Canal-neck interaction in vestibular nuclear neurons of the cat

Summary The convergence and interaction of horizontal semicircular canal and neck proprioceptive inputs were studied in neurons of the caudal two thirds of the vestibular nuclear complex. Extracellular neuron activity was recorded under muscle relaxation and slight anesthesia in chronically prepared cats. The following stimulations were applied: horizontal rotations of (a) the whole body (labyrinth stimulation), (b) the trunk vs. the stationary head (neck stimulation), and (c) the head vs. the stationary trunk (combined labyrinth and neck stimulation).Of 152 neurons investigated, 83 (55%) showed convergence of the two inputs. In about half of these neurons, the neck input was very weak and hardly affected the labyrinthine response during head rotation. Judged from the response pattern, several of these neurons presumably were related to vestibulo-oculomotor function (i.e., vestibular nystagmus). In the other half (i.e., 27% of all neurons), sensitivity of the two inputs was similar. Both labyrinthine and neck responses contained a dynamic (velocity) component; neck responses of more than half of these neurons had, in addition, a static (position) component. The dynamic components were either antagonistic or synergistic as to their convergence during head rotation. When applying this combined stimulation, the dynamic components summed linearly, yielding subtraction in case of antagonistic convergence and addition in case of synergistic convergence. In contrast, the static components of the neck responses remained largely unchanged during head rotation. However, the static head-to-trunk deflection determined the tonic discharge level in such neurons and thus facilitated or disfacilitated the dynamic responses to superimposed labyrinth stimulation.We suggest that the two patterns of labyrinthine-neck interaction observed in vestibular nuclear neurons, i.e., subtraction and addition, may be involved in the postural control of the trunk and head, respectively. In contrast, interference of the neck input with vestibule-oculomotor function appears to be almost negligible in the intact cat.Supported by Deutsche Forschungsgemeinschaft, SFB 70     

Modulation of the jaw-opening reflex by stimulation of the vestibular nuclear complex in rats

The effect of stimulation of the vestibular nuclear complex (VN) on the jaw-opening reflex (JOR) was studied in anesthetized rats. The JOR was evoked by electrical stimulation of the inferior alveolar nerve, and was recorded as the electromyographic responses of the anterior belly of the digastric muscle, bilaterally. Conditioning electrical stimulation of the medial (MVN), lateral (LVN) and superior (SVN) vestibular nuclei facilitated the JOR bilaterally. Microinjection of monosodium glutamate into the SVN, LVN and MVN also facilitated the JOR bilaterally. In contrast, conditioning electrical stimulation of the inferior vestibular nucleus (IVN) inhibited the JOR ipsilaterally. Microinjection of monosodium glutamate into the IVN also inhibited the JOR ipsilaterally. These results suggest that the VN is involved in modulation of the JOR and plays an important role in controlling the jaw movements.     

Neuronal activity in the vestibular nuclei of the alert monkey during vestibular and optokinetic stimulation

Summary Recordings from neurons of the vestibular nuclei were performed in alert monkeys. Type I and type II units were identified by rotating the monkey about a vertical axis. All neurons responded also when only the visual surround was rotated around the stationary monkey. The combination of visual and vestibular stimulation points towards non-algebraic summation characteristics for the two inputs, with each input dominating the response over a certain range.Supported by Swiss National Foundation for Scientific Research 3.044.76 and Emil-Barell-Foundation of Hoffmann-La Roche, Basel, Switzerland     

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

Summary 1. 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). 2. 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. 3. 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. Gain values of individual cells also significantly dropped from 2.07 ± 0.67 spikes · s⁻¹/deg · s⁻²2 (mean ± SD; normal cats) to 1.27 ± 0.68 spikes · s⁻²/deg · s⁻² (INC lesioned cats) at 0.15 Hz. When responses of vestibular neurons were studied during pitch rotations in the range of 0.044–0.49 Hz in these cats, a large decrease of the phase lag was observed at lower frequencies, whereas the slopes of phase lag curves of vestibular neurons in intact cats were rather flat. 4. Procaine infusion into the bilateral INCs not only resulted in a decrease of 20–50 deg in the phase lag in responses of vestibular neurons that had lagged head acceleration by 90–140 deg before procaine infusion, but also dropped the gain of the response to rotation by an average of 31%, whereas responses of neurons that had showed phase shifts near 0 deg were not influenced consistently. Simultaneous recording of the vestibular neurons and the vertical vestibuloocular reflex (VOR) indicated that the phase advance and gain drop of vestibular neurons occurred earlier than those of the VOR. These results exclude the possibility that the change in dynamic response of vestibular neurons after procaine infusion is due to depression of general brain stem activity that may lead to the phase advance of the VOR, and suggest that the decrease of the phase lag and gain drop in responses of the vestibular neurons was caused by removal of the phase-lagging, feedback signal coming from the INC to the vestibular nuclei.     

Modulation of the masseteric monosynaptic reflex by stimulation of the vestibular nuclear complex in rats

The effect of stimulation of the vestibular nuclear complex (VN) on the masseteric monosynaptic reflex (MMR) was studied in anesthetized rats. The MMR was evoked by electrical stimulation of the mesencephalic trigeminal nucleus and was recorded, bilaterally, as the electromyographic responses of the masseter muscles. Conditioning electrical stimulation of the medial vestibular nucleus (MVN) facilitated the MMR bilaterally, as did microinjection of monosodium glutamate into the MVN. In contrast, conditioning electrical stimulation of the inferior vestibular nucleus (IVN) inhibited the MMR bilaterally. Microinjection of monosodium glutamate into the IVN also inhibited the MMR bilaterally. Conditioning electrical stimulation of the lateral and superior vestibular nuclei did not modulate the MMR. These results suggest that the MVN and the IVN are involved in modulation of the MMR and plays an important role in controlling jaw movements.     

Mechanisms of compensation for vestibular deficits in the frog

Summary In hemilabyrinthectomized frogs excitatory responses of central vestibular neurons to electrical stimulation of the remaining vestibular nerve were recorded extra- and intracellularly at different stages (0, 3, and 60 days) after the operation.The output pattern of ipsilateral vestibular neurons sending an axon across the midline via the vestibular commissure to the deafferented nucleus did not change postoperatively.The synaptic efficacy of these commissural axons ending on partially deafferented vestibular neurons on the lesioned side increased with time. This enhanced synaptic potency was associated with a shortening in time to peak and duration and an increase in amplitude of the evoked EPSPs. As a result most vestibular neurons were readily excited by single shock stimulation of the contralateral vestibular nerve, a finding which was rarely observed in control animals.These plastic changes are explained by the assumption of reactive synaptogenesis. The consequences of this modification for the readjustment of static and dynamic vestibular reflexes are discussed.     

Immunoreactivity for calcium-binding proteins defines subregions of the vestibular nuclear complex of the cat

The vestibular nuclear complex (VNC) is classically divided into four nuclei on the basis of cytoarchitectonics. However, anatomical data on the distribution of afferents to the VNC and the distribution of cells of origin of different efferent pathways suggest a more complex internal organization. Immunoreactivity for calcium-binding proteins has proven useful in many areas of the brain for revealing structure not visible with cell, fiber or Golgi stains. We have looked at the VNC of the cat using immunoreactivity for the calcium-binding proteins calbindin, calretinin and parvalbumin. Immunoreactivity for calretinin revealed a small, intensely stained region of cell bodies and processes just beneath the fourth ventricle in the medial vestibular nucleus. A presumably homologous region has been described in rodents. The calretinin-immunoreactive cells in this region were also immunoreactive for choline acetyltransferase. Evidence from other studies suggests that the calretinin region contributes to pathways involved in eye movement modulation but not generation. There were focal dense regions of fibers immunoreactive to calbindin in the medial and inferior nuclei, with an especially dense region of label at the border of the medial nucleus and the nucleus prepositus hypoglossi. There is anatomical evidence that suggests that the likely source of these calbindin-immunoreactive fibers is the flocculus of the cerebellum. The distribution of calbindin-immunoreactive fibers in the lateral and superior nuclei was much more uniform. Immunoreactivity to parvalbumin was widespread in fibers distributed throughout the VNC. The results suggest that neurochemical techniques may help to reveal the internal complexity in VNC organization.     

Light microscopical study of nitric oxide synthase I-positive neurons, including fibres in the vestibular nuclear complex of the cat

Nitric oxide is a gaseous neurotransmitter that is synthesized by the enzyme nitric oxide synthase I (NOS I). At present, little is known of NOS I-positive neurons in the vestibular nuclear complex of the cat (VNCc). The aim of the present study was to examine the morphology, distribution patterns and interconnections of NOS I-positive neurons, including fibres in the VNCc. Five adult cats were used as experimental animals. All cats were anaesthetized and perfused transcardially. Brains were removed, postfixed, cut on a freezing microtome and stained in three different ways. Every third section was treated with the Nissl method, other sections were stained either histochemically for NADPH diaphorase or immunohistochemically for NOS I. The atlas of Berman (1928) was used for orientation in the morphometric study. NOS I-positive neurons and fibres were found in all parts of VNCc: medial vestibular nucleus (MVN); lateral vestibular nucleus (LVN); superior vestibular nucleus (SVN); inferior vestibular nucleus (IVN); X, Y, Z groups and Cajal's nucleus. The NOS I-positive neurons were classified according to their size (small, medium-sized, large neurons type I and type II) and their shape (oval, fusiform, triangular, pear-shaped, multipolar and irregular). In every nucleus, a specific neuronal population was observed. In SVN, a large number of interconnections between NOS I-positive neurons were identified. In MVN, chain-like rolls of small neurons were found. Tiny interconnections between MVN and mesencephalic reticular formation were present. Our data provide information on the morphology, distribution patterns and interconnections of NOS I-positive neurons in the VNCc and can be extrapolated to other mammals.     

Relationship of cat vestibular neurons to otolith-spinal reflexes

Summary The dynamics of neurons in the vestibular nuclei of canal-plugged, decerebrate cats were studied in response to lateral (roll) tilt. Forelimb and neck extensor reflexes recorded simultaneously develop a progressive phase lag above 0.1 Hz. Neurons which exhibited a muscle-like phase lag were excited during low frequency stimuli by ipsilateral side-up tilt (beta response). Neurons with alpha responses, excited during side-down tilt, exhibited a constant phase, without a high frequency lag. Vestibulospinal neurons were present in both of these response groups, as were units driven at monosynaptic latencies by electrical stimulation of the ipsilateral labyrinth. The phase-lagging beta responses are appropriate for contributing to the reflexes observed in the ipsilateral neck and contralateral forelimb.Partially supported by NASA grant NSG-2380 and NIH grant NS02619NIH Postdoctoral Fellowship PHS NS06128     

The velocity response of vestibular nucleus neurons during vestibular,visual, and combined angular acceleration

Summary In alert Rhesus monkeys neuronal activity in the vestibular nuclei was measured during horizontal angular acceleration in darkness, acceleration of an optokinetic stimulus, and combined visual-vestibular stimulation. The working ranges for visual input velocity and acceleration extend up to 60 °/s and 5 °/s². The corresponding working range for vestibular input acceleration is wider and time-dependent. During combined stimulation, that is acceleration of the monkey in the light, a linear relation between neuronal activity and velocity could be established for all neurons. Type I vestibular plus eye movement neurons displayed the greatest sensitivity and had a small linear range of operation. Other vestibular neurons were less sensitive but had a larger range of linear response to different values of acceleration. Accelerating the animal and visual surround, simultaneously but in opposite directions, results in neuronal activity proportional to relative velocity over a limited range.Supported by a grant from the Swiss National Foundation for Scientific Research 3.672-0.77     

Timing of bilateral cerebellar output evoked by unilateral vestibular stimulation in the frog

Electrical stimulation of one VIIIth nerve evoked simple spike activity in Purkinje cells located on either side of the cerebellum. This cerebellar output was delayed by ca. 10 ms with respect to its mossy fiberparallel fiber input. The onset of the cerebellar output occurs on the average simultaneously on either side of the corpus cerebelli. The delay is explained by slowly rising EPSPs in PC induced by primary afferent and by second and higher order vestibular fibers. The latter inputs are stronger and terminate ipsi- and contralaterally in the granular layer.Supported by Deutsche Forschungsgemeinschaft (Pr. 158/1)     

Lesion-induced vestibular plasticity in the frog: are N-methyl-D-aspartate receptors involved?

Summary The synaptic excitation of central vestibular neurons in the isolated superfused brainstem of chronic hemilabyrinthectomized (HL) frogs and of controls was studied electrophysiologically and pharmacologically. Central vestibular neurons were excited either through vestibular afferent fibers or through the vestibular commissural pathway by means of electrical stimulation of the ipsilateral or the contralateral VIIIth nerve. In chronic HL frogs, commissural field potential amplitudes were on the average larger than those of intact frogs and the shape parameters of intracellularly recorded commissural EPSPs of chronic animals were on the average shifted towards those of vestibular afferent EPSPs. In control frogs, vestibular afferent EPSPs were generated independently from N-methyl-D-aspartate (NMDA) receptors, whereas commissural EPSPs exhibited a delayed NMDA receptor mediated component. Commissural EPSPs of HL frogs exhibited a NMDA receptor mediated component as well. The size of this EPSP component was larger when the time to peak of the EPSP was longer. EPSPs with similar rise times exhibited NMDA mediated components of similar size, irrespective of whether they originated from chronic animals or controls. The tendency of these EPSPs towards shorter rise times in chronic animals was paralled by a similar decrease of the relative size of their NMDA receptor mediated component. It is concluded that the increased synaptic efficacy of commissural fibers observed in chronic HL frogs does not result from an increased NMDA receptor component.     

Increased projection of ascending dorsal root fibers to vestibular nuclei after hemilabyrinthectomy in the frog

Summary The projections from brachial, ascending dorsal root fibers were studied autoradiographically in controls and chronically (four months) hemilabyrinthectomized frogs. Comparison showed that projections into the partially denervated vestibular nuclear complex of chronically hemilabyrinthectomized animals were far more dense than in control animals. In the cerebellar granular layer, no obvious difference in the extent of dorsal root projections was observed between both groups of animals. Cerebellar areas such as the auricular lobe and the dorsal rim, which normally receive many terminals from vestibular but not from dorsal root afferents, were not invaded by dorsal root fibers in chronically hemilabyrinthectomized frogs.     

Responses of cat vestibular neurons to stimulation of the frontal cortex

Summary 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 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.     

Noradrenergic modulation of neuronal responses to glutamate in the vestibular complex

Increases in firing rate induced in secondary vestibular neurons by microiontophoretic application of glutamate were studied during long-lasting applications of noradrenaline (NA) and/or its antagonists and agonists. Sixty-nine percent of the tested neurons, scattered through all nuclei of the vestibular complex, modified their responsiveness to glutamate in the presence of NA. The effects were depressive in a majority (40%) and enhancing in a minority (29%) of cases. NA application depressed responses to glutamate more often than it increased them in lateral, medial and superior vestibular nuclei, while the reverse was true for the spinal nucleus. The mean intensities of NA-evoked effects were comparable in the various nuclei. The enhancing effects of NA were antagonized by application of the alpha2 receptor antagonist yohimbine, and their depressive effects were enhanced by the beta receptor antagonist timolol. It is concluded that NA exerts a control on the processing of vestibular information and that this modulation is exerted by at least two mechanisms involving alpha2 and beta noradrenergic receptors.     

Intracellular study of frog's vestibular neurons in relation to the labyrinth and spinal cord

Summary Field and intracellular potentials were recorded in the vestibular nuclei of the frog following stimulation of the anterior branch of the ipsilateral vestibular nerve and the spinal cord. The field potential induced by stimulation of the vestibular nerve consisted of an early positive-negative wave followed by a slow negativity and that recorded during spinal cord stimulation was composed of an antidromic potential followed by a slow negative wave. These potentials were most prominent in the ventral region of the stato-acoustic complex. Mono- and polysynaptic EPSPs were recorded from vestibular neurons following vestibular nerve stimulation. Short latency depolarizations of small amplitude preceded the monosynaptic EPSPs in some neurons. Spike-like partial responses were commonly superimposed on the EPSPs. These all-or-none depolarizations probably originated in the dendrites. In a group of vestibular neurons stimulation of the vestibular nerve evoked full action potentials with latencies ranging from 0.2 to 1.1 msec. They are presumably caused by antidromic activation of neurons which send their axons to the labyrinth. The presence of efferent neurons in the vestibular nuclei was confirmed by their successful staining with Procion Yellow following axonal electrophoresis.After stimulation of the spinal cord, antidromic spike potentials and EPSPs were recorded in vestibular neurons. In addition, short-latency depolarizing potentials (EDPs) were evoked by spinal stimulation, with latencies similar to those of antidromic potentials. The EDPs are suggested to be induced by electrotonic transmission from the neighboring cell and likely to be active spike potentials produced at some distance away from the soma.     

Synaptic responses of neurons in the parietal associative cortex of the cat to stimulation of the red nucleus

Acute experiments on anesthetized and immobilized cats using intracellular recording were used to study the responses of neurons in the parietal associative cortex to stimulation of the red nucleus. Efferent neurons of the parietal cortex were identified by antidromal activation on stimulation of the intrinsic nuclei of the pons and motor cortex. Oligo- and polysynaptic EPSP in response to stimulation of the red nucleus were seen. The results are discussed in the light of the morphological organization of the rubrothalamic and cerebellothalamocortical tracts. Laboratory for Central Nervous System Physiology (Director V. V. Fanardzhyan), L. A. Orbel' Institute of Physiology, Armenian National Academy of Sciences, Erevan. Translated from Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 81, No. 12, pp. 64–69, December, 1995.     

Conflicting visual-vestibular stimulation and vestibular nucleus activity in alert monkeys

Summary In alert Rhesus monkeys (Macaca mulatta) neuronal activity of vestibular nuclei was recorded during pure vestibular and conflicting visual-vestibular stimulation. Pure vestibular stimulation consisted of rotating the monkey about the vertical axis in complete darkness. During conflicting visual-vestibular stimulation the monkeys were rotated in the light within a vertically striped cylinder mechanically coupled to the turntable. The conflict is that although the monkey is accelerated, there is no relative movement between visual surrounding and the animal. In the conflict situation thresholds of neuronal modulation and of nystagmus were raised compared with those during pure vestibular stimulation. Nystagmus slow-phase velocity could always be dissociated from the neuronal activity, the nystagmus often being totally suppressed whereas the neuronal activity was only attenuated. This suggests a further information processing between vestibular and oculomotor nuclei in the generation of nystagmus.Supported in part by Swiss National Foundation for Scientific Research 3.672-0.77, and Emil Barell-Foundation of Hoffmann-La Roche, Basel, Switzerland     

Direct visualization by scanning electron microscopy of the preganglionic innervation and synapses on the true surfaces of neurons in the frog heart

A simple and flexible technique is reported for scanning electron microscopic observation of the true surfaces of neurons, and the distribution of glial cells, preganglionic nerve fibers and synapses on them. The interatrial septum of the frog was chosen as a convenient preparation, and connective tissue and glial cells were successively removed by incubation in collagenase and protease solutions. The three-dimensional relationships between presynaptic fibers and target neurons can be observed with high resolution over long distances. Some neurons have multiple innervation. Varicosities can be seen clearly, and where their interior has been exposed, synaptic vesicles can be discerned.