Anatomical and physiological characteristics of vestibular neurons mediating the vertical vestibulo-ocular reflexes of the squirrel monkey

The morphology of 35 vestibular neurons whose firing rate was related to vertical eye movements was studied by injection of horseradish peroxidase intracellularly into physiologically identified vestibular axons in alert squirrel monkeys. The intracellularly injected cells were readily classified into four main groups. One group of cells, down position-vestibular-pause neurons (down PVPs; N = 12), increased their firing rate during downward eye positions, paused during saccades, and were located in the medial vestibular nucleus (MV) and the adjacent ventrolateral vestibular nucleus (VLV). They had axons that crossed the midline and ascended in the medial longitudinal fasciculus (MLF) to terminate in the trochlear nucleus, the lateral aspect of the caudal oculomotor nucleus, and the dorsal aspect of the rostral oculomotor nucleus. A second group of cells (N = 15) were also located in the MV and VLV, but increased their firing rate during upward eye positions, and paused during saccades. These cells had axons that crossed the midline and ascended in the contralateral MLF to terminate in the medial aspect of the oculomotor nucleus. A third group of cells (N = 4) were located in the superior vestibular nucleus, generated bursts of spikes during upward saccades, and increased their tonic firing rate during upward eye positions. These cells had axons that ascended laterally to the ipsilateral MLF to terminate in regions of the trochlear and oculomotor nuclei similar to those in which down PVPs terminated. A fourth group of cells (N = 4), located in the VLV, had axons that projected to the spinal cord, although they had firing rates that were significantly correlated with vertical eye position. Electrical stimulation of the vestibular nerve evoked spikes at monosynaptic latencies in each of the above classes of cells, six of which were injected with horseradish peroxidase. Each group of cells had collateral projections to other areas of the brainstem. Some of the neurons that projected to the contralateral trochlear and oculomotor nuclei had collaterals that crossed the midline to terminate in the oculomotor nucleus ipsilateral to the soma, and some gave rise to small collaterals that terminated in the abducens nucleus. Other areas of the brainstem that received collateral inputs from neurons projecting to oculomotor and trochlear nuclei included the interstitial nucleus of Cajal, the caudal part of the dorsal raphe nucleus, the nucleus raphe obscurus, Roller's nucleus, the intermediate and caudal interstitial nuclei of the MLF, and the nucleus prepositus.     

Morphology of physiologically identified second-order vestibular neurons in cat, with intracellularly injected HRP

The morphology of horizontal canal second-order type I neurons was investigated by intracellular staining with horseradish peroxidase (HRP) and three-dimensional reconstruction of the cell bodies and axons. Axons penetrated in and around the abducens nucleus were identified as originating from type I neurons by their characteristic firing pattern to horizontal rotation and by their monosynaptic response to stimulation of the ipsilateral vestibular nerve. A total of 47 type I neurons were stained. The cell bodies were located in the rostral portion of the medial vestibular nucleus (MVN) and were large or medium sized and had rather elongated shapes and rich dendritic arborizations. The neurons were divided into two groups: those which projected to the contralateral side of the brain stem (type Ic neurons) and those which projected to the ipsilateral side of the brainstem (type Ii neurons). All stem axons of type Ic neurons crossed the midline and bifurcated into rostral and caudal branches in the contralateral medial longitudinal fasciculus (MLF). Two or three collaterals arising close to this bifurcation distributed terminals in a relatively wide area in the contralateral abducens nucleus. Some of these collaterals projected further to the contralateral MVN and thus are vestibular commissural axons. Some of the rostral and caudal stem axons had collaterals which projected to the contralateral nucleus prepositus hypoglossi (PH), nucleus raphe pontis, or medullary reticular formation. There were at least six classes of type Ii neurons, most of which distributed to a relatively limited region in the ipsilateral abducens nucleus and they were categorized according to their future projections into the following categories: A) no further collaterals beyond the abducens nucleus; B) collaterals in the abducens nucleus and a branch descending and terminating in ipsilateral PH; C) projected to the abducens nucleus, PH, and an area rostral to the abducens nucleus; D) projected to the abducens nucleus and to ipsilateral reticular formation rostral and caudal to the abducens nucleus; E) collaterals in the abducens nucleus and a thick caudal stem axon entering and descending in ipsilateral MLF; F) a thick caudal stem axon entering and descending in ipsilateral MLF and no collaterals to the abducens nucleus. Some type Ii neurons also had recurrent collaterals which projected back to the ipsilateral MVN; these may inhibit type II neurons during ipsilateral rotation.     

Afferents to the abducens nucleus in the monkey and cat

The abducens nucleus is a central coordinating element in the generation of conjugate horizontal eye movements. As such, it should receive and combine information relevant to visual fixation, saccadic eye movements, and smooth eye movements evoked by vestibular and visual stimuli. To reveal possible sources of these signals, we retrogradely labeled the afferents to the abducens nucleus by electrophoretically injecting horseradish peroxidase into an abducens nucleus in four monkeys and two cats. The histologic material was processed by the tetramethyl benzidine (TMB) method of Mesulam. In both species the largest source of afferents to the abducens nucleus was bilateral projections from the ventrolateral vestibular nucleus and the rostral pole of the medial vestibular nucleus. Scattered neurons were also labeled in the middle and caudal levels of the medial vestibular nucleus. Large numbers of neurons were labeled in the ventral margin of the nucleus prepositus hypoglossi in the cat and in the common margin of the nucleus prepositus and the medial vestibular nucleus in the monkey, a region we call the marginal zone. Substantial numbers of retrogradely labeled neurons were found in the dorsomedial pontine reticular formation both caudal and rostral to the abducens nuclei. In the monkey, large numbers of labeled neurons were present in the contralateral medial rectus subdivision of the oculomotor complex, while smaller numbers occurred in the ipsilateral medial rectus subdivision and elsewhere in the oculomotor complex. In the cat, large numbers of retrogradely labeled cells were present in a small periaqueductal gray nucleus immediately dorsal to the caudal pole of the oculomotor complex, and a few labeled neurons were also dispersed through the caudal part of the oculomotor complex. Occasional labeled neurons were present in the contralateral superior colliculus in both species. The size and distribution of the labeled neurons within the intermediate gray differed dramatically in the two species. In the cat, the retrogradely labeled neurons were very large and occurred predominantly in the central region of the colliculus, while in the monkey, they were small to intermediate in size and were distributed more uniformly within the middle gray. Among the afferent populations present in the monkey, but not in the cat, was a group of scattered neurons in the ipsilateral rostral interstitial nucleus of the medial longitudinal fasciculus and a denser, bilateral population in the interstitial nucleus of Cajal.(ABSTRACT TRUNCATED AT 400 WORDS)     

Anatomical connections of the nucleus prepositus of the cat

The afferent and efferent connections of the nucleus prepositus hypoglossi with brainstem nuclei were studied using anterograde and retrograde axonal transport techniques, and by intracellular recordings and injections of horseradish peroxidase into prepositus hypoglossi neurons. The results of experiments in which horseradish peroxidase was injected into the prepositus hypoglossi suggest that the major inputs to the prepositus hypoglossi arise from the ipsi- and contralateral perihypoglossal nuclei (particularly the prepositus hypoglossi and intercalatus), vestibular nuclei (particularly the medial, inferior, and ventrolateral nuclei), the paramedian medullary and pontine reticular formation, and from the cerebellar cortex (flocculus, paraflocculus, and crus I; the nodulus was not available for study). Regions containing fewer labeled cells included the interstitial n. of Cajal, the rostral interstitial n. of the medial longitudinal fasciculus, the n. of the posterior commissure, the superior colliculus, the n. of the optic tract, the extraocular motor nuclei, the spinal trigeminal n., and the central cervical n. The efferent connections of the prepositus hypoglossi were studied by injecting 3H-leucine into the prepositus hypoglossi, and by following the axons of intracellularly injected prepositus hypoglossi neurons. The results suggest that in addition to the cerebellar cortex, the most important extrinsic targets of prepositus hypoglossi efferents are the vestibular nuclei (particularly the medial, inferior, and ventrolateral nuclei, and the area X), the inferior olive (contralateral dorsal cap of Kooy and ipsilateral subnucleus b of the medial accessory olive), the paramedian medullary and pontine reticular formation, the reticular formation surrounding the parabigeminal n., the contralateral superior colliculus and pretectum, the extraocular motor nuclei (particularly the contralateral abducens nucleus and the ipsilateral medial rectus subdivision of the oculomotor nucleus), the ventral lateral geniculate n., and the central lateral thalamic nucleus. Other areas which were lightly labeled in the autoradiographic experiments were the contralateral spinal trigeminal n., the n. raphe pontis, the Edinger Westphal n., the zona incerta, and the paracentral thalamic n. Many of the efferent connections of the prepositus hypoglossi appear to arise from principal prepositus hypoglossi neurons whose axons collateralize extensively in the brainstem. On the other hand, small prepositus hypoglossi neurons project to the inferior olive, and multidendritic neurons project to the cerebellar flocculus, apparently without collateralizing in the brainstem.(ABSTRACT TRUNCATED AT 400 WORDS)     

Central oculomotor circuits

Recent data and hypotheses concerning the central oculomotor pathways are reviewed. Lateral and vertical eye movements are discussed successively, beginning in each case with the final common pathway and then progressing step by step along the main supranuclear tracts selectively involved in the 3 types of eye movements: vestibular movements, saccades and smooth pursuit. It is now established that the final common pathway of lateral eye movements in frontal-eyed species is the abducens nucleus, which controls not only the ipsilateral lateral rectus, but also, through the internuclear neurons, almost all the conjugate lateral activity of the opposite medial rectus. The ascending tract of Deiters, providing direct excitatory vestibular signals to the medial rectus motoneurons, could either have totally regressed in man or would play only a minor functional role. Likewise, a direct inhibition of the medial rectus motoneurons now seems unlikely or ineffective, the relaxation of this muscle resulting essentially from the disfacilitation mediated by the abducens internuclear neurons. This particular mechanism could be explained by the fact that the medial rectus motoneurons also receive messages of convergence, a slow disjunctive movement independent of lateral eye movements. Convergence is performed by excitatory reticular neurons near the oculomotor nucleus and by inhibitory pathways projecting onto the abducens motoneurons, perhaps passing through the internuclear neurons of the oculomotor nucleus. The premotor relay of horizontal reflex eye movements is the medial vestibular nucleus (M.V.N.) which contains excitatory and inhibitory neurons projecting onto the contralateral and ipsilateral abducens nuclei respectively. Afferences of the M.V.N. arise from: the labyrinth, through the vestibular nerve (vestibulo-ocular reflex); the neck, through the dorsal part of the medullary tegmentum (cervico-ocular reflex); the peripheral retina and the visual pathways (for the vestibular contribution of optokinetic nystagmus), perhaps via the pretectum, the nucleus reticularis tegmenti pontis (N.R.T.P.) and/or the nucleus prepositus hypoglossi (N.P.H.) (visuo-ocular reflex). The premotor relay for all ipsilateral saccades is the paramedian pontine reticular formation (P.R.F.) which excites the ipsilateral abducens nucleus and inhibits the contralateral abducens nucleus, via the burst inhibitory neurons located ventrally to the ipsilateral abducens nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)     

The origin of brainstem afferents of the paramedian pontine reticular formation in the cat

Transcannular microinjections of horseradish peroxidase (HRP) were made into the paramedian pontine reticular formation (PPRF) in adult cats to determine the origin of the principal sources of inputs to this important preoculomotor center for the production of saccadic eye movements. Retrogradely labeled cells were observed in numerous oculomotor-related structures, including the prerubral field (rostral interstitial nucleus of the medial longitudinal fasciculus), nucleus of Darkschewitsch, nucleus of the posterior commissure, deep superior colliculus, supraoculomotor ventral periaqueductal gray, contralateral paramedian pontine reticular formation, pontine raphe and dorsal medial pontine tegmentum medial to the abducens nucleus (purported to contain omnipause neurons), cell group Y, and the perihypoglossal complex (nucleus prepositus hypoglossi). Other sources of afferents to the region included the zona incerta, lateral and medial habenular nuclei, medial hypothalamus, medial mammillary nucleus, nucleus cuneiformis, medial medullary reticular formation, and the medial and lateral cerebellar nuclei. The results are discussed in terms of the potential influence of these nuclei on the control of eye movement.     

Eye movement related neurons in the cat pontine reticular formation: projection to the flocculus

This study examines projection to the cerebellar flocculus of eye movement-related neurons in the median and paramedian part of the cat pontine tegmentum between the trochlear and the abducens nucleus. They were identified by rhythmic activity related to horizontal vestibular nystagmus induced by sinusoidal rotation. These neurons were classified into several groups by their discharge patterns during nystagmus, using criteria of earlier studies on saccadic eye movements and vestibular nystagmus in the monkey. Electrical stimulation of the ipsilateral flocculus elicited antidromic spike responses in a number of burst-tonic neurons and long-lead and medium-lead burst neurons. These neurons were located in and around the medial longitudinal fasciculus, the nucleus raphe pontis and the nucleus reticularis tegmenti pontis. A few neurons tested were also activated antidromically by stimulation of the contralateral flocculus. In contrast, no pauser neurons were activated from the ipsi-lateral flocculus. It is concluded that eye movement-related neurons in the medial pontine tegmentum, except for pauser neurons, directly project to the flocculus and may convey information about eye movements of visual and vestibular origins to the flocculus.     

The one-and-a-half syndrome--a unilateral disorder of the pontine tegmentum: a study of 20 cases and review of the literature

The one-and-a-half syndrome is a clinical disorder of extraocular movements characterized by a conjugate horizontal gaze palsy in one direction plus an internuclear ophthalmoplegia in the other. The syndrome is usually due to a single unilateral lesion of the paramedian pontine reticular formation or the abducens nucleus on one side (causing the conjugate gaze palsy), with interruption of internuclear fibers of the ipsilateral medial longitudinal fasciculus after it has crossed the midline from its site of origin in the contralateral abducens nucleus (causing failure of adduction of the ipsilateral eye). Twenty cases are reported; 14 had multiple sclerosis.     

Location of abducens afferent neurons in the cat.

The sites of afferent neurons to the abducens nucleus were determined by the use of horseradish peroxidase hydraulically placed into the VIth nucleus in nine cats. Labeled neurons were found bilaterally in the medial vestibular nuclei, the prepositus nucleus of the hypoglossus, the contralateral dorsomedial gigantocellular tegmental field, the contralateral reticular formation, and the ipsilateral vestibular nerve ganglion.     

Anatomy and physiology of saccadic burst neurons in the alert squirrel monkey. I. Excitatory burst neurons

Saccadic burst neurons in the pontine reticular formation have been implicated in the generation of saccades in the horizontal plane on the basis of lesion and extracellular recording studies in the cat and monkey. In the present study, saccadic burst neurons were anatomically and physiologically characterized with intraaxonal recording and injection of horseradish peroxidase in the alert squirrel monkey. A population of burst neurons were found that appear analogous to the excitatory burst neurons (EBNs) described previously in the cat. All neurons are located in the caudal pontine reticular formation and have a major axonal projection to the ipsilateral abducens nucleus. Additional projections were found to the medial vestibular nucleus, the nucleus prepositus, and regions of the pontine and medullary reticular formation rostral, ventral, and caudal to the abducens. All neurons fire exclusively during saccades and have a discharge pattern similar to that of medium-lead burst neurons described previously in the cat and monkey. In most neurons the saccadic burst begins 5-15 msec before saccade onset. Linear relationships exist between burst duration and saccade duration, number of spikes in the burst and saccade amplitude, and firing frequency and instantaneous velocity. Physiological activity of each neuron shows the closest relationship with the amplitude of the saccade component in a particular direction. For all neurons, this on-direction is in the ipsilateral hemifield and is predominantly horizontal, but may have either an upward or downward vertical component. These results support a major role for the EBNs in the monkey in generating the saccadic burst in abducens motoneurons, as well as in contributing to the oculomotor activity in other classes of premotor neurons.     

Anatomy and physiology of saccadic burst neurons in the alert squirrel monkey. II. Inhibitory burst neurons

Electrophysiological and intracellular labelling studies in the cat have identified a population of saccadic burst neurons in the medullary reticular formation that have an inhibitory, monosynaptic projection to the contralateral abducens nucleus. In the present study, intraaxonal recording and injection of horseradish peroxidase were used to identify and characterize the corresponding population of inhibitory burst neurons (IBNs) in the alert squirrel monkey. Squirrel monkey IBNs are located in the reticular formation ventral and caudal to the abducens nucleus and project contralaterally to the abducens. Additional contralateral projections are present to the vestibular nuclei, the nucleus prepositus, and the pontine and medullary reticular formation rostral and caudal to the abducens. All neurons fire a burst of spikes during saccades and are silent during fixation. In most neurons the burst begins 5-15 msec before saccade onset. The number of spikes in the saccadic burst is linearly related to the amplitude of the component of the saccade in the neuron's on-direction. Linear relationships also exist between burst duration and saccade duration and between firing frequency and instantaneous eye velocity. For all neurons, the on-direction is in the ipsilateral hemifield, with a vertical component that may be either upward or downward. Neurons with projections to the vertically related descending and superior vestibular nuclei tend to have on-directions with larger vertical components than neurons that lack these projections. These results, together with those on excitatory burst neurons reported in the preceding paper, demonstrate a reciprocal organization of burst neuron input to the abducens in the monkey similar to that found in the cat and indicate a major role for these neurons in generating the oculomotor activity in motoneurons as well as in other classes of premotor neurons.     

Evidence for glycine as an inhibitory neurotransmitter of vestibular, reticular, and prepositus hypoglossi neurons that project to the cat abducens nucleus

The localization and distribution of brain-stem afferent neurons to the cat abducens nucleus has been examined by high-affinity uptake and retrograde transport of 3H-glycine. Injections of 3H-glycine selectively labeled (by autoradiography) only neurons located predominantly in the ipsilateral medial vestibular and contralateral prepositus hypoglossi nuclei, and in the contralateral dorsomedial reticular formation, the latter corresponding to the location of inhibitory burst neurons. The specificity of uptake and retrograde transport of 3H-glycine was indicated by the absence of labeling of the dorsomedial medullary reticular neurons ipsilateral and in close proximity to the injection site, where local uptake by diffusion could have occurred. The selectivity of uptake and transport was demonstrated by the absence of retrograde labeling following injections of 3H-GABA or 3H-leucine into the abducens nucleus. The immunohistochemical localization of glycine and GABA revealed a differential distribution of the 2 inhibitory neurotransmitter candidates in the extraocular motor nuclei. Glycine-immunoreactive staining of synaptic endings in the abducens nucleus was dense with a widespread soma-dendritic distribution but was sparse in the trochlear and oculomotor nuclei. By contrast, GABA-immunoreactive staining within the oculomotor and trochlear nuclei was associated with synaptic endings that were particularly prominent on the somata of motoneurons. GABA-immunoreactive staining in the abducens nucleus, however, was sparse. These differences between glycine- and GABA-immunoreactive staining in the extraocular motor nuclei were correlated with differences in the immunoreactivity of axons in the descending (glycine) and ascending (GABA) limbs of the medial longitudinal fasciculus. Glycine-immunoreactive neurons, furthermore, were observed in the same locations as neurons that were labeled autoradiographically by retrograde transport of 3H-glycine from the abducens nucleus. Electrophysiological recordings from abducens motoneurons and internuclear neurons revealed a marked reduction in the slow positivity of the orthodromic extracellular potential elicited by ipsilateral vestibular nerve stimulation following systemic administration of strychnine, an antagonist of glycine. Intracellular recordings demonstrated that the vestibular-evoked disynaptic inhibitory postsynaptic potentials in abducens neurons were effectively blocked by strychnine but were unaffected by picrotoxin, an antagonist of GABA.(ABSTRACT TRUNCATED AT 400 WORDS)     

One-and-a-half syndrome in ischaemic locked-in state: a clinico-pathological study.

Five patients with "locked-in" syndrome and dysconjugate palsy of horizontal gaze were studied. In all cases internuclear ophthalmoplegia due to dysfunction or destruction of the median longitudinal fasciculus was combined with an ipsilateral gaze palsy, producing the "one-and-a-half" syndrome. Clinical and electro-oculographic examination suggested involvement of the paramedian pontine reticular formation when all ipsilateral saccades were abolished, when exotropia of the contralateral eye was present, and when vestibular stimulation showed full conjugate deviation to the damaged side. Involvement of the abducens nucleus was suggested when the palsy of ipsilateral gaze was not dissociated on vestibular stimulation. In three cases these clinical deductions were confirmed by the pathological study, which showed a corresponding destruction of the median longitudinal fasciculus, paramedian pontine reticular formation and abducens nucleus. In one case the one-and-a-half syndrome evolved into a total horizontal gaze palsy, which corresponded to involvement of the abducens nucleus contralateral to the initially destroyed paramedian pontine reticular formation. Vertical oculocephalic response disappeared, because of destruction of the median longitudinal fasciculus on both sides (bilateral internuclear ophthalmoplegia). Patients with the locked-in syndrome provide a unique situation in which complex pontine oculomotor disturbances may be studied, because consciousness is preserved. In these patients, dissociated and dysconjugate oculomotor palsy may have been underestimated.     

Anatomy of physiologically identified eye-movement-related pause neurons in the cat: pontomedullary region

Pause neurons (PNs) are inhibitory neurons close to the midline at the pontomedullary junction that fire tonically and then cease firing just prior to quick eye movements of visual or vestibular origin. Previous physiological evidence has shown that these neurons have a role of central importance in the generation of rapid eye movements in any direction and all major models of ocular motor control incorporate PNs as major elements. In this study in cats, we injected horseradish peroxidase intracellularly into somata or axons of physiologically identified PNs. After appropriate tissue preparation, cell body and axonal reconstructions were performed, with the aid of a camera lucida-equipped microscope. Fifty-three PNs were stained and reconstructed. These consisted of 17 cell bodies and dendrites and 36 axons. Seven of these included both cell bodies and axons. PN somas lay close to the midline in the nucleus raphe pontis and centralis superior, had extensive dendritic arborizations tending to arise from either pole of the elongated soma, and had axons which typically crossed the midline and bifurcated into long branches which extended rostrally and caudally, inferior to the medial longitudinal fasciculus. There were major terminal arborizations and boutons in areas just rostral and caudal to the abducens nucleus in areas where two types of premotor neurons, excitatory and inhibitory burst neurons, are concentrated. Many axosomatic contacts were noted. Other terminal arborizations and boutons were found close to the midline in a region rostral to abducens nucleus containing other neurons known to burst prior to quick eye movements, and in the nucleus reticularis gigantocellularis. Rostral stem axons could be traced to the level of the trochlear nucleus and inferior to the medial longitudinal fasciculus. The caudal stem axons could be traced parallel to the midline and inferior to the medial longitudinal fasciculus and as far caudally as the hypoglossal nucleus.     

Frontal eye field efferents in the macaque monkey: II. Topography of terminal fields in midbrain and pons

Frontal eye field (FEF) projections to the midbrain and pons were studied in nine macaque monkeys that were used to study FEF projections to the striatum and thalamus (Stanton et al.: J. Comp. Neurol. 271:473-492, '88). Injections of tritiated amino acids or WGA-HRP were made into FEF cortical locations where low-level microstimulation (less than or equal to 50 microA) elicited saccadic eye movements, and anterograde axonal labeling was mapped. The injections were made into the anterior bank of the arcuate sulcus from dorsomedial sites where large saccades were evoked (lFEF) to ventrolateral sites where small saccades were evoked (sFEF). The largest terminal fields of FEF fibers were located in the ipsilateral superior colliculus (SC). Projections to SC were topographically organized: lFEF sites projected to intermediate and deep layers of caudal SC, sFEF sites projected to intermediate and superficial layers of rostral SC, and FEF sites between these extremes projected to intermediate locations in SC. Patches of terminal labeling were located ipsilaterally in the lateral mesencephalic reticular formation near the parabigeminal nucleus and the ventrolateral pontine reticular formation. These patches were larger from lFEF injections. Small, dense terminal patches were seen in the ipsilateral pontine gray, mostly along the medial and dorsal borders of these nuclei but occasionally in central and dorsolateral regions. Patches of label like those in the pontine nuclei were located ipsilaterally in the reticularis tegmenti pontis nucleus in lFEF cases and bilaterally in sFEF cases. Small terminal patches were found in the nucleus of Darkschewitsch and dorsal and medial parts of the parvicellular red nucleus in most FEF cases. In the pretectal region, labeled terminal patches were consistently found in the nucleus limitans of the posterior thalamus, but we could not determine if label in the nucleus of the pretectal area and dorsal parts of the nucleus of the posterior commissure marked axon terminals or fibers of passage. We found small, lightly labeled terminal patches in the pontine raphe between the rootlets of the abducens nerve (three cases) or in the adjacent paramedian pontine reticular formation (one case). Omnipauser cells in this region are important in initiating saccades. In one sFEF case, very small patches of label were located in the supragenual nuclei anterior to the abducens nuclei and in the ipsilateral nucleus prepositus hypoglossi posterior to the abducens nucleus. Presaccadic burster neurons in the periabducens region are known to fire immediately before horizontal saccades.(ABSTRACT TRUNCATED AT 400 WORDS)     

Internuclear ophthalmoplegia of abduction: clinical and electrophysiological data on the existence of an abduction paresis of prenuclear origin.

Three patients showed unilateral and five bilateral abduction paresis. Five had associated adduction nystagmus of the contralateral eye. Electrophysiological testing of masseter and blink reflexes indicated an ipsilateral rostral pontine or mesencephalic lesion, and excluded a lesion of the infranuclear portion of the abducens nerve. Abduction paresis was attributed to impaired inhibition of the tonic resting activity of the antagonistic medial rectus muscle. The prenuclear origin of the disorder is based on morphological and neurophysiological evidence of an ipsilateral inhibitory connection between the paramedian pontine reticular formation and the oculomotor nucleus running close to but separated from the medial longitudinal fasciculus.     

Afferent connections of the oculomotor nucleus in the chick

Horseradish peroxidase was injected into the oculomotor nucleus of the chick in order to locate and characterize the neurons projecting to this nucleus. In the rostral mesencephalon, 120-180 neurons were labelled in the medial area of the ipsilateral nucleus campi Foreli; 190-220 in the interstitial nucleus of Cajal (most of them contralateral); and smaller numbers bilaterally in the medial mesencephalic reticular formation, the nucleus of the basal optic root complex, and the central grey matter. More caudally, numerous neurons were labelled in the contralateral abducens nucleus and the vestibular complex and a few in the nucleus reticularis pontis caudalis. Labelled neurons appeared ipsilaterally in the caudal region of the nucleus vestibularis superior and in the rostral tip of the nucleus descendens just lateral to the tractus lamino-olivaris. In the contralateral vestibular complex, a group of labelled cells observed in the dorsolateral area may be homologous to the mammalian cell group Y. At the level of the contralateral abducens nucleus, the most numerous group of cells (625-700) projecting to the oculomotor nucleus formed a lateromedial fringe that affected the nucleus tangentialis, the rostral tip of the nucleus descendens, and the ventrolateral region of the nucleus medialis. Only a few labelled neurons were seen in the contralateral nucleus vestibularis superior, the ipsilateral cell group A, and the ipsilateral nucleus vestibularis medialis.     

Vestibulo-oculomotor connections in an elasmobranch fish,the atlantic stingray,Dasyatis sabina

In elasmobranch fishes, including the Atlantic stingray, the medial rectus muscle is innervated by the contralateral oculomotor nucleus. This is different from most vertebrates, in which the medial rectus is innervated by the ipsilateral oculomotor nucleus. This observation led to the prediction that the excitatory vestibulo-extraocular motoneuron projections connecting each semicircular canal to the appropriate muscle should use a contralateral projection from the vestibular nuclei to the motoneurons. This hypothesis was examined in the Atlantic stingray by injecting horseradish peroxidase unilaterally into the oculomotor nucleus. It was found that vestibulo-oculomotor projections arise from the ipsilateral anterior octaval nucleus and the contralateral descending octaval nucleus. The same pattern was observed when the trochlear nucleus was involved in the injection. There were no cells labeled in the region of the abducens nucleus, and no candidate for a nucleus prepositus hypoglossus was identified. The presence of compensatory eye movements, the directional sensitivity of the semicircular canals, the location of the motoneurons innervating each eye muscle, and our results indicate that the excitatory input to the extraocular motoneurons is derived from the contralateral descending octaval nucleus, and the inhibitory input is derived from the ipsilateral anterior octaval nucleus. The absence of both abducens internuclear interneurons and a nucleus prepositus hypoglossus suggests that eye movements, particularly those in the horizontal plane, are controlled differently in elasmobranchs than in other vertebrates examined to date. © 1994 Wiley-Liss, Inc.     

Midbrain paresis of horizontal gaze

Unilateral paramedian involvement of the midbrain tegmentum causes monocular paralysis of adduction in the ipsilateral eye, paresis of contralateral saccades in the opposite eye, and conjugate paresis of ipsilateral smooth pursuit. The adduction paralysis can be nuclear, or internuclear from a lesion in the medial longitudinal fasciculus. This distinctive midbrain syndrome of horizontal gaze paresis is exemplified by means of quantitative infrared oculographic, radiological, and neuropathological correlation in two patients with predominantly paramedian midbrain tumors involving the mesencephalic reticular formation and the oculomotor nucleus. Binocular paralysis of elevation provided evidence that one human oculomotor nucleus contains axons to both superior rectus muscles, as does the simian oculomotor nucleus. The midbrain tectum was spared. These pathophysiological correlations indicate that the mesencephalic reticular formation contains pathways that control contralateral saccades and ipsilateral smooth pursuit.     

Brainstem afferents to the lateral mesencephalic tegmental region of the cat

The lateral mesencephalic tegmental region (LTR) is a part of the midbrain reticular formation characterized by the presence of neurons exhibiting head movement-related discharge modulation. In addition, the LTR contains directionally selective visual units. Possible sources for these vestibular and visual signals were studied by retrograde axonal transport of horseradish peroxidase and three different fluorescent tracers (rhodamine, fast blue, and fluorogold) injected into various parts of the LTR. All injections into the LTR traced afferents from the vestibular nuclei and from the nucleus prepositus hypoglossi. Predominant projections were derived from the ipsilateral nucleus prepositus hypoglossi and the ipsilateral medial vestibular nucleus, whereas the observed inputs from the inferior, lateral, and superior vestibular nuclei were much weaker. Further inputs to the LTR originated in the deep and intermediate layers of the ipsilateral superior colliculus and the ipsilateral periaqueductal gray, the contralateral LTR, and the contralateral marginal nucleus of the brachium conjunctivum. Tracer deposits in medial parts of the tegmentum neighboring the LTR never produced the pattern of afferents observed after injections into the LTR. Our results suggest that afferents from the deeper layers of the superior colliculus are probably the source of visual signals in the LTR and that head movement-related responses are likely to be derived from the nucleus prepositus hypoglossi and the medial vestibular nucleus. © 1995 Wiley-Liss, Inc.