Bilateral Vestibular Nuclei Research Articles

Neuronal response sensitivity to bidirectional off-vertical axis rotations: a dimension of imbalance in the bilateral vestibular nuclei of cats after unilateral labyrinthectomy

In decerebrate cats after acute hemilabyrinthectomy, the response sensitivity of extracellularly recorded vestibular nuclear neurons on the lesioned and labyrinth-intact sides were examined quantitatively during constant velocity off-vertical axis rotations with an aim to elucidate the functional contribution of otolithic inputs to the ipsilateral and contralateral vestibular nuclei. The bidirectional response sensitivity, δ, was determined as the ratio of the gain during clockwise to that during counterclockwise rotations. A continuum of response sensitivity was identified: one-dimensional neurons showed symmetrically bidirectional response patterns, while two-dimensional neurons showed asymmetrically bidirectional patterns that in some cases approached unidirectional patterns with change in velocity. The proportion of two-dimensional neurons was significantly increased after acute hemilabyrinthectomy. Two-dimensional neurons that responded only to one direction of rotation in at least one of the velocities tested were described as unidirectional neurons. This unidirectional response pattern was observed in one-third of the entire neuronal population studied, but not in cats with both labyrinths intact, thus suggesting that such prominent broadly tuned responses are normally masked by converging otolithic inputs from the contralateral side. These neurons were found in higher proportion on the lesioned side than on the labyrinth-intact side. Among the 70% of unidirectional neurons that exhibited bidirectional response at some velocities and unidirectional response at others, prominent shifts in δ values (i.e. between 0/∞ and finite values) with velocity can be computed for each neuron. The shifts in δ values correlated with large shifts in the response dynamics and spatial orientation as the response pattern changed with velocity. The response orientations of the unidirectional neurons pointed in all directions on the horizontal plane. When all the two-dimensional neurons (i.e. both the unidirectionally and bidirectionally responsive ones) were pooled, imbalances in the distribution of the response orientations and in response gain were found between the ipsilateral-side-down/head-down half-circle and the contralateral-side-down/head-up half-circle on the labyrinth-intact side, but not on the lesioned side. These results, derived from spatiotemporal processing of gravitational signals, reveal a novel dimension of imbalance between neuronal populations in the two vestibular nuclear complexes after acute lesion of one labyrinth. This feature would provide, on the one hand, deranged cues of spatial orientation and direction during slow head excursions and, on the other, a framework for the dynamic behavioral deficits associated with hemilabyrinthectomy.

Nitric oxide in the flocculus works the inhibitory neural circuits after unilateral labyrinthectomy

We previously reported that nitric oxide (NO) production in the unipolar brush (UB) cells is involved in vestibular compensation [T. Kitahara, N. Takeda, P.C. Emson, T. Kubo, H. Kiyama, Changes in nitric oxide synthase-like immunoreactivities in unipolar brush cells in the rat cerebellar flocculus after unilateral labyrinthectomy, Brain Res. 765 (1997) 1–6]. To further elucidate the role of NO-mediated signaling in flocculus after unilateral labyrinthectomy (UL), we examined UL-induced Fos expression, a marker of neural activity, in vestibular brainstem with continuous floccular infusions of Nω-nitro- l-arginine methyl ester ( l-NAME), an inhibitor of NO synthase (NOS). After UL with floccular l-NAME infusions, Fos expression appeared in bilateral medial vestibular (MVe) and prepositus hypoglossal (PrH) nuclei. After UL with floccular saline infusions, however, Fos expression was observed only in the ipsi-MVe and contra-PrH. Furthermore, it has been revealed that UL with l-NAME infusions caused more severe vestibulo-ocular disturbances than UL with saline infusions at the initial stage [Kitahara et al. Brain Res. 765 (1997) 1–6]. Therefore, it is suggested that UL with floccular l-NAME infusions activates the contra-MVe and ipsi-PrH neurons and causes more severe imbalance between intervestibular nuclear activities at the initial stage. NO-mediated signaling in flocculus could be a possible driving force of the flocculus-mediated inhibition on the contra-MVe and ipsi-PrH at the initial stage of vestibular compensation.

Unilateral Vestibular Deafferentation Induces Brain-Derived Neurotrophic Factor (BDNF) Protein Expression in the Guinea Pig Lateral but not Medial Vestibular Nuclei

The aim of the present study was to investigate the expression of the neurotrophin, brain-derived neurotrophic factor (BDNF), in the vestibular nucleus complex (VNC) following unilateral vestibular deafferentation (UVD). Using immunohistochemistry, BDNF expression was analyzed in the bilateral VNC of the guinea pig in sham animals, and immediately, 10 h, and 50 h post-UVD. No BDNF expression was seen in the VNC in sham animals or immediately or 10 h post-UVD. However, at 50 h post-UVD, clear BDNF expression was seen in the bilateral lateral vestibular nuclei (LVN) and in the bilateral abducens nuclei, but not in the medial vestibular nuclei (MVN). The significance of this late BDNF expression is nuclear.

Effects of the GABA agonists baclofen and THIP on long-term compensation in hemilabyrinthectomised rats

Horizontal eye movements, elicited by sinusoidal rotation in darkness, were recorded with a magnetic search coil technique in pigmented rats, hemilabyrinthectomised 8–12 weeks before the investigation. Separate gains during rotation towards the lesioned side (LS) and the intact side (IS) were calculated by a computer program, demonstrating an asymmetry. Systemic single administration of the GABA B agonist baclofen caused a dose-related temporary rebalancing of the compensatory eye movements to the LS and the IS. At an optimal dose of 14 μmol/kg b.wt symmetry was achieved by excitation of eye movements during rotation to the LS and depression during rotation to the IS. Administration of the GABA A agonist THIP did not obviously reduce the asymmetry. It is suggested that stimulation of GABA B receptors modifies the tonic imbalance between the bilateral vestibular nuclei and/or the central processing of the input from the peripheral sensory organs.

Evidence for reduced nitric oxide synthase (NOS) activity in the ipsilateral medial vestibular nucleus and bilateral prepositus hypoglossi following unilateral vestibular deafferentation in the guinea pig.

The aim of the present study was to examine, using a radioenzymatic assay technique, nitric oxide synthase (NOS) activity in the bilateral medial vestibular nuclei (MVN) and prepositus hypoglossi (PH), during the development of vestibular compensation for unilateral vestibular deafferentation (UVD) in the guinea pig. In the MVN ipsilateral to the UVD, and bilaterally in PH, NOS activity decreased following UVD compared to sham controls and did not recover significantly up to 50 h later, when a substantial degree of behavioural vestibular compensation had occurred. These results suggest that UVD causes a decrease in NOS activity in the ipsilateral MVN and the bilateral PH, and that a consequent decrease in NO may be responsible for some of the ocular motor and postural symptoms of UVD.

Vestibular Neuronal Function of Cats Following Unilateral Vertebral Artery Occlusion

Since vertigo is known to be caused by vertebrobasilar arterial occlusive disease (ex. vertebrobasilar insufficiency; VBI), an electrophysiological study was performed to examine the effects of a unilateral vertebral artery (VA) occlusion on the neuronal activities of the medial vestibular nucleus (MVN) and vestibular ganglion (VG) in f -chloralose-anesthetized cats. Single neuronal activity in the MVN and VG was recorded extracellularly with a glass-insulated silver wire microelectrode. When the unilateral VA was occluded for 5 min, the spontaneous firing rate in the MVN was altered as follows. The type A pattern exhibited a transient increase, followed by a decrease. The type B pattern showed a gradual decrease, and no further changes after the onset of occlusion. Type A and B patterns were observed respectively in MVN ipsi- and contra-lateral to the VA occlusion. In contrast, the VG neuronal firing rate was not affected by the occlusion. These results seem to support the clinical suggestion that the vertigo in VBI is produced by a difference in the function between the bilateral vestibular nuclei, rather than by the peripheral vestibular nerve.

The coding of head orientations in neurons of bilateral vestibular nuclei of cats after unilateral labyrinthectomy: response to off-vertical axis rotation.

In decerebrate cats that had been acutely hemilabyrinthectomized (HL), the extracellular activities of vestibular nuclear neurons on the lesioned and labyrinth-intact sides were studied during constant-velocity off-vertical axis rotations (OVAR) in the clockwise (CW) and counterclockwise (CCW) directions (at 10 degrees tilt). Over the range of 1.75-15 degrees/s, two types of neuronal responses were identified on both sides. Some neurons showed symmetric and velocity-stable bidirectional response sensitivity (delta defined as the CW gain over the CCW gain) while other neurons exhibited asymmetric and velocity-variable delta. The mathematically derived gain tuning ratios of these two groups of neurons were within the range of one-dimensional and two-dimensional neurons respectively. The best response orientations in one-dimensional neurons and the orientations of the maximum response vector, S(max), in two-dimensional neurons were found to point in all directions on the horizontal plane. On the labyrinth-intact side, both the one-dimensional and two-dimensional neurons showed asymmetry in the neuron numbers and/or the response gains between the two roll quadrants as well as between the two pitch quadrants. In addition, both the neuron number and gain were significantly higher for neurons in the head-down/ipsilateral-side-down half-circle than those in the head-up/contralateral-side-down half-circle. None of the aforementioned asymmetries was observed on the lesioned side. That a comparable pattern of distribution was observed in the one-dimensional and two-dimensional neurons suggests that these neurons maintain a common spatial reference frame in encoding head orientational signals arising from the ipsilateral and contralateral otoliths. Furthermore, a predominance of two-dimensional neurons that exhibited a greater gain with CW rotations was observed on both sides of HL cats. Of the response dynamics observed amongst neurons on the two sides of HL cats, no difference was found with regard to the response gain and the pattern of response lead. However, a difference in response lag was observed between neurons on the two sides of HL cats. These suggest that there is a segregation of otolithic signals to reach the ipsilateral and contralateral vestibular nuclei. Taken together, the present study demonstrates that one-dimensional and two-dimensional neuronal responses could be elicited with inputs arising solely from the ipsilateral or contralateral otoliths. The observed orientational tuning and the CW-CCW asymmetry to bidirectional rotation may provide the essential directional coding of head orientations. Further, the imbalance of spatial/dynamic response patterns between the bilateral vestibular nuclei following the restriction of otolith inputs by HL implies that converging otolithic inputs from the bilateral labyrinths are essential for producing the neuronal responses in control animals. The results are also discussed in terms of the possible contribution of the various neural asymmetries between neuronal subpopulations in the bilateral vestibular nuclei to the behavioral symptoms accompanying acute HL.

Temporal relationship between the expression of Fos, Jun and Krox-24 in the guinea pig vestibular nuclei during the development of vestibular compensation for unilateral vestibular deafferentation

Immediately following unilateral vestibular deafferentation (UVD), expression of the c-fos protein increased in both vestibular nucleus complexes (VNCs). By 10 h post-UVD, c-fos proteins were more evident in the bilateral medial vestibular nuclei (MVN), but their expression had decreased in the bilateral superior vestibular nuclei (SVN) and lateral vestibular nuclei (LVN) compared to immediately post-UVD. At 50 h post-UVD, c-fos protein expression was still evident in the bilateral VNC, but had decreased relative to immediately post-UVD. Immediately post-UVD, c-jun protein expression increased in the bilateral VNC, but was more noticeable at 10 h post-UVD. By 50 h post-UVD, c-jun protein expression had decreased markedly throughout the bilateral VNC. The krox-24 protein could not be seen anywhere in the immediate post-UVD condition. However, it was expressed at a low level in the bilateral MVN at 10 h post-UVD; by 50 h post-UVD, this expression had decreased. No evidence of apoptosis was observed following UVD.

Characteristics of Nystagmus Evoked by Electrical Stimulation of The Uvular/Nodular Lobules of the Cerebellum in Monkey

Electrical stimulation in the monkey vestibulocerebellum has previously been shown to produce ocular nystagmus, but large stimulating current values were used. Using long duration (< or = 10-second) stimulus pulse trains and low current values (< 50 microA), we studied the nystagmus evoked by microstimulation in the uvular/nodular regions of the cerebellum. In doing this, we found quantitative differences in the nystagmus evoked from these two regions. Stimulation of the nodulus typically produced a vigorous nystagmus with a contralateral slow phase and a prolonged afternystagmus in the same direction. In contrast, stimulation of the uvula typically produced a regular ipsilateral nystagmus pattern with a very short, if any, afternystagmus in the same direction. In addition, at some stimulation sites in the uvula we observed an adaptation in the slow phase eye velocity during the time that the stimulation remained on. This effect could result in a secondary nystagmus, with a slow phase velocity direction opposite to that first evoked by the stimulation, followed by a prolonged afternystagmus in the direction of the secondary nystagmus at stimulus offset. The nystagmus evoked by these cerebellar stimulations differs from both natural nystagmus produced by large field visual motion and from the nystagmus produced by electrical stimulation of the nucleus of the optic tract. The nystagmus produced by uvular and nodular stimulation shows a shorter latency and a more rapid slow phase eye velocity buildup. The uvula stimulations also showed a much shorter afternystagmus. Also, the same nystagmus was evoked whether the animal was in a lighted or dark surround. These characteristics and recent single-unit recording studies in the uvula seem to suggest that the uvula acts not as a direct input to the velocity storage mechanism, but instead perhaps as part of an internal regulator for balance between the bilateral vestibular nuclei which are normally part of the nystagmus response. On the other hand, the nodulus, with its prolonged afternystagmus in the same direction as the evoked nystagmus, may be involved as a part of the velocity storage mechanism.

Response of otolith-related neurons in bilateral vestibular nucleus of acute hemilabyrinthectomized cats to off-vertical axis rotations.

Annals of the New York Academy of SciencesVolume 656, Issue 1 p. 755-765 Response of Otolith-Related Neurons in Bilateral Vestibular Nucleus of Acute Hemilabyrinthectomized Cats to Off-Vertical Axis Rotationsa Y. S. CHAN, Y. S. CHAN Department of Physiology Faculty of Medicine University of Hong Kong 5 Sassoon Road Hong KongSearch for more papers by this authorY. M. CHEUNG, Y. M. CHEUNG Department of Physiology Faculty of Medicine University of Hong Kong 5 Sassoon Road Hong KongSearch for more papers by this author Y. S. CHAN, Y. S. CHAN Department of Physiology Faculty of Medicine University of Hong Kong 5 Sassoon Road Hong KongSearch for more papers by this authorY. M. CHEUNG, Y. M. CHEUNG Department of Physiology Faculty of Medicine University of Hong Kong 5 Sassoon Road Hong KongSearch for more papers by this author First published: May 1992 https://doi.org/10.1111/j.1749-6632.1992.tb25254.xCitations: 10 a This study was supported by research grants from the Lee Wing Tat Medical Research Fund, Sun Yat Sen Foundation Fund for Medical Research, Research Grants Committee of the University of Hong Kong, and University and Polytechnic Grants Committee (UPGC). AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume656, Issue1Sensing and Controlling Motion: Vestibular and Sensorimotor FunctionMay 1992Pages 755-765 RelatedInformation

Internuclear neurons in the ocular motor system of frogs

Medial and lateral rectus motoneurons of frogs were localized after retrograde labeling with horseradish peroxidase (HRP) injected in the medial rectus muscle or applied on the cut end of the abducens nerve. Coordinates of these cell columns were used as target areas for the injection of small amounts of HRP (20-60 nl) and [3H]leucine (25-40 nl) and as search areas for retrogradely and anterogradely labeled internuclear neurons (INT) in in vivo and in vitro experiments. HRP injection in the medial rectus subdivision of the oculomotor nucleus (n = 6) resulted in retrograde labeling of cell bodies in the contralateral principal abducens nucleus. On the average about 16 cells per animal were found. Somatic diameters were about 13.5 +/- 2.8 microns (n = 32). The number and the size of these abducens internuclear neurons (AbINT) are smaller than those of lateral rectus motoneurons (n = 75; diameter: 19 +/- 3.2 microns). A crossed projection of AbINT to medial rectus motoneurons in the contralateral oculomotor nucleus is further supported by autoradiographic results. Following injection of [3H]leucine into the abducens nucleus, a high density of silver grains was visible within the contralateral oculomotor nucleus, mainly in the caudal part of the oculomotor nucleus, where medial rectus motoneurons are located. Injection of [3H]leucine in vivo (n = 4) and in vitro (n = 3) resulted in a similar high density of silver grains within the contralateral oculomotor nucleus, but the background level of silver grains was significantly higher after in vitro (264 +/- 38/2,500 microns2) than after in vivo injections (195 +/- 17/2,500 microns2). HRP injection in the principal abducens nucleus (n = 9) resulted in retrograde labeling of cell bodies in the medial rectus subdivisions of the bilateral oculomotor nuclei. Ipsilateral projections predominated, with about 10 (+/- 8) labeled cells over contralateral projections (about 3 +/- 2). Average diameters of these oculomotor internuclear neurons (OcINT) were again smaller (10.8 +/- 2 microns; n = 18) than those of medial rectus motoneurons (14.4 +/- 3 microns; n = 52). In addition, retrogradely labeled cells were consistently encountered in the bilateral vestibular nuclei, the cerebellar nuclei, the dorsal brainstem caudal to the abducens nuclei, and ipsilaterally in the pretectum. Most of the vestibular neurons were located in the rostral part of the vestibular nuclear complex. These neurons might constitute part of the three-neuronal arc of the vestibulo-ocular reflex in the frog. Labeled cells in the pretectum were restricted to the ipsilateral posterior thalamic nucleus (P).(ABSTRACT TRUNCATED AT 400 WORDS)

The Mechanism of Irritative Nystagmus and Paralytic Nystagmus:A Histochemical Study of the Guinea Pig's Vestibular Organ and an Autoradiographic Study of the Vestibular Nuclei

To establish the difference of mechanism between irritative and paralytic nystagmus, alterations of Na-K-ATPase and succinic dehydrogenase activity in the vestibular sensorineural elements were investigated for 20 guinea pigs, and glucose uptake of the vestibular nuclei for 13 guinea pigs were measured by the [14C]-2-deoxy-D-glucose method. Irritative and paralytic nystagmus were experimentally provoked by introducing K+ into the perilymphatic space. From the results it was concluded that irritative nystagmus is provoked by increased excitability of vestibular sensory cells, while paralytic nystagmus is provoked by decreased excitability. However, the direction of nystagmus was eventually decided by the tonus imbalance between the bilateral vestibular nuclei. The ipsilateral vestibular nucleus was predominant during irritative nystagmus, while the contralateral vestibular nucleus was predominant during paralytic nystagmus.

Amino Acid Assay of Vestibular Nuclei 10 Months after Unilateral Labyrinthectomy in Squirrel Monkeys

Amino acids were assayed by HPLC in bilateral vestibular nuclei from normal squirrel monkeys (n = 3) and those 10-month post-unilateral labyrinthectomy (n = 4). Findings of vestibulo-spinal and vestibulo-oculomotor functions were identical for both groups. No left-right asymmetry of amino acids was found within either group, nor between groups with the exception of GABA: GABA was significantly reduced in the bilateral vestibular nuclei of the 10-month post-lesion animals. This may be indicative of a reduction of cerebello-vestibular inhibitory control which could be secondary to the reduction of excitatory inputs to the system.

Neuronal activity in the contralateral medial vestibular nucleus of the guinea pig following unilateral labyrinthectomy

The recovery of normal ocular motor and postural behavior following unilateral labyrinthectomy is termed vestibular compensation and it has been attributed to the return of normal resting activity to neurons in the bilateral vestibular nuclei. Previous studies in the cat have reported that approximately normal resting activity returns to type I neurons in the medial vestibular nucleus (MVN) contralateral to the deafferentiation by 4–8 weeks post-operation (post-op.), while the gain of the response of these neurons to horizontal angular acceleration remains lower than normal. The present data demonstrate that, in the guinea pig, normal resting activity returns to the contralateral MVN type I neurons by only 52–60 h post-op., while the gain and phase of the response of these neurons to horizontal sinusiodal acceleration remains abnormal up to 8–12 months post-op. By contrast with previous studies, the present data show that type II neurons in the contralateral MVN exhibit some increase in their frequency of occurrence and gain toward normal values during vestibular compensation (at 52–60 h and 8–12 months post-op.). The rapid recovery of normal type I resting activity correlates with the disappearance of spontaneous nystagmus and postural asymmetries in the guinea pig by 52 h post-op.

Neuronal activity in the ipsilateral medial vestibular nucleus of the guinea pig following unilateral labyrinthectomy

The recovery of normal ocular motor and postural behavior following unilateral labyrinthectomy (vestibular compensation) has been attributed to the return of normal resting activity to neurons in the bilateral vestibular nuclei. However, previous studies in the cat have reported that average resting activity recovers to no more than 50% of the normal value in neurons in the vestibular nucleus ipsilateral to the labyrinthectomy even after 4 months post-operation (post-op.), despite the fact that, for some symptoms, vestibular compensation is complete by this time. The present data demonstrate that in the guinea pig, normal average resting activity is restored to type I neurons in the ipsilateral medial vestibular nucleus (MVN) by 52–60 h post-op., although type I neurons remain scarce compared to normal. This recovery of resting activity correlates with the compensation of spontaneous nystagmus and postural asymmetries by 52 h post-op. which we have previously reported. In addition, the present data further confirm that the recovery of type I resting activity in the ipsilateral MVN is not due to recovery of resting activity in ipsilateral Scarpa's ganglion neurons or to input from the dorsal brainstem commissures.

Lesions in the cat prepositus complex: effects on the vestibulo-ocular reflex and saccades.

The effects of bilateral electrolytic lesions within and around the prepositus hypoglossi (p.h.) nucleus on horizontal saccades in the dark and on the horizontal sinusoidal vestibulo-ocular reflex (v.o.r.) in the dark were studied. After p.h. lesion, including its rostral part between P 7 and P 8, the v.o.r. showed a phase lead as much as about 90 deg at 0.10 Hz. A significant gain reduction paralleled that phase lead at lower frequencies. A large post-saccadic drift was also observed, the time constant of which ranged from 0.3 to 0.6 s. After p.h. lesion extending from P 8 to P 11 (but sparing the rostral part of the p.h.), no significant gain or phase lead change was observed. Post-saccadic drift was either missing or weak. A bilateral medial vestibular nucleus (m.v.n.) lesion from P 7 to P 11 produced a marked gain decrease, paralleled by a marked phase advance. A post-saccadic drift was observed (tau = 0.6 s). A surgical mid-line lesion from P 7 to P 11 (depth: about 2 mm) was followed by no remarkable change in the gain and in the phase of the v.o.r. No post-saccadic drift was observed after such lesion. It was concluded that (i) both the horizontal v.o.r. integration processing, and the horizontal saccadic integration processing were destroyed when an electrolytic lesion was made 'in the region of' the rostral part of the p.h. nucleus, and that (ii) the posterior four-fifths of the p.h. was the location of neither the horizontal v.o.r. integrator nor the horizontal saccadic integrator.

Validity of Ewald's law in the horizontal semicircular canals and the vestibular nuclei: electron microscopic observation and quantitative [14C]-2-deoxy-D-glucose study.

Two series of experiments were performed by using a total of 36 guinea pigs. The 1st series was to compare the influence of rotatostimulation on the bilateral cristae ampullare. After repeating the rotatostimulation with the cupulometric mode (the terminal turning velocity: 450 degrees/sec) for 48 hr, ultrastructural changes of sensory cells were found in 9 of 17 animals with transmission electron microscopy. They showed stronger influences on the ampullopetal flow side with the exception of one animal. In the 2nd series, the difference in excitability between the bilateral vestibular nuclei was investigated on 6 guinea pigs which were exposed to the clockwise constant angular acceleration of 1 degree/sec2 for 8 min. The autoradiographic study with [14C]-2-deoxy-D-glucose (the 2-DG) revealed that the rotation-induced glucose uptake was increased significantly in the superior and the medial vestibular nuclei of the ampulloptal flow side. The results of the two series of experiments suggest the predominance of the ampullopetal effect not only in the horizontal semicircular canals but also in the vestibular nuclei.

Commissural and ipsilateral internuclear connection of vestibular nuclear complex of the cat

Commissural and ipsilateral intrinsic connections of the vestibular nuclear complex of cats were investigated using retrograde transport of horseradish peroxidase (HRP). HRP was microiontophoretically injected into limited areas (0.2–0.5 mm in diameter) of the respective vestibular nuclei. In the commissural connections, major fibers were observed between the bilateral superior vestibular nuclei (SVN) and between the bilateral descending vestibular nuclei (DVN); a moderate number of fibers was found from the medial vestibular nucleus (MVN) to the contralateral MVN, SVN and lateral vestibular nucleus (LVN) and from the DVN to the contralateral LVN. Minor commissural connections were detected between the bilateral LVN. The ipsilateral internuclear connections of the vestibular nuclear complex were: (1) from the LVN, MVN and DVN to the SVN, (2) from the MVN and DVN to the LVN and (3) from the MVN to the DVN. Minor ipsilateral intrinsic connections were found from the SVN to the MVN.

Cerebellar, medullary and spinal afferent connections of the paramedian reticular nucleus in the cat

The topographic organization of afferent projections from the deep cerebellar nuclei, medulla oblongata and spinal cord to the paramedian reticular nucleus (PRN) of the cat was studied using the horseradish peroxidase (HRP) method of retrograde labelling. Discrete placements of HRP within each of the dorsal (dPRN) and ventral (vPRN) regions of the PRN showed some segregation of input. The deep cerebellar nuclei project in a predominantly contralateral fashion upon the PRN. A small but significant ipsilateral fastigial afferent component is also present. The fastigial and dentate nuclei contribute the majority of fibers to the dPRN whereas the interposed nucleus provides very little. The vPRN receives a relatively uniform input from all 3 cerebellar nuclei. Both lateral vestibular nuclei contribute the majority of fibers from the vestibular nuclear complex largely from their dorsal division. Additional input arises from bilateral medial and inferior vestibular nuclei. The vPRN receives relatively more fibers from the inferior vestibular nuclei than does the dPRN while inputs from the medial vestibular nuclei are comparably sparse. The PRN receives bilateral projections from the nucleus intercalatus (of Staderini). A significant projection to the contralateral PRN occurs from the ventrolateral subnucleus of the solitary complex and its immediate vicinity. Additional sources of medullary afferent input include the lateral, gigantocellular and magnocellular tegmental fields, the contralateral PRN and the raphe nuclei. Sites of origin of spinal afferents to the dPRN are bilaterally distributed mainly within Rexed's laminae VII and VIII of the cervical cord whereas those to the vPRN are confined largely to the medial portion of the contralateral lamina VI in the C1 segment. A few labelled cells are found in the thoracolumbar cord with those to the vPRN being more caudal. These data provide the neuroanatomical substrate for a better understanding of the functional role of the PRN in mediating cardiovascular responses appropriate to postural changes.

Connections of the rabbit abducens nucleus

Wheat germ agglutinated horseradish peroxidase was injected into the rabbit abducens nucleus under physiological control. Major projections from the reticular formation and the ipsilateral oculomotor, bilateral vestibular and bilateral perihypoglossal nuclei were demonstrated. A reciprocal connection with the ipsilateral perihhypoglossal nuclei was also shown. We suggest that the perihypoglossal nuclei and the reticular formation are important premotor centers involved with plasticity of eye movements.