[Pathways and ascending vestibular projections emanating from primary nuclei: radioautographic study (author's transl)]

Activation of the thalamic parafascicular nucleus by electrical stimulation of the peripheral vestibular nerve in rats

The parafascicular nucleus (PFN) of the thalamus is a primary structure in the feedback circuit of the basal ganglia-thalamo-cortical system, as well as in the neural circuit of the vestibulo-thalamo-striatal pathway. We investigated the characteristics of the functional connectivity between the peripheral vestibular system and the PFN in rats. A single electrical stimulation was applied to the horizontal semicircular canal nerve in the peripheral vestibular end-organs. This resulted in polysynaptic local field potentials (LFPs) in the PFN, which were composed of long-lasting multiple waves. The LFPs were prominently seen contralateral to the stimulation site. The PFN LFPs were suppressed by transient chemical de-afferentation of peripheral vestibular activity using a 5% lidocaine injection into the middle ear. The spontaneous firing rate of the single units increased after electrical stimulation to the horizontal canal nerve in a frequency-dependent manner. The induction of cFos protein was more prominent in the contralateral PFN than in the ipsilateral PFN following horizontal semicircular canal nerve stimulation. The functional vestibulo-parafascicular connection is a neural substrate for the transmission of vestibular sensory information to the basal ganglia.

The thalamocortical vestibular system in animals and humans

The vestibular system provides the brain with sensory signals about three-dimensional head rotations and translations. These signals are important for postural and oculomotor control, as well as for spatial and bodily perception and cognition, and they are subtended by pathways running from the vestibular nuclei to the thalamus, cerebellum and the "vestibular cortex." The present review summarizes current knowledge on the anatomy of the thalamocortical vestibular system and discusses data from electrophysiology and neuroanatomy in animals by comparing them with data from neuroimagery and neurology in humans. Multiple thalamic nuclei are involved in vestibular processing, including the ventroposterior complex, the ventroanterior-ventrolateral complex, the intralaminar nuclei and the posterior nuclear group (medial and lateral geniculate nuclei, pulvinar). These nuclei contain multisensory neurons that process and relay vestibular, proprioceptive and visual signals to the vestibular cortex. In non-human primates, the parieto-insular vestibular cortex (PIVC) has been proposed as the core vestibular region. Yet, vestibular responses have also been recorded in the somatosensory cortex (area 2v, 3av), intraparietal sulcus, posterior parietal cortex (area 7), area MST, frontal cortex, cingulum and hippocampus. We analyze the location of the corresponding regions in humans, and especially the human PIVC, by reviewing neuroimaging and clinical work. The widespread vestibular projections to the multimodal human PIVC, somatosensory cortex, area MST, intraparietal sulcus and hippocampus explain the large influence of vestibular signals on self-motion perception, spatial navigation, internal models of gravity, one's body perception and bodily self-consciousness.

Vestibular Cortical Area in the Periarcuate Cortex

Vestibular input to the periarcuate cortex in the Japanese monkey was examined by analyzing laminar field potentials evoked by electrical stimulation of the vestibular nerve. Vestibular-evoked potentials consisted of early-positive and late-negative potentials and early-negative and late-positive potentials in the superficial and deep layers of the cortex, respectively. They were distributed bilaterally in the periarcuate cortex around the junction of the spur and the arcuate sulcus. This vestibular-projecting area corresponded to the periarcuate area where retrogradely-labeled corticovestibular neurons were distributed after the injection of a tracer into the vestibular nuclei. Comparison of the vestibular-projection area with the distribution of smooth pursuit-related neurons in the same monkey revealed that such neurons existed in the vestibular-projecting area of the periarcuate cortex.

Medial vestibular connections with the hypocretin (orexin) system

The mammalian medial vestibular nucleus (MVe) receives input from all vestibular endorgans and provides extensive projections to the central nervous system. Recent studies have demonstrated projections from the MVe to the circadian rhythm system. In addition, there are known projections from the MVe to regions considered to be involved in sleep and arousal. In this study, afferent and efferent subcortical connectivity of the medial vestibular nucleus of the golden hamster (Mesocricetus auratus) was evaluated using cholera toxin subunit-B (retrograde), Phaseolus vulgaris leucoagglutinin (anterograde), and pseudorabies virus (transneuronal retrograde) tract-tracing techniques. The results demonstrate MVe connections with regions mediating visuomotor and postural control, as previously observed in other mammals. The data also identify extensive projections from the MVe to regions mediating arousal and sleep-related functions, most of which receive immunohistochemically identified projections from the lateral hypothalamic hypocretin (orexin) neurons. These include the locus coeruleus, dorsal and pedunculopontine tegmental nuclei, dorsal raphe, and lateral preoptic area. The MVe itself receives a projection from hypocretin cells. CTB tracing demonstrated reciprocal connections between the MVe and most brain areas receiving MVe efferents. Virus tracing confirmed and extended the MVe afferent connections identified with CTB and additionally demonstrated transneuronal connectivity with the suprachiasmatic nucleus and the medial habenular nucleus. These anatomical data indicate that the vestibular system has access to a broad array of neural functions not typically associated with visuomotor, balance, or equilibrium, and that the MVe is likely to receive information from many of the same regions to which it projects.

Three-dimensional analysis of cerebellar terminals and their postsynaptic components in the ventral lateral nucleus of the cat thalamus

Relationships among cerebellar terminals (CTs), dendrites of thalamocortical projection neurons (TCNs), and dendrites of local circuit neurons in the ventral lateral nucleus of the cat thalamus were analyzed quantitatively by observing several series of serial ultrathin sections and by using a computer-assisted program for the three-dimensional reconstruction from serial ultrathin sections. In pentobarbital-anesthetized cats, CTs were labeled either by injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the cerebellar nuclei or by intra-axonal injection of HRP after electrophysiological identification. By using two series of 133 and 73 serial sections, mutual relationships between 43 WGA-HRP-labeled CTs and their postsynaptic structures were analyzed based on their synaptic specializations and shapes of synaptic vesicles. Thirty-nine of these CTs formed a synapse with one TCN dendrite, whereas only four CTs formed synapses with two TCN dendrites. These CTs also synapsed on dendrites containing pleomorphic synaptic vesicles (presynaptic dendrites). Single CTs synapsed on 0-6 presynaptic dendrites (2.2 +/- 1.5, N = 43) through their whole extents, and about 40% of these presynaptic dendrites that were contacted by CTs established synaptic contacts with the same TCN dendrites on which the CTs synapsed. Thus, a CT, a presynaptic dendrite, and a TCN dendrite formed a triadic arrangement. Triadic arrangements were identified in approximately 60% of these 43 CTs. However, they rarely had a glomerulus-like appearance, as described previously in the ventral lateral nucleus and other main thalamic relay nuclei. In another series of 83 and 43 serial sections along dendrites of TCNs, observations were focused on the triadic arrangement. Triadic arrangements were located evenly on the primary and secondary dendrites of TCNs. Computer-assisted three-dimensional reconstructions were made on one WGA-HRP-labeled CT and two intra-axonally labeled CTs (a bouton en passant and a bouton terminal) with their surrounding neuronal elements, and complex spatial arrangement of neuronal processes became obvious. These results provide the quantitative assessment of synaptic arrangements among CTs, presynaptic dendrites, and TCN dendrites and reveal their spatial interrelations in the cat ventral lateral nucleus.

Subcortical projections to the centromedian and parafascicular thalamic nuclei in the cat

The primary objective of this study is to identify the totality of input to the centromedian and parafascicular (CM-Pf) thalamic nuclear complex. The subcortical projections upon the CM-Pf complex were studied in the cat with three different retrograde tracers. The tracers used were unconjugated horseradish peroxidase (HRP), horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP), and rhodamine-labeled fluorescent latex microspheres (RFM). Numerous subcortical structures or substructures contained labeled neurons with all three tracing techniques. These labeled structures included the central nucleus of the amygdala; the entopeduncular nucleus; the globus pallidus; the reticular and ventral lateral geniculate nuclei of the thalamus; parts of the hypothalamus including the dorsal, lateral, and posterior hypothalamic areas and the ventromedial and parvicellular nuclei; the zona incerta and fields of Forel; parts of the substantia nigra including the pars reticularis and pars lateralis, and the retrorubral area; the pretectum; the intermediate and deep layers of the superior colliculus; the periaqueductal gray; the dorsal nucleus of the raphe; portions of the reticular formation, including the mesencephalic, pontis oralis, pontis caudalis, gigantocellularis, ventralis, and lateralis reticular nuclei; the nucleus cuneiformis; the marginal nucleus of the brachium conjunctivum; the locus coeruleus; portions of the trigeminal complex, including the principal sensory and spinal nuclei; portions of the vestibular complex, including the lateral division of the superior nucleus and the medial nucleus; deep cerebellar nuclei, including the medial and lateral cerebellar nuclei; and lamina VII of the cervical spinal cord. Moreover, the WGA-HRP and rhodamine methods (known to be more sensitive than the HRP method) revealed several afferent sources not shown by HRP: the anterior hypothalamic area, ventral tegmental area, lateral division of the superior vestibular nucleus, nucleus interpositus, and the nucleus praepositus hypoglossi. Also, the rhodamine method revealed labeled neurons in laminae V and VI of the cervical spinal cord.

Vestibulo-thalamic neurons give off descending axons to the spinal cord

Vestibulo-thalamic (VT) neurons were physiologically studied in the anesthetized cat. Forty-seven VT neurons were recorded extracellularly. More than half of the VT neurons responded monosynaptically to vestibular nerve stimulation while the others responded polysynaptically. They were activated antidromically from one or two sites in the VPL. VPM, VL, VM, SG, and PO of the contralateral thalamus. Four fifths of the VT neurons were activated from the C1 segment of the spinal cord. Half of them were also activated from the C4 segment, but none were activated from the L5 segment. It is suggested that most VT neurons project descending axons to the cervical spinal cord. Axonal branching was shown by means of systematic microstimulation in the thalamus and the ventral horn in the C1 segment. The VT neurons were mainly located in the descending vestibular nucleus.

Extracellular recording of vestibulo-thalamic neurons projecting to the spinal cord in the cat

Forty vestibulo-thalamic (VT) neurons were recorded extracellularly in the vestibular nuclei of the anesthetized cat. More than half of the VT neurons responded monosynaptically to vestibular nerve stimulation; the others responded polysynaptically. The VT neurons were activated antidromically from one or two sites in the contralateral VPL, VPM, VL, VM, SG, and PO in the thalamus. Their axonal arborizations in the thalamus were likely restricted in narrow areas. About three quarters of the VT neurons were also activated antidromically from the ventral funiculus in the C1 segment. Axonal branchings were found in the contralateral C1 gray matter. The VT neurons were mainly localized in the descending vestibular nucleus.

Vestibular projections to the thalamus of the pigeon: an anatomical study

In a series of retrograde tracing studies involving the injection of WGA-HRP into the thalamus of the pigeon, labeled neurons were consistently observed in anterior regions of the vestibular nuclei. Following small dorsal thalamic injections, labeled neurons were located predominantly in rostroventrolateral regions of the superior vestibular nucleus, less numerously within the ventral part of the lateral vestibular nucleus, and least numerously within the medial vestibular nucleus. Following large dorsal thalamic injections, many more vestibular neurons were labeled, and these were distributed more extensively throughout anterior parts of the superior, lateral, and medial nuclei. No labeled neurons were found in the descending nucleus. Injections of tritiated amino acids into vestibular nuclei revealed a terminal field within the dorsal thalamic nucleus: dorsolateralis posterior, pars rostralis. The location of this field between auditory, somatosensory, and paleostriatally and neostriatally projecting nuclei suggests a general similarity to the organization of vestibulothalamic projections in mammals.

The vestibulothalamic connections in the rat: a morphological analysis using wheat germ agglutinin-horseradish peroxidase

The vestibulothalamic connections were studied in the rat using wheat germ agglutinin-horseradish peroxidase (WGA-HRP). The distributions of anterograde labelling of fibers and terminals in the brainstem and the thalamus were analyzed by injecting WGA-HRP into the superior (SVN) and lateral (LVN) vestibular nuclei, and the medial (MVN) and inferior (IVN) vestibular nuclei. The distributions of retrograde labelling of cells were analyzed in the vestibular nuclear complex by injecting WGA-HRP into the thalamus centered in the central lateral nucleus (CL), ventral posterolateral nucleus (VPL), and rostral part of the dorsal medial geniculate nucleus (rMGd). The vestibular projection to the CL via the medial longitudinal fasciculus (MLF) and the ascending tract of Deiters (ATD) originates mainly in the contralateral MVN and ipsilateral SVN. The vestibular projections to the VPL and the ventral lateral nucleus (VL) via MLF, ATD and superior cerebellar peduncle (SCP) originate mainly in the MVN and SVN, bilaterally. The projection to the rMGd via the lateral lemniscus-inferior collicular brachium, and MLF (and SCP) originates in the contralateral IVN.

Transneuronal transport in the vestibular and auditory systems of the squirrel monkey and the arctic ground squirrel. I. Vestibular system

Transneuronal transport of [3H]proline, [3H]fucose, and [3H]leucine in various combinations from pledgets implanted in the ampulla of a single semicircular duct was studied in the squirrel monkey and arctic ground squirrel after long survival periods. Tritiated amino acids implanted in any single ampulla resulted in labeling of nearly all vestibular and auditory receptors, nearly all cells of the vestibular and spiral ganglia and central transport via nearly all root fibers of both nerves. Primary vestibular fibers were distributed to the vestibular nuclei (VN) and specific parts of the cerebellum in the pattern previously described. Transneuronal transport of [3H]proline by vestibular neurons was present in all known secondary pathways, except those projecting to thalamic nuclei. Observations were similar in both species, except for small differences in commissural vestibular projections. Major commissural transport was to all parts of the opposite medial vestibular nucleus (MVN) and to peripheral parts of the superior vestibular nucleus (SVN), but some transport was present in all contralateral VN, including ventral cell group y. Descending transneuronal transport was evident in vestibulospinal tract (VST) ipsilaterally and in the medial longitudinal fasciculus (MLF) bilaterally. Both [3H]proline and [3]fucose were transported transneuronally to the ipsilateral abducens nucleus (AN); with long survivals [3H]proline was transported peripherally via the ipsilateral abducens nerve root. Ascending transport in the MLF was bilateral, asymmetric and greatest contralaterally. Fibers entered the contralateral MLF near the AN and the lateral wing of the ipsilateral MLF rostral to most of the VN. Terminals in the trochlear nuclei (TN) were bilateral and greatest contralaterally. In the monkey terminals in ipsilateral oculomotor complex (OMC) were distributed uniformly in all subdivisions, except for the medial rectus subdivision (MRS), where terminals were more numerous. The greatest density of terminals was present contralaterally in the superior rectus subdivision (SRS) of the OMC; only sparse terminals were present in the MRS on that side. Transport in the ipsilateral abducens nerve roots in the monkey and the virtual absence of transport to the MRS of the contralateral OMC suggested transneuronal transport to abducens motor neurons, but not to internuclear neurons (AIN). The AIN project only to the MRS of the contralateral OMC and do not appear to receive vestibular input. Comparable observations were made in the AN, TN and OMC of the ground squirrel, although the representation of the extraocular muscles in the OMC is unknown.(ABSTRACT TRUNCATED AT 400 WORDS)

The cerebellar and vestibular nuclear complexes in the turtle. I. Projections to mesencephalon, rhombencephalon, and spinal cord

Cerebellar and vestibular projections were investigated in the turtle Pseudemys scripta elegans following injection of 35S-methionine into the cerebellar and vestibular nuclear complexes at various locations. Fibers arising from the cerebellar nuclei were traced via the cerebellar commissure to the contralateral vestibular nuclear complex (particularly the n. vestibularis inferior and n. vestibularis ventrolateralis) and caudal rhombencephalic tegmentum. Ascending projections crossing the midline in the ventral isthmomesencephalic tegmentum terminated in the contralateral red nucleus and nuclei of the fasciculus longitudinalis medialis (f lm). Vestibular projections ascending mainly via the f lm terminated in the nuclei of the f lm, the nuclei of the posterior commissure, and particularly the extraocular motor nuclei. Vestibulo-ocular projections arising from the rostral vestibular nuclear complex were almost exclusively ipsilateral; those from the caudal vestibular nuclear complex were bilateral. Evidence for a topographic organization of the projections to the trochlear and oculomotor nuclei was also obtained. There were some vestibular projections to the contralateral rhombencephalic tegmentum and n. vestibularis inferior. Spinal projections coursing within the ipsilateral ventral descending tract and the ipsilateral fasciculus longitudinalis medialis were found to arise from both rostral and caudal vestibular regions. The caudal vestibular nuclear complex in addition gave rise to fibers descending in the contralateral fasciculus longitudinalis medialis. Evidence for the existence of labeled fibers crossing at spinal levels was also obtained. Vestibulospinal terminations appeared restricted to the ventral horn.

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

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

Afferent connections of the zona incerta: a horseradish peroxidase study in the rat

Restricted microelectrophoretic injections either of free horseradish peroxidase or of horseradish peroxidase conjugated with wheat germ agglutinin were given to albino rats in order to study the afferent connections of structures of the subthalamic region. The results suggest that the zona incerta receives its main input from several territories of the cerebral cortex, the mesencephalic reticular formation, deep cerebellar nuclei, regions of the sensory trigeminal nuclear complex and the dorsal column nuclei. Substantial input to the zona incerta appears to come from the superior colliculus, the anterior pretectal nucleus and the periaqueductal gray substance, whereas many other structures, among which hypothalamic nuclei, the locus coeruleus, the raphe complex, the parabrachial area and medial districts of the pontomedullary reticular formation, seem to represent relatively modest but consistent additional input sources. The afferentation of neurons in Forel's fields H1 and H2 appears to conform to the general pattern outlined above. As pointed out in the Discussion, the present results provide hodological support for the classic concept according to which the zona incerta can be regarded as a rostral extent of the midbrain reticular core. Some of the possible physiological correlates of the fiber connections of the zona incerta in the context of the sleep-waking cycle, ingestive behaviors, somatic motor mechanisms, visual functions and nociceptive behavior are briefly discussed.

Cortical and brain stem afferents to the ventral thalamic nuclei of the cat demonstrated by retrograde axonal transport of horseradish peroxidase

After horseradish peroxidase (HRP) injections into various parts of the ventral thalamic nuclear group and its adjacent areas, the distribution of labeled neurons was compared in the cerebral cortex, basal ganglia, and the brain stem. The major differences in distribution patterns were as follows: Injections of HRP into the lateral or ventrolateral portions of the ventroanterior and ventrolateral nuclear complex of the thalamus (VA-VL) produced retrogradely labeled neurons consistently in area 4 gamma (lateral part of the anterior and posterior sigmoid gyri, lateral sigmoid gyrus and the lateral fundus of the cruciate sulcus), the medial division of posterior thalamic group (POm), suprageniculate nucleus (SG) and anterior pretectal nucleus ipsilaterally, and in the nucleus Z of the vestibular nuclear complex bilaterally. Injections into the medial or dorsomedial portion of the VA-VL resulted in labeled neurons within the areas 6a beta (medial part of the anterior sigmoid gyrus), 6a delta (anterior part of ventral bank of buried cruciate sulcus), 6 if. fu (posterior part of the bank), fundus of the presylvian sulcus (area 6a beta), medial part of the nucleus lateralis posterior of thalamus and nucleus centralis dorsalis ipsilaterally, and in the entopeduncular nucleus (EPN) and medial pretectal nucleus bilaterally. Only a few neurons were present in the contralateral area 6a delta. After HRP injections into the ventral medial nucleus (VM), major labeled neurons were observed in the gyrus proreus, area 6a beta (mainly in the medial bank of the presylvian sulcus), and EPN ipsilaterally, and in the medial pretectal nucleus and substantia nigra bilaterally. Following HRP injections into the centre médian nucleus (CM), major labeled neurons were found in the areas 4 gamma, 6a beta, and the orbital gyrus ipsilaterally, and in the EPN, rostral and rostrolateral parts of the thalamic reticular nucleus, locus ceruleus, nucleus reticularis pontis oralis et caudalis and nucleus prepositus hypoglossi bilaterally. The contralateral intercalatus nucleus also possessed labeled neurons. With HRP injections into the paracentral and centrolateral nuclei, labeled neurons were observed in the gyrus proreus and the cortical areas between the caudal presylvian sulcus and anterior rhinal sulcus ipsilaterally, and in the nuclei interstitialis and Darkschewitsch bilaterally. Minor differences in the distribution pattern were observed in the superior colliculus, periaqueductal gray, mesencephalic and medullary reticular formations, and vestibular nuclei in all cases of injections.

Thalamic projections to the anterior suprasylvian and posterior sigmoid cortex: an HRP study of the "vestibular areas" of the cerebral cortex in the cat

We have confirmed electrophysiologically the existence of an oligosynaptic vestibular projection to the cortex surrounding the rostral end of the anterior suprasylvian sulcus ( ASsS ). However, we failed to confirm a similar projection to area 3a in the posterior sigmoid gyrus. We studied the thalamic projections to each of these cortical regions by injecting small amounts of HRP in the cortex and looking for neurons retrogradely labeled throughout the thalamus. The exact location of the cortical injections was assessed cytoarchitectonically. The heaviest neuronal labeling after injections in the banks of ASsS was obtained in Po (including in this complex GMmc ). A moderate number of projections was found from VPi, VPm and VPl (the labeling in the latter being particularly prominent in a case injected in the lower bank of ASsS ), and also from VL. Occasional labeled neurons were found in the rostro-ventral part of LP. After injections in area 3a in the posterior sigmoid gyrus, which affected to a minor degree either area 3b or 4, many labeled cells appeared in the rostral and dorsal part of VPl, and in the central and lateral parts of VL. Fewer labeled cells were found in VPi, Po and LP. In most cases some occasional labeled cell was observed also in the intralaminar nuclei and in Vm.

Afferent and efferent connections of the medial, inferior and lateral vestibular nuclei in the cat and monkey

Attempts were made to determine the afferent and efferent connections of the medial (MVN), inferior (IVN) and lateral (LVN) vestibular nuclei (VN) in the cat and monkey using retrograde and anterograde axoplasmic transport technics. Injections of HRP and [3H]amino acids were made selectively into MVN, IVN and LVN and into: (1) MVN and IVN, (2) LVN and IVN and (3) all 4 VN. Contralateral afferents to MVN arise from (1) the nuclei prepositus (NPP) and intercalatus (NIC), (2) all parts of MVN and cell group 'y' and (3) parts of the superior vestibular nucleus (SVN), IVN and the fastigial nucleus (FN). Ipsilateral projections to MVN arise from: (1) a central band of the flocculus and the nodulus and uvula, (2) the interstitial nucleus of Cajal (INC), and (3) visceral nuclei of the oculomotor nuclear complex (OMC). Efferent projections of MVN are to: (1) the ipsilateral supraspinal nucleus (SSN), and (2) the contralateral central cervical nucleus (CCN), MVN, SVN, cell group 'y', the rostroventral region of LVN, the trochlear nucleus (TN) and the INC. Projections to the abducens nuclei (AN) and the OMC are bilateral. Some ascending fibers in the cat cross within the OMC. In the monkey fibers from MVN end in a central band of the ipsilateral flocculus. Afferents to IVN arise ipsilaterally from SVN, the nodulus, the uvula and the anterior lobe vermis. Contralateral afferents arise from: (1) parts of CCN, MVN, SVN, IVN and cell group 'y' and (2) the central third of the FN. IVN receives bilateral projections from the perihypoglossal nuclei (PH) and the visceral nuclei of the OMC. Efferents from IVN project: (1) ipsilaterally to nucleus beta of the inferior olive, (2) contralaterally to parts of MVN, SVN and cell group 'y' and (3) bilaterally to the paramedian reticular nuclei. No commissural fibers interconnect cell groups 'f' and 'x'. Ascending fibers from IVN terminate contralaterally in the TN and the OMC. In the monkey fibers from IVN terminate in the ipsilateral nodulus, uvula and anterior lobe vermis; no fibers project to FN in either the cat or the monkey. Afferents to the LVN arise primarily from the ipsilateral anterior lobe vermis and bilaterally from rostral parts of the FN. No commissural fibers interconnect the LVN. Projections of the LVN are primarily to spinal cord via the vestibulospinal tract (VST); collaterals of the VST terminate in the lateral reticular nucleus (LRN). Ascending uncrossed projections from LVN in the cat terminate in the medial rectus subdivision of the OMC.(ABSTRACT TRUNCATED AT 400 WORDS)

Spatial relationships between the terminations of somatic sensory motor pathways in the rostral brainstem of cats and monkeys. II. Cerebellar projections compared with those of the ascending somatic sensory pathways in lateral diencephalon

Previous studies have shown that ascending somatic sensory pathways arising from the dorsal column nuclei, lateral cervical nucleus and spinothalamic tract terminate in parts of the thalamus adjacent to those which receive cerebellar terminations. This termination pattern creates a border between the ventroposterolateral nucleus (VPL) and the ventrolateral nucleus (VL) in the cat and between the caudal and oral parts of VPL (VPLc and VPLo, respectively) in the monkey. Since it is not clear how sharp these borders are, a double orthograde labeling strategy was used in the present study to make direct comparisons of the projections to the thalamus from these sources of input. It was found that there was a change in the sources of afferent input to the different target areas that paralleled changes in cytoarchitecture. Moving caudally to rostrally, VPL in the cat and VPLc in the monkey received projections predominantly from the middle, dorsal (clusters) portion of the dorsal column nuclei. These projections were gradually replaced near the VPL-VL border in the cat and VPLc-VPLo border in the monkey first by input from the lateral cervical nucleus (cat only) and the rostral and ventral portions of the dorsal column nuclei and then by spinothalamic projections. Towards VL in the cat and the rostral parts of VPLo in the monkey (referred to as Vim by Hassler, '59 and Mehler, '71), these projections were in turn replaced by those from the cerebellum. This sequence resulted in a complex pattern (summarized in Fig. 10) where some thalamic territories received input predominantly from one source and others received converging input from several sources. The major region receiving converging ascending somatic sensory and cerebellar terminations was located at the border between VPL and VL in the cat and in the caudal parts of Olszewski's ('52) VPLo in the monkey (that is, between VPLc and Vim). In general, the results in the cat were similar to those in the monkey. One notable difference was that the domain containing terminals from the cerebellum and the rostral-ventral parts of the dorsal column nuclei was located medially between VPLc and Vim in the monkey, whereas it extended across the entire mediolateral border between VPL and VL in the cat. In both species, thalamic neurons received input predominantly from one afferent source and only minor input, if any, from other sources.(ABSTRACT TRUNCATED AT 400 WORDS)

The superior vestibular nucleus: an intracellular HRP study in the cat. II. Non-vestibulo-ocular neurons

Superior vestibular neurons were penetrated with horseradish peroxidase (HRP)-loaded glass microelectrodes in anesthetized cats and identified electrophysiologically following electrical stimulation of the vestibular nerves and oculomotor complex. Neurons that were not antidromically activated from the oculomotor complex were stained by intracellular injection of horseradish peroxidase. Three types of neurons are identified according to their initial axonal trajectories into the cerebellum, the dorsal pontine reticular formation, or the brachium conjunctivum. Ipsilateral vestibular nerve input to all neurons is primarily monosynaptic and excitatory, whereas the contralateral is inhibitory. The neurons are located in the periphery of the superior vestibular nucleus. Soma diameters range from 20.5 micrometers to 44 micrometers. Most neurons exhibit globular and ovoid cell bodies. The dendritic arbors are intermediate between iso- and allodendritic branching patterns. The few spines and dendritic appendages present are distributed mainly distally on the dendrites. Soma size does not correlate with axon diameter, number of dendrites, or dendritic territories.

The superior vestibular nucleus: an intracellular HRP study in the cat. I. Vestibulo-ocular neurons

Superior vestibular neurons were penetrated with horseradish peroxidase (HRP)-loaded glass microelectrodes in anesthetized cats. Responses to electrical stimulation of the oculomotor complex and the vestibular nerves were characterized and selected neurons were injected with HRP. Neurons antidromically activated by oculomotor complex stimulation were generally monosynaptically excited by the ipsilateral vestibular nerve. Notable was the absence of strong commissural inhibition by stimulation of the contralateral vestibular nerve. Light microscopy of antidromically identified injected cells demonstrated that these cells are predominantly located at the central levels of the superior vestibular nucleus along the incoming vestibular nerve fibers but a few are found at more caudal levels. Cell bodies, elongated or pyramidal, are mainly medium-sized to large (30-50 micrometers). Dendritic trees extend in a plane at an acute to the collaterals of the vestibular nerve fibers. Dendrites remain within the nuclear territory and generally display an isodendritic branching pattern. Dendritic spines and appendages are mainly distributed on secondary and distal dendrites. A few terminal enlargements similar to growth cones are observed in these neurons. Axons of these neurons project rostrally via the medial longitudinal fasciculus, while a minor projection via the brachium conjunctivum is also found. Axon collaterals, when present, originate in the nucleus itself and in the pontine reticular formation.

Corticothalamic projections from postcruciate area 4 in the dog

Corticothalamic projections from postcruciate area 4, located on the rostral part of the posterior sigmoid gyrus, were traced with the autoradiographic technique in the dog. Injections of tritiated amino acids were made into the lateral and medial parts of area 4 in regions corresponding to the forelimb and hindlimb areas of the primary motor cortex, respectively. In cases with injections placed in the lateral part of area 4, dense accumulations of label were present in the lateral part of the ventral anterior nucleus (VA), the central part of the ventral lateral nucleus (VL), the ventral half of the ventral posterior inferior nucleus (VPI), the caudal part of the central lateral nucleus (CL), and the centrum medianum (CM). Lighter label was also present in the lateral part of the cytoarchitectonically distinct VL region bordering the ventrobasal complex (VB), as well as in the ventrolateral part of the mediodorsal nucleus (MD), and in the lateral posterior nucleus (LP). In one case in which the injection site involved an adjacent part of area 3a, label was also seen ventrally in the medial division of the posterior nuclear group (POm). However, no detectable differences in VL, MD, or intralaminar labeling patterns were noted between this case and the four other cases with injections confined to the lateral part of area 4. In two cases with injections restricted to the medial part of area 4, dense label was present in the lateralmost part of VL, the ventral part of VPI, the caudal part of CL, and CM. Lighter label was also present in the VL region bordering the dorsolateral edge of VB and in LP. An additional case in which the injection also involved the rostral border of area 3a showed a similar pattern of thalamic labeling. Projections from both the lateral and medial parts of area 4 were also noted in the subthalamic nucleus, zona incerta, and nucleus of Darkschewitsch. These results suggest that corticothalamic projections from postcruciate area 4 to VL are organized topographically such that projections from the lateral part of area 4 project centrally within VL while those from the medial part of area 4 project more laterally. Both parts of area 4 also project topographically to a cytoarchitectonically distinct region of VL located immediately adjacent to VB. In contrast, the projections to the intralaminar nuclei do not appear to be topographically organized. The data from cases involving spread of the injection into area 3a suggest that projection patterns from area 3a to ventral, intralaminar, and medial thalamic nuclei are similar to those from area 4. However, it appears that at least the lateral part of area 3a also projects to POm.

The vestibulothalamic pathway: contribution of the ascending tract of Deiters

Axoplasmic transport techniques were used to determine the contribution of the ascending tract of Deiters (ATD) to the vestibulothalamic projection in cats. Large injections of HRP into the thalamus centered on the border region between the ventrobasal complex and the caudal ventrolateral nucleus resulted in bilateral retrograde labeling of cells in the vestibular nuclear complex and the nucleus prepositus hypoglossi (PH). Similar thalamic injections were also made in animals with extensive bilateral lesions of the medial longitudinal fasciculus (MLF) and the brachium conjunctivum (BC). HRP-positive neurons in these cases were localized principally to the ventral lateral vestibular nucleus and adjacent superior vestibular nucleus ipsilateral to the thalamic injection, evidence that vestibulothalamic neurons in these nuclei may project to the thalamus over the unlesioned ATD. Injections of [35S]methionine into the rostral vestibular nuclear complex in animals with MLF and BC lesions confirmed these findings, demonstrating orthograde transport of radiolabel in the ATD with termination in thalamus. These experiments document a contribution of the ATD to the ipsilateral vestibulothalamic projection; other sources of the vestibulothalamic pathway (PH, Y group) likely travel through projection systems destroyed in the lesions made in the present study.

[Identification of the vestibular projections in the oculomotor nuclei in the cat by autoradiography and electron microscopy]

The projection of vestibular pathways to the oculomotor nuclei ws investigated by electron microscopic radioautography. Unilateral injection of tritiated amino acids into the rostral vestibular complex was used in order to characterize the location and to identify the different types of labeled synaptic terminals involved in these pathways. In the normal oculomotor nuclei, 4 types of synaptic boutons were identified. Following the labeling of the vestibular synapses, in the ipsilateral oculomotor nucleus, types I and II boutons are the most prominent group and make up 75% of the synaptic vesicles, they are distributed on the cellular soma and the large dendrites of the oculomotor neurons. In contrast, in the contralateral oculomotor nucleus, type III boutons which are smaller and have larger diameter synaptic vesicles were predominant; they are prevalent on the distal part of the dendritic tree. From the results obtained, a relationship between the present anatomical findings and previously published physiological studies is established. The following conclusion is suggested: the inhibitory vestibular inputs probably terminate on the oculomotor neurons by these large types I and II boutons and the excitatory vestibular inputs by the smaller type III boutons. Also discussed is the complexity of the pattern of afferentation and the functional arrangement of the oculomotor nuclei.

An axonal transport study of the ascending projection of medial lemniscal neurons in the rat

The pattern of projection of the rat medial lemniscus was studied by axonal transport labeling following injections of tritiated leucine, proline and/or adenosine, or of horseradish peroxidase for retrograde identification of the neurons of origin. The vast majority of neurons in the gracile, cuneate, and principal trigeminal nuclei contribute to an almost totally crossed projection primarily to the thalamic ventrobasal complex. Additional thalamic components were traced to specific sites within the "posterior group," including a medial component largely traversed by lemniscal axons and a caudolateral component lying between the principal nucleus of the medial geniculate and ventral nucleus of the lateral geniculate. We have designated this latter zone "intermediate geniculate," distinguishing a somatosensory portion of the geniculate group on the basis of its myelo- and cytoarchitecture, as well as its connections. Other projections replicated in several animals included the zona incerta and nearby sectors of the substantia nigra; three distinct mesencephalic arrangements within the deep layers of the superior colliculus, the external nucleus of the inferior colliculus, and the intercollicular nucleus; the anterior pretectal nucleus; dorsal sectors of the inferior olivary complex and the ipsilateral cerebellar cortex. The results are compared with findings in other species (with emphasis on the caudal thalamic region) in an attempt to resolve some of the apparent inconsistencies in nomenclature.

The afferent projections to the centrum medianum of the cat as demonstrated by retrograde transport of horseradish peroxidase

The afferent projections of nucleus centrum medianum (CM) of the thalamus were studied, in the cat, by means of retrograde transport of electrophoretically ejected horseradish peroxidase. Several variations of method--survival time, fixatives, substrates, etc.--were tried to improve the amount of visible reaction product. Labeled neurons were localized primarily in two categories of nuclei in the brain. The first consisted of structures making up or closely related to the basal ganglia: the entopeduncular nucleus, the pars reticulata of the substantia nigra, and motor cortex. The second category was made up of nuclei closely related to postural and orienting functions: the deep layers of the superior colliculus ipsilaterally, and the medial and lateral vestibular nuclei bilaterally. Other nuclei containing retrogradely labeled neurons were the periaqueductal gray and locus coeruleus. Brain stem reticular projections were sparse and widely scattered. These results identify CM as an important element in the loop system linking medial thalamus and neostriatrum; the probable attention and orientation related functions of this system are discussed.

Vestibular projections to the monkey thalamus: an autoradiographic study

Vestibulothalamic projections were studied in the monkey (macaca mulatta) by injecting anerograde trace substances (radioactive leucine and proline) into the vestibular nuclear complex. Terminal labelling was found bilaterally mainly in the nucleus ventroposterior lateralis pars oralis (VPLo) and to a lesser extent in the nucleus ventroposterior inferior (VPI) and nucleus ventralis lateralis pars caudalis (VLc). The labelling was sparse, and scattered over wide areas. The vestibular origin of this projection was confirmed by injecting retrograde tracer substances (horseradish peroxidase and 125I wheat germ agglutinin) into VPLo. In the autoradiographic study no labelling was found in the posterior group.

An anatomical reinvestigation of the termination of the spinothalamic tract in the monkey

The projections of the spinothalamic tract in the macaque monkey have been reinvestigated using the Wiitanen modification of the Fink-Heimer technique. In agreement with previous studies in the monkey (mehler, Bowsher, Kerr) it was found that the spinothalamic tract ascends outside the medial lemniscus, enters the thalamus just dorsal to this structure, and terminates in the posterior, intralaminar and ventral regions, as well as in the zona incerta. The posteromedial nucleus (POm) receives a dense spinothalamic projection medially and ventromedially; elsewhere in the POm the projection is more scattered. The fibers to the intralaminar region terminate in the nucleus centralis lateralis (CL) with a distinct pattern of the distribution. The nucleus centralis medialis (CeM) has a minute projection. There was no evidence for somatotopic organization in the projections to the POm or to the intralaminar region. The distribution of the terminal degeneration in the ventral region was more complex. Although present in the whole nucleus ventralis posterolateralis (VPL), the degeneration was unevenly distributed and also extended beyond the VPL. So-called clusters of dense degeneration lay in the outskirts of the forelimb and hindlimb representation areas, namely at its ventral, ventrolateral, dorsolateral, and medial borders. Centrally the degeneration was scattered. Thus, most of the VPL receives only a sparse spinothalamic projection, but a small portion contains dense networks of terminal spinal fibers. A somatotopic pattern was evident, for after low thoracic lesions most of the medial VPL lacked degeneration. Spinothalamic fibers pass beyond the VPL to terminate in a zone of transition (nucleus ventralis intermedius of V.im of Hassler, '59; Mehler, '71) between the rostral pole of the VPL and the nucleus ventralis lateralis (VL). This zone also reportedly receives cerebellar and vestibular afferent fibers. Observations suggesting that the evolution of the spinothalamic tract and the spino-cervico-thalamic pathway in carnivores and primates may be linked are discussed. The spinothalamic clusters in the monkey's VPL appear to be homologous to much of the cervicothalamic tract projection to the VPL in the cat.