Responses of vestibular neurons to stimulation of the interstitial nucleus of Cajal in the cat

Physiology of midbrain head movement neurons in cervical dystonia

BACKGROUND

Early theories for cervical dystonia, as promoted by Hassler, emphasized the role of the midbrain interstitial nucleus of Cajal. Focus then shifted to the basal ganglia, and it was further supported with the success of deep brain stimulation. Contemporary theories suggested the role of the cerebellum, but even more recent hypotheses renewed interest in the midbrain. Although the pretectum was visited on several occasions, we still do not know about the physiology of midbrain neurons in cervical dystonia.

METHODS

We analyzed the unique database of pretectal neurons collected in the 1970s and 1980s during historic stereotactic surgeries aimed to treat cervical dystonia. This database is valuable because such recordings could otherwise never be obtained from humans.

RESULTS

We found the following 3 types of eye or neck movement sensitivity: eye-only neurons responded to pure vertical eye movements, neck-only neurons were sensitive to pure neck movements, and the combined eye-neck neurons responded to eye and neck movements. There were the 2 neuronal subtypes: burst-tonic and tonic. The eye-neck or eye-only neurons sustained their activity during eccentric gaze holding. In contrast, the response of neck-only and eye-neck neurons exponentially decayed during neck movements.

CONCLUSIONS

Modern quantitative analysis of a historic database of midbrain single units from patients with cervical dystonia might support novel hypotheses for normal and abnormal head movements. This data, collected almost 4 decades ago, must be carefully viewed, especially because it was acquired using a less sophisticated technology available at that time and the aim was not to address specific hypothesis, but to make an accurate lesion providing optimal relief from dystonia. © 2017 International Parkinson and Movement Disorder Society.

Neck muscle synergies during stimulation and inactivation of the interstitial nucleus of Cajal (INC)

The interstitial nucleus of Cajal (INC) is thought to control torsional and vertical head posture. Unilateral microstimulation of the INC evokes torsional head rotation to positions that are maintained until stimulation offset. Unilateral INC inactivation evokes head position-holding deficits with the head tilted in the opposite direction. However, the underlying muscle synergies for these opposite behavioral effects are unknown. Here, we examined neck muscle activity in head-unrestrained monkeys before and during stimulation (50 muA, 200 ms, 300 Hz) and inactivation (injection of 0.3 mul of 0.05% muscimol) of the same INC sites. Three-dimensional eye and head movements were recorded simultaneously with electromyographic (EMG) activity in six bilateral neck muscles: sternocleidomastoid (SCM), splenius capitis (SP), rectus capitis posterior major (RCPmaj.), occipital capitis inferior (OCI), complexus (COM), and biventer cervicis (BC). INC stimulation evoked a phasic, short-latency ( approximately 5-10 ms) facilitation and later ( approximately 100-200 ms) a more tonic facilitation in the activity of ipsi-SCM, ipsi-SP, ipsi-COM, ipsi-BC, contra-RCPmaj., and contra-OCI. Unilateral INC inactivation led to an increase in the activity of contra-SCM, ipsi-SP, ipsi-RCPmaj., and ipsi-OCI and a decrease in the activity of contra-RCPmaj. and contra-OCI. Thus the influence of INC stimulation and inactivation were opposite on some muscles (i.e., contra-OCI and contra-RCPmaj.), but the comparative influences on other neck muscles were more variable. These results show that the relationship between the neck muscle responses during INC stimulation and inactivation is much more complex than the relationship between the overt behaviors.

Three-Dimensional Eye–Head Coordination After Injection of Muscimol Into the Interstitial Nucleus of Cajal (INC)

The interstitial nucleus of Cajal (INC) is thought to be the “neural integrator” for torsional/vertical eye position and head posture. Here, we investigated the coordination of eye and head movements after reversible INC inactivation. Three-dimensional (3-D) eye–head movements were recorded in three head-unrestrained monkeys using search coils. INC sites were identified by unit recording/electrical stimulation and then reversibly inactivated by 0.3 μl of 0.05% muscimol injection into 26 INC sites. After muscimol injection, the eye and head 1) began to drift (an inability to maintain stable fixation) torsionally: clockwise (CW)/counterclockwise (CCW) after left/right INC inactivation respectively. 2) The eye and head tilted torsionally CW/CCW after left/right INC inactivation, respectively. Horizontal gaze/head drifts were inconsistently present and did not result in considerable position offsets. Vertical eye drift was dependent on both vertical eye position and the magnitude of the previous vertical saccade, as in head-fixed condition. This correlation was smaller for gaze and head drift, suggesting that the gaze and head deficits could not be explained by a first-order integrator model. Ocular counterroll (OC) was completely disrupted. The gain of torsional vestibuloocular reflex (VOR) during spontaneous eye and head movements was reduced by 22% in both CW/CCW directions after either left or right INC inactivation. Our results suggest a complex interdependence of eye and head deficits after INC inactivation during fixation, gaze shifts, and VOR. Some of our results resemble the symptoms of spasmodic torticollis (ST).

Abnormal interaction between vestibular and voluntary head control in patients with spasmodic torticollis

The functional status of vestibulo-collic reflexes in the sternocleidomastoid (SCM) muscles was investigated in 24 patients with spasmodic torticollis using small, abrupt 'drops' of the head. None had been treated with botulinum toxin injections during at least 4 months preceding the study. Eight of the patients, four of whom had been studied before surgery, were also studied after selective peripheral denervation of neck muscles. The reflex was of normal latency and duration in the 'passive drop' condition, in which subjects were instructed not to oppose the fall of the head. To study voluntary interaction with the reflex response, subjects were then asked to flex the neck as quickly as possible after onset of the head drop ('active drop'). In this condition, voluntary responses in patients were delayed, smaller and less effective in counteracting the head fall than in normal subjects. The same abnormalities were also found in patients after surgery when the head posture was improved. Somatosensory/auditory voluntary reaction times in SCM were normal, as was the latency of the startle reflex. We conclude that voluntary interaction with the vestibulo-collic reflex is disrupted in patients with spasmodic torticollis, a finding which corroborates the patients' aggravation of their symptoms by head or body perturbations. Lack of effective interaction between two major systems controlling head position may contribute to torticollis.

Unilateral muscimol inactivations of the interstitial nucleus of Cajal in the alert rhesus monkey do not elicit seesaw nystagmus

Seesaw-nystagmus (SSN) is a unique form of nystagmus with disconjugate vertical and conjugate torsional eye movements. Although rare, this disorder serves as a model for neuronal binocular control of the alignment of vertical-torsional eye movements of both eyes. The pathomechanism of SSN, however, is unclear. Studies in patients have suggested that the jerk SSN is associated with a midbrain lesion, i.e. a lesion of the interstitial nucleus of Cajal (iC), a center of integration of vertical and torsional eye movements. To test this hypothesis, we examined three dimensional binocular eye movements after reversible local inactivations of the iC and its immediate vicinity in the midbrain of the alert monkey. Inactivations were induced by muscimol microinjections. Eye movements were recorded with binocular scleral search coils. Isolated inactivations of neither the iC nor its immediate vicinity in the midbrain (including the adjacent rostral interstitial nucleus of the medial longitudinal fascicle, riMLF) elicited a disconjugate vertical/torsional nystagmus (SSN). However, there was a direction-specific right/left asymmetry in which a larger vertical amplitude was associated with the contralesional eye and a larger torsional amplitude with the ipsilesional eye, indicating a vestibular imbalance. We conclude that, first, iC lesions do not elicit SSN and, second, that apart from the gaze holding deficit a vestibular imbalance contributes to the vertical/torsional nystagmus after iC lesions.

Projections and firing properties of down eye-movement neurons in the interstitial nucleus of Cajal in the cat

To clarify the role of the interstitial nucleus of Cajal (INC) in the control of vertical eye movements, projections of burst-tonic and tonic neurons in and around the INC were studied. This paper describes neurons with downward ON directions. We examined, by antidromic activation, whether these down INC (d-INC) neurons contribute to two pathways: a commissural pathway to the contralateral (c-) INC and a descending pathway to the ipsilateral vestibular nucleus (i-VN). Stimulation of the two pathways showed that as many as 74% of neurons were activated antidromically from one of the pathways. Of 113 d-INC neurons tested, 44 were activated from the commissural pathway and 40 from the descending pathway. No neurons were activated from both pathways. We concluded that commissural and descending pathways from the INC originate from two separate groups of neurons. Tracking of antidromic microstimulation in the two nuclei revealed multiple low-threshold sites and varied latencies; this was interpreted as a sign of existence of axonal arborization. Neurons with commissural projections tended to be located more dorsally than those with descending projections. Neurons with descending projections had significantly greater eye-position sensitivity and smaller saccadic sensitivity than neurons with commissural projections. The two groups of INC neurons increased their firing rate in nose-up head rotations and responded best to the rotation in the plane of contralateral posterior/ipsilateral anterior canal pair. Neurons with commissural projections showed a larger phase lag of response to sinusoidal rotation (54.6 +/- 7.6 degrees ) than neurons with descending projections (45.0 +/- 5.5 degrees ). Most neurons with descending projections received disynaptic excitation from the contralateral vestibular nerve. Neurons with commissural projections rarely received such disynaptic input. We suggest that downward-position-vestibular (DPV) neurons in the VN and VN-projecting d-INC neurons form a loop, together with possible commissural loops linking the bilateral VNs and the bilateral INCs. By comparing the quantitative measures of d-INC neurons with those of DPV neurons, we further suggest that integration of head velocity signals proceeds from DPV neurons to d-INC neurons with descending projections and then to d-INC neurons with commissural projections, whereas saccadic velocity signals are processed in the reverse order.

Vestibular integrators in the oculomotor system

The interstitial nucleus of Cajal (INC) and the nucleus prepositus hypoglossi (nph) are key elements in the vertical and horizontal oculomotor neural integrators, respectively. In this article, we attempt to develop possible circuits for these vestibular integrators by synthesizing recent information on the properties and connections of neurons involved in the integration process. We also examine how the cerebellar flocculus could play a role in the vertical integrator and vestibulo-ocular reflex (VOR) as well as in the modulation and plasticity of the VOR. We suggest that the circuitry for the vertical integrator involves the cerebellar flocculus in addition to the already proposed circuits distributed between the INC and the vestibular nuclei. The horizontal vestibular integrator is also distributed and seems to be characterized by functional compartmentalization. Both integrators play a wider role than simply transforming velocity-coded signals into position commands and may be pivotal in the short- and long-term modulation of the various oculomotor subsystems.

The reticulovestibular projection in the rabbit: an experimental study with the retrograde horseradish peroxidase method

The reticulovestibular projections of the brainstem in the rabbit were studied by the retrograde transport of horseradish peroxidase (HRP). After selective iontophoretic injections of the tracer into various subdivisions of the vestibular nuclear complex (VNC), labeled neurons were found in defined regions of the reticular formation (RF) of the caudal pons and the rostral medulla. The results indicate that all four vestibular nuclei receive projection from RF. This projection is bilateral with a contralateral predominance. The major projection originates from dorsal and dorsolateral regions of the caudal pontine reticular nucleus (RPc) and the gigantocellular reticular nucleus (RGc) at the transitional level between them. A modest projection originates from pars alpha of the caudal pontine reticular nucleus (RPc alpha), the parvocellular reticular nucleus (Rpc) and pars alpha of the parvocellular nucleus (Rpc alpha), mostly from their ventral regions. A small projection arises from pars alpha of the gigantocellular reticular nucleus (RGc alpha), as well as from the ventral reticular subnucleus (Rv) and cell group a in the caudal aspect of the medulla. No clear-cut topical relationship was noted between the location of neurons in RF and projection site in VNC. The superior vestibular nucleus (SV) and the medial vestibular nucleus (MV) receive projections exclusively from RPc and RGc, whereas the lateral reticular nucleus (LV) and the inferior vestibular nucleus (IV) receive additional projections from the remaining RF nuclei. The termination areas of reticular fibers within SV and IV seem to be diffuse but in MV and LV there is a clear preponderance to the regions located ventrally. The present study has established cells of origin for the reticulovestibular projections from the pontomedullary RF to individual VNC nuclei in the rabbit.

The origin of high and regular discharge rates of eye-movement-related neurons in the region of the interstitial nucleus of Cajal

Burst-tonic neurons in the region of the interstitial nucleus of Cajal (INC) that show a close correlation to vertical eye movement have been known to exhibit high and regular discharge rates, not only during fixation in alert animals, but also during sleep. Since they receive major input from vertical semicircular canals, we examined in this study whether or not the source of the high and regular discharge rates was the primary vestibular afferents. Infusion of lidocaine into the middle ear bilaterally resulted in a significant decrease of mean discharge rates and an increase in the coefficient of variation of the mean rates. However, burst-tonic neurons in cats that had received bilateral labyrinthectomy 6 weeks previously still exhibited high and regular discharge rates similar to those of normal cats. These results indicate that high and regular discharges of eye-position-related INC cells are maintained largely by input from primary vestibular afferents in normal cats. However, such characteristic discharges could also be maintained centrally in the brainstem without peripheral vestibular input in labyrinthectomized cats.

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

Bilateral lesions of the midbrain reticular formation within, and in the close vicinity of, the interstitial nucleus of Cajal (INC) result in the severe impairment of the ability to hold eccentric vertical eye position after saccades, phase advance and decreased gain of the vestibulo-ocular reflex (VOR) induced by sinusoidal vertical rotation. In addition, the INC region of alert animals contains many burst-tonic and tonic neurons whose activity is closely correlated with vertical eye movement, not only during spontaneous saccades, but also during the VOR, smooth pursuit and optokinetic eye movements. Although their activity is closely related to these conjugate vertical eye movements, it is different from the oculomotor motor neuron activity. These results indicate that the INC region is involved in, and indispensable for, some aspects of eye position generation during vertical eye movement. Further comparison of INC neuron discharge with eye movements during two special conditions indicates that the INC region alone cannot produce eye position signals. First INC neuron discharge shows no response or an 80 degrees phase advance (close to the expected value if there is no integration) in the dark compared to the light during sinusoidal vertical linear acceleration in alert cats. Second, during rapid-eye-movement (REM) sleep, the discharge of INC neurons is no longer correlated with eye position. These results imply that the INC is not the entire velocity-to-position integrator, but that it has to work with other region(s) to perform the integration. A close functional linkage has been described between vertical-eye-movement-related neurons in the INC region and vestibulo-ocular relay neurons related to the vertical semicircular canals in the vestibular nuclei. It has been suggested that both are the major constituents of the common neural integrator circuits for vertical eye movements.

Comparative topography of projections from the mesodiencephalic junction to the inferior olive, vestibular nuclei, and upper cervical cord in the cat

Distributions of neurons located in the central rostral mesencephalon and caudal diencephalon that project to the upper cervical spinal cord, vestibular nuclei, or inferior olive were studied in the cat by using retrograde axonal transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Afferent sources to all of these targets were observed in the interstitial nucleus of Cajal (INC), the region surrounding the fasciculus retroflexus (PF), and the nucleus of the fields of Forel (NFF). Three-dimensional reconstruction revealed differences in densities of cells projecting from these common areas. Spinal projecting cells were present in slightly greater numbers in the caudal two-thirds of the INC, whereas those projecting to the vestibular complex were more numerous in the rostral two-thirds of this nucleus. A relatively smaller number of olivary projecting cells were dispersed throughout the INC. Olivary afferent sources outnumber those with spinally directed or vestibularly directed axons in the PF region. In the fields of Forel, cells projecting to the vestibular nuclei or inferior olive were concentrated medially, whereas cells projecting to the spinal cord appeared both medially and laterally. Each type of afferent source was also seen in the nucleus of the posterior commissure and the posterior ventral lateral hypothalamic area. Unique sources of afferents to the inferior olive were observed in the parvicellular red nucleus (ipsilateral to the injections) and the anterior and posterior pretectal nuclei. A large number of labeled neurons was seen in the nucleus of Darkschewitsch after injections of tracer into the inferior olive, but this projection did not appear to be unique, as small numbers of labeled cells were also seen after injections into the cervical spinal cord. The Edinger-Westphal nucleus and the adjacent somatic oculomotor nucleus contained cells which projected separately to the spinal cord or the vestibular complex, and the superior colliculus contained cells which projected separately to the contralateral spinal cord or the contralateral inferior olive. In this study, it was also noted that neurons in the medial terminal nucleus of the accessory optic tract projected to the ipsilateral inferior olive or to the contralateral vestibular complex. These differences in locations and densities of cells projecting to the cervical spinal cord, vestibular complex, and inferior olive may underlie functional specializations in these areas in relation to vertical eye and head movement control and to neural systems controlling postural adjustments accompanying limb movements.

Mesodiencephalic projections to the vestibular complex in the cat

The distribution of cells in the rostral medial mesencephalon and caudal diencephalon which project to the vestibular complex was mapped in the cat by using retrograde axonal transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Subsequent experiments using anterograde transport of WGA-HRP clarified the position of the terminations of the mesodiencephalic-derived afferents in the vestibular complex. After large injections which involved the entire vestibular complex, retrogradely labeled cells were seen in both the ipsilateral and contralateral interstitial nucleus of Cajal (INC) and were more numerous in its rostral pole. Labeled cells also occurred in the perifascicular region, both immediately adjacent to the fasciculus retroflexus and rostroventral to it. Fusiform midline cells of the Edinger-Westphal nucleus were also labeled, as well as a number of cells in the adjacent somatic portion of the oculomotor complex (OMC). Another group of labeled cells was observed within the contralateral medial terminal nucleus of the accessory optic tract (MTN) and in the posterior hypothalamic nucleus. Injections limited to subregions of the vestibular complex resulted in similar but slightly varying distributions and numbers of retrogradely labeled cells. After injections covering the caudal half of the medial vestibular nucleus (MVN) and descending vestibular nucleus (DVN), labeled cells in the INC and tegmentum dorsal to it were especially prominent, but none was seen in the MTN or OMC. Injections placed in the rostral MVN, lateral vestibular nucleus, y group, and superior vestibular nucleus resulted in a distribution of labeled cells similar to that seen following global vestibular injections, but these cells were fewer in number. After an injection confined to the y group, a small number of retrogradely labeled cells were seen in the rostral pole of the INC and immediately ventral to the fasciculus retroflexus. Projections from the rostral medial mesencephalon and caudal diencephalon to the MVN, DVN, and y group were confirmed by using anterograde transport of WGA-HRP. Direct projections from the INC-perifascicular regions and somatic neurons of the OMC to the caudal vestibular complex could play a role in eye-head coordination. Those projections from the rostral INC and MTN to the rostral vestibular complex may play a role in vertical eye movements and responses to visual stimuli which move in the vertical plane.

Effects of lesion of the interstitial nucleus of Cajal on vestibular nuclear neurons activated by vertical vestibular stimulation

Experiments were performed in cats anesthetized with nitrous oxide to study the effects of INC lesions on responses of vestibular nuclear neurons during sinusoidal rotations of the head in the vertical (pitch) plane. Responses of neurons in the INC region were recorded during pitch rotations at 0.15 Hz. A great majority of these neurons did not respond to static pitch tilts, and they seemed to respond either to anterior or to posterior semicircular canal inputs with a peak phase lag of 140 deg (re head acceleration). Responses of vestibular nuclei neurons in intact cats were recorded during pitch rotations at the same frequency (0.15 Hz). Neurons that seemed to respond to vertical semicircular canal inputs showed peak phase lags of 90 deg relative to head acceleration, whereas neurons that responded to static pitch tilts showed peak phase shifts near 0 deg. These results indicate that responses of neurons in the INC region lag those of vestibular neurons by about 50 deg, suggesting that the former neurons possess a phase-lagging (i.e. integrated) vestibular signal. Responses of vestibular neurons in cats that had received electrolytic lesions of bilateral INCs 1-2 weeks previously were recorded during pitch rotations at the same frequency (0.15 Hz). Neurons that presumably responded to vertical semicircular canal inputs showed a peak phase lag of 60 deg relative to head acceleration, a significant decrease of the phase lag compared to normal, whereas responses near 0 deg were unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional differences of brain glucose metabolic compensation after unilateral labyrinthectomy in rats: a [14C]2-deoxyglucose study

A unilateral labyrinthectomy was performed on anesthetized adult albino rats. Brain [14C]2-deoxyglucose (2DG) uptake was measured autoradiographically 3.5 h to 20 days later and compared to sham-operated controls. In the vestibular nuclei (nn.) of labyrinthectomized subjects, large left-right differences of 2DG uptake occurred, which decreased over time. The equalization of vestibular nuclear 2DG uptake paralleled behavioral compensation of body, neck and head postural abnormalities, and known equalization of vestibular nuclear cell firing rates during compensation. There was a small difference of 2DG uptake in medial and lateral vestibular nn. 20 days after lesions when animals had a residual head tilt and tonic eye deviation. In the oculomotor nn., trochlear nn. and interstitial n. of Cajal, large left-right differences of 2DG uptake occurred, which did not change over time. The higher 2DG uptake in these nn. occurred ipsilateral to the labyrinthine lesion and did not correlate with the onset and cessation of nystagmus. The persistent asymmetry did appear to correlate with ipsilateral downward and contralateral upward eye deviation which continued for long periods after the lesion. We hypothesize that the non-compensating metabolic asymmetry in the oculomotor and trochlear nn. could be due to lesioned otolithic input to the vestibular nn. which relays to trochlear and oculomotor nn.

Projections of the group y of the vestibular nuclei and the dentate and fastigial nuclei of the cerebellum to the interstitial nucleus of Cajal

Experiments were performed to study the projection of the group y of the vestibular nuclei and the dentate and fastigial nuclei of the cerebellum to the interstitial nucleus of Cajal (INC) in cats by using retrograde axonal transport of horseradish peroxidase (HRP) and electrophysiological methods; and to study the vestibular responses of such projection neurons. Following injections of HRP into the unilateral INC, with partial involvement of the surrounding reticular formation, including the nucleus of Darkschewitsch (ND), many retrogradely labeled neurons were found in the dorsal part of the group y nucleus contralateral to the injection site. Labeled cells were also seen in the contralateral dentate nucleus, frequently in its caudal-ventral part, and in the contralateral fastigial nucleus at all rostrocaudal levels, but most frequently in its caudal part. In electrophysiological experiments performed on cats anesthetized with alpha-chloralose or N2O and paralyzed with gallamine, group y, dentate and fastigial nuclei neurons were antidromically activated by weak stimuli that were confined to the contralateral INC. Depth-threshold curves for antidromic activation of such neurons revealed that the lowest threshold points were within the INC, but not in the ND. The INC-projecting neurons in the group y and dentate nuclei did not respond to electrical stimulation of the ipsilateral or contralateral vestibular nerve, indicating that they do not receive direct labyrinthine inputs. On the contrary, many fastigial neurons projecting to the INC responded to labyrinthine stimulation, suggesting that they may be involved in the vestibular reflexes. These results suggest a difference in properties of INC-projecting neurons in these nuclei.

Connections and oculomotor projections of the superior vestibular nucleus and cell group 'y'

Attempts were made to determine brainstem and cerebellar afferent and efferent projections of the superior vestibular nucleus (SVN) and cell group 'y' ('y') in the cat using axoplasmic tracers. Injections of HRP, WGA-HRP and [3H]amino acids were made into SVN and 'y' using two different infratentorial stereotaxic approaches. Controls were provided by unilateral HRP injections involving the oculomotor nuclear complex (OMC), the interstitial nucleus of Cajal (INC) and the deep cerebellar nuclei (DCN). Large injections of SVN almost invariably involved 'y' and dorsal parts of the lateral vestibular nucleus (LVN). Smaller injections involved central and ventral peripheral parts of SVN. Discrete injections of 'y' involved small dorsal parts of LVN. Afferents to SVN are derived mainly from the vestibular nuclei (VN) and parts of the vestibulocerebellum. SVN receives afferents: bilaterally from caudal portions of the medial (MVN) and inferior (IVN) vestibular nuclei and 'y'; contralaterally from ventral and lateral parts of SVN and rostral MVN; and ipsilaterally from the nodulus, uvula and medial parts of the flocculus. Purkinje cells (PC) in medial parts of the flocculus project to central regions of SVN, while PC in the nodulus and uvula appear to project mainly to dorsal peripheral regions of SVN. SVN receives sparse projections from the ipsilateral INC, the contralateral central cervical nucleus (CCN) and virtually no projections from the reticular formation. SVN projects via the medial longitudinal fasciculus (MLF) to the ipsilateral trochlear nucleus (TN), the inferior rectus subdivision of the OMC, the INC, the nucleus of Darkschewitsch (ND) and the rostral interstitial nucleus of the MLF (RiMLF). Contralateral projections of SVN cross in the ventral tegmentum caudal to most of the decussating fibers of the superior cerebellar peduncle and terminate in the dorsal rim of the TN and the superior rectus and inferior oblique subdivisions of the OMC; sparse crossed projections enter the INC and the ND. Cerebellar projections of SVN end as mossy fibers in the ipsilateral nodulus, uvula and in medial parts of the flocculus bilaterally. Retrograde transport from unilateral injections of the OMC indicate that afferents from SVN arise ipsilaterally from central and dorsal regions and contralaterally from dorsal peripheral regions. Ventral cell group 'y' receives small numbers of afferent fibers from caudal central parts of the ipsilateral flocculus. No fibers from ventral 'y' could be traced to other vestibular nuclei, the OMC or the cerebellum.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of lesion of the interstitial nucleus of Cajal on vestibular horizontal canal neurons in the cat

Effects of procaine infusion into the interstitial nucleus of Cajal (INC) on vestibular nuclear neurons related to the horizontal canal were studied in cats anesthetized with nitrous oxide and paralyzed with gallamine. Neurons that responded to sinusoidal horizontal rotation (at 0.18 Hz) were recorded extracellularly in the medial and descending vestibular nuclei. Spontaneous activity of type I neurons increased, whereas that of type II neurons decreased following procaine infusion into the ipsilateral INC. Gain of the neuronal response to horizontal rotation decreased after the ipsilateral INC infusion, but there was no consistent effect on phase. Infusion into the contralateral INC seemed less effective. Similar effects were obtained with electrolytic lesions that were confined to the ipsilateral INC area. These results suggest that the INC influences type I neurons through inhibitory action of type II neurons and that it eventually controls the gain, but not the phase, of the horizontal vestibular reflexes.

Interstitial-vestibular interaction in the control of head posture

Experiments were performed in cats to determine whether the head tilt following a unilateral lesion of the interstitial nucleus of Cajal (INC) can be attributed to removal of interstitiospinal fibers which have direct excitatory synaptic connections with ipsilateral neck extensor (biventer cervicis-complexus) and flexor (sternocleidomastoid, SCM) motoneurons. Unilateral INC lesions were made either electrolytically or reversibly by procaine infusion into the INC, and electromyographic activity was recorded bilaterally from biventer (BIV), splenius (SP) and SCM muscles. In both groups of lesions, activity of the ipsilateral SP and BIV was higher than that of the contralateral ones. When procaine was infused into the INC of awake cats, an increase of activity of the ipsilateral SP began before the cats presented the typical head tilt to the opposite side. Bilateral INC lesions caused dorsiflexion of the head. These results indicate that the head tilt resulting from unilateral INC lesions can not be explained by simple removal of the ipsilateral, direct excitatory interstitioneck impulses. When unilateral INC lesions were combined with hemilabyrinthectomies, cats that were given labyrinthectomies on the side opposite to the previous INC lesions showed very severe head tilt, whereas cats that received labyrinthectomies on the same side did not show obvious head tilt. Furthermore, it took a much longer time for the cats of the former group to compensate the head tilt than it took those that had single lesions of the INC or labyrinth.(ABSTRACT TRUNCATED AT 250 WORDS)

Responses of cat vestibular neurons to stimulation of the frontal cortex

To study the neural basis for the regulation of vestibulocollic reflexes during voluntary head movements, the effects of stimulation of the precruciate cortex near the presylvian sulcus (neck area of the motor cortex) and the frontal eye fields (FEF) on vestibular neurons were studied in cerebellectomized cats anesthetized with alpha chloralose. Neurons were recorded in the medial and descending vestibular nuclei and antidromically identified from C1. Stimulation of the FEF and precruciate cortex fired 29 and 13% of neurons that did not exhibit spontaneous activity. About 80% of spontaneously discharging neurons were influenced by stimulation of either of the two. Stimulation of the precruciate cortex or FEF suppressed or facilitated labyrinthine evoked monosynaptic activation of vestibulospinal neurons, suggesting that the frontal cortical neurons have the properties to regulate the vestibulocollic reflexes.