuloocular neurons was on the soma or proximal dendrites of the target extraocular motoneurons. 5. In contrast, we did not find conclusive evidence that single vestibuloocular neurons receiving input from the horizontal semicircular canal give off axon collaterals to motoneurons innervating both the contralateral lateral rectus and the ipsilateral medial rectus muscles. Projection of horizontal vestibuloocular neurons to motoneurons supplying individual muscles might be useful for convergence during horizontal head movement.