Functional properties of burster-driving neurons in the cat.

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Single unit recordings were made from burster-driving neurons (BDNs) in the prepositus hypoglossi nucleus and the underlying dorsomedial reticular formation caudal to the abducens nucleus in the aleart cat. BDNs were identified by their activation from the contralateral vestibular nerve with short latency, type II response to horizontal head rotation, and characteristic firing pattern during vestibular nystagmus. In response to contralateral head rotation, BDNs exhibited a burst of activity at each quick phase directed to the contralateral side following a tonic type II activity during the slow phase. BDNs exhibited bursts during contralateral saccades as well. The number of spikes in the burst was closely correlated with amplitude of horizontal component of eye movements. Similarly, intra-burst firing rate was related to horizontal eye velocity in the contralateral direction. During ipsilateral quick phases and saccades, BDNs tended to decrease their firing rate. When nystagmus was absent, firing responses of BDNs to sinusoidal head rotations at 0.05-0.5 Hz were approximately in phase with head velocity. The firing rate of BDNs during fixation was not correlated with eye position. BDNs increased their firing rate in a burst fashion in response to visual stimuli moving to the contralateral side even when no eye movements were observed. BDNs were polysynaptically activated following stimulation of the ipsilateral superior colliculus. The response was facilitated during contralatral head rotation and supressed during ipsilateral rotation, indicating convergence of tectal and vestibular inputs on BDNs. Intracellular injection of HRP into identified BDNs revealed their contralateral axonal projection and somatic location on the ventral border of the prepositus hypoglossi nucleus and the adjacent reticular formation.