The interstitial nucleus of Cajal in the midbrain reticular formation and vertical eye movement

Noradrenergic current responses of neurons in rat oculomotor neural integrators

The prepositus hypoglossi nucleus (PHN) and the interstitial nucleus of Cajal (INC) are involved in the control of horizontal and vertical gaze, respectively. A previous study showed that PHN neurons exhibit depolarized or hyperpolarized responses to noradrenaline (NA). However, the adrenoceptor types that participate in NA-induced responses and the effects of NA on INC neurons have not yet been investigated. Furthermore, the relationship between NA-induced responses and neuron types defined by neurotransmitter phenotypes has not been determined. In this study, we investigated NA-induced current responses in PHN and INC neurons and the relationships between these responses and neuron types using whole cell recordings in wild-type and transgenic rat brainstem slices. Local application of NA to the cell soma induced slow inward (SI) and slow outward (SO) currents that were mainly mediated by α1 and α2 adrenoceptors, respectively. These current responses were observed in both PHN and INC neurons, although the proportion of INC neurons that responded to NA was low. Analyses of the distributions of the current responses revealed that in the PHN, all fluorescently identified inhibitory neurons exhibited SI currents, whereas glutamatergic and cholinergic neurons exhibited both SI and SO currents. In the INC, glutamatergic and inhibitory neurons preferentially exhibited SI and SO currents, respectively. When the PHN and INC neurons were characterized by their firing pattern, we found that the proportions of the currents depended on their firing pattern. These results suggest that various modes of noradrenergic modulation in horizontal and vertical neural integrators are dependent on neuron type.NEW & NOTEWORTHY Noradrenergic modulation of oculomotor neural integrators involved in gaze control has not been elucidated. Here, we report that noradrenaline (NA)-induced slow inward (SI) and outward (SO) currents are mediated mainly by α1 and α2 adrenoceptors in neurons that participate in horizontal and vertical gaze control. The NA-induced current responses differed depending on the neurotransmitter phenotype and firing pattern. These results suggest various modes of noradrenergic modulation in horizontal and vertical integrator neurons.

Brainstem Neural Circuits Triggering Vertical Saccades and Fixation

Neurons in the nucleus raphe interpositus have tonic activity that suppresses saccadic burst neurons (BNs) during eye fixations, and that is inhibited before and during saccades in all directions (omnipause neurons, OPNs). We have previously demonstrated via intracellular recording and anatomical staining in anesthetized cats of both sexes that OPNs are inhibited by BNs in the medullary reticular formation (horizontal inhibitory BNs, IBNs). These horizontal IBNs receive monosynaptic input from the caudal horizontal saccade area of the superior colliculus (SC), and then produce monosynaptic inhibition in OPNs, providing a mechanism to trigger saccades. However, it is well known that the neural circuits driving horizontal components of saccades are independent from the circuits driving vertical components. Thus, our previous results are unable to explain how purely vertical saccades are triggered. Here, we again apply intracellular recording to show that a disynaptic vertical IBN circuit exists, analogous to the horizontal circuit. Specifically, we show that stimulation of the SC rostral vertical saccade area produces disynaptic inhibition in OPNs, which is not abolished by midline section between the horizontal IBNs. This excludes the possibility that horizontal IBNs could be responsible for the OPN inhibition during vertical saccades. We then show that vertical IBNs in the interstitial nucleus of Cajal, which receive monosynaptic input from rostral SC, are responsible for the disynaptic inhibition of OPNs. These results indicate that a similarly functioning SC-IBN-OPN circuit exists for both the horizontal and vertical oculomotor pathways. These two IBN-mediated circuits are capable of triggering saccades in any direction.Significance Statement Saccades shift gaze to objects of interest, moving their image to the central retina, where it is maintained for detailed examination (fixation). During fixation, high gain saccade burst neurons (BNs) are tonically inhibited by omnipause neurons (OPNs). Our previous study showed that medullary horizontal inhibitory BNs (IBNs) activated from the caudal superior colliculus (SC) inhibit tonically active OPNs in order to initiate horizontal saccades. The present study addresses the source of OPN inhibition for vertical saccades. We find that OPNs monosynaptically inhibit vertical IBNs in the interstitial nucleus of Cajal during fixation. Those same vertical IBNs are activated by the rostral SC, and inhibit OPN activity to initiate vertical saccades.

Serotonergic current responses of neurons in rat oculomotor neural integrators

The prepositus hypoglossi nucleus (PHN) and the interstitial nucleus of Cajal (INC) are involved in controlling horizontal and vertical gaze, respectively. Previous studies have shown that PHN neurons exhibit depolarized or hyperpolarized responses to serotonin (5-hydroxytryptamine, 5-HT). However, serotonergic modulation of INC neurons has not been examined. Furthermore, the relationship between 5-HT-induced responses and neuron types based on neurotransmitter phenotypes has not been clarified. In this study, we investigated 5-HT-induced current responses in PHN and INC neurons and the distributions of distinct current responses in different neuron types, using whole cell recordings of wild-type and transgenic rat brain stem slices. Local application of 5-HT to the cell soma confirmed that slow inward (SI) and slow outward (SO) currents were mediated by 5-HT₂ and 5-HT_1A receptors, respectively. Furthermore, fast inward (FI) currents that were mediated by 5-HT₃ receptors were observed. These three current responses were observed in both PHN and INC neurons. Analyses of the distributions of the three current responses revealed that fluorescently identified glutamatergic and inhibitory neurons in the PHN showed high proportions of SI and SO currents, respectively, whereas glutamatergic and inhibitory neurons in the INC showed mainly SO currents. When PHN and INC neurons were characterized on the basis of firing patterns, the proportions of the currents depended on the firing patterns. The different distributions of 5-HT-induced currents suggest distinct serotonergic modulation modes specific to horizontal and vertical gaze control.NEW & NOTEWORTHY Serotonergic modulation of vertical gaze control (interstitial nucleus of Cajal, INC) is less understood than that of horizontal gaze control (prepositus hypoglossal nucleus, PHN). Here, we report 5-HT-induced fast inward currents in addition to the previously reported slow inward and outward currents. The distributions of these currents in INC neurons based on neurotransmitter phenotypes differ from those in PHN neurons. These results suggest distinct serotonergic modulation modes in horizontal and vertical gaze control centers.

Microstimulation of Interstitial Nucleus of Cajal Evokes Directionally Disconjugate Eye Movements in Monkeys With Pattern Strabismus

Purpose

Pattern strabismus is characterized by a horizontal misalignment of the eyes that varies with vertical eye position. This disorder has traditionally been described, and treated, as overaction or underaction of the oblique muscles. In recent years, evidence has accumulated that indicate that the disorder is associated with abnormal cross-talk between brainstem pathways that contribute to the horizontal and vertical components of eye movements. The present study was designed to investigate the hypothesis that the key abnormalities are at the level of, or downstream from, the interstitial nucleus of Cajal (INC).

Methods

Microstimulation was applied to the INC in two mature rhesus monkeys with "A" pattern strabismus that was experimentally induced in infancy. We asked whether the evoked movements would be vertical and conjugate, as has been previously reported in normal monkeys, or would be directionally disconjugate (i.e. with oblique or horizontal movement observed for at least one eye).

Results

Evoked movements were conjugate and vertical for a minority of sites but, for most sites, the evoked movements were directionally disconjugate. Moreover, there was typically a convergent change in horizontal strabismus when the evoked movements were upward and a divergent change when the evoked movements were downward.

Conclusions

Microstimulation of INC in monkeys with A-pattern strabismus evokes movements with the expected directional disconjugacies, implying that the key neural abnormalities are within, or downstream from, this structure. High site-to-site variability in the conjugacy/disconjugacy of evoked movements rules out the hypothesis that the abnormalities are solely peripheral.

Conserved subcortical processing in visuo-vestibular gaze control

Gaze stabilization compensates for movements of the head or external environment to minimize image blurring. Multisensory information stabilizes the scene on the retina via the vestibulo-ocular (VOR) and optokinetic (OKR) reflexes. While the organization of neuronal circuits underlying VOR is well-described across vertebrates, less is known about the contribution and evolution of the OKR and the basic structures allowing visuo-vestibular integration. To analyze these neuronal pathways underlying visuo-vestibular integration, we developed a setup using a lamprey eye-brain-labyrinth preparation, which allowed coordinating electrophysiological recordings, vestibular stimulation with a moving platform, and visual stimulation via screens. Lampreys exhibit robust visuo-vestibular integration, with optokinetic information processed in the pretectum that can be downregulated from tectum. Visual and vestibular inputs are integrated at several subcortical levels. Additionally, saccades are present in the form of nystagmus. Thus, all basic components of the visuo-vestibular control of gaze were present already at the dawn of vertebrate evolution.

Distinct purinergic receptor-mediated currents of rat oculomotor integrator neurons characterized by different firing patterns

The prepositus hypoglossi nucleus (PHN) and the interstitial nucleus of Cajal (INC) are oculomotor neural integrators involved in the control of horizontal and vertical gaze, respectively. We previously reported that local application of adenosine 5'-trisphosphate (ATP) to PHN neurons induced P2X receptor-mediated fast inward currents, P2Y receptor-mediated slow inward currents, and/or adenosine P1 receptor-mediated slow outward currents. In contrast to the findings on PHN neurons, the expression of functional purinergic receptors in INC neurons has not been examined. In this study, we investigated ATP-induced current responses in INC neurons and the distributions of the three current types across distinct firing patterns in PHN and INC neurons using whole cell recordings of rat brainstem slices. The application of ATP induced all three current types in INC neurons. Pharmacological analyses indicated that the fast inward and slow outward currents were mainly mediated by the P2X and P1 subtypes, respectively, corresponding to the receptor subtypes in PHN neurons. However, agonists of the P2Y subtype did not induce the slow inward current in INC neurons, suggesting that other subtypes or mechanisms are responsible for this current. Analysis of the distribution of the three current types in PHN and INC neurons revealed that the proportions of the currents were distinctly dependent on the firing patterns of PHN neurons whereas the proportion of the fast inward current was higher during all firing patterns of INC neurons. The different distributions of ATP-induced currents suggest distinct modes of purinergic modulation specific to horizontal and vertical integrators.NEW & NOTEWORTHY The roles of purinergic signaling on vertical (mediated by the interstitial nucleus of Cajal; INC) and horizontal (prepositus hypoglossal nucleus; PHN) gaze control are not understood. Here, we report three current types induced by ATP in INC neurons; the distribution of these current types across different types of INC neurons is different from that in PHN neurons. These results suggest distinct modes of purinergic modulation in horizontal and vertical gaze control centers.

Different Activation Mechanisms of Excitatory Networks in the Rat Oculomotor Integrators for Vertical and Horizontal Gaze Holding

Gaze holding in the horizontal and vertical directions is separately controlled via the oculomotor neural integrators, the prepositus hypoglossi nucleus (PHN) and the interstitial nucleus of Cajal (INC), respectively. Our previous in vitro studies demonstrated that transient, high-frequency local stimulation of the PHN and the INC increased the frequency of spontaneous EPSCs that lasted for several seconds. The sustained EPSC response of PHN neurons was attributed to the activation of local excitatory networks primarily mediated via Ca²⁺-permeable AMPA (CP-AMPA) receptors and Ca²⁺-activated nonselective cation (CAN) channels. However, the contribution of CP-AMPA receptors to the activation of INC excitatory networks appeared to be small. In this study, we clarified the mechanisms of excitatory network activation in the PHN and INC using whole-cell recordings in rat brainstem slices. Although physiological and histological analyses showed that neurons that expressed CP-AMPA receptors existed not only in the PHN but also in the INC, the effect of a CP-AMPA receptor antagonist on the sustained EPSC response was significantly weaker in INC neurons than in PHN neurons. Meanwhile, the effect of an NMDA receptor antagonist on the sustained EPSC response was significantly stronger in INC neurons than in PHN neurons. Furthermore, the current and the charge transfer mediated via NMDA receptors were significantly larger in INC neurons than in PHN neurons. These results strongly suggest that these excitatory networks are activated via different synaptic mechanisms: a CP-AMPA receptor and CAN channel-dependent mechanism and an NMDA receptor-dependent mechanism in horizontal and vertical integrators, respectively.

Abnormal Eye Position Signals in Interstitial Nucleus of Cajal in Monkeys With "A" Pattern Strabismus

Purpose

Pattern strabismus is characterized by a cross-axis pattern of horizontal and vertical misalignments. In A-pattern strabismus, for example, a divergent change in the horizontal misalignment occurs on downgaze. Work with nonhuman primate models has provided evidence that this disorder is associated with abnormal cross-talk between brainstem pathways that normally encode horizontal and vertical eye position and velocity. Neurons in the interstitial nucleus of Cajal (INC) are normally sensitive to vertical eye position; in the present study, we test the hypothesis that, in monkeys with pattern strabismus, some INC neurons will show an abnormal sensitivity to horizontal eye position.

Methods

Monkeys were rewarded for fixating a visual target that stepped to various locations on a tangent screen. Single neurons were recorded from INC in one normal monkey, and two with A-pattern strabismus. Multiple linear regression analysis was used to estimate the preferred direction for each neuron.

Results

In the normal monkey, all INC neurons had preferred directions within 20° of pure vertical (either up or down). The preferred directions were significantly more variable in the monkeys with pattern strabismus, with a minority being more sensitive to horizontal eye position than vertical eye position. In addition, the vertical eye position sensitivity was significantly less in the monkeys with strabismus.

Conclusions

In pattern strabismus, neurons in INC show neurophysiological abnormalities consistent with a failure to develop normal tuning properties. Results were consistent with the hypothesis that, in pattern strabismus, INC receives an abnormally strong signal related to horizontal eye position.

Differences in eye movement range based on age and gaze direction

PURPOSE

To determine the range of eye movement in normal human subjects and to investigate the effect of age and gaze direction on eye movement.

PATIENTS AND METHODS

A prospective observational study. We enrolled 261 healthy subjects, 5-91 years of age. Photographs were obtained in the cardinal gaze positions and processed using Photoshop. The processed images were analyzed using the Image J program to measure the angle of eye movement. The angle of eye movement was quantified using a modified limbus test. We measured the angle of eye movement in adduction, abduction, elevation, and depression.

RESULTS

The ranges of eye movement were 44.9 ± 7.2° in adduction, 44.2 ± 6.8° in adduction, 27.9 ± 7.6° in elevation, and 47.1 ± 8.0° in depression. The ranges of eye movement in the younger group were higher than that in the older group in adduction, abduction, and elevation (P < 0.001, P = 0.013, and P < 0.001, respectively), except in depression (P = 0.790). There were significant negative correlations between the angles of horizontal and upward gazes and age (R = -0.294 in adduction, R = -0.355 in abduction, and R = -0.506 in elevation, all P < 0.001). However, the angle of downward gaze was not significantly correlated with age (R = 0.017, P = 0.722).

CONCLUSIONS

The angle of upward gaze most rapidly decreased with age than the angle of other gaze. Unlike the age-related decline of range in horizontal and upward gazes, only downward gaze was not impaired by increasing age. Differences in eye movement range based on gaze direction and their associated aging mechanisms should be considered when assessing eye movements.

Corollary discharge in precerebellar nuclei of sleeping infant rats

In week-old rats, somatosensory input arises predominantly from external stimuli or from sensory feedback (reafference) associated with myoclonic twitches during active sleep. A previous study suggested that the brainstem motor structures that produce twitches also send motor copies (or corollary discharge, CD) to the cerebellum. We tested this possibility by recording from two precerebellar nuclei—the inferior olive (IO) and lateral reticular nucleus (LRN). In most IO and LRN neurons, twitch-related activity peaked sharply around twitch onset, consistent with CD. Next, we identified twitch-production areas in the midbrain that project independently to the IO and LRN. Finally, we blocked calcium-activated slow potassium (SK) channels in the IO to explain how broadly tuned brainstem motor signals can be transformed into precise CD signals. We conclude that the precerebellar nuclei convey a diversity of sleep-related neural activity to the developing cerebellum to enable processing of convergent input from CD and reafferent signals.

Whole-Brain Calcium Imaging during Physiological Vestibular Stimulation in Larval Zebrafish

The vestibular apparatus provides animals with postural and movement-related information that is essential to adequately execute numerous sensorimotor tasks. In order to activate this sensory system in a physiological manner, one needs to macroscopically rotate or translate the animal's head, which in turn renders simultaneous neural recordings highly challenging. Here we report on a novel miniaturized, light-sheet microscope that can be dynamically co-rotated with a head-restrained zebrafish larva, enabling controlled vestibular stimulation. The mechanical rigidity of the microscope allows one to perform whole-brain functional imaging with state-of-the-art resolution and signal-to-noise ratio while imposing up to 25° in angular position and 6,000°/s² in rotational acceleration. We illustrate the potential of this novel setup by producing the first whole-brain response maps to sinusoidal and stepwise vestibular stimulation. The responsive population spans multiple brain areas and displays bilateral symmetry, and its organization is highly stereotypic across individuals. Using Fourier and regression analysis, we identified three major functional clusters that exhibit well-defined phasic and tonic response patterns to vestibular stimulation. Our rotatable light-sheet microscope provides a unique tool for systematically studying vestibular processing in the vertebrate brain and extends the potential of virtual-reality systems to explore complex multisensory and motor integration during simulated 3D navigation.

Brain Stem Neural Circuits of Horizontal and Vertical Saccade Systems and their Frame of Reference

Sensory signals for eye movements (visual and vestibular) are initially coded in different frames of reference but finally translated into common coordinates, and share the same final common pathway, namely the same population of extraocular motoneurons. From clinical studies in humans and lesion studies in animals, it is generally accepted that voluntary saccadic eye movements are organized in horizontal and vertical Cartesian coordinates. However, this issue is not settled yet, because neural circuits for vertical saccades remain unidentified. We recently determined brainstem neural circuits from the superior colliculus to ocular motoneurons for horizontal and vertical saccades with combined electrophysiological and neuroanatomical techniques. Comparing well-known vestibuloocular pathways with our findings of commissural excitation and inhibition between both superior colliculi, we proposed that the saccade system uses the same frame of reference as the vestibuloocular system, common semicircular canal coordinate. This proposal is mainly based on marked similarities (1) between output neural circuitry from one superior colliculus to extraocular motoneurons and that from a respective canal to its innervating extraocular motoneurons, (2) of patterns of commissural reciprocal inhibitions between upward saccade system on one side and downward system on the other, and between anterior canal system on one side and posterior canal system on the other, and (3) between the neural circuits of saccade and quick phase of vestibular nystagmus sharing brainstem burst neurons. In support of the proposal, commissural excitation of the superior colliculi may help to maintain Listing's law in saccades in spite of using semicircular canal coordinate.

Comparisons of Neuronal and Excitatory Network Properties between the Rat Brainstem Nuclei that Participate in Vertical and Horizontal Gaze Holding

Gaze holding is primarily controlled by neural structures including the prepositus hypoglossi nucleus (PHN) for horizontal gaze and the interstitial nucleus of Cajal (INC) for vertical and torsional gaze. In contrast to the accumulating findings of the PHN, there is no report regarding the membrane properties of INC neurons or the local networks in the INC. In this study, to verify whether the neural structure of the INC is similar to that of the PHN, we investigated the neuronal and network properties of the INC using whole-cell recordings in rat brainstem slices. Three types of afterhyperpolarization (AHP) profiles and five firing patterns observed in PHN neurons were also observed in INC neurons. However, the overall distributions based on the AHP profile and the firing patterns of INC neurons were different from those of PHN neurons. The application of burst stimulation to a nearby site of a recorded INC neuron induced an increase in the frequency of spontaneous EPSCs. The duration of the increased EPSC frequency of INC neurons was not significantly different from that of PHN neurons. The percent of duration reduction induced by a Ca²⁺-permeable AMPA (CP-AMPA) receptor antagonist was significantly smaller in the INC than in the PHN. These findings suggest that local excitatory networks that activate sustained EPSC responses also exist in the INC, but their activation mechanisms including the contribution of CP-AMPA receptors differ between the INC and the PHN.

Imaging correlates of neural control of ocular movements

The purpose of oculomotor movements is maintenance of clear images on the retina. Beyond this oversimplification, it requires several different types of ocular movements and reflexes to focus objects of interest to the fovea-the only portion of retina capable of sharp and clear vision. The different movements and reflexes that execute this task are the saccades, smooth pursuit movements, fixation, accommodation, and the optokinetic and vestibulo-ocular reflexes. Many different centres in the cerebrum, cerebellum, brainstem and thalami, control these movements via different pathways. At the outset, these mechanisms appear dauntingly complex to a radiologist. However, only a little effort could make it possible to understand these neural controls and empower the reading session. The following review on ocular movements and their neural control will enable radiologists and clinicians to correlate lesions with clinical deficits effectively without being swamped by exhaustive detail. Key Points • Knowledge of cortical and subcortical areas controlling ocular movements is important. • Understanding of neural control of ocular movements makes a good foundation. • Awareness of anatomic areas controlling ocular movements helps in clinico-radiologic correlation.

Cortical and Subcortical Substrates of Cranial Nerve Function

The pivotal role of cranial nerves in a wholesome life experience cannot be overemphasized. Research has opened new avenues to understand cranial nerve function. Classical concept of strict bilateral cortical control of cranial nerves has given way to concepts of hemispheric dominance and hemispheric lateralization. An astute Neuroradiologist should keep abreast of these concepts and help patients and referring physicians by applying this knowledge in reading images. This chapter provides an overview of cranial nerve function and latest concepts pertaining to their cortical and subcortical control.

Input-output organization of inhibitory neurons in the interstitial nucleus of Cajal projecting to the contralateral trochlear and oculomotor nucleus

Neurons in the interstitial nucleus of Cajal (INC) that are known to be involved in eye and head movements are excitatory. We investigated the input-output organization of inhibitory INC neurons involved in controlling vertical saccades. Intracellular recordings were made in INC neurons activated antidromically by stimulation of the contralateral trochlear or oculomotor nucleus, and their synaptic input properties from the superior colliculi (SCs) and the contralateral INC were analyzed in anesthetized cats. Many INC neurons projected to the contralateral trochlear nucleus, Forel's field H, INC, and oculomotor nucleus, and mainly received monosynaptic excitation followed by disynaptic inhibition from the ipsi- and contralateral SCs. After sectioning the commissural connections between the SCs, these neurons received monosynaptic excitation from the ipsilateral medial SC and disynaptic inhibition via the INC from the contralateral lateral SC. Another group of INC neurons were antidromically activated from the contralateral oculomotor nucleus, INC and Forel's field H, but not from the trochlear nucleus, and received monosynaptic excitation from the ipsilateral lateral SC and disynaptic inhibition from the contralateral medial SC. The former group was considered to inhibit contralateral trochlear and inferior rectus motoneurons in upward saccades, whereas the latter was considered to inhibit contralateral superior rectus and inferior oblique motoneurons in downward saccades. The mutual inhibition existed between these two groups of INC neurons for upward saccades on one side and downward saccades on the other. This pattern of input-output organization of inhibitory INC neurons suggests that the basic neural circuits for horizontal and vertical saccades are similar.

The nucleus pararaphales in the human, chimpanzee, and macaque monkey

The human cerebral cortex and cerebellum are greatly expanded compared to those of other mammals, including the great apes. This expansion is reflected in differences in the size and organization of precerebellar brainstem structures, such as the inferior olive. In addition, there are cell groups unique to the human brainstem. One such group may be the nucleus pararaphales (PRa); however, there is disagreement among authors about the size and location of this nucleus in the human brainstem. The name "pararaphales" has also been used for neurons in the medulla shown to project to the flocculus in the macaque monkey. We have re-examined the existence and status of the PRa in eight humans, three chimpanzees, and four macaque monkeys using Nissl-stained sections as well as immunohistochemistry. In the human we found a cell group along the midline of the medulla in all cases; it had the form of interrupted cell columns and was variable among cases in rostrocaudal and dorsoventral extent. Cells and processes were highly immunoreactive for non-phosphorylated neurofilament protein (NPNFP); somata were immunoreactive to the synthetic enzyme for nitric oxide, nitric oxide synthase, and for calretinin. In macaque monkey, there was a much smaller oval cell group with NPNFP immunoreactivity. In the chimpanzee, we found a region of NPNFP-immunoreactive cells and fibers similar to what was observed in macaques. These results suggest that the "PRa" in the human may not be the same structure as the flocculus-projecting cell group described in the macaque. The PRa, like the arcuate nucleus, therefore may be unique to humans.

The anatomical identification of saccadic omnipause neurons in the rat brainstem

Omnipause neurons (OPNs) represent a crucial component for the generation of saccadic eye movements. They inhibit saccadic premotor neurons in the paramedian pontine reticular formation (PPRF) as well as in the rostral interstitial nucleus of the medial longitudinal fascicle (RIMLF) during the intersaccadic interval. In turn, inhibition of OPNs is a prerequisite in order to generate saccadic eye movements. Although the anatomy of the saccadic system including the OPNs has been extensively studied in primates and cats, no detailed anatomical description of these neurons in rats has been performed so far. The aim of the present study was the identification of putative OPNs in the rat brainstem based on their projection target, localization, and histochemical characteristics. Stereotactic tract-tracer injections into the rostral mesencephalon including the RIMLF in rat resulted in back-labeling of a neuron group adjacent to the midline at the level of traversing fibers of the abducens nerve, which are considered as OPNs lying in the nucleus raphe interpositus. Combined immunohistochemical staining for various markers revealed in these neurons the expression of parvalbumin, chondroitin sulfate proteoglycan, and glycine, but a lack of serotonin. The results of our study demonstrate the striking similarity between individual elements of the premotor saccadic network in rats and primates. The exact knowledge of their location in rats provides a basis for in vitro studies of the OPNs in rat brainstem slices.

Three-dimensional transformations for goal-directed action

Much of the central nervous system is involved in visuomotor transformations for goal-directed gaze and reach movements. These transformations are often described in terms of stimulus location, gaze fixation, and reach endpoints, as viewed through the lens of translational geometry. Here, we argue that the intrinsic (primarily rotational) 3-D geometry of the eye-head-reach systems determines the spatial relationship between extrinsic goals and effector commands, and therefore the required transformations. This approach provides a common theoretical framework for understanding both gaze and reach control. Combined with an assessment of the behavioral, neurophysiological, imaging, and neuropsychological literature, this framework leads us to conclude that (a) the internal representation and updating of visual goals are dominated by gaze-centered mechanisms, but (b) these representations must then be transformed as a function of eye and head orientation signals into effector-specific 3-D movement commands.

Endoscopic third ventriculotomy improves parkinsonism following a ventriculo-peritoneal shunt in a patient with non-communicating hydrocephalus secondary to idiopathic aqueduct stenosis

We report a 47-year-old woman who manifested ocular motility disorder, bilateral pyramidal signs, and severe parkinsonism after a ventriculo-peritoneal shunt for non communicating hydrocephalus secondary to idiopathic aqueduct stenosis. The ocular motility disorder consisted of severe vertical gaze palsy and convergence retraction nystagmus. Parkinsonism included not only bradykinesia but also resting tremor and cogwheel rigidity. On the other hand, striatal uptake did not decrease in (18)F-dihydroxyphenylalanine positron emission tomography, and anti-Parkinsonian drugs were not effective. 99mTc-ethyl cysteinate dimer bicisate single-photon emission computed tomography and F-18 fluorodeoxyglucose positron emission tomography revealed wide-ranged frontal cerebral cortical dysfunction due to midbrain dysfunction. Moreover, endoscopic third ventriculotomy markedly improved the clinical symptoms as well as the frontal cerebral cortical flow. A neural network formation known as the 'cortico-basal ganglia loop,' which intimately connects the frontal lobe with the basal ganglia, is possibly associated with the Parkinsonism observed in our patient.

Adaptation of the vestibulo-ocular reflex for forward-eyed foveate vision

To maintain visual fixation on a distant target during head rotation, the angular vestibulo-ocular reflex (aVOR) should rotate the eyes at the same speed as the head and in exactly the opposite direction. However, in primates for which the 3-dimensional (3D) aVOR has been extensively characterised (humans and squirrel monkeys (Saimiri sciureus)), the aVOR response to roll head rotation about the naso-occipital axis is lower than that elicited by yaw and pitch, causing errors in aVOR magnitude and direction that vary with the axis of head rotation. In other words, primates keep the central part of the retinal image on the fovea (where photoreceptor density and visual acuity are greatest) but fail to keep that image from twisting about the eyes' resting optic axes. We tested the hypothesis that aVOR direction dependence is an adaptation related to primates' frontal-eyed, foveate status through comparison with the aVOR of a lateral-eyed, afoveate mammal (Chinchilla lanigera). As chinchillas' eyes are afoveate and never align with each other, we predicted that the chinchilla aVOR would be relatively low in gain and isotropic (equal in gain for every head rotation axis). In 11 normal chinchillas, we recorded binocular 3D eye movements in darkness during static tilts, 20-100 deg s(1) whole-body sinusoidal rotations (0.5-15 Hz), and 3000 deg s(2) acceleration steps. Although the chinchilla 3D aVOR gain changed with both frequency and peak velocity over the range we examined, we consistently found that it was more nearly isotropic than the primate aVOR. Our results suggest that primates' anisotropic aVOR represents an adaptation to their forward-eyed, foveate status. In primates, yaw and pitch aVOR must be compensatory to stabilise images on both foveae, whereas roll aVOR can be under-compensatory because the brain tolerates torsion of binocular images that remain on the foveae. In contrast, the lateral-eyed chinchilla faces different adaptive demands and thus enlists a different aVOR strategy.

Concomitant presentation of three rare mesencephalic syndromes: case report

We describe a unique case of concomitant presentation of three rare mesencephalic syndromes. A 48-year-old man with an acute stoke was found to have an unusual combination of three rare mesencephalic syndromes after detailed neuro-ophthalmic evaluation: the plus-minus lid syndrome, the vertical one-and-a-half syndrome, and Claude's syndrome. We discuss the clinical and anatomical localization of these syndromes. This was corroborated by magnetic resonance imaging (MRI) which revealed areas of infarction at the thalamo-mesencephalic junction and the right rostral midbrain involving the third nerve fascicle and the red nucleus. Our case highlights the importance of a careful ocular motility examination as a tool which has a highly localizing value in the diagnosis of stroke.

Computational approaches to spatial orientation: from transfer functions to dynamic Bayesian inference

Spatial orientation is the sense of body orientation and self-motion relative to the stationary environment, fundamental to normal waking behavior and control of everyday motor actions including eye movements, postural control, and locomotion. The brain achieves spatial orientation by integrating visual, vestibular, and somatosensory signals. Over the past years, considerable progress has been made toward understanding how these signals are processed by the brain using multiple computational approaches that include frequency domain analysis, the concept of internal models, observer theory, Bayesian theory, and Kalman filtering. Here we put these approaches in context by examining the specific questions that can be addressed by each technique and some of the scientific insights that have resulted. We conclude with a recent application of particle filtering, a probabilistic simulation technique that aims to generate the most likely state estimates by incorporating internal models of sensor dynamics and physical laws and noise associated with sensory processing as well as prior knowledge or experience. In this framework, priors for low angular velocity and linear acceleration can explain the phenomena of velocity storage and frequency segregation, both of which have been modeled previously using arbitrary low-pass filtering. How Kalman and particle filters may be implemented by the brain is an emerging field. Unlike past neurophysiological research that has aimed to characterize mean responses of single neurons, investigations of dynamic Bayesian inference should attempt to characterize population activities that constitute probabilistic representations of sensory and prior information.

Neural correlates of forward and inverse models for eye movements: evidence from three-dimensional kinematics

Inverse and forward dynamic models have been conceptually important in computational motor control. In particular, inverse models are thought to convert desired action into appropriate motor commands. In parallel, forward models predict the consequences of the motor command on behavior by constructing an efference copy of the actual movement. Despite theoretical appeal, their neural representation has remained elusive. Here, we provide evidence supporting the notion that a group of premotor neurons called burst-tonic (BT) cells represent the output of the inverse model for eye movements. We show that BT neurons, like extraocular motoneurons but different from the evoked eye movement, do not carry signals appropriate for the half-angle rule of ocular kinematics during smooth-pursuit eye movements from eccentric positions. Along with findings of identical response dynamics as motoneurons, these results strongly suggest that BT cells carry a replica of the motor command. In contrast, eye-head (EH) neurons, a premotor cell type that is the target of Purkinje cell inhibition from the cerebellar flocculus/ventral paraflocculus, exhibit properties that could be consistent with the half-angle rule. Therefore, EH cells may be functionally related to the output of a forward internal model thought to construct an efference copy of the actual eye movement.

Interstitial Nucleus of Cajal Encodes Three-Dimensional Head Orientations in Fick-Like Coordinates

Two central, related questions in motor control are 1) how the brain represents movement directions of various effectors like the eyes and head and 2) how it constrains their redundant degrees of freedom. The interstitial nucleus of Cajal (INC) integrates velocity commands from the gaze control system into position signals for three-dimensional eye and head posture. It has been shown that the right INC encodes clockwise (CW)-up and CW-down eye and head components, whereas the left INC encodes counterclockwise (CCW)-up and CCW-down components, similar to the sensitivity directions of the vertical semicircular canals. For the eyes, these canal-like coordinates align with Listing’s plane (a behavioral strategy limiting torsion about the gaze axis). By analogy, we predicted that the INC also encodes head orientation in canal-like coordinates, but instead, aligned with the coordinate axes for the Fick strategy (which constrains head torsion). Unilateral stimulation (50 μA, 300 Hz, 200 ms) evoked CW head rotations from the right INC and CCW rotations from the left INC, with variable vertical components. The observed axes of head rotation were consistent with a canal-like coordinate system. Moreover, as predicted, these axes remained fixed in the head, rotating with initial head orientation like the horizontal and torsional axes of a Fick coordinate system. This suggests that the head is ordinarily constrained to zero torsion in Fick coordinates by equally activating CW/CCW populations of neurons in the right/left INC. These data support a simple mechanism for controlling head orientation through the alignment of brain stem neural coordinates with natural behavioral constraints.

Polysynaptic inputs to vestibular efferent neurons as revealed by viral transneuronal tracing

The Bartha strain of the alpha-herpes pseudorabies virus (PrV) was used as a retrograde transneuronal tracer to map synaptic inputs to the vestibular efferent neurons of the Mongolian gerbil, Meriones unguiculatus. Although previous experiments have shown that vestibular efferent neurons respond to visual motion and somatosensory stimuli, the anatomic connections mediating those responses are unknown. PrV was injected unilaterally into the horizontal semicircular canal neuroepithelium of gerbils, where it was taken up by efferent axon terminals. The virus was then retrogradely transported to efferent cell bodies, replicated, and transported into synaptic endings projecting onto the efferent cells. Thirty animals were sacrificed at approximately 5-h increments between 75 and 105 h post-infection after determining that shorter time points had no central infection. Infected cells were visualized immunohistochemically. Temporal progression of neuronal infection was used to determine the nature of primary and higher order projections to the vestibular efferent neurons. Animals sacrificed at 80-94 h post-inoculation exhibited immunostaining in the dorsal and ventral group of vestibular efferent neurons, predominately on the contralateral side. Neurons within the medial, gigantocellular, and lateral reticular formations were among the first cells infected thereafter. At 95 h, additional virus-labeled cell groups included the solitary, area postrema, pontine reticular, prepositus, dorsal raphe, tegmental, the subcoeruleus nuclei, the nucleus of Darkschewitsch, and the inferior olivary beta and ventrolateral subnuclei. Analysis beyond 95 h revealed virus-infected neurons located in the vestibulo-cerebellar and motor cortices. Paraventricular, lateral, and posterior hypothalamic cells, as well as central amygdala cells, were also labeled. Spinal cord tissue exhibited no labeling in the intermediolateral cell column, but scattered cells were found in the central cervical nucleus. The results suggest functional associations among efferent feedback regulation of labyrinthine sensory input and both behavioral and autonomic systems, and support a closed-looped vestibular feedback model with additional open-loop polysynaptic inputs.

Present concepts of oculomotor organization

This chapter gives an introduction to the oculomotor system, thus providing a framework for the subsequent chapters. This chapter describes the characteristics, and outlines the structures involved, of the five basic types of eye movements, for gaze holding ("neural integrator") and eye movements in three dimensions (Listing's law, pulleys).

The three-dimensional vestibulo-ocular reflex evoked by high-acceleration rotations in the squirrel monkey

The aim of this study was to determine if the angular vestibulo-ocular reflex (VOR) in response to pitch, roll, left anterior-right posterior (LARP), and right anterior-left posterior (RALP) head rotations exhibited the same linear and nonlinear characteristics as those found in the horizontal VOR. Three-dimensional eye movements were recorded with the scleral search coil technique. The VOR in response to rotations in five planes (horizontal, vertical, torsional, LARP, and RALP) was studied in three squirrel monkeys. The latency of the VOR evoked by steps of acceleration in darkness (3,000 degrees /s(2) reaching a velocity of 150 degrees /s) was 5.8+/-1.7 ms and was the same in response to head rotations in all five planes of rotation. The gain of the reflex during the acceleration was 36.7+/-15.4% greater than that measured at the plateau of head velocity. Polynomial fits to the trajectory of the response show that eye velocity is proportional to the cube of head velocity in all five planes of rotation. For sinusoidal rotations of 0.5-15 Hz with a peak velocity of 20 degrees /s, the VOR gain did not change with frequency (0.74+/-0.06, 0.74+/-0.07, 0.37+/-0.05, 0.69+/-0.06, and 0.64+/-0.06, for yaw, pitch, roll, LARP, and RALP respectively). The VOR gain increased with head velocity for sinusoidal rotations at frequencies > or =4 Hz. For rotational frequencies > or =4 Hz, we show that the vertical, torsional, LARP, and RALP VORs have the same linear and nonlinear characteristics as the horizontal VOR. In addition, we show that the gain, phase and axis of eye rotation during LARP and RALP head rotations can be predicted once the pitch and roll responses are characterized.

GABAergic neurons in the rostral mesencephalon of the macaque monkey that control vertical eye movements

The mesencephalic reticular formation is important for the generation of vertical eye movements, but up until now the location of inhibitory premotor neurons is not known in primates. With tract-tracer methods combined with immunocytochemistry or in situ hybridization, we investigated the location of GABAergic premotor neurons in the rostral interstitial nucleus of the medial longitudinal fascicle (riMLF) and interstitial nucleus of Cajal (iC) in macaque monkeys. In the present work, only the premotor pathways of the downward pulling eye muscles, superior oblique (SO) and inferior rectus (IR), were studied. We found that very few, small GABAergic neurons are present in the riMLF, and none of them was found to project to the oculomotor nuclei, suggesting the presence of exclusively excitatory projections from the riMLF to the oculomotor neurons. However, in the iC, medium-sized and large GABAergic neurons were identified projecting contralaterally to the SO and IR motoneurons, and presumably the iC of the other side. These commissural GABAergic projections are well suited to inhibit the SO and IR motoneurons and possibly premotor down-burst-tonic neurons during upward eye movements.

Gravity modulates Listing's plane orientation during both pursuit and saccades

Previous studies have shown that the spatial organization of all eye orientations during visually guided saccadic eye movements (Listing's plane) varies systematically as a function of static and dynamic head orientation in space. Here we tested if a similar organization also applies to the spatial orientation of eye positions during smooth pursuit eye movements. Specifically, we characterized the three-dimensional distribution of eye positions during horizontal and vertical pursuit (0.1 Hz, +/-15 degrees and 0.5 Hz, +/-8 degrees) at different eccentricities and elevations while rhesus monkeys were sitting upright or being statically tilted in different roll and pitch positions. We found that the spatial organization of eye positions during smooth pursuit depends on static orientation in space, similarly as during visually guided saccades and fixations. In support of recent modeling studies, these results are consistent with a role of gravity on defining the parameters of Listing's law.

Neural control of 3-D gaze shifts in the primate

The neural mechanisms that specify target locations for gaze shifts and then convert these into desired patterns of coordinated eye and head movements are complex. Much of this complexity is only revealed when one takes a realistic three-dimensional (3-D) view of these processes, where fundamental computational problems such as kinematic redundancy, reference-frame transformations, and non-commutativity emerge. Here we review the underlying mechanisms and solutions for these problems, starting with a consideration of the kinematics of 3-D gaze shifts in human and non-human primates. We then consider the neural mechanisms, including cortical representation of gaze targets, the nature of the gaze motor command used by the superior colliculus, and how these gaze commands are decomposed into brainstem motor commands for the eyes and head. A general conclusion is that fairly simple coding mechanisms may be used to represent gaze at the cortical and collicular level, but this then necessitates complexity for the spatial updating of these representations and in the brainstem sensorimotor transformations that convert these signals into eye and head movements.

The deep mesencephalic nucleus as an output center of basal ganglia: morphological and electrophysiological similarities with the substantia nigra

The deep mesencephalic nucleus (DMN) is a large midbrain reticular region between the superior colliculus, the substantia nigra compacta, the periaqueductal gray, and the medial geniculate body. Although some data suggest that it is involved in nociception and visceral control, its functions remain unclear. In the present study, by using morphological (combination of anterograde and retrograde tracers with immunocytochemistry and in situ hibrydization) and electrophysiological (firing activity and transynaptic response to striatal stimulation) methods, we show that a subpopulation of DMN cells shares many morphological and electrophysiological characteristics with those of the substantia nigra reticulata (SNR). These similarities include the following: 1) firing rate, firing pattern, and conduction velocity; 2) expression of GAD65, GAD67, and PV; 3) excitatory and inhibitory inputs from the striatum; and 4) projections to the ventral thalamus, superior colliculus, and pedunculopontine tegmental nucleus. Some differences were also found. In comparison with SN, DMN cells and striatal afferents are more sparsely distributed and they show conspicuous contralateral projections to the thalamus and superior colliculus. This suggests that, similarly to the SNR, the DMN acts as an output center of basal ganglia and probably facilitates the inter-hemispheric regulation of these centers.

Vestibular-pursuit interactions: gaze-velocity and target-velocity signals in the monkey frontal eye fields

Visual information about a moving object is obtained by accurate tracking with the eyes using the smooth pursuit system, which must interact with the vestibular system during head movement. Such pursuit-vestibular interactions require calculation of gaze (i.e., eye in space) in order to match eye velocity in space to actual target velocity, using vestibular, retinal-image velocity, and eye-velocity information. To understand the role the frontal eye fields (FEFs) play in pursuit-vestibular interactions, we examined responses of pursuit-related neurons near the arcuate sulcus in head-stabilized monkeys during visual tracking tasks that dissociate eye movement in the orbit from that in space. The activity of the majority of pursuit-related neurons was related to gaze velocity. They also responded to passive body rotation in complete darkness. When the monkeys fixated the stationary target, similar modulation was observed, reflecting the velocity signal of a second test target. Muscimol infusion into the FEF pursuit areas severely impaired smooth gaze tracking. These results suggest that the region near the arcuate sulcus coordinates its various inputs to provide signals for target velocity in space and accurate gaze-velocity command during pursuit-vestibular interactions.

Anatomy and physiology of the primate interstitial nucleus of Cajal. II. Discharge pattern of single efferent fibers

Anatomy and physiology of the primate interstitial nucleus of Cajal. II. Discharge pattern of single efferent fibers. J. Neurophysiol. 80: 3100-3111, 1998. Single efferent fibers of the interstitial nucleus of Cajal (NIC) were characterized physiologically and injected with biocytin in alert behaving monkeys. Quantitative analysis demonstrated that their discharge encodes a constellation of oculomotor variables. Tonic and phasic signals were related to vertical (up or down) eye position and saccades, respectively. Depending on how they encoded eye position, saccade velocity, saccade size, saccade duration, and smooth-pursuit eye velocity, fibers were characterized as regular or irregular, bi- or unidirectionally modulated, more or less sensitive, and reliable or unreliable. Further, fibers that did not burst for saccades (tonic) and fibers the eye-position and saccade-related signals of which increased in the same (in-phase) or in the opposite (anti-phase) directions were encountered. A continuum of discharge properties was the rule. We conclude that NIC efferent fibers send a combination of eye-position, saccade-, and smooth-pursuit-related signals, mixed in proportions that differ for different fibers, to targets of the vertical neural integrator such as extraocular motoneurons.

Trigeminocerebellar and trigemino-olivary projections in rats

Retrograde and anterograde neuronal tracers (HRP, biocytin, biotinylated dextran-amine) were used to study the organisation of trigeminocerebellar and trigemino-olivary connections, focusing on the connectivity between trigeminal nuclear regions and the sagittal zones of the rat cerebellar cortex. Trigeminocerebellar projections were bilateral, but mostly ipsilateral. Direct trigeminocerebellar fibres originated mostly in the principal trigeminal nucleus (VP) and pars oralis (Vo), pars interpolaris (Vi), and to a lesser extent in pars caudalis (Vc) of the spinal trigeminal nucleus. Consistent projections were found from the Vc to cerebellar lobules IX and X. The trigeminal fibres terminated in the cerebellum in an organised fashion. The ventral part of the VP, Vo and Vi projected to the medial A zone and the C3 and D2 subzones, whereas the dorsal part of the nuclei projected to the lateral A zone and the C2, D0 and D1 subzones. In lobules IX and X, the organisation was different. The medial half of the VP, Vo, Vi and Vc projected to the lateral aspects of these lobules whereas their lateral part projected to their medial aspects. Trigeminal projections to the deep cerebellar nuclei were also present. Projections to a given sagittal zone concomitantly reached its corresponding nuclear target. Trigemino-olivary projections were principally contralateral. The Vo, Vi and Vc projected to the rostromedial dorsal accessory olive, the adjacent dorsal leaf and the dorsomedial part of the ventral leaf of the principal olive, which are known to project subzones C3, D0 and D1 of the rat cerebellar cortex.

Morphology and soma-dendritic distribution of synaptic endings from the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) on motoneurons in the oculomotor and trochlear nuclei in the cat

The morphology and soma-dendritic distribution of anterograde biocytin-labelled rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) synaptic endings in the oculomotor and trochlear nuclei have been examined by electron microscopy by using both preembedding immunoperoxidase and postembedding immunogold methods. The results indicate that three morphological types of riMLF synaptic endings are distinguishable on the basis of synaptic vesicle morphology (spheroidal, pleiomorphic, or ellipsoidal) and postsynaptic membrane specializations (asymmetrical or symmetrical). All three morphological types of riMLF synaptic endings establish synaptic connections predominantly with dendrites. Synaptic endings that contain ellipsoidal synaptic vesicles have a more proximal soma-dendritic distribution than those that contain either spheroidal or pleiomorphic synaptic vesicles. Furthermore, all three morphological types of synaptic endings are encountered in the same motoneuron subdivisions of the oculomotor and trochlear nuclei in the same experiments. The findings suggest that subregions of the riMLF contain coexistent populations of excitatory and inhibitory premotor neurons that are related to opposite directions of vertical saccadic eye movements but that project to the same motoneuron subgroups on the ipsilateral side. Both the morphology and the mode, pattern, and soma-dendritic distribution of saccade-related riMLF synaptic endings that establish synaptic connections with vertical motoneurons differ from those of excitatory and inhibitory second-order vertical vestibular synaptic endings. These differences in the synaptic organization of riMLF and second-order vestibular inputs to oculomotor and trochlear motoneurons may be related to differences in the information transferred by each source, the riMLF input conveying eye-velocity signals, and the vestibular input conveying eye-position signals.

Further evidence for the specific involvement of the flocculus in the vertical vestibulo-ocular reflex (VOR)

We examined the simple-spike activity of floccular Purkinje (P) cells during sinusoidal pitch rotation and vertical optokinetic stimuli in alert, head-fixed cats. The great majority of pitch-responding P cells also responded to optokinetic stimuli with increased activity when the directions of the resultant eye movements were the same. During rapid modification of the VOR induced by visual pattern movement, modulation amplitudes of the cells tested increased together with the eye velocity increase. Maximal activation directions of these cells studied during vertical rotation in many planes were near the vertical canal planes. These results suggest that the activity of the majority of pitch-responding P cells contains a vertical eye velocity component during vestibular or optokinetic stimuli in addition to canal inputs during pitch rotation.

Ascending projections of posterior canal-activated excitatory and inhibitory secondary vestibular neurons to the mesodiencephalon in cats

The axonal projections of 62 posterior canal (PC)-activated excitatory and inhibitory secondary vestibular neurons were studied electrophysiologically in cats. PC-related neurons were identified by monosynaptic activation elicited by electrical stimulation of the vestibular nerve and activation following nose-up rotation of the animal's head. Single excitatory and inhibitory neurons were identified by antidromic activation following electrical stimulation of the contralateral and ipsilateral medial longitudinal fasciculus, respectively. The oculomotor projections of identified neurons were confirmed with a spike-triggered averaging technique. The axonal projections of the identified neurons were then studied by systematic, antidromic stimulation of the mesodiencephalon. Excitatory neurons showed two main types of axonal projections. In one type, axonal branches were issued to the interstitial nucleus of Cajal, central gray, and thalamus including the ventral posterolateral, ventral posteromedial, ventral lateral, ventral medial, centromedian, central lateral, lateral posterior, and ventral lateral geniculate nuclei. The other type was more frequently observed, giving off axon collaterals to the above-mentioned regions and to Forel's field H as well. Inhibitory neurons issued axonal branches to limited areas which included the central gray, interstitial nucleus of Cajal, its adjacent reticular formation and caudalmost part of Forel's field H, but not the rostral part of the Forel's field H and the thalamus. These results suggest that PC-related excitatory neurons participate in the genesis of vertical eye movements and in the perception of the vestibular sensation, and that PC-related inhibitory neurons seem to take part only in the genesis of vertical eye movements.

The interstitial nucleus of Cajal is involved in generating downward fast eye movement in alert cats

We examined whether or not the interstitial nucleus of Cajal (INC) is involved in generating vertical fast eye movement. We used a gamma-aminobutyric acid (GABA) agonist (muscimol) to deactivate the INC cell bodies bilaterally in alert head-fixed cats. The INC was identified by recording vertical eye position-related burst-tonic neurons. Following muscimol infusion into the bilateral INC, the cats were unable to hold eccentric upward eye position after saccades, and the mean time constant of postsaccadic drift was 0.4 sec. In addition, downward saccades were virtually lost without clear impairment of horizontal saccades. Muscimol infusion into the Forel's field H resulted in loss of both upward and downward fast eye movement as reported previously. Bilateral electrolytic INC lesions produced results similar to those after muscimol infusion into the INC, suggesting that the cutting of passing fibers in the INC alone cannot explain the selective loss of downward fast eye movement. These results indicate that the INC itself is involved in generating downward fast eye movement. We propose that the loss of activity in possible downward burster-driving neurons in the INC can explain the selective loss of downward fast eye movement after bilateral INC lesions.

Ocular torsion produced by unilateral chemical inactivation of the interstitial nucleus of Cajal in chronically labyrinthectomized cats

Unilateral lesions of the rostral midbrain involving the interstitial nucleus of Cajal (INC) have been known to produce ocular torsion in alert animals including humans, which has been assumed to be the result of an impaired otolith-ocular reflex. We examined the effects of chemical deactivation of the INC using a gamma-aminobutyric acid (GABA) agonist (muscimol) in cats that had received bilateral labyrinthectomy, and compared the results with those in normal cats. Ocular torsion with a magnitude similar to that observed in normal cats appeared in chronically labyrinthectomized cats after unilateral muscimol infusion into the INC, indicating that ocular torsion following unilateral INC deactivation can be produced by a mechanism independent of the otolith-ocular reflex.

Possible downward burster-driving neurons related to the anterior semicircular canal in the region of the interstitial nucleus of Cajal in alert cats

It is well known that vestibular nystagmus evoked by head rotation occurs in the plane specific to that in which head rotation was applied in three-dimensional space. Although burster-driving neurons (BDN) have been demonstrated for a quick phase of horizontal nystagmus, it is not yet known where the counterpart for vertical nystagmus is located. We analyzed the activity of a class of neurons in the region within, and in the close vicinity of, the interstitial nucleus of Cajal (INC) in alert cats. Their activity gradually increased during an upward slow phase evoked by nose-down pitch. This increased activity was further followed by burst discharge shortly before and during the downward quick phase. Gradually increased activity was also evoked by contralateral roll. These results suggest that the gradually increased activity was evoked by activation of the contralateral anterior canal. Many of these cells were fired by electrical stimulation of the contralateral vestibular nerve with short latencies. These cells also showed burst discharge shortly before and during downward saccades induced by visual stimuli, and the number of spikes during bursts was correlated with saccade amplitudes. Although all had irregular resting discharges, eye-position-related activity was rarely obtained. The characteristic behavior of these cells is very similar, except for their on-directions, to the behavior of horizontal BDNs, suggesting that these INC cells are a candidate for downward BDNs related to the anterior canal.

Latencies of response of eye movement-related neurons in the region of the interstitial nucleus of Cajal to electrical stimulation of the vestibular nerve in alert cats

Recent studies have shown that the interstitial nucleus of Cajal (INC) in the midbrain reticular formation is involved in the conversion of vertical semicircular canal signals into eye position during vertical vestibulo-ocular reflexes. Secondary vestibulo-ocular relay neurons related to the vertical canals, which constitute the majority of output neurons sending signals from the vestibular nuclei directly to the oculomotor nuclei, have been shown to project axon collaterals to the region within and near the INC. To understand how the INC is involved in the signal conversion, latencies of response of neurons in the INC region to electrical stimulation of the vestibular nerve were examined in alert cats. The responses of 96 cells whose activity was clearly modulated by sinusoidal pitch rotation (at 0.31 Hz) were analyzed. These included 41 cells whose activity was closely correlated with vertical eye movement (38 burst-tonic and 3 tonic neurons), and 55 other cells (called pitch cells as previously). Twenty nine of the 96 cells (30%) were activated at disynaptic latencies following single shock stimulation of the contralateral vestibular nerve. Disynaptically activated cells were significantly more frequent for pitch cells than for eye movement-related cells (25/55 = 45% vs 4/41 = 10%; p less than 0.001, Chi-square test). Conversely, cells that did not receive short-latency activation (less than 6 ms) were more frequent among eye movement-related cells than pitch cells (26/41 = 63% vs 13/55 = 24%; p less than 0.001, Chi-square test).(ABSTRACT TRUNCATED AT 250 WORDS)