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Vestibular Unit Activity during Nystagmus
Vestibular Unit Activity during Nystagmus H. SHIMAZU Department of Neurophysiology, Institute of Brain Research, School of Medicine, University of Tokyo, 7-3-1Hongo, Bunkyo-ku, Tokyo (Japan)
Vestibular nucleus neurons whose activity is closely related to nystagmus have first been found in the rabbit by Duensing and Schaefer (1958). Subsequent studies have confirmed the existence of eye movement-related neurons in the vestibular nuclei of the cat (Horcholle and TyE-Dumont, 1968) and the monkey (Luschei and Fuchs, 1972; Miles, 1974; Keller and Daniels, 1975; Keller and Kamath, 1975; Fuchs and Kimm, 1975; Waespe et al., 1977). A question arises whether the particular vestibular neurons recorded from are immediate premotor, or their eye movement-related activity is merely corollary or represents a fraction of bulbopontine activity indirectly related to the motor output. As an approach to resolving this problem, unit spikes of presynaptic axons identified as originating from the vestibular nuclei were recorded within the abducens nucleus and their discharge pattern and timing were correlated with abducens nerve activity during nystagmus (Maeda et al., 1971). It was found that identified units exhibited spike activity related to both the slow and quick phases of nystagrnus, suggesting that vestibular nucleus neurons are immediate premotor and participate in nystagmic modulation of ocular activity. A similar suggestion was made by observing in the trochlear nucleus presynaptic axon spikes of vestibular nucleus neurons correlated with trochlear motor activity during nystagmus (Baker and Berthoz, 1974). Mergner and Pompeiano (1977) extended this line of study to drug-induced saccadic eye movements (REM) and concluded that the vestibular nuclear complex represents one of the premotor structures responsible for the REM. The present report will describe a correlative study of presynaptic impulses within the abducens nucleus and postsynaptic potentials in motoneurons during vestibular nystagmus. The results suggest that vestibular nucleus neurons projecting to the abducens nuclei contribute to generation of the rapid change in the postsynaptic potentials in motoneurons at the quick phase. This possibility has been ascertained by recording unit activity of neurons in the medial vestibular nucleus and identifying their direct connection with abducens rnotoneurons by electrophysiological techniques. NYSTAGMUS-RELATED DISCHARGE WITHIN THE ABDUCENS NUCLEUS OF PRESYNAPTIC AXONS OF VESTIBULAR NUCLEUS NEURONS Unit spikes of axons were recorded within the abducens nucleus in the endphale is016 cat under local anesthesia. Those units which were not activated antidromically
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from the abducens nerve were selected for study. Units were further selected by the presence of their response to horizontal angular acceleration of the head and by their nioiiosyiiuptic activation following electrical stimulation of the vestibular nerve on the left or right side. These criteria indicate that the units under study are spikes of axons which originate from the secondary vestibular neurons in the horizontal canal system and project to either the ipsilateral or contralateral abducens nucleus. Nystagmus was induced by high frequency (400 pulses/sec) electrical stimulation of the vestibular nerve. In order to correlate the timing of discharges of presynaptic axons with the postsynaptic potential changes in motoneurons during nystagmus, it was investigated how the extracellular field potentials reflected postsynaptic potentials of the population of motoneurons. The temporal relationship between intra- and extracellular potentials at the quick phase was examined for a large number of nystagmic beats (for detail, see Hikosaka et al., 1977). Fig. 1A exemplifies, from top to bottom, simultaneous recording of a steep depolarization in an abducens motonenron, the negative deflection of the field potential in the abducens nucleus and abducens nerve discharges on the same side at the quick excitatory phase of the motoneuron. Fig. 1B shows similar recording at the quick inhibitory phase of the motoneuron. The results showed that the onset of the negative or positive field potential at the quick phase was synchronous with the mean onset time of the steep depolarization or hyperpolarization of motoneurons measured in a large number of nystagmic beats. The field potential can therefore be utilized as an indicator to determine the onset time of the steep change in the membrane potential of motoneurons at the quick phase. Axons monosynaptically activated from the contralateral vestibular nerve (Fig. 2D) are presumed to originate from excitatory vestibular type I neurons. They fired invariably in phase with abducens nerve discharges on the same side during nystagmus. Tonic discharges of the axon during the slow phase were abruptly suppressed at the onset of steep positive field potential which reflected intracellular hyperpolarization of motoneurons (Fig. 2A, F). Assuming that the axon terminates on motoneurons, an abrupt decrease in excitatory effects of the axon should cause motoneuronal hyperpolarization due to disfacilitation. In fact, evidence was given in a previous study for the existence of disfacilitation in motoneurons at their quick inhibitory phase (Maeda et al., 1972).
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Fig. 1. Temporal relation between intracellular potentials of an abducens motoneuron and extracellular field potentials in the abducens nucleus at the quick phase of nystagmus. A) Simultaneous recording of intracellular (top), extracellular potentials (middle) and abducens nerve discharges (bottom) on the same side at the quick excitatory phase. B) Same arrangement as in A, but for the quick inhibitory phase. (From Hikosaka et al., 1977.)
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Fig. 2. Nystagmus-related discharge pattern of an axon monosynaptically activated from the contralateral vestibular nerve. A) Simultaneous recording of axonal spikes (a) and the field potential (b) in the abducens nucleus, and abducens nerve discharges with slow excitation followed by quick inhibition (c). All recordings were made o n the same side. B) Same as in A, but the direction of nystagmus was reversed. C-E) Responses to single shock stimulation of the ipsilateral vestibular nerve inducing positive field potential alone (C), that of the contralateral vestibular nerve evoking spikes superimposed on negative field potential (D) and that of the abducens nerve inducing antidromic field potential alone (E). F) Dot plots of spike discharges in 10 nystagmic beats during the slow excitatory phase. G) Same as in F, but for the quick excitatory phase. Vertical broken lines in F and G represent the onset time of positive or negative field potential, resp. (From Hikosaka et al., 1977.)
When the direction of nystagmus was reversed (Fig. 2B, G), the same axon was activated during the quick excitatory phase. The firing started usually later than the onset of negative field potential or was often not activated. The synaptic action of these axons presumably contributes to a part of the EPSPs produced in motoneurons at the quick phase described by Maeda et al. (1972). Axons monosynaptically activated form the ipsilateral vestibular nerve (Fig. 3C) are presumed to originate from inhibitory vestibular type I neurons. During nystagmus they fired periodically in phase with silent period of abducens nerve activity on the same side. The spike frequency usually increased gradually during the slow phase and was abruptly suppressed at the onset of steep negative deflection of the field potential which was a counterpart of intracellular depolarization (Fig. 3A, F). Given that these axons terminate on motoneurons, gradually increasing IPSPs in motoneurons during their slow inhibitory phase are attributed to synaptic action of these axons. Abrupt suppression of tonic discharges of these axons should cause disinhibition which exists in motoneurons at the quick excitatory phase (Maeda et al., 1972). A remarkable functional role of disinhibition in bringing about facilitation of motoneurons has been well known since the work of Wilson and Burgess (1962). Abrupt suppression of spike activity of inhibitory vestibular neurons
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Fig. 3. Nystagnius-related discharge pattern of an axon monosynaptically activated from the ipsilateral vestibular nerve. Same arrangement as in Fig. 2, but spikes are induced during the silent phase of abducens nerve activity. (From Hikosaka et al., 1977.)
therefore fulfils an immediate premotor function for the generation of quick excitation of motoneurons. The same axon as in Fig. 3A was activated during the quick inhibitory phase of motoneurons as well (Fig. 3B, G). Spike initiation of this axon at the quick phase tended to be slightly later than the onset of positive field potential, but they fired until the end of silent period of abducens nerve activity (more than 100 msec). The IPSPs observed in motoneurons at the quick inhibitory phase (Maeda et al., 1972) are in part attributed to the synaptic action of these vestibular neurons. NYSTAGMUS-RELATED DISCHARGE OF PREMOTOR NEURONS IN THE MEDIAL VESTIBULAR NUCLEUS In the study described above there was no evidence for the termination of nystagmus-related axons in the abducens nucleus. Moreover, the site of origin of these axons was presumed to be the vestibular nuclei because of their monosynaptic activation from the vestibular nerve, but the precise location of individual neurons in the vestibular nuclear complex could not be determined. Type I neuron In a recent study of Schor et al. (1977), extracellular unit spikes were recorded from neurons in the rostra1 part of the medial vestibular nucleus and were identified as secondary vestibular type I neurons in the horizontal canal system on the basis of their response to rotation and their monosynaptic activation following stimulation of the ipsilateral vestibular nerve. Neurons sending their axons to the contralateral abducens
473 nucleus were selected by their antidromic response to microstimulation (less than 15 PA) in the nucleus. This technique, however, could not distinguish between an axon which terminated within the abducens nucleus and a passing fiber which merely traversed the nucleus. During successive tracks through the contralateral abducens nucleus, the vestibular type I neuron could typically be activated from a variety of scattered sites within the nucleus, with intervening ineffective sites. This suggested that the axon branched extensively and terminated within the nucleus. More direct evidence for the excitatory connection of a projecting vestibular neuron to abducens motoneurons was obtained by the use of postspike averaging of abducens nerve discharges triggered from spikes of a single vestibular neuron. The validity of this technique was described elsewhere in detail (Hikosaka e t al., 1978). These identified vestibular type I neurons projecting to the contralateral abducens nucleus were found almost invariably to exhibit a nystagmic modulation of their spike activity (Fig. 4A). The discharge pattern was similar to that of presynaptic axons in the abducens nucleus shown in Fig. 2A. A similar study was performed on secondary type I neurons in the medial vestibular nucleus which projected to the ipsilateral abducens nucleus. By the aid of antidromic microstimulation technique for tracing the course of the axon and postspike averaging of abducens nerve discharges, the direct inhibitory action of these vestibular neurons on ipsilateral abducens motoneurons was confirmed. Most of the neurons thus identified showed a nystagmic modulation of firing. The discharges were in phase with the inhibitory period of ipsilateral abducens motoneurons and exhibited a similar pattern to that of presynaptic axons in the abducens nucleus shown in Fig. 3.
Type 11 neuron The nystagmic modulation of activity in the secondary vestibular neurons was caused by a periodic production of IPSPs at the onset of the quick phase (Hikosaka, Nakao and Shimazu, unpublished observation). As a candidate for inhibitory interneurons responsible for this periodic inhibition, vestibular type I1 neurons were selected for study (Schor et al., 1977), since they have been suggested as inhibitory neurons acting on type I neurons in the same nucleus (Shimazu and Precht, 1966). Type I1 neurons in the medial vestibular nucleus, characterized by their response to
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Fig. 4. Nystagmic modulation of spike activity of horizontal type I and type I1 neurons recorded in the medial vestibular nucleus. A) Spikes of a type I neuron projecting to the contralateral abducens nucleus. B) Spikes of a type I1 neuron. Bottom record in A and B represents abducens nerve discharges on the contralateral side. (From Nakao, Schor and Shimazu, unpublished observation.)
474 horizontal rotation (increased firing with contralateral angular acceleration and decreased firing with ipsilateral acceleration), were further selected by their activation at short latencies from the contralateral labyrinth. Most of these neurons had a nystagmic rhythm, showing an abrupt increase in discharge frequency at the onset of the quick inhibitory phase of contralateral abducens motoneurons, when type I neurons were showing a quick suppression of activity (Fig. 4B). The firing pattern of vestibular type I1 neurons during nystagmus was consistent with the hypothesis that they are contributing to inhibition of type I activity at the quick phase. This was more directly confirmed by postspike averaging of the membrane potential of a type I neuron triggered from spikes of a single type I1 neuron, revealing an IPSP with a monosynaptic latency. It is therefore concluded that the vestibular type I1 neurons mediating commissural inhibition play a role in nystagmic modulation of activity observed in type I neurons which make direct connections with abducens motoneurons and contribute to the generatiQn of nystagmic rhythm of abducens nerve activity. DISCUSSION The role of the vestibular nuclei in production of the quick eye movements has long been a subject of dispute. Until very recently, many oculomotor physiologists have been concerned about the pontine reticular formation as the site of origin of quick eye movements (6.Raphan and Cohen, 1978). The term of “site of origin” of movements may need some explanation. An explicit working definition has been given by Cohen and Henn (1972) who state that by site of origin is meant the location of the immediate supranuclear neural mechanism which generates these movements. Given this definition, the site of origin (Le., the location of “immediate premotor neurons”) of quick phases of nystagmus may not necessarily be a single site, but comprise multiple structures which make direct connections with ocular motoneurons to lead to excitation or inhibition related to nystagmic rhythm of motor output. The present results have shown that immediate premotor neurons participating in generation of not only the slow phase but also the quick phase of nystagmus are located in the vestibular nuclei. Obviously, however, this does not exclude other premotor neurons responsible for the quick phases. In fact, recent studies have provided evidence that interneurons in the abducens nucleus projecting to the contralateral oculomotor nucleus are also premotor neurons causing excitation of medial rectus motoneurons during both slow and quick eye movements (Delgado-Garcia et al., 1977; Nakao and Sasaki, 1978). A group of burst neurons in the dorsomedial reticular formation caudal to the abducens nucleus, are also immediate premotor neurons to give rise to inhibition of contralateral abducens motoneurons at the quick phase (Hikosaka and Kawakami, 1977; Hikosaka et al., 1978). The paramedian pontine reticular formation has also been proposed as a site of origin of quick phases of nystagmus in the horizontal plane (Bender and Shanzer, 1964; Cohen and Henn, 1972). This hypothesis is supported by lesion and stimulation experiments. Unit activity of burst neurons recorded in this area is closely related to saccades and quick phases of nystagmus. Since anatomical studies have shown that this area projects to the ipsilateral abducens nucleus (Biittner-Ennever and Henn, 1976; Graybiel, 1977), burst neurons in this area may make direct connections with
475
abducens motoneurons. It is of importance to prove this possibility by examining specific connections between abducens motoneurons and individual burst neurons. Finally, the well coordinated timing of impulse frequency change observed in vestibular and reticular neurons at the quick phase of nystagmus may imply the existence of significant vestibuloreticular or reticulovestibular interaction. To understand the mechanism of generation of nystagmus-related pattern in various premotor neurons, it seems therefore essential to know how functionally identified neurons in these two structures are coupled with each other. SUMMARY 1. Unit spikes of axons were recorded within the abducens nucleus in the enckphale isolC cat under local anesthesia. Units were identified by the absence of antidromic activation from the abducens nerve and the presence of response to horizontal rotation and monosynaptic activation from the vestibular nerve, indicating that the units under study were spikes of axons of vestibular nucleus neurons projecting to the abducens nucleus. Axons monosynaptically activated from the contralateral vestibular nerve fired in phase with abducens nerve activity and those from the ipsilateral vestibular nerve fired during the silent period of abducens nerve activity. Abrupt suppression of their tonic discharges occurred at the quick phase coincidently with disfacilitation or disinhibition in motoneurons. 2. Type I neurons in the medial vestibular nucleus were selected by the response to horizontal rotation and monsynaptic activation from the ipsilateral labyrinth. Neurons sending their axons to the contralateral o r ipsilateral abducens nucleus were identified by their antidromic response to microstimulation in the nucleus and their excitatory or inhibitory synaptic connection with motoneurons was confirmed by postspike averaging of abducens nerve discharges triggered from spikes of a single vestibular neuron. Spikes of thus identified vestibular type I neurons showed a nystagmic modulation, similar to that of presynaptic axons in the abducens nucleus. 3. Type I1 neurons in the medial vestibular nucleus were identified by their response to horizontal rotation and their activation at short latencies from the contralateral labyrinth. Most of these neurons had a nystagmic rhythm, showing a spike burst when type I neuron activity was suppressed. 4. It is concluded that type I neurons in the medial vestibular nucleus are immediate premotor neurons which participate in generation of nystagmic rhythm in abducens motoneurons. Type I1 neurons which mediate commissural inhibition play a role in nystagmic modulation of type I neuron activity by their periodic spike burst at the quick phase. REFERENCES Baker, R. and Berthoz, A. (1974) Organization of vestibular nystagmus in oblique oculomotor system. J . Neurophysiol ., 37: 195-217. Bender, M.B. and Shanzer, S. (1964) Oculomotor pathways defined by electrical stimulation and lesions in the brainstem of monkey. In The Oculomotor System, M.B. Bender (Ed.), Harper and Row, New York, pp. 81-1 40. Biittner-Ennever, J.A. and Henn, V. (1976) An autoradiographic study of the pathways from the pontine reticular formation involved in horizontal eye movements. Brain Res., 108: 155-164.
476 Cohen, B. and Henn, V. (1972) The origin of quick phases of nystagmus in the horizontal plane. Bibl. ophthul., (Basel), 82: 36-55. Delgado-Garcia, J., Baker, R. and Highstein, S.M. (1977) The activity of internuclear neurons identified within the abducens nucleus of the alert cat. In Control of Gaze by Bruin Stem Neurons, R. Baker and A. Berthoz (Eds.), Elsevier, Amsterdam-New York, pp. 291-300. Duensing, F. and Schaefer, K.P. (1958) Die Aktivitat einzelner Neurone im Bereich der Vestibulariskerne bei Horizontalbeschleunigungen unter besonderer Beriicksichtigung des vestibularen Nystagmus. Arch. Psychiut. Nervenkr., 198: 225-252. Fuchs, A.F. and Kimm, J. (1975) Unit activity in vestibular nucleus of the alert monkey during horizontal angular acceleration and eye movement. J. Neurophyswl., 38: 1140-1 161. Graybiel, A.M. (1977) Direct and indirect preoculomotor pathways of the brainstern: an autoradiographic study of the pontine reticular formation in the cat. J. comp. Neurol., 175: 37-78. Hikosaka, 0.and Kawakami, T. (1977) Inhibitory reticular neurons related to the quick phase of vestibular nystagmus - their location and projection. Exp. Bruin Res., 27: 377-396. Hikosaka, O., Igusa, Y., Nakao, S. and Shimazu, H. (1978) Direct inhibitory synaptic linkage of pontomedullary reticular burst neurons with abducens motoneurons in the cat. Exp. Brain Res., 33: 337-352. Hikosaka, O., Maeda, M., Nakao, S., Shimazu, H. and Shinoda, Y. (1977) Presynaptic impulses in the abducens nucleus and their relation to postsynaptic potentials in motoneurons during vestibular nystagmus. Exp. Bruin Res., 27: 355-376. Norcholle, G . and TyE-Dumont, S. (1968) ActivitBs unitaires des neurones vestibulaires et oculomoteurs an cows du nystagmus. Exp. Brain Res., 5: 16-31. Keller, E.L. and Daniels, P.D. (1975) Oculomotor related interaction of vestibular and visual stimulation in vestibular nucleus cells in alert monkey. Exp. Neurol., 46: 187-198. Keller, E.L. and Kamath, B.Y. (1975) Characteristics of head rotation and eye movement-related neuron8 in alert monkey vestibular nucleus. Bruin Res., 100: 182-187. Luschei, E.S. and Fuchs, A.F. (1972) Activity of brain stem neurons during eye movements of alert monkeys. J. Neurophyswl., 35: 445461. Maeda, M., Shimazu, H. and Shinoda, Y. (1971) Rhythmic activities of secondary vestibular efferent fibers recorded within the abducens nucleus during vestibular nystagmus. Brain Res., 34: 361-365. Maeda, M., Shimazu, H. and Shinoda, Y. (1972) Nature of synaptic events in cat abducens motoneurons at slow and quick phase of vestibular nystagmus. J. Neurophysiol., 35: 279-296.. Mergner, T. and Pompeiano, 0. (1977) Neurons in the vestibular nuclei related to saccadic eye movements in the decerebrate cat. I n Control of Gaze by Brain Stem Neurons, R.Baker and A. Berthoz (Eds.), Elsevier, Amsterdam-New York, pp. 243-251. Miles, F.A. (1974) Single unit firing patterns in the vestibular nuclei related to voluntary eye movements and passive body rotation in conscious monkeys. Brain Res., 71: 215-224. Nakao, S. and Sasaki, S. (1978) Firing pattern of interneurons in the abducens nucleus related to vestibular nystagmus in the cat. Bruin Res., 144: 389-394. Raphan, T. and Cohen, B. (1978) Brainstem mechanisms for rapid and slow eye movements. Ann. Rev. Physiol., 40: 527-552. Schor, R.H., Nakao, S. and Shimazu, H. (1977) Responses of medial vestibular nucleus neurons during vestibular nystagmus. Neurosci. Abstr., 3: 545. Shimazu, H. and Precht, W. (1966) Inhibition of central vestibular neurons from the contralateral labyrinth and its mediating pathway. J. Neurophysiol., 29: 467-492. Waespe, W.,Henn, V. and Miles, T.S. (1977) Activity in the vestibular nuclei of the alert monkey during spontaneous eye movements and vestibular or optokinetic stimulation. In Control of Gaze by Bruin Stem Neurons, R.Baker, and A Berthoz (Eds.), Elsevier, Amsterdam-New York, pp. 269-278. Wilson, V.J. and Burgess, P.R. (1962) Disinhibition in the cat spinal cord. J. Neurophysiol., 25: 392404.
Vestibular nucleus neurons whose activity is closely related to nystagmus have first been found in the rabbit by Duensing and Schaefer (1958). Subsequent studies have confirmed the existence of eye movement-related neurons in the vestibular nuclei of the cat (Horcholle and TyE-Dumont, 1968) and the monkey (Luschei and Fuchs, 1972; Miles, 1974; Keller and Daniels, 1975; Keller and Kamath, 1975; Fuchs and Kimm, 1975; Waespe et al., 1977). A question arises whether the particular vestibular neurons recorded from are immediate premotor, or their eye movement-related activity is merely corollary or represents a fraction of bulbopontine activity indirectly related to the motor output. As an approach to resolving this problem, unit spikes of presynaptic axons identified as originating from the vestibular nuclei were recorded within the abducens nucleus and their discharge pattern and timing were correlated with abducens nerve activity during nystagmus (Maeda et al., 1971). It was found that identified units exhibited spike activity related to both the slow and quick phases of nystagrnus, suggesting that vestibular nucleus neurons are immediate premotor and participate in nystagmic modulation of ocular activity. A similar suggestion was made by observing in the trochlear nucleus presynaptic axon spikes of vestibular nucleus neurons correlated with trochlear motor activity during nystagmus (Baker and Berthoz, 1974). Mergner and Pompeiano (1977) extended this line of study to drug-induced saccadic eye movements (REM) and concluded that the vestibular nuclear complex represents one of the premotor structures responsible for the REM. The present report will describe a correlative study of presynaptic impulses within the abducens nucleus and postsynaptic potentials in motoneurons during vestibular nystagmus. The results suggest that vestibular nucleus neurons projecting to the abducens nuclei contribute to generation of the rapid change in the postsynaptic potentials in motoneurons at the quick phase. This possibility has been ascertained by recording unit activity of neurons in the medial vestibular nucleus and identifying their direct connection with abducens rnotoneurons by electrophysiological techniques. NYSTAGMUS-RELATED DISCHARGE WITHIN THE ABDUCENS NUCLEUS OF PRESYNAPTIC AXONS OF VESTIBULAR NUCLEUS NEURONS Unit spikes of axons were recorded within the abducens nucleus in the endphale is016 cat under local anesthesia. Those units which were not activated antidromically
470
from the abducens nerve were selected for study. Units were further selected by the presence of their response to horizontal angular acceleration of the head and by their nioiiosyiiuptic activation following electrical stimulation of the vestibular nerve on the left or right side. These criteria indicate that the units under study are spikes of axons which originate from the secondary vestibular neurons in the horizontal canal system and project to either the ipsilateral or contralateral abducens nucleus. Nystagmus was induced by high frequency (400 pulses/sec) electrical stimulation of the vestibular nerve. In order to correlate the timing of discharges of presynaptic axons with the postsynaptic potential changes in motoneurons during nystagmus, it was investigated how the extracellular field potentials reflected postsynaptic potentials of the population of motoneurons. The temporal relationship between intra- and extracellular potentials at the quick phase was examined for a large number of nystagmic beats (for detail, see Hikosaka et al., 1977). Fig. 1A exemplifies, from top to bottom, simultaneous recording of a steep depolarization in an abducens motonenron, the negative deflection of the field potential in the abducens nucleus and abducens nerve discharges on the same side at the quick excitatory phase of the motoneuron. Fig. 1B shows similar recording at the quick inhibitory phase of the motoneuron. The results showed that the onset of the negative or positive field potential at the quick phase was synchronous with the mean onset time of the steep depolarization or hyperpolarization of motoneurons measured in a large number of nystagmic beats. The field potential can therefore be utilized as an indicator to determine the onset time of the steep change in the membrane potential of motoneurons at the quick phase. Axons monosynaptically activated from the contralateral vestibular nerve (Fig. 2D) are presumed to originate from excitatory vestibular type I neurons. They fired invariably in phase with abducens nerve discharges on the same side during nystagmus. Tonic discharges of the axon during the slow phase were abruptly suppressed at the onset of steep positive field potential which reflected intracellular hyperpolarization of motoneurons (Fig. 2A, F). Assuming that the axon terminates on motoneurons, an abrupt decrease in excitatory effects of the axon should cause motoneuronal hyperpolarization due to disfacilitation. In fact, evidence was given in a previous study for the existence of disfacilitation in motoneurons at their quick inhibitory phase (Maeda et al., 1972).
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Fig. 1. Temporal relation between intracellular potentials of an abducens motoneuron and extracellular field potentials in the abducens nucleus at the quick phase of nystagmus. A) Simultaneous recording of intracellular (top), extracellular potentials (middle) and abducens nerve discharges (bottom) on the same side at the quick excitatory phase. B) Same arrangement as in A, but for the quick inhibitory phase. (From Hikosaka et al., 1977.)
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Fig. 2. Nystagmus-related discharge pattern of an axon monosynaptically activated from the contralateral vestibular nerve. A) Simultaneous recording of axonal spikes (a) and the field potential (b) in the abducens nucleus, and abducens nerve discharges with slow excitation followed by quick inhibition (c). All recordings were made o n the same side. B) Same as in A, but the direction of nystagmus was reversed. C-E) Responses to single shock stimulation of the ipsilateral vestibular nerve inducing positive field potential alone (C), that of the contralateral vestibular nerve evoking spikes superimposed on negative field potential (D) and that of the abducens nerve inducing antidromic field potential alone (E). F) Dot plots of spike discharges in 10 nystagmic beats during the slow excitatory phase. G) Same as in F, but for the quick excitatory phase. Vertical broken lines in F and G represent the onset time of positive or negative field potential, resp. (From Hikosaka et al., 1977.)
When the direction of nystagmus was reversed (Fig. 2B, G), the same axon was activated during the quick excitatory phase. The firing started usually later than the onset of negative field potential or was often not activated. The synaptic action of these axons presumably contributes to a part of the EPSPs produced in motoneurons at the quick phase described by Maeda et al. (1972). Axons monosynaptically activated form the ipsilateral vestibular nerve (Fig. 3C) are presumed to originate from inhibitory vestibular type I neurons. During nystagmus they fired periodically in phase with silent period of abducens nerve activity on the same side. The spike frequency usually increased gradually during the slow phase and was abruptly suppressed at the onset of steep negative deflection of the field potential which was a counterpart of intracellular depolarization (Fig. 3A, F). Given that these axons terminate on motoneurons, gradually increasing IPSPs in motoneurons during their slow inhibitory phase are attributed to synaptic action of these axons. Abrupt suppression of tonic discharges of these axons should cause disinhibition which exists in motoneurons at the quick excitatory phase (Maeda et al., 1972). A remarkable functional role of disinhibition in bringing about facilitation of motoneurons has been well known since the work of Wilson and Burgess (1962). Abrupt suppression of spike activity of inhibitory vestibular neurons
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Fig. 3. Nystagnius-related discharge pattern of an axon monosynaptically activated from the ipsilateral vestibular nerve. Same arrangement as in Fig. 2, but spikes are induced during the silent phase of abducens nerve activity. (From Hikosaka et al., 1977.)
therefore fulfils an immediate premotor function for the generation of quick excitation of motoneurons. The same axon as in Fig. 3A was activated during the quick inhibitory phase of motoneurons as well (Fig. 3B, G). Spike initiation of this axon at the quick phase tended to be slightly later than the onset of positive field potential, but they fired until the end of silent period of abducens nerve activity (more than 100 msec). The IPSPs observed in motoneurons at the quick inhibitory phase (Maeda et al., 1972) are in part attributed to the synaptic action of these vestibular neurons. NYSTAGMUS-RELATED DISCHARGE OF PREMOTOR NEURONS IN THE MEDIAL VESTIBULAR NUCLEUS In the study described above there was no evidence for the termination of nystagmus-related axons in the abducens nucleus. Moreover, the site of origin of these axons was presumed to be the vestibular nuclei because of their monosynaptic activation from the vestibular nerve, but the precise location of individual neurons in the vestibular nuclear complex could not be determined. Type I neuron In a recent study of Schor et al. (1977), extracellular unit spikes were recorded from neurons in the rostra1 part of the medial vestibular nucleus and were identified as secondary vestibular type I neurons in the horizontal canal system on the basis of their response to rotation and their monosynaptic activation following stimulation of the ipsilateral vestibular nerve. Neurons sending their axons to the contralateral abducens
473 nucleus were selected by their antidromic response to microstimulation (less than 15 PA) in the nucleus. This technique, however, could not distinguish between an axon which terminated within the abducens nucleus and a passing fiber which merely traversed the nucleus. During successive tracks through the contralateral abducens nucleus, the vestibular type I neuron could typically be activated from a variety of scattered sites within the nucleus, with intervening ineffective sites. This suggested that the axon branched extensively and terminated within the nucleus. More direct evidence for the excitatory connection of a projecting vestibular neuron to abducens motoneurons was obtained by the use of postspike averaging of abducens nerve discharges triggered from spikes of a single vestibular neuron. The validity of this technique was described elsewhere in detail (Hikosaka e t al., 1978). These identified vestibular type I neurons projecting to the contralateral abducens nucleus were found almost invariably to exhibit a nystagmic modulation of their spike activity (Fig. 4A). The discharge pattern was similar to that of presynaptic axons in the abducens nucleus shown in Fig. 2A. A similar study was performed on secondary type I neurons in the medial vestibular nucleus which projected to the ipsilateral abducens nucleus. By the aid of antidromic microstimulation technique for tracing the course of the axon and postspike averaging of abducens nerve discharges, the direct inhibitory action of these vestibular neurons on ipsilateral abducens motoneurons was confirmed. Most of the neurons thus identified showed a nystagmic modulation of firing. The discharges were in phase with the inhibitory period of ipsilateral abducens motoneurons and exhibited a similar pattern to that of presynaptic axons in the abducens nucleus shown in Fig. 3.
Type 11 neuron The nystagmic modulation of activity in the secondary vestibular neurons was caused by a periodic production of IPSPs at the onset of the quick phase (Hikosaka, Nakao and Shimazu, unpublished observation). As a candidate for inhibitory interneurons responsible for this periodic inhibition, vestibular type I1 neurons were selected for study (Schor et al., 1977), since they have been suggested as inhibitory neurons acting on type I neurons in the same nucleus (Shimazu and Precht, 1966). Type I1 neurons in the medial vestibular nucleus, characterized by their response to
0.1s u
Fig. 4. Nystagmic modulation of spike activity of horizontal type I and type I1 neurons recorded in the medial vestibular nucleus. A) Spikes of a type I neuron projecting to the contralateral abducens nucleus. B) Spikes of a type I1 neuron. Bottom record in A and B represents abducens nerve discharges on the contralateral side. (From Nakao, Schor and Shimazu, unpublished observation.)
474 horizontal rotation (increased firing with contralateral angular acceleration and decreased firing with ipsilateral acceleration), were further selected by their activation at short latencies from the contralateral labyrinth. Most of these neurons had a nystagmic rhythm, showing an abrupt increase in discharge frequency at the onset of the quick inhibitory phase of contralateral abducens motoneurons, when type I neurons were showing a quick suppression of activity (Fig. 4B). The firing pattern of vestibular type I1 neurons during nystagmus was consistent with the hypothesis that they are contributing to inhibition of type I activity at the quick phase. This was more directly confirmed by postspike averaging of the membrane potential of a type I neuron triggered from spikes of a single type I1 neuron, revealing an IPSP with a monosynaptic latency. It is therefore concluded that the vestibular type I1 neurons mediating commissural inhibition play a role in nystagmic modulation of activity observed in type I neurons which make direct connections with abducens motoneurons and contribute to the generatiQn of nystagmic rhythm of abducens nerve activity. DISCUSSION The role of the vestibular nuclei in production of the quick eye movements has long been a subject of dispute. Until very recently, many oculomotor physiologists have been concerned about the pontine reticular formation as the site of origin of quick eye movements (6.Raphan and Cohen, 1978). The term of “site of origin” of movements may need some explanation. An explicit working definition has been given by Cohen and Henn (1972) who state that by site of origin is meant the location of the immediate supranuclear neural mechanism which generates these movements. Given this definition, the site of origin (Le., the location of “immediate premotor neurons”) of quick phases of nystagmus may not necessarily be a single site, but comprise multiple structures which make direct connections with ocular motoneurons to lead to excitation or inhibition related to nystagmic rhythm of motor output. The present results have shown that immediate premotor neurons participating in generation of not only the slow phase but also the quick phase of nystagmus are located in the vestibular nuclei. Obviously, however, this does not exclude other premotor neurons responsible for the quick phases. In fact, recent studies have provided evidence that interneurons in the abducens nucleus projecting to the contralateral oculomotor nucleus are also premotor neurons causing excitation of medial rectus motoneurons during both slow and quick eye movements (Delgado-Garcia et al., 1977; Nakao and Sasaki, 1978). A group of burst neurons in the dorsomedial reticular formation caudal to the abducens nucleus, are also immediate premotor neurons to give rise to inhibition of contralateral abducens motoneurons at the quick phase (Hikosaka and Kawakami, 1977; Hikosaka et al., 1978). The paramedian pontine reticular formation has also been proposed as a site of origin of quick phases of nystagmus in the horizontal plane (Bender and Shanzer, 1964; Cohen and Henn, 1972). This hypothesis is supported by lesion and stimulation experiments. Unit activity of burst neurons recorded in this area is closely related to saccades and quick phases of nystagmus. Since anatomical studies have shown that this area projects to the ipsilateral abducens nucleus (Biittner-Ennever and Henn, 1976; Graybiel, 1977), burst neurons in this area may make direct connections with
475
abducens motoneurons. It is of importance to prove this possibility by examining specific connections between abducens motoneurons and individual burst neurons. Finally, the well coordinated timing of impulse frequency change observed in vestibular and reticular neurons at the quick phase of nystagmus may imply the existence of significant vestibuloreticular or reticulovestibular interaction. To understand the mechanism of generation of nystagmus-related pattern in various premotor neurons, it seems therefore essential to know how functionally identified neurons in these two structures are coupled with each other. SUMMARY 1. Unit spikes of axons were recorded within the abducens nucleus in the enckphale isolC cat under local anesthesia. Units were identified by the absence of antidromic activation from the abducens nerve and the presence of response to horizontal rotation and monosynaptic activation from the vestibular nerve, indicating that the units under study were spikes of axons of vestibular nucleus neurons projecting to the abducens nucleus. Axons monosynaptically activated from the contralateral vestibular nerve fired in phase with abducens nerve activity and those from the ipsilateral vestibular nerve fired during the silent period of abducens nerve activity. Abrupt suppression of their tonic discharges occurred at the quick phase coincidently with disfacilitation or disinhibition in motoneurons. 2. Type I neurons in the medial vestibular nucleus were selected by the response to horizontal rotation and monsynaptic activation from the ipsilateral labyrinth. Neurons sending their axons to the contralateral o r ipsilateral abducens nucleus were identified by their antidromic response to microstimulation in the nucleus and their excitatory or inhibitory synaptic connection with motoneurons was confirmed by postspike averaging of abducens nerve discharges triggered from spikes of a single vestibular neuron. Spikes of thus identified vestibular type I neurons showed a nystagmic modulation, similar to that of presynaptic axons in the abducens nucleus. 3. Type I1 neurons in the medial vestibular nucleus were identified by their response to horizontal rotation and their activation at short latencies from the contralateral labyrinth. Most of these neurons had a nystagmic rhythm, showing a spike burst when type I neuron activity was suppressed. 4. It is concluded that type I neurons in the medial vestibular nucleus are immediate premotor neurons which participate in generation of nystagmic rhythm in abducens motoneurons. Type I1 neurons which mediate commissural inhibition play a role in nystagmic modulation of type I neuron activity by their periodic spike burst at the quick phase. REFERENCES Baker, R. and Berthoz, A. (1974) Organization of vestibular nystagmus in oblique oculomotor system. J . Neurophysiol ., 37: 195-217. Bender, M.B. and Shanzer, S. (1964) Oculomotor pathways defined by electrical stimulation and lesions in the brainstem of monkey. In The Oculomotor System, M.B. Bender (Ed.), Harper and Row, New York, pp. 81-1 40. Biittner-Ennever, J.A. and Henn, V. (1976) An autoradiographic study of the pathways from the pontine reticular formation involved in horizontal eye movements. Brain Res., 108: 155-164.
476 Cohen, B. and Henn, V. (1972) The origin of quick phases of nystagmus in the horizontal plane. Bibl. ophthul., (Basel), 82: 36-55. Delgado-Garcia, J., Baker, R. and Highstein, S.M. (1977) The activity of internuclear neurons identified within the abducens nucleus of the alert cat. In Control of Gaze by Bruin Stem Neurons, R. Baker and A. Berthoz (Eds.), Elsevier, Amsterdam-New York, pp. 291-300. Duensing, F. and Schaefer, K.P. (1958) Die Aktivitat einzelner Neurone im Bereich der Vestibulariskerne bei Horizontalbeschleunigungen unter besonderer Beriicksichtigung des vestibularen Nystagmus. Arch. Psychiut. Nervenkr., 198: 225-252. Fuchs, A.F. and Kimm, J. (1975) Unit activity in vestibular nucleus of the alert monkey during horizontal angular acceleration and eye movement. J. Neurophyswl., 38: 1140-1 161. Graybiel, A.M. (1977) Direct and indirect preoculomotor pathways of the brainstern: an autoradiographic study of the pontine reticular formation in the cat. J. comp. Neurol., 175: 37-78. Hikosaka, 0.and Kawakami, T. (1977) Inhibitory reticular neurons related to the quick phase of vestibular nystagmus - their location and projection. Exp. Bruin Res., 27: 377-396. Hikosaka, O., Igusa, Y., Nakao, S. and Shimazu, H. (1978) Direct inhibitory synaptic linkage of pontomedullary reticular burst neurons with abducens motoneurons in the cat. Exp. Brain Res., 33: 337-352. Hikosaka, O., Maeda, M., Nakao, S., Shimazu, H. and Shinoda, Y. (1977) Presynaptic impulses in the abducens nucleus and their relation to postsynaptic potentials in motoneurons during vestibular nystagmus. Exp. Bruin Res., 27: 355-376. Norcholle, G . and TyE-Dumont, S. (1968) ActivitBs unitaires des neurones vestibulaires et oculomoteurs an cows du nystagmus. Exp. Brain Res., 5: 16-31. Keller, E.L. and Daniels, P.D. (1975) Oculomotor related interaction of vestibular and visual stimulation in vestibular nucleus cells in alert monkey. Exp. Neurol., 46: 187-198. Keller, E.L. and Kamath, B.Y. (1975) Characteristics of head rotation and eye movement-related neuron8 in alert monkey vestibular nucleus. Bruin Res., 100: 182-187. Luschei, E.S. and Fuchs, A.F. (1972) Activity of brain stem neurons during eye movements of alert monkeys. J. Neurophyswl., 35: 445461. Maeda, M., Shimazu, H. and Shinoda, Y. (1971) Rhythmic activities of secondary vestibular efferent fibers recorded within the abducens nucleus during vestibular nystagmus. Brain Res., 34: 361-365. Maeda, M., Shimazu, H. and Shinoda, Y. (1972) Nature of synaptic events in cat abducens motoneurons at slow and quick phase of vestibular nystagmus. J. Neurophysiol., 35: 279-296.. Mergner, T. and Pompeiano, 0. (1977) Neurons in the vestibular nuclei related to saccadic eye movements in the decerebrate cat. I n Control of Gaze by Brain Stem Neurons, R.Baker and A. Berthoz (Eds.), Elsevier, Amsterdam-New York, pp. 243-251. Miles, F.A. (1974) Single unit firing patterns in the vestibular nuclei related to voluntary eye movements and passive body rotation in conscious monkeys. Brain Res., 71: 215-224. Nakao, S. and Sasaki, S. (1978) Firing pattern of interneurons in the abducens nucleus related to vestibular nystagmus in the cat. Bruin Res., 144: 389-394. Raphan, T. and Cohen, B. (1978) Brainstem mechanisms for rapid and slow eye movements. Ann. Rev. Physiol., 40: 527-552. Schor, R.H., Nakao, S. and Shimazu, H. (1977) Responses of medial vestibular nucleus neurons during vestibular nystagmus. Neurosci. Abstr., 3: 545. Shimazu, H. and Precht, W. (1966) Inhibition of central vestibular neurons from the contralateral labyrinth and its mediating pathway. J. Neurophysiol., 29: 467-492. Waespe, W.,Henn, V. and Miles, T.S. (1977) Activity in the vestibular nuclei of the alert monkey during spontaneous eye movements and vestibular or optokinetic stimulation. In Control of Gaze by Bruin Stem Neurons, R.Baker, and A Berthoz (Eds.), Elsevier, Amsterdam-New York, pp. 269-278. Wilson, V.J. and Burgess, P.R. (1962) Disinhibition in the cat spinal cord. J. Neurophysiol., 25: 392404.