RESPONSE OF COMMISSURAL AND OTHER UPPER CERVICAL VENTRAL HORN NEURONSTO VESTIBULAR STIMULI IN VERTICAL PLANES

1. To study their contribution to the vestibulocollic reflex, we have studied, in decerebrate paralyzed cats, the effect of sinusoidal vesti bular stimulation in multiple vertical planes on the spontaneous activ ity of neurons in the C-3 ventral horn. Antidromic microstimulation wa s used to identify 17/42 neurons as commissural; 10 of these were conf irmed to have a projection to the contralateral ventral horn. 2. Dynam ics of the responses of spontaneously firing neurons were studied with 0.05-1 Hz sinusoidal stimuli delivered near the plane of rotation tha t produced maximal modulation of neuron activity (response vector orie ntation). On the basis of their responses, we classified 38 neurons as receiving otolith, semicircular canal, or otolith+canal input. All th ree response types were found among commissurae and nonantidromic neur ons. 3. Two-thirds of neuron response vector orientations pointed cont ralaterally. They were either near the anterior or posterior canal pla nes or in the roll quadrant. In the case of neurons with input from ca nals, the latter indicates convergence from the vertical canals on the same side. There were almost no vectors in the pitch quadrants. The d istribution of response vector orientations resembles that seen in the vestibular nuclei and pontomedullary reticular formation, suggesting that commissural neurons may not make a new contribution to spatial pr ocessing in the vertical vestibulocollic reflex. 4. It is presumed tha t commissural neurons are premotor. If so, some have the properties to be in the pathway between the contralateral utricle and neck motoneur ons. More generally, their actions could modify the effectiveness of v estibulospinal and reticulospinal fibers that have similar spatial pro perties and make synapses with neck motoneurons.