CAT MEDIAL PONTINE NEURONS IN VESTIBULAR NYSTAGMUS

Two main events take place in abducens motoneurons at the quick phase of vestibular nystagmus. One is the high-frequency h r s t s of agonist abducens motoneurons, which results in a quick ipsilateral deviation of the eye. The other event is the abrupt cessation of firing of the contralateral ahducens rnotoneurons, which results in termination of the slow phase to the opposite side. In the normal human and animal, these events are highly coordinated and synchronous. The high-frequency discharge in the agonist motoneurons could be caused by several groups of neurons. Several classes of neurons in the medial pontine reticular formation (PRF) that discharge in Ilursts have been suggested to provide the necessary excitatory drive.'-3 Recently Igusa et ul. have shown that certain pontine medium-lead hurst neurons make monosynaptic connections with neurons in the ipsilateral ahductms niicleus.4 Monosynaptic Vc axons in the abducens nucleus, whose cell hotlies lie in the opposite vestibular nucleus, do have rhythmic discharges with nystagmus; but they usually begin to fire several milliseconds after the aliducc?ns rnotoneurons begin abrupt depolarization, which ushers in the spike t ~ u r s t . ~ Monosynaptic Vi neurons, inhibitory neurons in the ipsilateral vestibular nucleus, do cease firing at the quick phase and result in d i~ inhib i t ion ,~ .~ but active excitation appears to he a more powerful influence. Termination of tht? discharge of ahducens motoneurons responsible for the slow phase of vestibular nystagmus is hrought about by burst inhibitory neurons (RINs), premotor neurons that are predominantly located caudal to one al)ducens nucleus and that terminate on contralateral abducens motoneurons and possibly ahducens interneurons. These neurons have been demonstrated to be inhihiWhich neurons regulate the behavior of RINs? Suggestions have been made that pause neurons (PNs), a class of neurons that fire tonically but cease firing just prior to a quick eye movement in any direction, play a role in regulating saccadic discharges of eye m ~ t o n e i i r o n s . ~ ~ ~ The present paper analyzes the firing patterns of a number of groups of pontine reticular formation neurons in relation to vestibular nystagmus in the cat and analyzes the behavior of neural firing at the quick phase.