Integration of vestibular and neck afferent signals in the central cervical nucleus

Manual cervical therapy and vestibular migraine: A case series

Background

Vestibular migraine (VM) is a relatively new diagnostic entity with incomplete knowledge regarding pathophysiological mechanisms and therapeutic guidelines. By reporting the effect of manual cervical therapy (MCT) on people with VM, we suggest a possible role for upper cervical afferents in VM treatment and/or pathogenesis. The objective was to describe the change in clinical presentation and self-reported symptoms of VM corresponding to MCT and followed up to six months.

Methods

A nonrandomised *ABA design was utilised to consecutively and prospectively evaluate selected patients with diagnosed VM. Symptom characteristics (frequency and intensity) were recorded along with standardised patient-reported outcomes (PROs) to document the response to MCT.

Results

Three patients were recruited who met the diagnostic criteria for VM. All three patients demonstrated improvement in both migraine attack and interictal symptom frequency. These improvements mirrored changes in PROs and were sustained over a six-month follow-up period.

Conclusions

The improvement that coincided with the intervention including MCT was rapid, observable and sustained. This suggests that the upper cervical spine could be a therapeutic target in VM and may have implications for future research into the pathogenesis of VM.

Proprioceptive Cervicogenic Dizziness: A Narrative Review of Pathogenesis, Diagnosis, and Treatment

Basic science and clinical evidence suggest that cervical spine disorders can lead to dizziness. The cervical spine has highly developed proprioceptive receptors, whose input information is integrated with the visual and vestibular systems in the central nervous system, acting on the neck and eye muscles to maintain the coordinative motion of the head, eyes, neck, and body through various reflex activities. When the cervical proprioceptive input changes due to the mismatch or conflict between vestibular, visual, and proprioceptive inputs, cervicogenic dizziness may occur. The diagnosis of cervicogenic dizziness can be determined based on clinical features, diagnostic tests, and the exclusion of other possible sources of dizziness. The cervical torsion test appears to be the best diagnostic method for cervicogenic dizziness. Based on the available evidence, we first developed the diagnostic criteria for cervicogenic dizziness. Treatment for cervicogenic dizziness is similar to that for neck pain, and manual therapy is most widely recommended.

The neurochemically diverse intermedius nucleus of the medulla as a source of excitatory and inhibitory synaptic input to the nucleus tractus solitarii

Sensory afferent signals from neck muscles have been postulated to influence central cardiorespiratory control as components of postural reflexes, but neuronal pathways for this action have not been identified. The intermedius nucleus of the medulla (InM) is a target of neck muscle spindle afferents and is ideally located to influence such reflexes but is poorly investigated. To aid identification of the nucleus, we initially produced three-dimensional reconstructions of the InM in both mouse and rat. Neurochemical analysis including transgenic reporter mice expressing green fluorescent protein in GABA-synthesizing neurons, immunohistochemistry, and in situ hybridization revealed that the InM is neurochemically diverse, containing GABAegric and glutamatergic neurons with some degree of colocalization with parvalbumin, neuronal nitric oxide synthase, and calretinin. Projections from the InM to the nucleus tractus solitarius (NTS) were studied electrophysiologically in rat brainstem slices. Electrical stimulation of the NTS resulted in antidromically activated action potentials within InM neurons. In addition, electrical stimulation of the InM resulted in EPSPs that were mediated by excitatory amino acids and IPSPs mediated solely by GABA(A) receptors or by GABA(A) and glycine receptors. Chemical stimulation of the InM resulted in (1) a depolarization of NTS neurons that were blocked by NBQX (2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonoamide) or kynurenic acid and (2) a hyperpolarization of NTS neurons that were blocked by bicuculline. Thus, the InM contains neurochemically diverse neurons and sends both excitatory and inhibitory projections to the NTS. These data provide a novel pathway that may underlie possible reflex changes in autonomic variables after neck muscle spindle afferent activation.

Dorsal Neck Muscle Vibration Induces Upward Shifts in the Endpoints of Memory-Guided Saccades in Monkeys

Producing a movement in response to a sensory stimulus requires knowledge of the body's current configuration, and spindle organs embedded within muscles are a primary source of such kinesthetic information. Here, we sought to develop an animal model of kinesthetic illusions induced by mechanically vibrating muscles as a first step toward a mechanistic understanding of how kinesthesia is integrated into neural plans for action. We elected to examine the effects of mechanical vibration of dorsal neck muscles in head-restrained monkeys performing memory-guided saccades requiring them to look to the remembered location of a flashed target only after an imposed delay. During the delay on one-half of all trials, mechanical vibration (usually 1,500 ms in duration, 200 μm in amplitude, 100 Hz in frequency) was applied to the dorsal aspect on one side of the monkey's neck. We compared the metrics of such vibration saccades to control saccades without vibration during the delay interval. Relative to control saccades, the endpoints of vibration saccades were shifted consistently upward, even though the variability in saccadic endpoints was unaltered. Although the stability of the eye was compromised during the delay interval of vibration trials, as evidenced by an increased incidence of upward drifts and downward microsaccades, vibration saccades displayed different metrics than control saccades, including an upwardly deviated radial direction and increased vertical amplitude. The influence of variations in the duration (500–2,500 ms), amplitude (100–300 μm), or frequency (75–125 Hz) of vibration scaled well with the presumed change in spindle activity entrained by vibration. Comparisons of the profile of these results are made to the human literature. We conclude that neck muscle vibration induces alterations in oculomotor performance in monkeys consistent with a central interpretation of illusory neck flexion and downward gaze deviation due to increased activation in the spindles of neck extensor muscles.

Dissociating self-generated from passively applied head motion: neural mechanisms in the vestibular nuclei

The ability to distinguish sensory inputs that are a consequence of our own actions from those that result from changes in the external world is essential for perceptual stability and accurate motor control. To accomplish this, it has been proposed that an internal prediction of the consequences of our actions is compared with the actual sensory input to cancel the resultant self-generated activation. Here, we provide evidence for this hypothesis at an early stage of processing in the vestibular system. Previous studies have shown that neurons in the vestibular nucleus, which receive direct inputs from vestibular afferent fibers, are responsive to passively applied head movements. However, these same neurons do not reliably encode head velocity resulting from self-generated movements of the head on the body. In this study, we examined the mechanism that underlies the selective elimination of vestibular sensitivity to active head-on-body rotations. Individual neurons were recorded in monkeys making active head movements. The correspondence between intended and actual head movement was experimentally controlled. We found that a cancellation signal was gated into the vestibular nuclei only in conditions in which the activation of neck proprioceptors matched that expected on the basis of the neck motor command. This finding suggests that vestibular signals that arise from self-generated head movements are inhibited by a mechanism that compares the internal prediction of the sensory consequences by the brain to the actual resultant sensory feedback. Because self-generated vestibular inputs are selectively cancelled early in processing, we propose that this gating is important for the computation of spatial orientation and control of posture by higher-order structures.

Glutamate and AMPA receptor immunoreactivity in Ia synapses with motoneurons and neurons of the central cervical nucleus

Axonal tracing and high resolution immunocytochemistry were used to identify transmitter content and postsynaptic receptors in synapses between Ia primary afferents and motoneurons and in neurons of the central cervical nucleus (CCN), respectively, in the rat. The terminals, as well as the target neurons, were identified by postembedding immunogold detection of transganglionically or retrogradely, respectively, transported cholera toxin B subunit (CTB), and in adjacent sections postembedding immunogold was employed to demonstrate glutamate and AMPA receptors in the same synapses. A total of 390 CTB-labelled Ia boutons in apposition to CTB-labelled motoneurons, CCN neurons or unlabelled dendrites in the surrounding neuropil were traced in section series from two animals. A third animal was used as a control. In the motor nucleus, a majority of the synapses were with medium-sized dendrites, whereas in the CCN the distribution was skewed towards fine-calibre dendrites. In both nuclei, somatic and juxtasomatic synapses were quite infrequent (<10%). All of the CTB-labelled Ia boutons recovered in the sections incubated for glutamate (n=323) were enriched with glutamate immunoreactivity. One hundred and fifty of these disclosed synaptic contact in at least two ultrathin sections. In this sample, 50% (33-59%) appeared immunoreactive to receptor sub-units GluR1-4 in at least two ultrathin sections, whereas 35% were labelled in one section only. Distribution of gold particles relative to presynaptic and postsynaptic membrane profiles (n=23) revealed a close correlation between AMPA immunoreactivity and the postsynaptic membrane of the synapse. Finally, immunogold particles signalling GluR1 were observed much less frequently than particles signalling GluR2/3 or GluR4. Our results provide additional strong evidence that chemical transmission at Ia synapses is mediated by glutamate and identify GluR2/3 and GluR4 as important postsynaptic receptors.

GABA-, glycine-, and glutamate-immunoreactive bouton profiles in apposition to neurons of the central cervical nucleus in the rat

The neurons of the central cervical nucleus (CCN) convey information about the position and movements of the head, and receive excitatory input from dorsal neck muscles and the labyrinth. Both of these afferent sources form glutamatergic synaptic contacts with CCN neurons. However, these sensory afferent sources can also inhibit CCN neurons. To further elucidate the synaptic organization, we made an electron microscopic investigation, identifying and evaluating the relative frequency of bouton profiles containing the inhibitory transmitters GABA and glycine in apposition to identified CCN neurons. In addition, labeling for glutamate was performed. The identification of the CCN neurons was made possible by injections of retrograde tracer substances into the cerebellum. These substances were made visible by preembedding immunocytochemistry or postembedding immunogold staining. Such staining was also used to detect the three amino acids that were found in boutons apposed to the identified neurons (cf. Ornung et al., J. Comp. Neurol. 1996;365:413-426; Lindå et al., J. Comp. Neurol. 2000;425:10-23). Due to the relatively poor transport of the tracer substances into dendrites of the CCN neurons, the analysis was restricted to the cell body and included bouton profiles in direct apposition to the soma membrane. Data from 10 CCN neurons revealed that about 50% of the apposing bouton profiles were immunoreactive for GABA, and about 34% for glycine. In four neurons, the degree of colocalization of GABA and glycine was determined to be close to 30%. Thus, the vast majority of glycine-labeled profiles also contained GABA, while a considerable fraction of the profiles were immunoreactive for only GABA. The values for glycine immunoreactive bouton profiles presented here may represent somewhat low estimates, depending on the method used. Data from four neurons showed that about 18% of the profiles were labeled for glutamate. The large fraction of purely GABA immunoreactive profiles, or at least a substantial group of them, is suggestive of their derivation from axons descending from the brainstem.

Vestibulospinal control of reflex and voluntary head movement

Secondary canal-related vestibulospinal neurons respond to an externally applied movement of the head in the form of a firing rate modulation that encodes the angular velocity of the movement, and reflects in large part the input "head velocity in space" signal carried by the semicircular canal afferents. In addition to the head velocity signal, the vestibulospinal neurons can carry a more processed signal that includes eye position or eye velocity, or both (see Boyle on ref. list). To understand the control signals used by the central vestibular pathways in the generation of reflex head stabilization, such as the vestibulocollic reflex (VCR), and the maintenance of head posture, it is essential to record directly from identified vestibulospinal neurons projecting to the cervical spinal segments in the alert animal. The present report discusses two key features of the primate vestibulospinal system. First, the termination morphology of vestibulospinal axons in the cervical segments of the spinal cord is described to lay the structural basis of vestibulospinal control of head/neck posture and movement. And second, the head movement signal content carried by the same class of secondary vestibulospinal neurons during the actual execution of the VCR and during self-generated, or active, rapid head movements is presented.

Spinovestibular projections in the rat, with particular reference to projections from the central cervical nucleus to the lateral vestibular nucleus

Projections from the spinal cord to the vestibular nuclei were examined following injections of Phaseolus vulgaris-leucoagglutinin, cholera toxin subunit B, or biotinylated dextran at various levels of the spinal cord in the rat. Labeled terminals were abundant after injections of the tracers into the C2 and C3 segments containing the central cervical nucleus. Labeled terminals were seen in the descending vestibular nucleus and the parvocellular, magnocellular, and caudal parts of the medial vestibular nucleus throughout its rostrocaudal extent. Labeled terminals were most numerous in the lateral vestibular nucleus throughout its rostrocaudal extent. The projections from the central cervical nucleus to the vestibular nuclei were exclusively contralateral to the cells of origin because the axons of the central cervical nucleus neurons cross in the spinal cord. Following tracer injections in the cervical enlargement, many labeled terminals were seen in the magnocellular part of the medial vestibular nucleus, but a few were seen in the lateral and the descending vestibular nucleus. Injections into more caudal segments resulted in sporadic terminal labeling in the magnocellular part of the medial vestibular nucleus, the descending vestibular nucleus, and the caudal part of the lateral vestibular nucleus. The results indicate that primary neck afferent input relayed at the central cervical nucleus is mediated directly to the contralateral vestibular nuclei. It is suggested that this projection serves as an important linkage from the upper cervical segments to the lateral vestibulospinal tract in the tonic neck reflex.

Projections from the central cervical nucleus to the cerebellar nuclei in the rat, studied by anterograde axonal tracing

Projections from the central cervical nucleus (CCN) to the cerebellar nuclei were examined following injections of Phaseolus vulgaris-leucoagglutinin or cholera toxin subunit B into the C1-C3 segments in the rat. Labeled axons and terminals were immunohistochemically demonstrated. Labeled spinocerebellar fibers arising from the CCN entered the cerebellum through the inferior and the superior cerebellar peduncles. Labeled mossy fiber terminals were seen in lobules I-VI, sublobule VIIb, lobules VIII and IX, and the copula pyramidis of the cerebellar cortex. Labeled axons ran toward the cerebellar cortex, through and between the medial and the interpositus nuclei, and gave off collateral axons and terminal axons to the cerebellar nuclei. The projections to the cerebellar nuclei were predominantly contralateral to the cells of origin. Labeled terminals were distributed from the medial to the ventrolateral part of the middle subdivision of the medial nucleus throughout its rostrocaudal extent. Labeled terminals were also seen in the lateral part of the medial nucleus and in the border region between the medial nucleus and the interpositus nuclei, which corresponds to the rostromedial extension of the posterior interpositus nucleus. In the anterior interpositus nucleus, labeled terminals were distributed dorsoventrally in the middle third of the mediolateral extent. They were more numerous in the rostrodorsal part of this area. Labeled terminals were distributed dorsally and caudally in the medial third of the posterior interpositus nucleus. No labeled terminals were seen in the caudomedial subdivision and the dorsolateral protuberance of the medial nucleus, the dorsolateral hump region and the lateral nucleus. The present study demonstrates that the CCN projects to specific areas of the cerebellar cortex and the medial and the interpositus nuclei.

Morphology of single vestibulospinal collaterals in the upper cervical spinal cord of the cat: I. Collaterals originating from axons in the ventromedial funiculus contralateral to their cells of origin

Vestibulospinal neurons in the medial and descending vestibular nuclei have widespread bilateral terminations in the upper cervical spinal cord. These terminations arise from axons travelling in several funiculi, including the ventromedial, ventrolateral, lateral, and dorsolateral funiculi in addition to the dorsal columns. The purpose of the present study was to examine the morphology of single vestibulospinal collaterals which terminate in the upper cervical spinal cord and which originate from axons located in one of these funicular pathways, the ventromedial funiculus, contralateral (cVMF) to their cells of origin in the vestibular nuclei. The 32 collaterals described were selected from two separate sets of experiments which took advantage of different techniques. Nineteen of the collaterals were labelled following Phaseolus vulgaris leucoagglutinin (PHA-L) injections into the medial vestibular nucleus and medial regions of the descending vestibular nucleus. The remaining 13 collaterals originated from physiologically identified vestibulospinal axons that were stained after intra-axonal injections of horseradish peroxidase (HRP). The combined projection of all cVMF axon collaterals spread from laminae V to IX, and included the central cervical nucleus. There was a high degree of variability in the pattern of terminations of individual collaterals. This variability was more pronounced among PHA-L-labelled collaterals than HRP-labelled collaterals whose terminations were restricted to laminae VIII and IX. Some PHA-L-labelled collaterals had terminations which were focused within a single lamina, whereas others had termination zones spanning as many as four laminae. The differences between collaterals were compounded when the characteristics of branching patterns were considered. Some collaterals which occupied similar termination zones had different branching structures.(ABSTRACT TRUNCATED AT 250 WORDS)

Morphology of single vestibulospinal collaterals in the upper cervical spinal cord of the cat. II. Collaterals originating from axons outside the ventral funiculi

Recent studies have shown that vestibulospinal axons reach the upper cervical spinal cord of the cat via several different funicular routes. The purpose of this study was to describe the projections of those axons travelling outside the well-recognized pathways in the ventral funiculi. These axons are located in the dorsal columns, dorsolateral funiculi, and lateral funiculi. Collaterals of these axons were stained following extracellular injections of Phaseolus vulgaris leucoagglutinin in the medial and descending vestibular nuclei. The trajectories of individual collaterals were reconstructed from serial histological sections. Collaterals arising from axons in the same funiculus usually had the same characteristic appearance. Axons in the lateral funiculi, ipsilateral or contralateral to their cells of origin, gave rise to collaterals that had a simple structure and usually followed a horizontal trajectory across laminae VII and VIII. The boutons of these collaterals were distributed throughout the mediolateral extent of laminae VI and VII and the dorsal half of lamina VIII. In contrast, axons in the dorsolateral funiculi, ipsilateral or contralateral to their cells of origin, terminated primarily in laminae IV and V. Many collaterals of these axons projected either rostrally or caudally and had a narrow mediolateral distribution. The combined distribution of boutons from collaterals originating from axons in the dorsal columns included the dorsal horn and intermediate zone. Although these collaterals were less common and formed a heterogeneous group, they were easily distinguished from collaterals originating from axons travelling in other funiculi. These results indicate that vestibulospinal axons travelling outside the ventral funiculi comprise several distinct systems. Each system travels by a different funicular route and is distinguished by differences in collateral morphology and termination zones.(ABSTRACT TRUNCATED AT 250 WORDS)

Connections from the lateral vestibular nucleus to the upper cervical spinal cord of the cat: a study with the anterograde tracer PHA-L

The projections of neurons in the lateral vestibular nucleus (LVN) to the upper cervical spinal cord of the cat were investigated by means of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L). At the junction of C1 and C2, axons were distributed bilaterally in the ventromedial funiculi, and ipsilaterally in the ventrolateral and lateral funiculi. The majority of boutons were found ipsilateral to the injection sites and most of these boutons were found at the base of the ventral horn and throughout the medial two-thirds of lamina VIII. A more modest termination zone was found along the ventral border of lamina VII and a small number of boutons were scattered in the dorsal horn. Contralateral termination zones were similar to the ipsilateral projections. There were significant changes in the distribution of vestibulospinal axons and density of boutons at the junction of C3 and C4. At this level, most vestibulospinal axons travelled ipsilaterally and were found along the medial border of the ventromedial funiculus and the ventral margin of the ventrolateral funiculus. The overall distribution of boutons near the border of C3 and C4 was similar to the pattern seen at the junction of C1 and C2. However, bouton density fell by a factor of three. Large zones of the grey matter were devoid of boutons in individual experiments. These results demonstrate that the projections of neurons in the LVN to the upper cervical spinal cord are densest in the regions containing motoneurons supplying suboccipital muscles. This result suggests that monosynaptic connections to those motoneurons may be an important part of the neural circuitry responsible for vestibulocollic reflexes. However, the large number of boutons found in regions dorsal to motoneuron nuclei in all upper cervical segments indicates that the primary path from vestibulospinal axons to neck motoneurons may be indirect and involve relays via spinal interneurons.

Multiplicity of vestibulospinal projections to the upper cervical spinal cord of the cat: a study with the anterograde tracer Phaseolus vulgaris leucoagglutinin

The distribution and frequency of vestibulospinal axons and boutons in the upper cervical spinal cord of the cat were investigated. The anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) was injected into discrete regions of the vestibular nuclei, including the medial and descending nuclei, as well as small regions of the lateral vestibular nucleus along its medial border with the medial vestibular nucleus. In contrast to previous reports, labelled vestibulospinal axons were not found to be restricted to the ventromedial and ventrolateral funiculi, but were also observed bilaterally in the lateral funiculi, the dorsolateral funiculi and the dorsal columns. The diameter of these axons ranged from 0.5 to 7.4 microns. Labelled boutons were found bilaterally from lamina IV to IX as well as in lamina X. Contralateral to the injection site, boutons were frequently found as far dorsal as lamina II. Ipsilaterally, boutons were found this far dorsal in only one experiment. There was a dense projection to the contralateral central cervical nucleus, while very few, if any, boutons were observed in the ipsilateral central cervical nucleus. In each experiment, the density of boutons was greater in the rostral cervical segments than in more caudal segments. The "new" vestibulospinal projections to the dorsal horn and central cervical nucleus were confirmed in separate experiments using retrograde transport of horseradish peroxidase. These results show that vestibulospinal axons project to the upper cervical spinal cord via multiple funicular paths. The rich terminations of these axons outside of the ventral horn, as well as in the neck motoneuron nuclei, indicate that vestibulospinal projections must play a wide variety of functions in addition to their well-documented role in the direct control of head movement.