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Basic and AppliedAspects ofVestibular FUllCtio1& I.C. Hwang, N.G. Daunton and V.I. Wilson (Eds.)© Hong Kong University Press, Hong Kong, 1988 PHYSIOLOGICAL AND MORPHOLOGICAL PROPERTIES OF BRAIN STEM NEURONS RELATED TO VESTIBULAR AND SACCADIC EYE MOVEMENTS IN THE CAT T. Kitama*, Y. Ohki**, H. Shimazu*** and K. Yoshida* *Department ofPhysiology, Institute ofBasic Medical Sciences, University ofTsukuba, Tsukuba-city, Ibaraki 305, Japan; **Department ofNeurophysiology, Institute ofBrain Research, School ofMedicine, University ofTokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan; and *** Department ofNeurobiology, Tokyo Metropolitan Institute for Neurosciences, 2-6 Musashidai, Fuchu-city, Tokyo 183, Japan. Abstract Single lDlit recordings were made from burster-driving neurons (BDNs) in the dorsomedial brain stem caudal to the abducens nucleus in the cat. BDNs were identified by their activation from the contralateral vestibular nerve, type II response to horizontal rotation, and characteristic firing pattern during nystagmus . 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. BDNs exhibited a burst of spikes for saccades as well as for the quick phase of nystagmus directed to the contralateral side. The firing rate of BDNs during fixation was not correlated with eye position. Intracellular injection of HRP into identified BDNs revealed their contralateral axonal projections, and somatic location on the ventral border of the prepositus hypoglossi nucleus and the -adjacent reticular formation. In recent years, anatomical and electrophysiological studies have demonstrated that the prepositus hypoglossi nucleus is significantly involved in the vestibulooculomotor system (Baker and Berthoz, 1975; Baker et al., 1975; Fukushima et ai., 1977; Hikosaka et aI., 1978; Lopez-Barneo et al.,1982; McCrea and Baker,1985). It receives powerful disynaptic vestibular connections which are primarily reciprocal in nature; i.e. contralateral excitation and ipsilateral inhibition (Baker and Berthoz, 1975). Single unit recordings in the preposiKeywords : prepositus hypoglossi nucleus, medullary reticular formation, vestibular nystagmus, saccade, horizontal rotation, burster-driving neurons 46 Kitama et al. tus nucleus have revealed neuronal activity closely related to eye movements in both the horizontal and vertical directions (Baker et ai., 1975; Lopez-Barneo et ai., 1982). In particular, it has been well documented that many prepositus neurons with a horizontal directionality show fIring characteristics of burst-tonic neurons and resemble abducens motoneurons on the same side; they increase their fIring rate with both ipsilateral eye position and velocity. These motoneuron-type (Mn-type) cells display a type II modulation in response to head rotation, and phase of the response is similar to that of the abducens nerve (Fukushima et al., 1977). Activity of some prepositus neurons is related primarily to eye position. In addition, a recent study (Ohki et ai., 1988) has revealed the existence of a new class of neurons in the prepositus hypoglossi nucleus and the underlying medullary reticular formation, which exhibits a unique fIring pattern during horizontal vestibular nystagmus. These neurons also receive short latency excitation from the contralateral horizontal canal and display type-II response during horizontal rotation. Their firing response during nystagmus is, however, quite distinct from that of Mn-type neurons: they exhibit a burst of activity in association with the quick phase directed to the contralateral side instead of the ipsilateral side. Since these neurons project contralaterally to the area where there is a concentration of excitatory premotor burst neurons (EBNs) (lgusa et ai., 1980; Sasaki and Shimazu, 1981), and their burst activity coincides with that ofEBNs, it is suggested that they send excitatory input to EBNs, thus being called burster-driving neurons (BDNs) . In the present study, we have analysed in acute preparations the dynamic characteristics of single BDNs in response to sinusoidal head rotation and compared the results with those of abducens motoneurons and Mn-type neurons in the prepositus nucleus and reticular formation to examine if any quantitative differences might exist. Since EBNs are thought to be the major source causing an increase in activity ofabducens motoneurons not only during quick phases of nystagmus (lgusa et al., 1980; Sasaki and Shimazu, 1981) but also during saccades (Kaneko et ai., 1981), we analysed the activity of BDNs in the alert cat to elucidate their possible functional role in the generation of saccade. Some preliminary morphological data obtained by intracellular injection of HRP will also be presented. The response of BDNs to head rotation was examined with cats prepared under halothane anesthesia. Methods for recording from the abducens nerve and for stimulation of the vestibular nerves were the same as those described in a previous paper (Nakao et...

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