The termination of spinovestibular fibres in the cat

Effects of body orientation on maximum voluntary arm torques

INTRODUCTION

Increased reliance on bulbospinal motor systems has been implicated in individuals with chronic stroke during maximum voluntary arm joint torque generation.

METHODS

Maximum isometric single-joint and multi-joint arm strength was observed in two body orientations (sitting and supine) while maintaining identical head/neck/trunk/extremity joint configurations in order to identify bulbospinal contributions to maximum joint torque generation in 11 individuals with stroke and 10 individuals without stroke.

RESULTS

During sitting, shoulder flexion was greater for both groups, whereas shoulder extension and elbow flexion, part of the "flexion synergy," were greater only in individuals with stroke.

CONCLUSIONS

Body orientation influenced isometric arm strength, notably the constituents of flexion synergy in individuals with stroke, suggesting bulbospinal motor pathway involvement. From a practical perspective, clinical evaluation of single joint strength in the supine position may underestimate strength available during activities of daily living that are performed in an upright orientation.

Afferent pathways to the region of the vestibular nuclei that participates in cardiovascular and respiratory control

Prior experiments have shown that a region of the medial and inferior vestibular nuclei contributes to cardiovascular and respiratory regulation. In addition to labyrinthine inputs, the majority of neurons in this region of the vestibular nuclei receive signals from the skin, muscle, and viscera, although the pathways conveying these nonlabyrinthine inputs to the vestibular nucleus neurons are unknown. To gain further insight into the afferent pathways to this functionally distinct subdivision of the vestibular complex, we combined monosynaptic mapping with viral transneuronal tracing in the ferret. First order afferent projections were defined by retrograde transport of the beta-subunit of cholera toxin (CTbeta), and the extended polysynaptic circuitry was defined in the same animals by injection of a recombinant of pseudorabies virus Bartha (PRV) into the contralateral vestibular nuclei. Neurons containing CTbeta or infected by retrograde transneuronal transport and replication of PRV were distributed throughout the spinal cord, but were 10 times more prevalent in the cervical cord than the lumbar cord. The labeled spinal neurons were most commonly observed in Rexed's laminae IV-VI and the dorsal portions of laminae VII-VIII. Both the CTbeta and PRV injections also resulted in labeling of neurons in all four vestibular nuclei, the prepositus hypoglossi, the reticular formation, the inferior olivary nucleus, the medullary raphe nuclei, the spinal and principal trigeminal nuclei, the facial nucleus, and the lateral reticular nucleus. Following survival times >/=3 days, PRV-infected neurons were additionally present in nucleus solitarius and the gracile and cuneate nuclei. These data show that an anatomical substrate is present for somatosensory and visceral inputs to influence the activity of cells in the autonomic region of the vestibular nuclei and suggest that these signals are primarily transmitted through brainstem relay neurons.

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.

Vestibulospinal and reticulospinal neuronal activity during locomotion in the intact cat. II. Walking on an inclined plane

The experiments described in this report were designed to determine the contribution of vestibulospinal neurons (VSNs) in Deiters' nucleus and of reticulospinal neurons (RSNs) in the medullary reticular formation to the modifications of the walking pattern that are associated with locomotion on an inclined plane. Neuronal discharge patterns were recorded from 44 VSNs and 63 RSNs in cats trained to walk on a treadmill whose orientation was varied from +20 degrees (uphill) to -10 degrees (downhill), referred to as pitch tilt, and from 20 degrees roll tilt left to 20 degrees roll tilt right. During uphill locomotion, a majority of VSNs (25/44) and rhythmically active RSNs (24/39) showed an increase in peak discharge frequency, above that observed during locomotion on a level surface. VSNs, unlike some of the RSNs, exhibited no major deviations from the overall pattern of the activity recorded during level walking. The relative increase in discharge frequency of the RSNs (on average, 31.8%) was slightly more than twice that observed in the VSNs (on average, 14.4%), although the average absolute change in discharge frequency was similar (18.2 Hz in VSNs and 21.6 Hz in RSNs). Changes in discharge frequency during roll tilt were generally more modest and were more variable, than those observed during uphill locomotion as were the relative changes in the different limb muscle electromyograms that we recorded. In general, discharge frequency in VSNs was more frequently increased when the treadmill was rolled to the right (ear down contralateral to the recording site) than when it was rolled to the left. Most VSNs that showed significant linear relationships with treadmill orientation in the roll plane increased their activity during right roll and decreased activity during left roll. Discharge activity in phasically modulated RSNs was also modified by roll tilt of the treadmill. Modulation of activity in RSNs that discharged twice in each step cycle was frequently reciprocal in that one burst of activity would increase during left roll and the other during right roll. The overall results indicate that each system contributes to the changes in postural tone that are required to adapt the gait for modification on an inclined surface. The characteristics of the discharge activity of the VSNs suggest a role primarily in the overall control of the level of electromyographic activity, while the characteristics of the RSNs suggest an additional role in determining the relative level of different muscles, particularly when the pattern is asymmetric.

Vestibulospinal and reticulospinal neuronal activity during locomotion in the intact cat. I. Walking on a level surface

To examine the function of descending brain stem pathways in the control of locomotion, we have characterized the discharge patterns of identified vestibulo- and reticulospinal neurons (VSNs and RSNs, respectively) recorded from the lateral vestibular nucleus (LVN) and the medullary reticular formation (MRF), during treadmill walking. Data during locomotion were obtained for 44 VSNs and for 63 RSNs. The discharge frequency of most VSNs (42/44) was phasically modulated in phase with the locomotor rhythm and the averaged peak discharge frequency ranged from 41 to 165 Hz (mean = 92.8 Hz). We identified three classes of VSNs based on their discharge pattern. Type A, or double peak, VSNs (20/44 neurons, 46%) showed two peaks and two troughs of activity in each step cycle. One of the peaks was time-locked to the activity of extensor muscles in the ipsilateral hindlimb while the other occurred anti-phase to this period of activity. Type B, or single pause, neurons (13/44 neurons, 30%) were characterized by a tonic or irregular discharge that was interrupted by a single pronounced and brief period of decreased activity that occurred just before the onset of swing in the ipsilateral hindlimb; some type B VSNs also exhibited a brief pulse of activity just preceding this decrease. Type C, or single peak, neurons (9/44 neurons, 23%) exhibited a single period of increased activity that, in most cells, was time-locked to the burst of activity of either extensor or flexor muscles of a single limb. The population of RSNs that we recorded included neurons that showed phasic activity related to the activity of flexor or extensor muscles [electromyographically (EMG) related, 26/63, 41%], those that were phasically active but whose activity was not time-locked to the activity of any of the recorded muscles (13/63, 21%) and those that were completely unrelated to locomotion (24/63, 38%). Most of the EMG-related RSNs showed one (15/26) or two (11/26) clear phasic bursts of activity that were temporally related to either flexor or extensor muscles. The discharge pattern of double-burst RSNs covaried with ipsilateral and contralateral flexor muscles. Peak averaged discharge activity in these EMG-related RSNs ranged from 4 to 98 Hz (mean = 35.2 Hz). We discuss the possibility that most VSNs regulate the overall activity of extensor muscles in the four limbs while RSNs provide a more specific signal that has the flexibility to modulate the activity of groups of flexor and extensor muscles, in either a single or in multiple limbs.

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.

Morphometric analysis of the human vestibular nuclei

BACKGROUND

Cytoarchitectural investigations of the vestibular nuclei have been undertaken in different species of mammals. These data provide a description of the general architecture of the nuclei but limited information about quantitative characteristics of their cell population. We have recently obtained data about the morphometric parameters of the vestibular nuclei neurons in some species. The application of quantitative image analysis techniques to the research of the cellular morphology in the vestibular area of humans might provide basic information to compare with data from animal studies, taking into account the observed correlation between physiological and morphological properties of vestibular neurons.

METHODS

The characteristics of the major vestibular nuclei in humans have been studied with light microscopic techniques in serially cut sections. Camera lucida drawings of the vestibular nuclei and their neurons were made and subjected to computerized image analysis. For each vestibular nucleus, information was obtained about topography, morphological characteristics (i.e., location, volume, and length), and the number and morphometric parameters of their neurons (cross-sectional areas, maximum and minimum diameters). Morphometric data about cell parameters were statistically analyzed by comparing the populations within different parts of each nucleus and from different nuclei.

RESULTS

Among the vestibular nuclei, the medial, which is the largest, has the greatest number of neurons, and the interstitial, the least. The lateral and interstitial nuclei contain the largest cells, and the descending nucleus has the smallest cells. The superior nucleus contains cells of intermediate size. The size of cells decreases in a rostrocaudal direction in the medial, lateral, and descending nuclei, the opposite trend being observed in the superior nucleus. Within the superior and medial nuclei, there are discrete areas with cells with distinctive characteristics.

CONCLUSIONS

These results suggest that, just as most of the anatomical characteristics of the second-order neurons found in animals have been preserved in humans, so the physiological mechanisms observed in the vestibular system of animals should apply to humans.

Rostrocaudal and ventrodorsal change in neuronal cell size in human medial vestibular nucleus

BACKGROUND

The present paper describes the cytoarchitectonic, morphometric, and three-dimensional characteristics of the human medial vestibular nucleus (MVN). We also studied the regional distribution, in size, of the different neurons and its possible relationship with a functional polarization of the different regions of the nucleus.

METHODS

Nine adult human brainstems (30-50 years of age) without neurological problems were used. Specimens were obtained from necropsy and fixed in 4% paraformaldehyde and 5% acetic acid in distilled water. After fixation, blocks were washed, dehydrated, and embedded in paraffin and serial sectioned at 20 microns. Sections were stained with formaldehydethionin, dehydrated, cleared in eucalyptol, and mounted with Eukitt. MVN neurons were drawn with the aid of a camera lucida at 200-micron intervals at 390 x magnification. Serial 50-micron frozen sections were used to determine the volume of the MVN. The three-dimensional reconstruction of MVN was accomplished with a drawing program in a Macinthosh II computer and an AVS on a Stardent workstation computer.

RESULTS

In the three-dimensional reconstruction, the human MVN shows a pyramidal form. The base of this pyramid constitutes the rostral limit, and its vertex forms the caudal border of the MVN. The estimated volume is 30.44 +/- 0.85 mm3, with a neuronal population of 127,737 cells and 4,136 neurons/mm3 in density. The average neuronal cross-section changes from one minimum at caudal level (212.46 +/- 2.04 microns 2) to one maximum at rostral level (491.47 +/- 5.08 microns 2). Four cell types, small (< 200 microns 2), medium (200-500 microns 2), large (500-1000 microns 2), and giant (> 1,000 microns 2) cells, were observed. Medium cells constitute 66%, small cells 18%, and large and giant cells 15% and 1% of the neuronal population.

CONCLUSIONS

The MVN shows a variation in neuronal size, and it has the highest neuronal density of all the human vestibular nuclei. Large cells predominate in rostral regions of the MVN, with significant differences in the area and diameter of the cells among rostral, central, and caudal regions. Furthermore, the largest cells are grouped in the ventrolateral part of the nucleus, close to its boundaries with the inferior and the lateral vestibular nuclei. The morphological polarization, with respect to the neuronal size of the MVN, can be related to a functional polarization of rostral and caudal regions of this nucleus.

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 of the individual vestibular end-organs in the brain stem of the squirrel monkey

The central nervous system (CNS) projections of primary afferent neurons from individual vestibular receptors were studied using horseradish peroxidase (HRP) or biocytin labeling in 14 ears from 7 adult squirrel monkeys using the technique developed in the chinchilla (Lee et al., 1989, 1992). The specificity of labeling was verified by examining the location of the labeled fibers and cell bodies in the vestibular nerve and Scarpa's ganglion. Labeled fibers and cells were restricted to nerves and areas belonging to groups of cells in either the superior or the inferior ganglion of the vestibular nerve. In the vestibular nerve root, labeled primary afferent fibers also exhibited a receptor-dependent segregation at the entrance to the medulla. Fibers from the HSC and the SSC were found rostrally and those from the PSC and the SAC were found in the caudal area. The UTR fibers were situated intermediate between these two groups of fibers. (A bundle of fibers, probably vestibular efferents, was identified immediately rostrally and ventromedially to the UTR fibers.) The primary afferent fibers bifurcated into secondary ascending and descending fibers at the lateral border of the vestibular nuclei, forming a longitudinal rostrocaudal vestibular tract. The secondary fibers from individual end-organs occupied specific locations in the tract: the UTR fibers were dorsal to the SSC and the HSC fibers, PSC fibers were found most medially, and the SAC fibers occupied the lateralmost area. The secondary UTR fibers overlapped considerably with those of the SSC and the HSC. The orderly receptor-dependent segregation of fibers was more prominent in the descending tracts than in the ascending tracts. In the vestibular nuclei complex the location of the tertiary branches of various end-organs exhibited considerable overlap within the major vestibular nuclei (SN, superior nucleus; LN, lateral nucleus; MN, medial nucleus; DN, descending nucleus). There were still differences, however, in the projection pattern. Fibers from the SAC ran primarily in the lateral area, fibers from the SSC and the UTR were found ventromedially to the SAC fibers, and the HSC projected slightly medially to the fibers from the SSC. The PSC fibers projected most medially. The UTR and SAC sent numerous fibers to the cerebellum. Fibers from the semicircular canals projected through the rostrodorsal region of the SN and presumably also projected to the cerebellum. The precise termination of fibers was evaluated by studying the location of labeled boutons, which were identified in all major vestibular nuclei. Labeled boutons from all the receptors were in the rostral and central areas of the SN, and in the MN mainly in the rostral two-thirds. In the LN, boutons from all the receptors were in the rostroventral part, most of which were from the UTR and SAC. No labeled boutons were in the caudodorsal part of this nucleus. Labeled boutons in the DN primarily surrounded the descending tract fibers and were particularly prominent medially. In specimens in which superior vestibular nerve receptor organs were scratched vestibular efferent fibers were also labeled. These fibers traveled in the most ventral part of the vestibular nerve root and projected in the ventral aspect of the LN to labeled soma in the ipsilateral and contralateral brain stem. Specificity the in projection patterns of efferent fibers from different end-organs could not be ascertained.

The reticulovestibular projection in the rabbit: an experimental study with the retrograde horseradish peroxidase method

The reticulovestibular projections of the brainstem in the rabbit were studied by the retrograde transport of horseradish peroxidase (HRP). After selective iontophoretic injections of the tracer into various subdivisions of the vestibular nuclear complex (VNC), labeled neurons were found in defined regions of the reticular formation (RF) of the caudal pons and the rostral medulla. The results indicate that all four vestibular nuclei receive projection from RF. This projection is bilateral with a contralateral predominance. The major projection originates from dorsal and dorsolateral regions of the caudal pontine reticular nucleus (RPc) and the gigantocellular reticular nucleus (RGc) at the transitional level between them. A modest projection originates from pars alpha of the caudal pontine reticular nucleus (RPc alpha), the parvocellular reticular nucleus (Rpc) and pars alpha of the parvocellular nucleus (Rpc alpha), mostly from their ventral regions. A small projection arises from pars alpha of the gigantocellular reticular nucleus (RGc alpha), as well as from the ventral reticular subnucleus (Rv) and cell group a in the caudal aspect of the medulla. No clear-cut topical relationship was noted between the location of neurons in RF and projection site in VNC. The superior vestibular nucleus (SV) and the medial vestibular nucleus (MV) receive projections exclusively from RPc and RGc, whereas the lateral reticular nucleus (LV) and the inferior vestibular nucleus (IV) receive additional projections from the remaining RF nuclei. The termination areas of reticular fibers within SV and IV seem to be diffuse but in MV and LV there is a clear preponderance to the regions located ventrally. The present study has established cells of origin for the reticulovestibular projections from the pontomedullary RF to individual VNC nuclei in the rabbit.

Afferent innervation of the vestibular nuclei in the chinchilla. II. Description of the vestibular nerve and nuclei

The morphological characteristics of the vestibular nuclei of the chinchilla were studied in horizontally cut serial sections of the brain stem. Horseradish peroxidase labeling allowed unambiguous delineation of the vestibular nuclei and areas of innervation by the vestibular afferent fibers. The cytoarchitecture of the vestibular nuclei was documented with the aid of camera lucida drawings and quantitatively evaluated with computerized methodology. The cellular groups identified in other species were found in the chinchilla. The superior vestibular nucleus (SN) originated ventromedial to the mesencephalic tract and nuclei of the trigeminal nerve. This nucleus contained medium-sized cells with a central group of larger cells (20-34 microns in diameter). It received its maximum vestibular innervation caudally in the ventrolateral and dorsal aspects of the nucleus. Fibers projected to the SN in bundles with thick fibers surrounded by thin ones. The lateral vestibular nucleus (LN) originated 0.9-1.2 mm below the rostral aspect of the vestibular area. It was ventrocaudal to the SN and contained many large cells with diameters of 45-60 microns. The LN was innervated mainly in the ventrocaudal aspect by oblique and transverse fibers that formed a dense mesh. The medial vestibular nucleus (MN) originated 0.3-0.6 mm caudal to the beginning of the SN, adjacent to the floor of the IVth ventricle. It extended for 3-4 mm along the SN, LN and descending vestibular nucleus (DN). The MN contained the densest and most homogeneous cells, which had diameters of 10-20 microns. This nucleus received its greatest innervation at the level of the vestibular root. Thin fibers traveled to the MN through the SN and LN. The caudal pole of the nucleus did not receive fibers. The DN originated 1.8-2.5 mm caudal to the origination of the SN, between the caudal LN and the MN. Caudally it replaced the LN. Most of the cells of the DVN were medium-sized, with diameters of 10-20 microns. The main vestibular innervation of the DN was in the lateral aspect of the nucleus. Tertiary fibers projected in small, separate bundles of uniform-sized thick fibers. The interstitial nucleus originated 1.1-1.4 mm from the beginning of the SN. It occupied the center of the vestibular root, 0.8-0.9 mm medial to the root entry zone. It contained a few large cells (greater than 20 microns in diameter), many medium-sized cells, and some small cells.(ABSTRACT TRUNCATED AT 400 WORDS)

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

Phaseolus vulgaris leucoagglutinin and wheat germ agglutinin-horseradish peroxidase were iontophoretically injected into different parts of the vestibular nuclear complex (VNC) of the rat. Injections centered into the caudal part of the medial vestibular nucleus revealed a vestibulospinal projection predominantly to the dorsal horn of the cervical spinal cord, besides the expected projection to the intermediate zone (IZ) and ventral horn (VH). While most of the anterogradely labelled fibres could be localized in laminae III to V, some scattered fibres were also seen in laminae I and VI. Lamina II remained free of labelling. The dorsal horn (DH) area with detectable anterograde labelling showed a rostrocaudal extension from C1-C6. Injections into other parts of the VNC labelled fibres and terminals in the IZ and VH while the DH remained almost free of labelling. Additionally, fluorogold and wheat germ agglutinin-horseradish peroxidase were pressure- or iontophoretically injected at different levels into the spinal cord to confirm the projection to the dorsal horn by means of retrograde tracing. Labelled neurons in the area of the medial vestibular nucleus (MVN), from which anterograde labelling in the DH was obtained, were only detectable after fluorogold and wheat germ agglutinin-horseradish peroxidase injections into the cervical spinal cord, in particular its DH. This projection from the caudal medial vestibular nucleus to the dorsal horn of the cervical spinal cord probably enables the VNC to influence sensory processing in the DH, in addition to its well-established influence on posture and locomotion via projections to the intermediate zone and ventral horn.

WGA-HRP studies on the fiber connections from the spinal cord to the Deiters' nucleus

The labyrinthine-spinal reflexes are influenced by the inputs from the cervical and lumbal propriocepters. We studied the afferent route by the retrograde WGA-HRP method in cats. After the injection of WGA-HRP into either the dorsal or the ventral Deiters' nucleus, labeled neurons were investigated in the spinal cord, the cerebellum, the contralateral vestibular nucleus complex and the brain stem nucleus. In this paper, we report the results in the spinal cord of cats. i) When WGA-HRP was injected into the dorsal Deiters' nucleus, labeled neurons in the spinal cord were found mainly extending from the cervical to the lumbosacral area of the spinal cord. A number of labeled cells were located predominantly in the contralateral cervical segments, while a small number of labeled cells was found ipsilaterally in the lumbosacral segments. ii) In the case of the ventral Deiters' nucleus, labeled neurons were found extending from the cervical to the upper thoracic area of the spinal cord. Localization of labeled neurons in the spinal cord was limited mainly to Rexed's laminae VII and VIII. These results suggest that the afferent fibers from the spinal cord to the Deiters' nucleus are closely related to the labyrinthine-spinal reflex.

Projections ascending from the spinal cord to the brain in petromyzontid and myxinoid agnathans

The course of projections ascending through the rostral spinal cord to nuclei in the brains of petromyzontid and myxinoid agnathans was examined with silver staining of anterograde degeneration and horseradish peroxidase histochemistry. As in jawed vertebrates, the ascending spinal projections of lampreys and hagfishes appear to be organized into two major systems, the spinal lemniscal and dorsal column pathways. The spinal lemniscal pathway, extending rostrally along the ventrolateral margin of the spinal and medullary central gray, consists of a spinoreticular and possibly a spinovestibular projection in both aganthan groups. In Pacific hagfish, spinal lemniscal fibers reach the ipsilateral mesencephalic tectum, but no spinal projection to the thalamus was evident. The spinal lemniscus of lampreys ascends to the region of the isthmus and may extend into the mesencephalic tegmentum. Anterograde and retrograde tracing methods indicate that a very small population of cells in the far rostral cord of lampreys may project to the optic tectum and diencephalon; however, spinotectal and spinothalamic projections, if present, are limited in extent. The dorsal column pathway in agnathans, consisting in part of primary spinal afferents, ascends in the dorsal funiculus of the cord. The dorsal column fibers of agnathans, like those of some other anamniotes, continue beyond the spinomedullary junction through the length of the hindbrain, possibly conveying ascending somatosensory input to the sensory nuclei of the alar medulla.

Effects of motor cortex and single muscle stimulation on neurons of the lateral vestibular nucleus in the rat

The neuronal responses to stimulation of motor cortical sites and of forelimb single muscles were studied in the lateral vestibular nucleus of anaesthetized rats. Of the 228 neurons tested for response to stimulation of contralateral motor cortex, 63% responded to cortical sites controlling extensor muscles and 30% to those controlling flexors. The corresponding figures for responders to ipsilateral stimulation were 34 and 21%. Vestibulospinal units responded to cortical sites controlling extensor and flexor muscles whereas the remaining lateral vestibular nucleus neurons, very reactive to cortical sites controlling extensor muscles, responded little to contralateral and not at all to ipsilateral cortical sites controlling flexor muscles. The effects evoked by contralateral cortical sites controlling extensors varied, those induced by cortical sites controlling flexors were inhibitory in 77% of cases. The responses to ipsilateral motor cortex stimulation differed not so much by cortical sites controlling extensor or flexor muscles as by whether the neuron was in the dorsal or ventral zone of the lateral vestibular nucleus: mixed in the former, all inhibitory in the latter. Of the lateral vestibular nucleus units tested for response to stimulation of ipsilateral or contralateral forelimb distal muscles, only 11% responded. All the vestibulospinal units responsive to muscle stimulation lay in the dorsal zone of the nucleus. The remainder, dorsal or ventral, were not responsive to contralateral muscles. Single lateral vestibular nucleus cells influenced both by ipsilateral muscle and by contralateral motor cortex made up 24% of the pool, vestibulospinal and non-vestibulospinal. They fell into three groups: responsive to one or both structures but responding more strongly to combined stimulation; responsive to each of the two structures but showing a response to combined stimulation not significantly different from that evoked by the cortex alone; responsive only to combined stimulation. The lateral vestibular nucleus units included in these three groups accounted for 29% of those tested for response to extensor muscles and cortical sites controlling extensors and 15% of those tested for response to flexor muscles and cortical sites controlling flexors. Twenty-five per cent of the vestibulospinal neurons responded both to contralateral muscles and to ipsilateral motor cortex stimulation but none of the non-vestibulospinal neurons responded to both. All the responders to both were in the dorsal zone of the lateral vestibular nucleus and responded to extensor stimuli, always in the same way. These results indicate that motor cortex output exerts a major influence on lateral vestibular nucleus discharges, while the muscle afferents have a modulatory influence on the lateral vestibular nucleus responses to cortex.(ABSTRACT TRUNCATED AT 400 WORDS)

Primary vestibular projections in the chinchilla

The central projections of fibers from the vestibular nerve were studied in 19 chinchillas after horseradish peroxidase labelling. In addition, the limits of the vestibular nuclei and the anatomical characteristics of their neurons were also studied. All five vestibular nuclei received primary afferents, but there were extensive areas of them that received very little or no projections at all, such as the rostral part of the superior vestibular nucleus, the dorsocaudal part of the lateral vestibular nucleus, the caudal half of the medial vestibular nucleus and the caudalmost aspect of the dorsal vestibular nucleus.

A reinvestigation of the spinovestibular projection in the cat using axonal transport techniques

There are numerous discrepancies within the literature concerning the sources of spinovestibular fibers and their distribution in the vestibular complex. Sources of afferents from all spinal levels were sought using the retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase. Following injections of this tracer in all portions of the vestibular complex retrograde labelling was densest at upper cervical levels, especially within the contralateral central cervical nucleus. Labelling was also observed in laminae VI (ipsilaterally), IV, V, VII, and VIII (bilaterally). At progressively more caudal levels, numbers of labelled cells decreased but were similarly distributed in these laminae. The terminal distribution of spinal efferent fibers within the vestibular complex was revealed by injecting wheat germ agglutinin conjugated to horseradish peroxidase or tritiated amino acids into various levels of the spinal cord. These studies showed that all spinal levels project to the descending vestibular nucleus and group x. The f-tail of the descending vestibular complex receives projections from upper cervical and thoracic levels. Terminations within the medial vestibular nucleus arise from both upper cervical and lumbar levels. No conclusive evidence was found supporting the presence of substantial direct spinal projections to the lateral vestibular nucleus, superior vestibular nucleus, or group z. Possible functional roles for the spinovestibular projection in posture and gaze are discussed.

Comparisons of the immunocytochemical localization of choline acetyltransferase in the vestibular nuclei of the monkey and rat

Immunocytochemical studies of the brainstem were done in the squirrel monkey and rat using the same polyclonal antisera for choline acetyltransferase (ChAT). Cells immunoreactive for ChAT (ChATir) were evident in large numbers in visceral and motor cranial nerve nuclei in both species, but virtually no ChATir cells were seen in the vestibular nuclear complex of the rat. In the monkey ChATir cells were distributed in caudal parts of the medial (MVN) and in dorsal parts of the inferior (IVN) vestibular nuclei. Only a few immunoreactive cells were seen in the rostral MVN and none were found in cell group f of the IVN. Nearly all cells of group z and x, which do not receive primary vestibular afferents, were immunoreactive to ChAT. None of the cells in the superior and lateral vestibular nuclei, cell group y, the infracerebellar nucleus or the interstitial nucleus of the vestibular nerve were immunoreactive for ChAT. Cells immunoreactive to ChAT were present in large numbers in the rostral part of the nucleus prepositus in the monkey, but not in the rat. The relatively small number and distribution of ChATir cells in the MVN suggested they could constitute only a small fraction of the MVN neurons that contribute to a massive commissural system. Significant differences in cholinergic vestibular neurons appear to exist between the rat and the monkey.

Influence of standing on vestibular neuronal activity in awake cats

Single-unit activity of vestibular nuclear neurons was recorded in chronically prepared, awake cats. To examine the influence of tonic activation of limb proprioception on vestibular function, the vestibular modulation by horizontal rotation (0.2 Hz, 17 deg) and head-tilt (0.1 Hz, 7 deg) was recorded in animals with freely hanging limbs and was compared with the modulation in standing cats. Of 29 examined cells responding to horizontal rotation, only about 30% were affected during standing, with most exhibiting a decrease of the mean discharge rate and gain. In contrast, about 70% of the 28 tilt-modulated cells showed pronounced effects during standing with a decrease of the gain and an increase of the mean discharge rate. The increase of the mean discharge rate in tilt cells may be caused by the excitatory spinovestibular afferent fibers or by the efferent vestibular system. For the observed inhibitory effects on the gain different mechanisms may be responsible: cerebellar inhibition and/or efferent vestibular receptor control. This control of labyrinthine information by somatosensory afferent fibers may serve for the stability of equilibrium in the moving animal.

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

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

Observations on the secondary vestibulocerebellar projections in the macaque monkey

The distribution of retrogradely labeled cells in the nuclei of the vestibular nuclear complex following injections of horseradish peroxidase in various parts of the cerebellar cortex (except the nodulus and paraflocculus) has been mapped in the macacus rhesus monkey. In the main the findings correspond to those made in other mammalian species (cf. Table 1). The flocculus receives afferents bilaterally from the superior, medial and descending vestibular nucleus, group y, the interstitial nucleus of the vestibular nerve and also from the abducent nucleus. The projection to the posterior vermis (lobules VIII and IX), especially to lobule IX, is more abundant than that to lobules VI-VII. The projection to the anterior lobe vermis appears to be modest. Evidence for projections to the cerebellar hemispheres was not obtained. Whether the lateral vestibular nucleus projects to the cerebellum in the macaque is uncertain. The regular occurrence of weakly labeled cells among heavily labeled ones suggests that many of the cerebellar projecting cells may have axonal branches passing to other destinations. The findings lend support to the notion that there are precise topical relations within the entire secondary vestibulocerebellar projection. For example, in the medial nucleus the sites of origin of fibers to the flocculus and uvula are different. Surprisingly, many cells in group z were found to project to the uvula and - to a lesser extent - to lobule VIII. The group z may, therefore, not be a pure relay nucleus in a spinothalamic pathway, as generally assumed. The rather marked cerebellar projection of the abducent nucleus, especially to the flocculus, is of interest for the analysis of cerebellar control of eye movements in the macaque.

The vestibular nuclei in the macaque monkey

The topography and the main features of the cytoarchitecture of the vestibular nuclear complex in the macaque monkey have been studied in serially cut, Nissl-stained, transverse, parasagittal and horizontal sections. In addition to the four main vestibular nuclei, the topographically closely related small cell groups (f, l, x, y, and z), distinguished by Brodal and Pompeiano ('57) in the cat, have been considered and illustrated. The vestibular nuclear complex in the macaque in general corresponds in topography and architecture to the situation described in some other mammals on which information is available, such as opossum, rabbit, cat, Galago, and man. Some dissimilarities in detail are found. For example, in man the lateral vestibular nucleus differs somewhat from the general pattern, especially in its position, and the small group f, fusing with the descending nucleus, appears to be indistinct; likewise the group y. The latter and the group z appear to be particularly well developed and easily distinguished in the macaque. The question of whether cytoarchitectonic areal differences within the vestibular nuclear complex can be correlated with differences in connections is discussed. Also in this respect there appears to be a general similarity between observations in the macaque and in other mammals. A correlation is most evident in the superior vestibular nucleus, and is rather clear in the medial and lateral vestibular nuclei and for the groups f,x,y, and z, whereas no such correlation can be found in the descending (inferior) nucleus. For several reasons it is difficult to draw reliable conclusions about comparative anatomical trends in the phylogenesis of the vestibular nuclear complex.

Distribution of primary vestibular fibers in the brainstem and cerebellum of the monkey

Attempts were made to determine the central projections of ganglion cells innervating individual semicircular ducts in the monkey by implanting or injecting tritiated amino acids (leucine and/or proline), or horseradish peroxidase (HRP), selectively into a single ampulla. Central transport via the vestibular ganglion in animals receiving isotope implants or injections fell into three categories: (1) transport from ganglion cells innervating all receptive elements of the labyrinth, (2) transport from ganglion cells innervating the three semicircular ducts, and (3) transport from cells of the inferior vestibular ganglion innervating the posterior semicircular duct. Transneuronal transport of isotope was observed in secondary vestibular fibers in animals where proline was used and survival exceeded 12 days. Transneuronal labeling of secondary auditory fibers was independent on the [3H]amino acid used, and occurred with survivals of 10 or more days. HRP implanted into the ampulla of the lateral semicircular duct in several animals produced retrograde transport to efferent vestibular and cochlear neurons, but did not result in transganglionic labeling of primary vestibular or auditory fibers. Primary vestibular fibers terminate throughout the superior (SVN) and medial vestibular nuclei (MVN). Within SVN, terminals are most pronounced in its central large-celled portion, but extend into peripheral parts of the nucleus, except for a small medial area near its junction with the oral pole of MVN. Primary projections to MVN are homogenously distributed throughout the nucleus excepting a small circular area of sparse terminals along its ventral margin. Primary vestibular afferents terminate mainly in rostral and caudal portions of the inferior vestibular nucleus (IVN), but do not reach cell group 'f'. Projections to the lateral vestibular nucleus (LVN) are restricted to its ventral part. Primary projections to the accessory vestibular nuclei reach the interstitial nucleus of the vestibular nerve (NIVN) and cell group 'y'. Fibers project beyond the vestibular nuclei (VN) to terminate ipsilaterally in the accessory cuneate nucleus (ACN), the subtrigeminal lateral reticular nucleus (SLRN), and well-defined portions of the reticular formation (RF). Projections to SVN and MVN are derived primarily from ganglion cells innervating the semicircular ducts, while projections to caudal IVN, cell group 'y' and ACN are related mainly to macular portions of the vestibular ganglion. NIVN receives both macular and duct afferents.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

Electrophysiological analysis of cerebellar corticovestibular and fastigiovestibular projections to the lateral vestibular nucleus in the cat

In the lateral vestibular nucleus, vestibulospinal tract (VST) neurons were surveyed with microelectrodes in cats anesthetized with sodium pentobarbital. The VST neurons (n = 450) were classified by their properties; axonal courses (LVST and MVST). spinal segmental levels of their axonal termination (C1-3, C4-8, T1-13, L1-4, and L5-neurons), their orthodromic activation by the primary vestibular nerve (second-order and non-second-order vestibular neurons), and their location in the LVN. Inhibitory and excitatory effects of cerebellar stimulation on these classified VST neurons were investigated. 84% (259/308) neurons were observed to receive cerebellar corticovestibular inhibition. The rate was high, and almost the same among classified neurons; C1-3 to L5-neurons, and second-order and non-second-order neurons. However, the rate with MVST neurons (69%) was significantly lower than with LVST cells (87%). These neurons which received cerebellar inhibition were distributed in all areas even deep in the rostroventral region of the LVN, while neurons which did not receive were distributed in the ventral region of the LVN. Electrical stimulation of ipsi- and contralateral fastigial nuclei evoked monosynaptic excitation of the classified VST neurons. Rate of occurrence of crossed fastigiovestibular excitation was higher with cervical neurons (86%) than with lumbar neurons (43%), and higher with second-order neurons (78%) than with non-second-order neurons (41%). Neurons which received monosynaptic excitation from crossed fastigiovestibular fibers were distributed in the ventral region of the LVN. In total, 73% of the neurons were identified to receive either ipsi- or contralateral fastigiovestibular excitation. The results indicated that there was relative scarcity of fastigiovestibular projections in the dorsal region of the LVN. Spinovestibular and other afferents to the LVN were also investigated.

Neuronal organization of the vestibulospinal system in the cat

In the lateral and descending vestibular nucleus, vestibulospinal neurons were surveyed extra- and intracellularly in cats anesthetized with sodium pentobarbital. The neurons were investigated both by their antidromic activation from the spinal cord (C1, C4, T1, L1 and L5 spinal levels) and from the oculomotor nucleus region, and by orthodromic activation from the vestibular nerve. Axonal courses of vestibulospinal neurons were determined electrophysiologically at C1 level, as medial (MVST) or lateral (LVST). By single and the same electrodes a number of neurons were recorded in wide regions of the lateral and descending vestibular nucleus from single cats. Thus, it became possible to investigate somatotopical localization systematically for the first time with microelectrode techniques. Neurons of origin of the LVST were localized in the lateral vestibular nucleus. Second-order vestibular neurons were localized in the ventral region of the lateral vestibular nucleus, and in the rostral region of the descending vestibular nucleus. Many second-order, double discharge MVST neurons were identified in the descending and lateral vestibular nucleus. Somatotopical localization were recognized, but non-second-order cervical neurons were identified in the dorsal region, although second-order lumbar neurons were identified in the ventral region of the lateral vestibular nucleus. Specific modes of vestibular activation of these vestibulospinal neurons were discussed, and the vestibulospinal systems in the cat and rabbit were discussed.

Dorsolateral spinal afferents to some medullary sensory nuclei. An anatomical study in the cat

The course and termination of afferents in the spinal dorsolateral fascicle to some medullary sensory nuclei were studied by tracing degeneration following lesions of spinal white matter. The main conclusions depend on successive degeneration experiments; other points were studied with single-stage lesions. The dorsal column nuclei were particularly studied; terminations in these nuclei following dorsolateral lesions followed a clear-cut pattern, with fibres arising from segments below T6 terminating in the gracile nucleus and those with more rostral origin solely in the cuneate nucleus. In both nuclei, the major terminations were in their rostral third with most fibres traversing deep caudal regions where some termination also occurred. Some fibres ended contralaterally. These restricted regions of termination contrasted with the wide-spread terminations seen after lesions of the dorsal column. A region at the cuneate rostral pole, adjacent to but clearly separable from nucleus z, receives a dense projection from both caudal and rostral spinal levels, the former fibres terminating in the dorsal part of the region, the latter extending more ventrally. We treat this as a separate subnucleus. The afferents to the dorsal column (together with those terminating in the other nuclei studied) were confined to the extreme dorsolateral white matter. Our observations confirm the established view that only afferents arising from caudal segments (below at least T 4-5) terminate in nucleus z, and that afferents terminating in group x arise from all levels (at least between C5 and L5): also that neither receives any afferents through the dorsal columns. Dorsolateral fibres arising from segments above at least T6 terminate in a clear-cut area at the lateral border of the external cuneate nucleus. Heavy terminal degeneration was also seen in the lateral cervical nucleus of afferents arising from both above and below T 4-5.

Plasticity of spinovestibular projections in response to hemicerebellectomy in newborn rats

Spinal cord projections to the lateral vestibular nucleus (LVN) are relatively sparse in normal adult black-hooded rats. However, in response to hemicerebellectomy in 2--3-day-old rats an increase in spinovestibular fibers to the ipsilateral lateral vestibular nucleus was observed. This increased input appears to be due to fibers which would normally be destined for the cerebellum but which alternatively project to the functionally related lateral vestibular nucleus. Considering the neuroanatomical relationships between the cerebellum and the vestibular nuclei and in view of the functional compensation frequently reported after cerebellar lesions, the observed remodelling may represent a partial anatomical basis for recovery of function.

Cerebellar afferent projections from the vestibular nuclei in the cat: an experimental study with the method of retrograde axonal transport of horseradish peroxidase

Details of cerebellar afferent projections from the vestibular nuclei were investigated by the method of retrograde axonal transport of horseradish peroxidase (HRP) in the cat. The distribution of labeled cells in the vestibular nuclei following HRP injections in various parts of the cerebellum indicates that vestibular neurons in the medial and descending nuclei and cell groups f and x project bilaterally to the entire cerebellar vermis, the flocculus, the fastigal nucleus and the anterior and posterior interpositus nuclei. In addition, labeled cells (giant, medium and small) were consistently found bilaterally in the superior and lateral vestibular nuclei following HRP injections in the nodulus, flocculus, fastigal nucleus, and following large injections in the vermis. No labeled cells were observed in cases of HRP injections in crus I and II, the paramedian lobule, paraflocculus and lateral cerebellar nuclei. The present findings indicate that secondary vestibulocerebellar fibers project to larger areas in the cerebellum and originate from more subdivisions and cell groups of the vestibular nuclear complex than previously known.

Connexions from large, ipsilateral hind limb muscle and skin afferents to the rostral main cuneate nucleus and to the nucleus X region in the cat

1. Evidence is presented for an input from ipsilateral hind limb group I muscle afferents and low threshold cutaneous afferents, to cells in the rostral division of the main cuneate nucleus (rMCN) and in the region of the descending vestibular nucleus and the nucleus X of Brodal & Pompeiano (1957a), the (DV-X). 2. Thirteen group I-rMCN cells were recorded from. The functional properties of these cells were similar to those of nueleus Z (Landgren & Silfvenius, 1971; Johansson & Silfvenius, 1977a, b). The cells were monosynaptically linked to spinal dorsolateral fascicle (DLF) fibres. Nine cells projected to the contralateral thalamus, i.e. a second group I hind limb bulbothalamic tract is described. Ten cells were synaptically activated from the ipsilateral cerebellum from the anterior projection zone of the dorsal spinocerebellar tract (DSCT). Axon-collateral activation by DSCT fibres was established for two of these cells. They were both bulbothalamic relay cells. For the remaining eight cells, activated from the cerebellum, this was not proven. These cells could, however, either be linked to DSCT fibres or to short axon-collaterals of a cell body of unknown location. A projection from the rMCN to the cerebellum is described and agrees with recent anatomical findings. Two cells were not excited from the cerebellum. 3. Four rMCN cells were activated by cutaneous afferents with their secondary axons in the DLF. Suggestive evidence for a bulbothalamic cutaneous hind limb path via the rMCN is presented. Two cells were activated from the cerebellum, presumably via axon-collaterals of nonsegmental cells. 4. Eight group I-DV-X cells were recorded from. They were monosynaptically linked to spinal DLF fibres and resembled functionally the nucleus Z and rMCN cells when stimulated from the periphery. Two cells projected to the contralateral thalamus, and two others were synaptically excited. Seven cells were activated from the ipsilateral cerebellum. Two of them projected to the cerebellum, and three were synapitcally activated by axon-collaterals of an undefined non-segmental cell. 5. Two DV-X cells which were activated by cutaneous afferents possibly had their spinal fibres deep in the dorsal column. Both were activated from the cerebellum, one by collaterals of a spinal axon. The functional organization of the three juxtaposed medullary nuclei, Z, rMCN and DV-X is discussed.

Neuronal activity in the lateral vestibular necleus of the cat. IV. Postsynaptic potentials evoked by stimulation of peripheral somatic nerves

The synaptic input to Deiters neurones evoked by stimulation of peripheral somatic nerves was measured by intracellular recordings. EPSPs with broad receptive fields and latencies which indicate polysynaptic connexions were commonly evoked from the FRA. In other cells, low threshold cutaneous afferents were effective at rather short latencies suggesting oligosynaptic connexions from fast ascending fibres. One example was found of EPSPs due to low threshold muscle afferents. IPSPs due to climging fibre activation of Purkinje cells as observed in most of the neurones were evoked by cutaneous volleys above 1.5-2.0T and muscle volleys above 5T (above 3-3.5T in case of Q). Often, IPSPs were evoked by stimulation of nerves, to the segmental level of which the the vestibulospinal neurone under investigation projected. A small proportion of cells received short latency IPSPs involving direct fast mossy fibre tracts, which were evoked from low threshold cutaneous afferents. IPSPs due to polysynaptic mossy fibre activation of Purkinje cells were evoked from the FRA bilaterally and from ipsilateral cutaneous afferents at 1.5-2.0T ("prolonged inhibition"). Prolonged excitatory/inhibitory events mediated by mossy fibre pathways may be involved in quadruped locomotion or other processes making use of a broad motor integration.

The reticulovestibular projection in the cat. An experimental study with silver impregnation methods

The distribution of degeneration in the vestibular nuclei (VN) has been studied in transversely cut sections from 9 cats with stereotaxically performed lesions in the main reticular formation (RF) of the brain stem (Nauta of Fink and Heimer method). No projection was found from the mesencephalic reticular formation (R.mes.) and nucleus reticularis ventralis (R.v.). However, the reticular nuclei gigantocellularis (R.gc.), parvocellularis (R.p.c.) and pontis oralis (R.p.o.) were found to project bilaterally onto the 4 main vestibular nuclei with an ipsilateral overweight. By far the greates contribution comes from the R.gc. and R.p.c. In cases with R.gc. lesions some degeneration was found in the small cell groups x and f. The latter is also supplied from the R.p.c. The distribution of degeneration within the vestibular complex is rather diffuse, but a certain pattern can be discovered. After R.p.c. lesions the maximal terminal field in the VN is found within the superior nucleus, while the lateral and medial nuclei are preferred sites of termination of fibers from R.gc. and R.p.c. The projections of the R.gc. and the R.p.c. may be more different than appears from our findings since lesions of one of them will most likely have affected some ascending or descending fibers emanating from the other. Since the areas of RF projecting to VN receive afferents from many sources, these sources have possibilities to act on the VN even if they do not possess direct connections with this nuclear complex. These possibilities should be remembered in physiological studies of responses in the VN following stimulation of many parts of the CNS.