Ipsilateral Nucleus Research Articles

Cholera Toxin B-HRP and Wheat Germ Agglutinin-HRP Tracing of Tensor Tympani Muscle Motorneurons and Processes in Rabbits

The brain stem position, organization and number of motoneurons innervating the rabbit tensor tympani muscle (TTM) were determined by retrograde axonal transport of cholera toxin B/horseradish peroxidase conjugate (CTB-HRP) and wheat germ agglutinin HRP conjugate (WGA-HRP) tracers. The synaptic input to the TTM motoneurons was examined with WGA-HRP. The results show the motoneurons of the TTM to be localized in a cluster ventro-lateral to the outer margin of the ipsilateral trigeminal motor nucleus (VMN) and dorso-lateral to the superior olive. The number of labeled cells was greater in the combined CTB-HRP/WGA-HRP injected cases. The TTM motoneurons were triangular and elongated in shape and smaller than those of the VMN. An extensive network of dendritic branches was present ventro-laterally in the vicinity of the superior olive. Similar, but less extensive collections of dendritic processes were observed to course dorso-medially, rostrally and caudally. Axons were observed to project first dorsally or laterally, towards the trigeminal motor root, then after a sharp turn coursed ventrally within the trigeminal motor root (VMR). Transneuronal transport of the WGA-HRP was not accomplished in any preparation, suggesting among other things, system or species differences in the effectiveness of the WGA-HRP conjugate as a transynaptic tracer. It is concluded that the TTM acoustic reflex in rabbits and other mammals, its threshold, prolonged contraction capacity, and its influence on middle ear sound transmission may be related to its demonstrated extensive synaptic field in the reflex chain, particularly in the area of the superior olive, while its many other physiological functions may be made possible by the number, location, and multi-dimensional orientation of its motoneurons and dendrites.

C-fos Expression in Gracilothalamic Tract Neurons after Electrical Stimulation of the Injured Sciatic Nerve in the Adult Rat

The number of c-fos protein-like immunoreactive (Fos-LI) cells in the gracile nucleus was determined after electrical stimulation at A alpha/A beta-fiber strength of the normal and of the previously injured sciatic nerve in adult rats. No Fos-LI cells were seen after electrical stimulation of the noninjured sciatic nerve, or after sciatic nerve injury without electrical stimulation. However, stimulation 21 days after sciatic nerve transection resulted in numerous Fos-LI cells in the ipsilateral gracile nucleus. Combined Fos immunocytochemistry and retrograde labeling from the thalamus showed that the majority (76%; range = 70-80%) of the cells in the gracile nucleus that expressed Fos-LI after nerve injury projected to the thalamus. The results indicate that morphological, biochemical, and physiological alterations in primary sensory central endings and second-order neurons, which have earlier been demonstrated in the dorsal column nuclei after peripheral nerve injury, are accompanied by changes in the c-fos gene activation pattern after stimulation of the injured sciatic nerve. A substantial number of the c-fos-expressing neurons project to the thalamus.

Electrical stimulation of the subdiaphragmatic vagus in rats: inhibition of heat-evoked responses of spinal dorsal horn neurons and central substrates mediating inhibition of the nociceptive tail flick reflex

Electrical stimulation of the subdiaphragmatic branch of the vagus nerve (SDVAS) inhibits the nociceptive tail flick (TF) reflex. The present experiments examined (1) the central substrates mediating SDVAS-produced inhibition of the TF reflex and (2) the effects of SDVAS on either background or noxious heat-evoked responses of spinal dorsal horn neurons. Microinjections of ibotenic acid in the ipsilateral nucleus tractus solitarius (NTS), nucleus raphe magnus (NRM), or bilateral locus coeruleus/subcoeruleus (LC/SC) significantly increased the intensity of SDVAS required to inhibit the TF reflex in lightly anesthetized rats. In studies of class-2 spinal dorsal horn neurons, SDVAS produced significant intensity-dependent inhibition of noxious heat-evoked responses of 17 25 (68%) units, facilitation of 4 25 (16%) units, and no effect on 4 25 (16%) units. In studies of class-3 spinal dorsal horn neurons, SDVAS produced significant intensity-dependent inhibition of noxious heat-evoked responses of 8 9 (89%) units. Noxious heat-evoked responses of 1 9 (11%) unit were facilitated by SDVAS. In general, the background activity of either class-2 or class-3 units was not significantly affected by SDVAS. SDVAS produced a significant rightward, parallel shift in the stimulus response function (SRF) of class-2 neurons to noxious, graded heat stimuli ranging from 40 to 52°C, while SDVAS produced a significant increase in the threshold and a significant reduction in the slope of the SRF of class-3 neurons. These data indicate that SDVAS generally inhibits noxious heat-evoked responses of lumbosacral spinal dorsal horn neurons in the rat, but does not significantly affect background activity of the same neurons. Furthermore, the inhibition of the TF reflex produced by SDVAS depends on central relays in the ipsilateral NTS, NRM, and bilateral LC/SC.

Induction of c-jun expression in vagal motoneurones following axotomy.

Using in situ hybridization, we have measured c-jun mRNA expression in the caudal medulla oblongata of the rat following sectioning of the cervical vagus nerve. Twenty four hours after either unilateral cervical vagotomy or nodose ganglionectomy, motoneurones in the ipsilateral dorsal motor nucleus of the vagus and nucleus ambiguus expressed increased (2-3 fold) levels of c-jun mRNA. After 7 days, c-jun mRNA expression was further elevated to levels 8-10-fold higher than basal. These results demonstrate an unusual, long-lasting activation of a member of the 'leucine zipper' class of immediate early gene, and suggest that immediate early genes such as c-jun may be responsible for the regulation of the expression of structural protein and neuropeptide genes that have been demonstrated to occur following nerve injury.

A circuit for coding interaural time differences in the chick brainstem

Third-order auditory neurons in the avian nucleus laminaris (NL) are the first to receive binaural input. In the chick, NL consists of a monolayer of neurons with polarized dendritic arbors oriented dorsally and ventrally. Afferents from second-order neurons in the ipsilateral nucleus magnocellularis (NM) innervate the dorsal dendrites of NL neurons, distributing processes of approximately equal length to NL neurons along an isofrequency band (roughly caudomedial to rostrolateral). Afferents from the contralateral NM innervate the ventral dendrites of NL neurons, distributing collateral branches sequentially as they proceed from caudomedial to rostrolateral along the isofrequency band of neurons. This innervation pattern could be the basis of a "delay line" circuit, as postulated in models of neural networks mediating sound localization. We examined this circuit by analyzing evoked field potentials using a brain slice preparation containing both NL and NM. The results were consistent with the previous anatomical findings. When the ipsilateral auditory nerve or ipsilateral NM was stimulated, there was no consistent variation in the latency of postsynaptic field potentials across the medial-to-lateral extent of NL. In contrast, when the contralateral NM or NM axons in the crossed dorsal cochlear tract were stimulated, a linear increase in the latency of postsynaptic potentials was observed from medial to lateral positions in NL. When stimulation amplitudes for both the ipsilateral and contralateral inputs were adjusted so as to produce little or no postsynaptic field potential, simultaneous bilateral stimulation evoked a pronounced response. Thus, NL neurons can act as "coincidence detectors." The amplitude of the postsynaptic response was dependent on the relative timing of stimulation of the two inputs. The optimal time difference changed systematically across the medial-to-lateral extent of NL. This system of delay lines and coincidence detectors could provide a mechanism for converting interaural time differences into a "place map" within NL.

Organizational and morphological features of cat sacrocaudal motoneurons.

We have investigated the organizational and morphological features of motoneurons from cat sacrocaudal spinal cord, the portion of the neuraxis that innervates the tail. This information is pertinent for development of a new model of spinal cord injury. An understanding of sacrocaudal circuitry is essential for physiological and behavioral assessment of the effects of sacrocaudal lesions. Observations from Nissl-stained sections corroborated Rexed's cytoarchitectural scheme. Putative motoneurons were located within two regions of the ventral horn: the ventromedial nucleus (lamina IX) and the nucleus commissuralis. To map motoneuron pools, cholera toxin-horseradish peroxidase conjugate was injected into each dorsal tail muscle. The dorsomedial muscle was innervated by ipsilateral nucleus commissuralis motoneurons. The dorsolateral and intertransversarius muscles were innervated by ipsilateral lamina IX and nucleus commissuralis motoneurons. Cell bodies of retrogradely labeled sacrocaudal motoneurons ranged from 22 to 82 microns in diameter; the unimodal distributions peaked between 45 and 50 microns. Dendritic trees of motoneurons, revealed by retrograde labeling or by intracellular injection with horseradish peroxidase, were extensive. Five to eight primary dendrites originated from the cell body. Dendritic branches extended throughout the ipsilateral ventral gray matter, with processes spreading into the surrounding white matter and the base of the dorsal horn. Dendrites from motoneurons with their soma in the lateral portion of lamina IX formed a longitudinal plexus at the gray/white border. Medial dendrites from motoneurons in the nucleus commissuralis formed bundles in the ventral gray commissure and spread throughout the contralateral ventral horn. It is speculated that contralateral dendrites subserve synchronized co-contraction of medial muscles from both sides of the tail.

Physiological Characterization of Pretectal Neurons Projecting to the Láteral Geniculate Nucleus in the Cat.

Single neurons in the pretectal nucleus of the optic tract and posterior pretectal nucleus were extracellularly recorded in anaesthetized cats and tested for antidromic activation after electrical stimulation of the ipsilateral dorsal lateral geniculate nucleus. Cells were further characterized by their response latencies to electrical stimulation of the optic nerve head and the optic chiasm, and by responses to various visual stimuli. 46 out of 188 neurons (24%) were antidromically activated from the lateral geniculate nucleus at response latencies of 0.6 - 2.6 ms. They had low spontaneous activities and preferred fast-moving visual stimuli. 29 of the antidromically activated neurons (63%) could be activated from the optic chiasm with response latencies of 4 - 10 ms. Together with the mean conduction time of 0.8 ms between the optic nerve head and the optic chiasm, this indicates that they receive an indirect retinal input via fast-conducting Y-fibres. Sometimes antidromically activated neurons spontaneously showed irregular burst activity. During unidirectional stimulation with a large moving visual stimulus, burst activity became more regular, and interburst intervals and the duration of single bursts decreased. After the stimulus was stopped, interburst intervals slowly increased until prestimulation activity was restored. The response properties of these neurons could reflect the transfer of saccade-related visual as well as oculomotor signals through the pretectum to the visual thalamus.

Synaptic connections of central carotid sinus afferents in the nucleus of the tractus solitarius of the rat. I. An electron microscopic study.

A transganglionic transport technique was used to study the synaptic connections of the central carotid sinus afferents in the nucleus of the tractus solitarius of the rat by electron microscopy. The caudal part of the nucleus was profusely innervated. Labelled fibres extended to the contralateral nucleus, and to the ipsilateral dorsal motor nucleus of the vagus nerve, nucleus ambiguus, spinal nucleus of the trigeminal nerve and the area postrema. The labelled terminals were densely packed with clear, predominantly spherical vesicles about 50 nm in diameter and a few often swollen mitochondria. The terminals synapsed on dendrites of various calibres, spindle- or pear-shaped somal profiles with short axes lesser than 8 microns, and axon terminals. In axo-axonal synapses, most labelled terminals appeared to be presynaptic. Frequently, profiles of labelled terminals were in direct apposition with one another. The latter may represent the morphological substrate of the interaction between baro- and chemoreceptor inputs in the nucleus of the tractus solitarius and warrants further study. The present results indicate that in addition to direct inputs, the carotid sinus afferents are able to influence second-order neurons in the nucleus of the tractus solitarius indirectly through presynaptic modulation.

A second auditory area in the non-cortical telencephalon of a reptile

Injections of horseradish peroxidase (HRP) into a caudocentral portion of the non-cortical telencephalon of Caiman known as the dorsolateral area (dorsal ventricular ridge) resulted in retrogradely labeled neurons throughout the entire extent of the ipsilateral nucleus reuniens. HRP-positive cells were most numerous in nucleus reuniens pars diffusa with only sparse labeling of neurons in nucleus reuniens pars centralis. The results of the present experiment, when compared with those of a prior study that determined the telencephalic connections of nucleus reuniens pars centralis, suggested that those two forebrain areas are separate. Staining with succinate dehydrogenase and acetylcholinesterase revealed that nucleus reuniens pars centralis and pars diffusa and their respective telencephalic projection areas can be differentiated on the basis of histochemical features. These findings in Caiman suggest that certain thalamic and telencephalic auditory areas in birds and crocodilians are most likely the result of common ancestry rather than examples of parallel evolution.

Anatomical evidence for an intergeniculate leaflet in Rana pipiens

The projections of the nucleus of Bellonci and the anterior thalamic nucleus in Rana pipiens appear to be remarkably similar to those that have been described for the mammalian intergeniculate leaflet. The connections of these nuclei were examined using both the anterograde and retrograde transport of horseradish peroxidase. Afferents to the neuropil of Bellonci and its nucleus include bilateral projections from the retina, the contralateral nucleus of Bellonci, and anterior thalamic nucleus as well as bilateral projections from the pretectum and the ipsilateral suprachiasmatic nucleus. Efferent projections observed following HRP injections in the anterior thalamus consist of three components: (1) a ventral hypothalamic-suprachiasmatic and commissural projection, (2) a dorsal descending tract to the pretectum and tectum, and (3) a ventral descending tract to the somatomotor brainstem.

Cisplatin-evoked induction of c- fos protein in the brainstem of the ferret: the effect of cervical vagotomy and the anti-emetic 5-HT 3 receptor antagonist granisetron (BRL 43694)

We have monitored the expression of c- fos protein in the medulla oblongata of the ferret, using immunocytochemistry, to identify the brainstem pathways involved in the mediation of nausea and vomiting caused by the antineoplastic drug cisplatin. Cisplatin administration resulted in c- fos-like immunoreactivity (FLI) in the area postrema, the nucleus of the solitary tract, and in scattered cells within the ependymal lining of the fourth ventricle. Unilateral cervical vagotomy greatly reduced FLI in the ipsilateral nucleus of the solitary tract but did not significantly affect reactivity in the contralateral solitary tract nucleus or in the area postrema. Pretreatment of the animals with the 5-HT 3 antagonist granisetron (BRL 43694) abolished the retching and vomiting caused by cisplatin and markedly reduced the cisplatin-evoked FLI in the nucleus of the solitary tract: treatment with this drug had no significant effect on cisplatin-evoked FLI in the area postrema. The results suggest that cisplatin induces c- fos gene expression in the nucleus of the solitary tract by an action involving vagal afferent pathways and also by a vagally independent, direct action on the area postrema. The anti-emetic 5-HT 3 antagonist drug granisetron mimicked the effect of vagotomy on c- fos protein induction suggesting that it may act via 5-HT 3 receptors known to be associated with vagal afferent terminals. The FLI seen in the area postrema was neither vagally dependent nor was it abolished by granisectron. FLI in the area postrema may reflect some vagus nerve activity but also a direct action of cisplatin on the ‘chemoreceptor trigger zone for vomiting’ which is considered to be in the area postrema, and may represent a central site of action for cisplatin-induced nausea and vomiting. On the other hand the observation that ependymal cells also show FLI following cisplatin administration raises the possibility that some c- fos protein induction in the area postrema arises from an action of cisplatin, such as DNA damage, which may not be directly associated with its emetic properties.

Induction of tyrosine hydroxylase in the rat substantia nigra by local injection of forskolin

Forskolin (FSK) was locally injected into the substantia nigra (SN) of anesthetised rats. The day after injection (24 and 36 hr), tyrosine hydroxylase (TH) activity increased locally in this structure but remained unmodified in the ipsilateral caudate nucleus (CN). The amount of messenger RNA for TH (TH-mRNA) was also increased in the SN 24 hr after the injection. However, TH protein content was modified neither locally in the SN nor in the ipsilateral CN. In addition, the decrease of the ratio between dopamine and its first metabolite in the CN and the SN suggested a decreased activity of the dopaminergic nigral cells. The absence of increase of the protein synthesis in spite of the fact that TH-gene transcription was initiated could be the consequence of the inhibition of dopaminergic cells by the drug. These results confirm that, in vivo, TH induction is cAMP-dependent and demonstrate that the TH-gene activity is not strictly coupled to the activity of dopaminergic cells in the SN.

Nigropedunculopontine projection in the rat: An Anterograde tracing study with phaseolus vulgaris‐leucoagglutinin (PHA‐L)

The termination of the substantia nigra pars reticulata efferents in the nucleus tegmenti pedunculopontinus was studied in the rat by using the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L). Both large and small injections of PHA-L in various portions of the substantia nigra pars reticulata labeled varicose fibers in the ipsilateral and contralateral nucleus tegmenti pedunculopontinus, subnucleus dissipatus as well as in the ipsilateral nucleus tegmenti pedunculopontinus, subnucleus compactus. However, the bulk of the nigral fibers appeared to terminate in the medial two-thirds of the ipsilateral subnucleus dissipatus of the pedunculopontine nucleus and exhibited a discrete dorsoventral topographical pattern. The terminal plexus displayed patches of uneven density, which was partly due to the numerous fiber bundles passing through the pedunculopontine nucleus, but also to an obvious preference of nigral fibers for some cells. Electron microscopic examination confirmed that nearly all of the varicosities observed in the light microscope contained synaptic vesicles and represented either terminal boutons or boutons en passant. The labeled boutons were elongated (average length: 1.5 microns) and consistently contained a prominent group of mitochondria. The results suggest that the nigral input to the nucleus tegmenti pedunculopontinus may be directed toward specific subpopulation(s) of pedunculopontine neurons and may influence not only cells in the subnucleus dissipatus, but also in the subnucleus compactus.

Labeling of human retinohypothalamic tract with the carbocyanine dye, DiI

The carbocyanine dye, DiI, was used to demonstrate human retinohypothalamic tract (RHT) projections in 6 normal human postmortem brains. In 5 of 6 brains, labeling was seen extending from the site of implantation in the distal optic nerve to both the ipsilateral and contralateral suprachiasmatic nuclei. This study confirms the presence of RHT projections in humans, and demonstrates the usefulness of DiI for neuronal tracing in human postmortem tissue.

Reduction of [ 125I]Bolton Hunter CCK8 and [ 3H]MK-329 (devazepide) binding to CCK receptors in the substantia nigra/VTA complex and its forebrain projection areas following MPTP-induced hemi-parkinsonism in the monkey

Cholecystokinin (CCK) receptors were visualized autoradiographically using [ 125I]Bolton Hunter CCK8 ([ 125I]BHCCK8) in the fore- and midbrain of 3 monkeys rendered hemi-parkinsonian by unilateral intra-carotid infusion of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). More specifically, CCK-A receptors were detected using [ 3H]MK-329 (devazepide), a peripheral-type (CCK-A) receptor antagonist. In the substantia nigra pars compacta, ipsilateral to the toxin infusion, where dopamine D 2 receptors (labelled with [ 3H]sulpiride) were lost, there was a decrease in the binding of both [ 125I]BHCCK8 and [ 3H]MK-329. Binding of the two CCK ligands was also reduced in the ipsilateral nucleus accumbens and most medial part of the caudate nucleus, whereas 3H-sulpiride binding was increased in the lateral caudate nucleus and putamen. These results indicate that CCK-A receptors may be located on dopaminergic cells within the substantia nigra, which are lost in the parkinsonian brain, and may also be present on dopaminergic terminals within restricted regions of nigral/ventral tegmental area projection sites.

Collateral sprouting of serotonergic fibers in the cingulate cortex and the septum following cortical-hippocampal lesions

The parietal cortex and dorsal hippocampus of adult rats were unilaterally ablated. One and 3 months after this operation, changes in serotonergic fiber distribution in the forebrain were studied immunohistochemically. At 1 month, increased numbers of serotonergic fibers were seen in the cingulate cortex and the medial and lateral septal nuclei of the lesioned side. This increase continued to 3 months in the ipsilateral cingulate cortex. In the present study, two different processes of serotonergic fiber collateral sprouting were noted. Plastic changes in serotonergic fibers in the ipsilateral septal nuclei and cingulate cortex may be attributable to the collateral sprouting underlying reactive synaptogenesis and the pruning effect, respectively.

Anticonvulsant effects of ipsilateral but not contralateral microinjections of the dopamine D2 agonist LY 171555 into the nucleus accumbens of amygdala-kindled rats

Recent radioligand binding studies demonstrated an increase in the density of dopamine D2 receptors in the nucleus accumbens ipsilateral to the stimulating electrode in amygdala- or hippocampal-kindled rats. In the present study we examined the anticonvulsant effect of dopamine agonists by unilateral microinjections into the nucleus accumbens in rats kindled from the right basolateral amygdaloid nucleus. Microinjections of the D2 agonist LY 171555 into the ipsilateral nucleus accumbens 15 min prior to the kindling stimulation in fully kindled rats decreased significantly kindling parameters such as seizure severity, seizure duration and afterdischarge duration, whereas the D1 agonist SKF 38393 had no anticonvulsant effects. After ipsilateral microinjection of 40 pmol LY 171555, focal and generalized kindled seizures were totally blocked in almost 50% of the rats. The anticonvulsant effect of LY 171555 could be completely antagonised by systemic administration of the D2 antagonist sulpiride. Microinjection of the D1 or D2 agonist into the nucleus accumbens contralateral to the stimulating electrode had no anticonvulsant effects. In accordance with other reports our data indicate a possible topographic limitation of D2 receptor mediated anticonvulsant effects to specific regions of the basal ganglia.

Catecholamine Metabolism in Locus Coeruleus Neurons: A Study of its Activation by Sciatic Nerve Stimulation in the Rat.

Sciatic nerve stimulation, which strongly activates noradrenergic locus coeruleus (NA-LC) neurons, was used in anaesthetized rats as a model to study the transneuronal control of catechol metabolism in this nucleus. We show, using in vivo electrochemistry and biochemical post-mortem assays, that a prolonged (20 min) unilateral sciatic nerve electrical stimulation led to a reversible enhancement (80 - 130%) of both endogenous and in vivo extracellular levels of 3,4-dihydroxyphenylacetic acid (DOPAC) within the contralateral LC region. An elevation in DOPAC levels was also observed in the ipsilateral nucleus but was always significantly lower. The response was abolished by a pretreatment with kynurenic acid, a non-selective excitatory amino acid (EAA) antagonist known to block footshock-induced excitations of NA-LC neurons: in antagonist-treated rats, the stimulation induced a non-significant effect (+ 30%) on endogenous DOPAC levels, which contrasted with the highly significant effect (+ 113%) observed in vehicle-treated animals. As the major source of EAA afferents to the LC originates in the nucleus paragigantocellularis, we made an attempt to suppress activation by a section of these fibres. An incision performed obliquely (45 degrees ) between LC and PGi greatly and significantly attenuated, but did not totally suppress, the increase in DOPAC endogenous content due to the stimulation. These experiments indicate that a peripheral stimulus provokes an activation of catecholamine metabolism within the soma - dendritic region of the NA-LC cells. They suggest that this effect may be mediated, at least in part, by afferent pathways originating from the medulla which utilize an EAA as transmitter.

Plastic synaptic reorganization in the sensorimotor cortex of adult cats after lesioning of the contralateral cerebellar nucleus interpositus

Intact cats and animals undergoing lesioning of the contralateral cerebellar nucleus interpositus between one and six months previously were used in this research, employing intracellular recording techniques and investigating the response of corticospinal neurons (CSN) to stimulating the aforementioned nucleus, the ipsilateral cerebellar nucleus interpositus, and the ventrolateral thalamic nucleus. A reduction was found in the stage of rise to peak in monosynaptic thalamocortical EPSP in CSN of operated animals, with a low axonal conduction velocity, pointing to distant terminal dendrosomatic sprouting and formation of new synapses at proximal sections of the CSN somatodendritic membrane. Findings are presented on formation of ipsilateral interpositothalamocortical projections duplicating similar contralateral projections in intact animals. Contralateral cortico-interposital collaterals were found in intact animals and similar sprouting of ipsilateral origin in those which had undergone surgery.

The Mechanism of Irritative Nystagmus and Paralytic Nystagmus:A Histochemical Study of the Guinea Pig's Vestibular Organ and an Autoradiographic Study of the Vestibular Nuclei

To establish the difference of mechanism between irritative and paralytic nystagmus, alterations of Na-K-ATPase and succinic dehydrogenase activity in the vestibular sensorineural elements were investigated for 20 guinea pigs, and glucose uptake of the vestibular nuclei for 13 guinea pigs were measured by the [14C]-2-deoxy-D-glucose method. Irritative and paralytic nystagmus were experimentally provoked by introducing K+ into the perilymphatic space. From the results it was concluded that irritative nystagmus is provoked by increased excitability of vestibular sensory cells, while paralytic nystagmus is provoked by decreased excitability. However, the direction of nystagmus was eventually decided by the tonus imbalance between the bilateral vestibular nuclei. The ipsilateral vestibular nucleus was predominant during irritative nystagmus, while the contralateral vestibular nucleus was predominant during paralytic nystagmus.