Midbrain and contralateral labyrinth influences on brain stem vestibular neurons in the cat

Cross-striolar and commissural inhibition in the otolith system

Neural connections from the saccular and utricular nerves to the ipsilateral vestibular neurons and the commissural effects were studied by using intracellular recordings of excitatory (E) and inhibitory (I) postsynaptic potentials (PSPs) in vestibular neurons of cats after focal stimulation of the saccular and the utricular maculae. Neural circuits from the maculae to vestibular neurons, termed cross-striolar inhibition, may provide a mechanism for increasing the sensitivity to linear acceleration and tilt of the head. It was examined whether secondary vestibular neurons activated by an ipsilateral otolith organ received a commissural inhibition from a contralateral otolith organ that occupied the same geometric plane. Results suggest that utricular-activated vestibular neurons receiving commissural inhibition may provide a mechanism for increasing the sensitivity to horizontal linear acceleration and tilt of the head. The commissural inhibition of the saccular system was much weaker than that of the utricular system.

Excitatory and inhibitory inputs from saccular afferents to single vestibular neurons in the cat

Connections from saccular afferents to vestibular neurons were studied by means of intracellular recordings of excitatory (E) and inhibitory (I) postsynaptic potentials (PSPs) in vestibular neurons after focal stimulation of the saccular macula in decerebrated cats. Focal stimulation was given to the saccular macula in two ways, in which the polarity of stimulus current via a pair of electrodes was changed. In group A, one of the electrodes was inserted into the ventral and the other into the dorsal edge of the saccular macula. The focal stimulation was across the striola so that the reversal of morphological polarization in hair cells was bridged by the pulse stimulus. In 22/36 vestibular neurons tested, the stimulation of the saccular macula evoked monosynaptic (</=1.2 ms) EPSPs, including EPSP-IPSP sequences, with one polarity of stimulation, and disynaptic (>/=1.5 ms) IPSPs when the polarity of the stimulus current was changed. In 14/36 neurons, the response pattern was the same regardless of the stimulus polarity; EPSPs (12/36) or IPSPs (2/36). In group B, a pair of electrodes was inserted into the dorsal edge of the saccular macula, so that the striola was not bridged by the current stimulus. In all of the vestibular neurons tested, the response pattern was always the same regardless of the polarity: mono- (22/31) and disynaptic (3/31) EPSPs or disynaptic IPSPs (6/31). In addition, the saccular nerve was stimulated after removing the macula in some cats (group C). The stimulation of the saccular nerve evoked EPSPs in 62 vestibular neurons (including EPSP-IPSP sequences in 31 neurons) and IPSPs in 19 vestibular neurons. Convergence between the saccular nerve and other vestibular nerves was studied by the intracellular recording of PSPs. Fifty-six percent (18/32) of the saccular-activated neurons had excitatory and/or inhibitory potentials evoked after stimulation of the utricular nerve and the horizontal and anterior semicircular canal nerves, and 44% (19/43) of the neurons received inputs from the posterior semicircular canal nerve. The results support the hypothesis that saccular afferents from one population of hair cells activate vestibular neurons monosynaptically and that afferents from another population of hair cells located on the opposite side of the striola appear to project to the same vestibular neurons disynaptically via inhibitory interneurons. Neural circuits from saccular afferents to vestibular neurons, which we term cross-striolar inhibition, thus may provide a mechanism for increasing the sensitivity to vertical linear acceleration. The circuit described is provided not only with high sensitivity but also with input noise-resistant characteristics.

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 influence of middle ear pressure on the lateral vestibulospinal tract neurons in cats

The influence of changes in middle ear pressure on the lateral vestibulospinal tract (LVST) neurons was investigated in anesthetized cats. The activities of some LVST neurons increased with negative pressure and decreased with positive pressure. This tendency was stronger when pressure changes were applied to the contralateral middle ear than to the ipsilateral side. The influence of changes in middle ear pressure on LVST neurons was opposite to that on the vestibular nerve, suggesting the existence of inhibitory inter- and commissural neurons.

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)

The commissural inhibition on secondary vestibulo-ocular neurons in the vertical semicircular canal systems in the cat

The commissural inhibition on secondary vestibulo-ocular neurons (VOns) from the contralateral (c-) vertical canal system in the same geometric plane was studied in the anesthetized cat. The secondary VOns were identified by their orthodromic responses to stimulation of the ampullary nerves of the anterior (ACN) or posterior (PCN) semicircular canals and also by their antidromic responses to stimulation of the IIIrd and IVth nuclei. The majority of ACN-activated excitatory VOns in the descending and medial nuclei (32/36, 89%) and in the superior nucleus (20/23, 87%), received commissural inhibition from the c-PCN, while only few ACN-activated inhibitory VOns (3/35, 9%) in the superior nucleus received commissural inhibition from the c-PCN. On the other hand, all of the PCN-activated excitatory (50/50) and inhibitory (30/30) VOns in the vestibular nuclei received commissural inhibition following c-ACN stimulation.

Neuronal mechanisms of the interaction of Deiters' nucleus with some brainstem structures

The effects of stimulation of the interstitial nucleus of Ramón y Cajal, as well as the nucleus of Darkschewitsch, inferior olive and nucleus reticularis tegmenti pontis on the neuronal activity of the lateral vestibular nucleus of Deiters were studied by means of an intracellular recording technique. Stimulation of these structures is shown to lead to antidromic and orthodromic activation of Deiters' neurons. Collaterals of vestibulospinal neurons entering these structures are revealed electrophysiologically. It was shown that stimulation of the rostral part of the inferior olive evoked in Deiters' neurons predominantly antidromic responses, whereas stimulation of the caudal part of the inferior olive leads mostly to synaptic activation. Stimulation of Ramón y Cajal's and Darkschewitsch's nuclei evokes mono- and/or polysynaptic excitatory and inhibitory postsynaptic potentials in Deiters' neurons. Mono-, oligo- and/or polysynaptic inhibitory postsynaptic potentials were also evoked by stimulation of nucleus reticularis tegmenti pontis, as well as the rostral and particularly, caudal parts of the inferior olive. Stimulation of the caudal part of the inferior olive evoked mono-, oligo- and/or polysynaptic excitatory postsynaptic potentials in Deiters' neurons. Convergence of influences from the stimulated structures on the vestibular neurons under study was shown. A topical correlation between Deiters' nucleus and the brainstem nuclei mentioned above was found. The presence of inhibitory and excitatory interaction of these structures with Deiters' nucleus was established.

Commissural and ipsilateral internuclear connection of vestibular nuclear complex of the cat

Commissural and ipsilateral intrinsic connections of the vestibular nuclear complex of cats were investigated using retrograde transport of horseradish peroxidase (HRP). HRP was microiontophoretically injected into limited areas (0.2-0.5 mm in diameter) of the respective vestibular nuclei. In the commissural connections, major fibers were observed between the bilateral superior vestibular nuclei (SVN) and between the bilateral descending vestibular nuclei (DVN); a moderate number of fibers was found from the medial vestibular nucleus (MVN) to the contralateral MVN, SVN and lateral vestibular nucleus (LVN) and from the DVN to the contralateral LVN. Minor commissural connections were detected between the bilateral LVN. The ipsilateral internuclear connections of the vestibular nuclear complex were: (1) from the LVN, MVN and DVN to the SVN, (2) from the MVN and DVN to the LVN and (3) from the MVN to the DVN. Minor ipsilateral intrinsic connections were found from the SVN to the MVN.

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.

Neural correlates of compensation after hemilabyrinthectomy

Vestibular brain stem neurons responsive to angular acceleration in the plane of the horizontal canal were examined in cats 30 to 52 days after contralateral labyrinthectomy and compared with similar units in cats with intact labyrinths and in other cats immediately after transection of the contralateral eighth nerve. In the compensated state, the mean spontaneous firing rate of type I neurons was 24 spikes/s, in contrast to the mean of 45/s observed immediately after contralateral labyrinthectomy. In intact cats, mean firing rate was 19 spikes/s. Sensitivity, as measured in spikes/s/deg/s2, was significantly lower immediately after labyrinthectomy than in intact controls and remained so in compensated cats. On the other hand, time constants and the ratio of adapting:nonadapting units was unchanged. Ablation of the midline cerebellum including vermis and fastigial nuclei did not materially affect these results. We concluded that (i) the main defects in static posture and nystagmus in the uncompensated state were due to the striking difference in resting firing rates between the ipsi- and contralateral vestibular nuclei; (ii) compensation in the static posture was the result of a tendency to equalize the resting firing rates in the two vestibular nuclei; and (iii) recovery in the dynamic, head-turning situation was due to partial recovery of sensitivity on the ipsilateral side, bringing it to the relatively constant, unchanging depressed sensitivity on the contralateral side.

Effects of stimulation of frontal cortex, superior colliculus, and neck muscle afferents on interstitiospinal neurons in the cat

Interstitiospinal neurons were activated by antidromic stimulation of the spinal cord ventromedial funiculus at C1 and C4 in cerebellectomized cats under chloralose anesthesia. Neurons responding only to C1 were classified as N cells and those responding both to C1 and C4 were classified as D cells, as in previous experiments (Fukushima et al. 1980a). Vestibular branching interstitiospinal and reticulospinal neurons were also identified as in the previous experiments. Stimulation of the ipsilateral pericruciate cortex evoked firing in 31% of N cells, 41% of D cells and 35% of the vestibular branching neurons, while stimulation of the contralateral cortex excited 6% of N cells, 29% of D cells and 14% of vestibular branching neurons. Response latencies ranged from 2 to 15 ms after the effective pulse. By measuring the thresholds of activation of these neurons while changing the depth of the stimulating electrodes, and by mapping the cortical areas, it was shown that the lowest threshold areas were in the frontal eye fields and the anterior sigmoid gyrus near the presylvian sulcus (Area 6). Stimulation of the latter area often evoked neck or shoulder muscles contraction. Stimulation in the deep layers of the ipsilateral superior colliculus evoked firing in about 20% of interstitiospinal neurons and about 42% of vestibular branching neurons, with typical latencies 2-3 ms after the effective pulse, while stimulation of the contralateral superior colliculus was rarely effective. N cells and D cells responded similarly. Thresholds for activation were high in the intermediate tectal layers and declined as the electrodes entered the underlying tegmentum. This suggests that the superior colliculus is not the main source of synaptic inputs to these neurons. Low threshold points were found above the deep fiber layer when stimulating electrodes were inserted into the pretectum. Stimulation of the C2 biventer cervicis nerve excited about 8% of N cells, 18% of D cells, and 15% of vestibular branching neurons bilaterally with typical latencies around 10 ms. Similar results were obtained when C2 splenius nerves were stimulated. The fibers responsible for such excitation are probably group II, since stimuli stronger than 1.8 times threshold of the lowest threshold fibers were needed to evoke excitation. Response decrement was often observed when stimuli were repeated at 1/s, while no such decrement was observed at the rate of 1/3 s. When the convergence of cortical and labyrinthine excitatory inputs was studied, 36% of interstitiospinal neurons received single inputs either from the pericruciate cortex or from the labyrinth, 22% of neurons received convergent excitation from both and the remaining 42% did not respond to either stimulus. Although vestibular branching neurons rarely received labyrinthine inputs, they frequently showed convergence of excitation to stimulation of the frontal cortex, superior colliculus and vestibular nuclei.

Vestibular responses and branching of interstitiospinal neurons

1. Interstitiospinal neurons were activated by antidromic stimulation of the ventromedial funiculus of the spinal cord at C1 and C4 in cerebellectomized cats under chloralose anesthesia. 46% of these neurons responded only at C1 (N cells) and the remaining 54% responded at C4 also (D cells). There is no topographical difference in the location of N and D cells. Conduction velocities of N cells were significantly slower than those of D cells. 2. Stimulation of the contralateral whole vestibular nerve evoked firing of 31% of both N and D cells; some responded early enough to suggest disynaptic connections, many responded late. Stimulation of the ipsilateral whole vestibular nerve evoked firing of several cells, one spontaneously discharging D cell was inhibited. 3. Stimulation of the contralateral individual semicircular canal nerves evoked firing of 33% of N cells and 13% of D cells. Most of these responses were late. N cells responded not only to the vertical canals but also to the horizontal canal, whereas D cells responded to the horizontal canal, but seldom to the vertical ones. Most canal responding neurons received specific input, only two N cells received convergent input from both the anterior and horizontal canals. Stimulation of the ipsilateral canals did not evoke excitation of any cells tested; one D cell was inhibited by stimulation of the horizontal canal nerve. 4. Stimulation of the rostral medial vestibular nucleus evoked characteristic negative field potentials centered in the contralateral interstitial nucleus of Cajal (INC). Approximately 60% of both N and D cells received excitation from the contralateral vestibular nuclei. About 17% of these responding neurons received monosynaptic excitation, most frequently from the rostral medial nucleus. Stimulation of the ipsilateral vestibular nuclei evoked firing of 12% of both N and D cells. 5. Twenty-nine neurons were fired antidromically by weak stimuli applied to the ipsilateral vestibular nuclei. Twenty-seven of the 29 were activated only from C1 and were found in the INC (10 cells) and in the reticular formation dorsal to the INC (19 cells). Measurement of the spread of the effect of stimulus current and comparison of latencies to stimulation of the vestibular nuclei and C1 indicated that these neurons have axon collaterals going to the ipsilateral vestibular nuclei. Only one of them received excitation from the contralateral posterior canal, others did not respond to the labyrinth. Some were activated by stimulation of the vestibular nuclei.

Properties of central vestibular neurons fired by stimulation of the saccular nerve

In 4 cats all vestibular afferents in one labyrinth except those innervating the saccular macula were transected and allowed to degenerate. 23--53 days after the initial surgery the central connections of the remaining saccular nerve were studied under chloralose anesthesia. Stimulation of the saccular nerve evoked N1 field potentials in the ipsilateral lateral and descending vestibular nuclei; little or no field potential activity was seen in the superior nucleus. The distribution of field potentials overlapped with that of neurons of origin of the vestibulospinal tracts. Forty-two neurons in the ipsilateral vestibular nuclei, many in the lateral nucleus, responded, often monosynaptically, to stimulation of the saccular nerve with single or double shocks; some of the neurons projected to the spinal cord. All saccular-fired neurons were tested for commissural actions by stimulation of the contralateral vestibular nerve. Many were facilitated, almost none were inhibited. In agreement with earlier work, we conclude that commissural inhibition may be a property of the canal system only.

The contribution of the contralateral labyrinth to second order vestibular neuronal activity in the cat

Vestibular brain stem units of the horizontal canal in the awake C1 transected cat were examined in acute experiments before and after the contralateral vestibular nerve was cut. The latter procedure resulted in an immediate increase in mean resting rate from 19 to 45 spikes/sec and greater spontaneous fluctuations in firing rate. Mean sensitivity to constant accelerations from 2 to 16 degrees/sec2 was significantly reduced. In the 4-6 degrees/sec2 range, for example, mean sensitivity fell from 5.7 +/- 3.2 spike/sec/deg/sec2 (range 0.7-13.6, n = 107) to 2.8 +/- 1.4 (range 0.2-5.5, n = 29). Mean sensitivity to 4-6 degrees/sec2 deceleration was reduced from about 4.5 to 3.0. On the other hand, time constants and the ratio of adapting to non-adapting units was unchanged. Removal of the midline cerebellum, including the vermis and fastigial nuclei in a portion of the animals, did not materially alter the above results. It is concluded that the contralateral labyrinth, acting via the vestibular brain stem commissural pathway, not only exerts a powerful effect on the resting activity of brain stem canal vestibular neurons, but also on their response to acceleration.

Labyrinthine input to the vestibular nuclei of the awake cat

The labyrinthine input to the vestibular nuclei was investigated in 24 awake cats. Stimulus consisted of electrical shocks given through bipolar silver wire electrodes, implanted in the utricular and lateral ampullar nerves. Throughout the vestibular nuclei, single units were recorded extracellularly with glass micropipettes filled with Fast Green. The tracts of the penetrating electrodes were identified histologically. In all four nuclei units responding to both labyrinths outnumbered unilaterally responding neurones with certain differences between the individual nuclei. Excitatory as well as inhibitory responses were observed, polysynaptic being more common than mono- or disynaptic ones. No monosynaptic contralateral responses were seen. The latency distribution of contralateral responses closely mirrored that of ipsilateral responses within each nucleus. Both excitatory and inhibitory responses fell into relatively segregated populations, based upon latency distribution. This implies separate pathways for labyrinthine input to the vestibular nuclei.

Crossed sacculo-ocular pathway via the Deiters' nucleus in cats

Single unit recordings were made in the Deiters' nucleus of cats in response to electrical stimulation of the superior area of the contralateral saccule and the ipsilateral oculomotor (IIIrd) nuclear complex. Correct placements of the stimulating and recording electrodes were identified by upward movements of the eyeballs associated with characteristic field potentials in the Deiters' nucleus. Stimulation of the contralateral saccule induced a negative N wave followed by positive P1 and P2 potentials. One hundred twenty-two units responded to contralateral saccular stimulation with a mean latency of 2.48 +/- 0.06 msec (SEM). Of these, only the responses of 31 cells could be made to collide with antidromic spikes evoked from ipsilateral IIIrd nuclear stimulation. This implies that the crossed sacculo-ocular pathway would involve at least four neurons with one commissural cell interposing between the bilateral vestibular nuclei.