A direct projection from the medial vestibular nucleus to the cervical spinal dorsal horn of the rat, as demonstrated by anterograde and retrograde tracing

The tracts, cytoarchitecture, and neurochemistry of the spinal cord

The human spinal cord can be described using a range of nomenclatures with each providing insight into its structure and function. Here we have comprehensively reviewed the key literature detailing the general structure, configuration of tracts, the cytoarchitecture of Rexed's laminae, and the neurochemistry at the spinal segmental level. The purpose of this review is to detail current anatomical understanding of how the spinal cord is structured and to aid researchers in identifying gaps in the literature that need to be studied to improve our knowledge of the spinal cord which in turn will improve the potential of therapeutic intervention for disorders of the spinal cord.

Baseline of upper teeth: (a) Control organ for spatial navigation? (b) Weak point for misaligned posture and pain?

Our observations question both the current doctrine of spatial orientation as processed by vestibular, visual and proprioceptive impressions as well as the horizontal alignment of the eye axis. Indeed our observations suggest spatial orientation as a physically based, largely mechanically transmitted interaction between individual and environment. It is controlled by an interface defined by the baseline of upper teeth. It simultaneously constitutes both body and environment acting as an integral part of that environment. Consequently, the baseline of upper teeth is part of the aforementioned environment. Instead of the eye axis during spatial orientation it aligns the true horizontal absolutely. This was tested by fixing a cross to upper teeth. While walking, running and jumping it did not deviate by more than 2° from the external axis. Subsequently, we inclined the baseline of upper teeth by inserting an asymmetric wafer so that it angulated the eye axis. Immediately, head, visual and vestibular axes tilted unstably with misaligned body posture. Only the indicative cross remained stably aligned to the external axes. The person felt "upright", not noticing his posture had changed. He was then instructed to straighten his shoulders and trunk until his posture was objectively nearly upright again. The voluntary correction caused the indicative cross to tilt. The person felt uneven while being more upright. We concluded that the automatic posture works by "synchronizing" the baseline of upper teeth to the external axis and that the synchronized position is supported by the vestibular system. Benefit of an interface is that the body's movements in the environment simultaneously happen within the baseline of upper teeth. Therein the vectors of the body and the environment are calculated to remain in balance. This model introduces the transmission of the vector information to postural muscles by the dura mater, controlled by tension between C0-C2. The information is skewed by bony dislocations between C0-C2 caused by an inclination of the interface. The resulting misalignments of posture are foreseeable and specifically correspond to the type of inclination. They occur in a broad section of the population. Diagnosed as muscular weakness, they may cause therapy resistant common diseases like back and joint pain after 5-10 years. Following our observations, the inclination of the baseline of upper teeth originates from inattentive changes in the length of upper teeth in dental treatment. Multiple treatments optimizing teeth length in long term patients improved the patients' situation.

Origin and neurochemical properties of bulbospinal neurons projecting to the rat lumbar spinal cord via the medial longitudinal fasciculus and caudal ventrolateral medulla

Bulbospinal systems (BS) originate from various regions of the brainstem and influence spinal neurons by classical synaptic and modulatory mechanisms. Our aim was to determine the brainstem locations of cells of origin of BS pathways passing through the medial longitudinal fasciculus (MLF) and the caudal ventrolateral medulla (CVLM). We also examined the transmitter content of spinal terminations of the CVLM pathway. Six adult rats received Fluorogold (FG) injections to the right intermediate gray matter of the lumbar cord (L1–L2) and the b-subunit of cholera toxin (CTb) was injected either into the MLF or the right CVLM (3 animals each). Double-labeled cells were identified within brainstem structures with confocal microscopy and mapped onto brainstem diagrams. An additional 3 rats were injected with CTb in the CVLM to label axon terminals in the lumbar spinal cord. Double-labeled cells projecting via the MLF or CVLM were found principally in reticular regions of the medulla and pons but small numbers of cells were also located within the midbrain. CVLM projections to the lumbar cord were almost exclusively ipsilateral and concentrated within the intermediate gray matter. Most (62%) of terminals were immunoreactive for the vesicular glutamate transporter 2 while 23% contained the vesicular GABA transporter. The inhibitory subpopulation was glycinergic, GABAergic or contained both transmitters. The proportions of excitatory and inhibitory axons projecting via the CVLM to the lumbar cord are similar to those projecting via the MLF. Unlike the MLF pathway, CVLM projections are predominantly ipsilateral and concentrated within intermediate gray but do not extend into motor nuclei or laminia VIII. Terminations of the CVLM pathway are located in a region of the gray matter that is rich in premotor interneurons; thus its primary function may be to coordinate activity of premotor networks.

Neurotransmitter phenotypes of descending systems in the rat lumbar spinal cord

Descending systems from the brain exert a major influence over sensory and motor processes within the spinal cord. Although it is known that many descending systems have an excitatory effect on spinal neurons, there are still gaps in our knowledge regarding the transmitter phenotypes used by them. In this study we investigated transmitter phenotypes of axons in the corticospinal tract (CST); the rubrospinal tract (RST); the lateral component of the vestibulospinal tract (VST); and the reticulospinal tract (ReST). They were labelled anterogradely by stereotaxic injection of the b subunit of cholera toxin (CTb) into the motor cortex, red nucleus, lateral vestibular nucleus and medial longitudinal fascicle (MLF) to label CST, RST, VST and ReST axons respectively. Neurotransmitter content of labelled axons was investigated in lumbar segments by using immunoflurescence; antibodies against vesicular glutamate transporters (VGLUT1 and VGLUT2) were used to identify glutamatergic terminals and the vesicular GABA transporter (VGAT) was used to identify GABA- and glycinergic terminals. The results show that almost all CST (96%) axons contain VGLUT1 whereas almost all RST (97%) and VST (97%) axons contain VGLUT2. Although the majority of ReST axons contain VGLUT2 (59%), a sizable minority contains VGAT (20%) and most of these terminals can be subdivided into those that are GABAergic or those that are glycinergic because only limited evidence for co-localisation was found for the two transmitters. In addition, there is a population of ReST terminals that apparently does not contain markers for the transmitters tested and is not serotoninergic. We can conclude that the CST, RST and VST are 'pure' excitatory systems whereas the ReST consists of a heterogeneous population of excitatory and inhibitory axons. It is anticipated that this information will enable inputs to spinal networks to be defined with greater confidence.

Effects of neck muscle activities during rhythmic jaw movements by stimulation of the medial vestibular nucleus in rats

This study first examines whether there is rhythmic activity of the neck muscles during cortically induced rhythmic jaw movements in rats anesthetized by urethane. Rhythmic jaw movements were induced by repetitive electrical stimulation of the orofacial motor cortex. An electromyogram in the splenius muscles (spEMG) showed rhythmic bursts during the jaw-opening phase, or during the transition from the jaw-opening phase to the jaw-closing phase. In the sternomastoid (stEMG), however, the electromyogram did not show any bursts during rhythmic jaw movements. A further study then examines whether stimulation of the medial vestibular nucleus (MVN) modulates the rhythmic activity of the neck muscles. Stimuli applied in the jaw-closing phase induced a transient burst in the stEMG, and the duration of activity in the spEMG was increased. Stimuli applied in the jaw-opening phase induced a transient burst in the stEMG and an inhibitory period in the spEMG. These results imply that the MVN is involved in the modulation of neck muscle activities during rhythmic jaw movements induced by stimulating the orofacial motor cortex.

Influence of cervical spine stabilization via Stiff Neck on the postural system in healthy patients: compensation or decompensation of the postural system?

Functional and structural disorders of the cervical spine are often regarded as the cause of non-specific vertigo. Pathogenetically, disorders of proprioceptive connections between neck muscles and vestibular cores as well as the proprioceptors in the cervical facette joints are presumed. According to a study by Hülse and Hölzl (HNO 48:295-301, 1), after manual therapeutic intervention in patients with functional disorders of the cervical spine 50% of the probands stated a significant reduction of their vertigo. This was backed up in posturography, which documented an improvement in vestibulospinal reactions. To date, the effects of artificial as well as surgical stabilization of the cervical spine on the balance system have not been explored yet. In a first pilot study, we examined the influence of artificial stabilization of the cervical spine via cervical collar Stiff Neck, manufactured by Ambu/Perfit ACE] on the balance system of 20 healthy probands. For this purpose, a posturography (Balance Master Systems, NeuroCom, Clackamas, OR, USA) was applied to 20 healthy probands (10 males, 10 females) with a mean age of 35 years who had no prior spine pathology. Posturography was analyzed under static and dynamic test situations with and without Stiff Neck cervical collar. The results were compared statistically to the Wilcoxon test. In the static test situation of the modified clinical test of sensory interaction on balance, a significantly improved standing stability occurred. In none of the dynamic tests did fixation of the cervical spine by Stiff Neck cuff lead to a measurable impairment of the movement coordination. All probands felt subjectively more stable when wearing the Stiff Neck. In healthy probands, a fixation of the cervical spine leads to a stabilization of the postural balance situation. This fixation seems to be helpful in compensating the malfunction of other components of balance information. In a next step, this same model of analysis is applied to patients with cervical instability. Standing stability and movement coordination before and after cervical fusion are being explored.

Expression of calcium-binding proteins and nNOS in the human vestibular and precerebellar brainstem

Information about the position and movement of the head in space is coded by vestibular receptors and relayed to four nuclei that comprise the vestibular nuclear complex (VNC). Many additional brainstem nuclei are involved in the processing of vestibular information, receiving signals either directly from the eighth nerve or indirectly via projections from the VNC. In cats, squirrel monkeys, and macaque monkeys, we found neurochemically defined subdivisions within the medial vestibular nucleus (MVe) and within the functionally related nucleus prepositus hypoglossi (PrH). In humans, different studies disagree about the borders, sizes, and possible subdivisions of the vestibular brainstem. In an attempt to clarify this organization, we have begun an analysis of the neurochemical characteristics of the human using brains from the Witelson Normal Brain Collection and standard techniques for antigen retrieval and immunohistochemistry. Using antibodies to calbindin, calretinin, parvalbumin, and nitric oxide synthase, we find neurochemically defined subdivisions within the MVe similar to the subdivisions described in cats and monkeys. The neurochemical organization of PrH is different. We also find unique neurochemical profiles for several structures that suggest reclassification of nuclei. These data suggest both quantitative and qualitative differences among cats, monkeys, and humans in the organization of the vestibular brainstem. These results have important implications for the analysis of changes in that organization subsequent to aging, disease, or loss of input.

Nonphosphorylated neurofilament protein is expressed by scattered neurons in the vestibular and precerebellar brainstem

Vestibular information is essential for the control of posture, balance, and eye movements. The vestibular nerve projects to the four nuclei of the vestibular nuclear complex (VNC), as well as to several additional brainstem nuclei and the cerebellum. We have found that expression of the calcium-binding proteins calretinin (CR) and calbindin (CB), and the synthetic enzyme for nitric oxide synthase (nNOS) define subdivisions of the medial vestibular nucleus (MVe) and the nucleus prepositus (PrH), in cat, monkey, and human. We have asked if the pattern of expression of nonphosphorylated neurofilament protein (NPNFP) might define additional subdivisions of these or other nuclei that participate in vestibular function. We studied the distribution of cells immunoreactive to NPNFP in the brainstems of 5 cats and one squirrel monkey. Labeled cells were scattered throughout the four nuclei of the VNC, as well as in PrH, the reticular formation (RF) and the external cuneate nucleus. We used double-label immunofluorescence to visualize the distribution of these cells relative to other neurochemically defined subdivisions. NPNFP cells were excluded from the CR and CB regions of the MVe. In PrH, NPNFP and nNOS were not colocalized. Cells in the lateral vestibular nucleus and RF colocalized NPNFP and a marker for glutamatergic neurons. We also found that the cholinergic cells and axons of cranial nerve nuclei 3, 4, 6, 7,10 and 12 colocalize NPNFP. The data suggest that NPNFP is expressed by a subset of glutamatergic projection neurons of the vestibular brainstem. NPNFP may be a marker for those cells that are especially vulnerable to the effects of normal aging, neurological disease or disruption of sensory input.

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.

Trigeminovestibular and trigeminospinal pathways in rats: retrograde tracing compared with glutamic acid decarboxylase and glutamate immunohistochemistry

This study identified neurons in the sensory trigeminal complex with connections to the medial (MVN), inferior (IVN), lateral (LVN), and superior (SVN) vestibular nuclei or the spinal cord. Trigeminovestibular and trigeminospinal neurons were localized by injection of retrograde tracers. Immunohistochemical processing revealed gamma-aminobutyric acid (GABA)- and glutamate-containing neurons in these two populations. Trigeminovestibular neurons projecting to the MVN and the IVN were in the caudal principal nucleus (5P), pars oralis (5o), interpolaris (5i), and caudalis (5c) and scattered throughout the rostral 5P. Projections were bilateral to the IVN, with an ipsilateral dominance to the MVN, except from the rostral 5P, which was contralateral. Neurons projecting to the LVN were numerous in the ventral caudal 5P and the 5o and less abundant in the rostral 5P, 5i, and 5c. Our results suggested that only 5P and 5o project to the dorsal LVN. Neurons projecting to the SVN were in the dorsal 5P, 5o, and 5i but not in 5c. Trigeminospinal neurons were mainly in the ventral 5o and 5i and in the lateral 5c, rarely or never in 5P. Among trigeminovestibular neurons, most of the somas were immunoreactive for glutamate, but some reacted for GABA. Among trigeminospinal neurons, the number of somas immunoreactive for each of the two amino acids was similar. Trigeminal terminals were observed in contact with vestibulospinal neurons in the IVN and LVN, giving evidence of a trigeminovestibulospinal pathway. Therefore, inhibitory and excitatory facial inputs may contribute through trigeminospinal or trigeminovestibulospinal pathways to the control of head/neck movements.

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.

Vestibulotrigeminal and vestibulospinal projections in rats: retrograde tracing coupled to glutamic acid decarboxylase immunoreactivity

Immunohistochemical experiments were performed using glutamic acid decarboxylase (GAD) to identify gamma-aminobutyric acid (GABA)ergic neurons in the vestibular nuclei (VN). VN neurons projecting to the sensory trigeminal complex (STC) or to the C1-C2 segments of the spinal cord were identified by injection of wheat germ agglutinin-apo-horseradish peroxidase coupled to colloidal gold (gold-HRP), a retrogradely transported tracer, in these structures. The experiments combining injection of gold-HRP in spinal cord and GAD immunohistochemistry revealed the existence in the medial, inferior and lateral VN of GAD immunoreactive neurons projecting to the spinal C1-C2 level. Experiments combining injection of gold-HRP in the STC and GAD immunohistochemistry demonstrated that, at least, 30-50% of the vestibulo-trigeminal neurons also contained GAD. Injections of two different retrograde tracers (gold-HRP and Biotinylated dextran amine) in the STC and the spinal cord demonstrated that some VN neurons project by axon collaterals to both structures. Because of the GABAergic spinal and STC vestibular projections we assume that these VN neurons with collateral projection are GABAergic. Therefore primary afferents from the face, neck or hindlimb could be modulated by inhibitory influences from GABAergic vestibular neurons.

Distribution of Fos labeling in the inferior olive following transient blockade of the VIIIth cranial nerve

The sodium channel blocker, tetrodotoxin (TTX), is an effective tool for blockade of action potentials in neurons. Unilateral transtympanic administration of 3 mM TTX produced behavioral symptoms paralleling those previously reported following unilateral vestibular ablation. Behavioral symptoms were evident as early as 15 min post-TTX. Fos immunocytochemistry revealed an initial bilateral distribution of Fos in the inferior olive (IO) followed by an almost exclusively unilateral distribution of Fos. By 1 h, Fos was predominantly localized in subdivisions of the IO contralateral to TTX treatment. Fos labeling in the IO was most pronounced at 2- and 6-h survival times and was localized in the contralateral IOA, IOB, IOC, IOBe, and IOK subdivisions and bilaterally in the IOM and IODM. Other regions of the brainstem including the vestibular nuclei, prepositus hypoglossi, dorsal paragigantocellular reticular nucleus, nucleus of the tractus solitarius and locus coeruleus also exhibited altered patterns of Fos labeling following TTX. The finding that Fos activity in the IO is initially bilateral and then rapidly becomes unilateral has not been reported for the traditional vestibular ablation models and may be unique to the TTX model. In addition, since altered Fos activity is readily detected in the IO at time-points prior to detectable changes in Fos in the central vestibular complex it is possible that the IO is particularly sensitive to events precipitated by unilateral vestibular disturbance.

Ascending and descending projections of the lateral vestibular nucleus in the frog Rana esculenta

The lectin Phaseolus vulgaris leucoagglutinin was injected into the frog lateral vestibular nucleus (LVN) to study its antero- and retrograde projections. The following new observations were made. 1) In the diencephalon, vestibular efferents innervate the thalamus in a manner similar to that of mammalian species. The projections show a preference for the anterior, central, and ventromedial thalamic nuclei. 2) In the mesencephalon, vestibular fibers terminate in the tegmental nuclei and the nucleus of medial longitudinal fascicle. 3) In the rhombencephalon, commissural and internuclear projections interconnect the vestibular nuclei. Some of the termination areas in the reticular formation can be homologized with the mammalian inferior olive and the nucleus prepositus hypoglossi. Another part of the vestibuloreticular projection may transmit vestibular impulses toward the vegetative centers of the brainstem. A relatively weak projection is detected in the spinal nucleus of the trigeminal nerve, dorsal column nuclei, and nucleus of the solitary tract. 4) In the spinal cord, vestibular terminals are most numerous in the ipsilateral ventral horn and in the triangular area of the dorsal horn. 5) The coincidence of retrogradely labeled cells with vestibular receptive areas suggests reciprocal interconnections between these structures and the LVN. 6) In seven places, the LVN projections overlap the receptive areas of proprioceptive fibers, suggesting a convergence of sensory modalities involved in the sense of balance.

Contributions of the vestibular nucleus and vestibulospinal tract to the startle reflex

The startle reflex is elicited by strong and sudden acoustic, vestibular or trigeminal stimuli. The caudal pontine reticular nucleus, which mediates acoustic startle via the reticulospinal tract, receives further anatomical connections from vestibular and trigeminal nuclei, and can be activated by vestibular and tactile stimuli, suggesting that this pontine reticular structure could mediate vestibular and trigeminal startle. The vestibular nucleus, however, also projects to the spinal cord directly via the vestibulospinal tracts, and therefore may mediate vestibular startle via additional faster routes without a synaptic relay in the hindbrain. In the present study, the timing properties of the vestibular efferent pathways mediating startle-like responses were examined in rats using electrical stimulation techniques. Transient single- or twin-pulse electrical stimulation of the vestibular nucleus evoked bilateral, startle-like responses with short refractory periods. In chloral hydrate-anesthetized rats, hindlimb electromyogram latencies recorded from the anterior biceps femoris muscle were shorter than those for stimulation of the trigeminal nucleus, and similar to those for stimulation of the caudal pontine reticular nucleus or ventromedial medulla. In awake rats, combining vestibular nucleus stimulation with either acoustic stimulation or trigeminal nucleus stimulation enhanced the whole-body startle-like responses and led to strong cross-modal summation without collision effects. In both chloral hydrate-anesthetized and awake rats, combining vestibular nucleus stimulation with ventromedial medulla stimulation produced a symmetrical collision effect, i.e. a loss of summation at the same positive and negative stimulus intervals, indicating a continuous connection between the vestibular nucleus and ventromedial medulla in mediating vestibular startle. By contrast, combining trigeminal nucleus stimulation with ventromedial medulla stimulation resulted in an asymmetric collision effect when the trigeminal nucleus stimulation preceded ventromedial medulla stimulation by 0.5 ms, suggesting that a monosynaptic connection between the trigeminal nucleus and ventromedial medulla mediates trigeminal startle. We propose that the vestibulospinal tracts participate strongly in mediating startle produced by activation of the vestibular nucleus. The convergence of the vestibulospinal tracts with the reticulospinal tract within the spinal cord therefore provides the neural basis of cross-modal summation of startling stimuli.

Uncrossed and crossed projections from the upper cervical spinal cord to the cerebellar nuclei in the rat, studied by anterograde axonal tracing

In the upper cervical spinal segments, neurons in the medial part of lamina VI give rise to uncrossed spinocerebellar axons, whereas the central cervical nucleus (CCN) and neurons in laminae VII and VIII give rise to crossed spinocerebellar axons. Using anterograde labeling with biotinylated dextran in the rat, we examined the projections of these neuronal groups to the cerebellar nuclei. Uncrossed and crossed projections were distinguished by cerebellar lesions placed on the side contralateral or ipsilateral to the tracer injections confined to the second and third cervical spinal segments (C2 and C3, respectively). Labeled terminals of uncrossed projections were seen in the middle, dorsal, and ventrolateral parts of the middle subdivision and in the ventral part of the caudomedial subdivision of the medial nucleus. In the anterior interpositus nucleus, terminals were seen in the middle of the mediolateral extent, whereas, in the posterior interpositus nucleus, they were seen in lateral and caudal parts. The terminals of crossed projections from the CCN were distributed ventrally in medial to ventrolateral parts of the middle subdivision of the medial nucleus. Some terminals were seen in the caudomedial subdivision of the medial nucleus. In the anterior interpositus nucleus, labeled terminals were seen mainly in rostromedial parts, whereas, in the posterior interpositus nucleus, they were seen in caudal and dorsal parts of the medial half. The present study suggests that the medial lamina VI group and the CCN in the upper cervical segments project to the different areas of the cerebellar nuclei and are concerned with different functions.

The normal distribution and projections of constitutive NADPH-d/NOS neurons in the brainstem vestibular complex of the rat

The vestibular system is a highly conserved sensory system in vertebrates that is largely responsible for maintenance of one's orientation in space, posture, and balance and for visual fixation of objects during motion. In light of the considerable literature indicating an involvement of nitric oxide (NO) in sensory systems, it is important to determine whether NO is associated with vestibular pathways. To study the relationship of NO to vestibular pathways, we first examined the normal distribution of constitutive NADPH-diaphorase (NADPH-d), a marker for nitric oxide synthase (NOS), in the vestibular complex (VC) and then examined its association with selected vestibular projection neurons. Survey of the four major vestibular nuclei revealed that only the medial vestibular nucleus contained significant numbers of perikarya stained for NADPH-d/NOS. By contrast, all the vestibular nuclei contained a network of fine processes that stained positive for NADPH-d, although the density of this network varied among the individual nuclei. To determine whether NADPH-d/NOS neurons project to vestibular efferent targets, injections of the retrograde tracer Fluoro-Gold were made into known targets of second-order vestibular neurons. Vestibular neurons containing constitutive NADPH-d/NOS were found to project predominantly to the oculomotor nucleus. A small number of neurons also participate in vestibulothalamic and intrinsic vestibular connections. These results indicate that NADPH-d/NOS neurons are prevalent in the MVN and that a subpopulation of these neurons project to the oculomotor complex. Nitric oxide is probably released locally from axons located throughout the vestibular complex but may play a particularly important role in vestibulo-ocular pathways.

Organisation of reciprocal connections between trigeminal and vestibular nuclei in the rat

In order to study the connection patterns between the sensory trigeminal and the vestibular nuclei (VN), injections of anterogradely and/or retrogradely transported neuronal tracers were made in the rat. Trigeminal injections resulted in anterogradely labelled fibres, with an ipsilateral preponderance, within the VN: in the ventrolateral part of the inferior nucleus (IVN), in the lateral part of the medial nucleus (MVN), in the lateral nucleus (LVN) with a higher density in its ventral half, and in the superior nucleus (SVN), more in the periphery than in the central part. Moderate trigeminal projections were observed in the small vestibular groups f, x and y/l and in the nucleus prepositus hypoglossi. Additional retrogradely labelled neurones were seen in the IVN, MVN, and LVN, in the same regions as those receiving trigeminal afferents. Morphological analysis of vestibular neurones demonstrated that vestibulo-trigeminal neurones are relatively small and belong to a different population than those receiving projections from the trigeminal nuclei. The trigeminovestibular and vestibulo-trigeminal relationships were confirmed by tracer injections in the VN. The results show that, in the VN, there is sensory information from facial receptors in addition to those reported from the neck and body. These facial afferents complement those from the neck and lower spinal levels in supplying important somatosensory information from the face and eye muscles. The oculomotor connections of the respective zones of the VN receiving trigeminal afferents suggest that sensory inputs from the face, including extraocular proprioception, may, through this pathway, influence the vestibular control of eye and head movements.

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.

Cervical primary afferent input to vestibulospinal neurons projecting to the cervical dorsal horn: an anterograde and retrograde tracing study in the cat

Vestibulospinal neurons in the caudal half of the medial and descending vestibular nuclei terminate in the cervical spinal cord, not only in the ventral horn and intermediate zone but also in the dorsal horn. The purpose of the present study was to examine whether the areas containing these vestibulospinal neurons are reached by cervical primary afferents. In one group of experiments, wheat germ agglutinin-horseradish peroxidase conjugate and horseradish peroxidase were pressure injected into spinal ganglia C2-C8 and revealed anterogradely labeled fibers and boutons in the caudal part (caudal to the dorsal cochlear nucleus) of the ipsilateral medial and descending vestibular nuclei. This projection was verified in experiments in which wheat germ agglutinin-horseradish peroxidase conjugate was microiontophoretically injected into the caudal half of either the medial or the descending vestibular nuclei and revealed retrogradely labeled cells only in ipsilateral spinal ganglia C2-C7, with a maximum of cells in C3. In another group of experiments, after microiontophoretic injections of Phaseolus vulgaris leucoagglutinin or Biocytin into either the medial or the descending vestibular nuclei, anterogradely labeled fibers and boutons were present in the cervical spinal cord, mainly bilaterally in the dorsal horn (laminae I-VI) but also, to a lesser extent, in the ventral horn and intermediate zone. The existence of a loop that relays cervical primary afferent information to vestibulospinal neurons projecting to the cervical spinal cord, in particular the dorsal horn, may have implications for vestibular control over local information processing in the cervical dorsal horn.

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.

Neurogenetical segregation of the vestibulospinal neurons in the rat

The time of origin of the vestibulospinal projection neurons was determined by a double-labeling method using 5-bromodeoxyuridine (BrdU), the thymidine analogue, and Fluoro-Gold (FG), a retrograde fluorescent tracer. Rat fetuses were exposed to BrdU in utero to label the vestibular neurons on one of the embryonic (E) days between E12 and E15. Upon reaching adulthood, the rats were given unilateral injections of FG into the cervical cord to identify the spinal projection neurons. Brainstem sections were immunohistochemically processed for BrdU and then examined for neurons that were both BrdU-positive and FG-positive in the vestibular nuclei. In the lateral vestibular nucleus (LVe), most of the vestibulospinal neurons were generated on E12. In the inferior vestibular nucleus (IVe), the vestibulospinal neurons were produced almost equally on both E12 and E13. In the medial vestibular nucleus (MVe), the vestibulospinal neurons were generated consistently on days between E12 and E14 with a mild peak on E13. The present results thus demonstrate that genesis of the vestibulospinal neurons occurs sequentially in the following order: firstly in the LVe, secondly in the IVe, and finally in the MVe. The different sequential generation of vestibulospinal neurons among the LVe, MVe and IVe may reflect the fact that the vestibulospinal projections are differentially organized depending on the nature of each subnucleus.