Abducens Nucleus


METHODS: We applied a phase-plane technique that compared each eye's velocity as a function of change in position (normalized displacement) in 22 patients with disease variously affecting the brainstem reticular formation, the abducens nucleus, the medial longitudinal fasciculus, the oculomotor nerve, the abducens nerve, the neuromuscular junction, or the extraocular muscles; 10 age-matched subjects served as controls.  

NPR-C immunoreactivity was detected in several regions, including the periaqueductal gray, oculomotor nucleus, red nucleus and trochlear nucleus of the midbrain; the pontine nucleus, dorsal tegmental nucleus, vestibular nucleus, locus coeruleus, trigeminal motor nucleus, nucleus of the trapezoid body, abducens nucleus and facial nucleus of the pons; and the dorsal motor nucleus of the vagus, hypoglossal nucleus, lateral reticular nucleus, nucleus ambiguus and inferior olivary nucleus of the medulla oblongata.  

We also observed glycine-immunoreactive populations in the optic tectum, the torus semicircularis and the midbrain tegmentum, the isthmus, the octavolateral area, the dorsal column nucleus, the abducens nucleus, the trigeminal motor nucleus, the facial motor nucleus, and the rhombencephalic reticular formation.  

Protein synthesis values were standardized to measurements from the abducens nucleus and compared with measurements from sham animals at similar age/survival times.  

Brain stem lesions involved the following: tectum (n = 5), tegmentum (n = 4), red nucleus (n = 3) of the midbrain, vestibular nucleus (n = 6), and a focal tegmental lesion involving the superior olivary nucleus (n = 6) and abducens nucleus (n = 4) of the pons and vestibular nucleus (n = 4) and inferior olivary nucleus (n = 1) of the medulla.  

Using extracellular stimulation, 62 individual motor units in the ferret abducens nucleus were evaluated for their contractile characteristics. The abducens nucleus motor units showed a twitch contraction time of 15.4 ms, a mean twitch tension of 30.2 mg, and an average fusion frequency of 154 Hz. The abducens nucleus was examined with horseradish peroxidase conjugated with the subunit B of cholera toxin histochemistry and found to contain an average of 183 motoneurons.  

Fourteen PMRF neurons located in a region ventral to the abducens nucleus produced 42 significant SpikeTA effects in arm and shoulder muscles.  

The pathological lesions, therefore, were considered to be a unilateral paramedian pontine reticular formation (PPRF) and abducens nucleus.  

The central mesencephalic reticular formation (cMRF) and paramedian pontine reticular formation (PPRF) are two brain stem regions that likely participate in the development of motor activity since both structures are anatomically connected to nuclei that encode movement goal (superior colliculus) and generate horizontal eye movements (abducens nucleus).  

Rat sagittal brainstem slices including the PH nucleus and the paramedian pontine reticular formation (PPRF) rostral to the abducens nucleus were used for recording intracellularly the synaptic activation of PH neurons from the PPRF.  

The pattern of the oculomotor deficit is consistent with bilateral dysplasia of the abducens nuclei with preserved internuclear neurons in the right abducens nucleus.  

In the monkey oculomotor, trochlear and abducens nucleus the motoneurons of multiply-innervated muscle fibers lie separated from those innervating singly-innervated muscle fibers and show different histochemical properties. The belly injections labeled the medial rectus muscle subgroup of the oculomotor nucleus or the greatest part of abducens nucleus, including some cells outside the medial border of abducens nucleus. In contrast, the distal injections labeled only a subset of the medial rectus muscle motoneurons and exclusively cells outside the medial border of abducens nucleus. The overall identification of multiply-innervated muscle fiber and singly-innervated muscle fiber motoneurons within the rat oculomotor nucleus, trochlear nucleus, and abducens nucleus revealed that the smaller multiply-innervated muscle fiber motoneurons tend to lie separate from the larger diameter singly-innervated muscle fiber motoneurons.  

Segmental variation of the abducens nucleus remains the sole example of efferent neuronal homeosis during vertebrate hindbrain evolution.  

The MIF motoneurons lie around the periphery of the oculomotor nucleus (III), trochlear nucleus (IV), and abducens nucleus (VI), slightly separated from the SIF subgroups.  

In monkeys, the motoneurons of SIFs lie within the oculomotor, trochlear, and abducens nucleus, whereas the motoneurons of MIFs appear in separate subgroups in the periphery of the classical nuclei borders. An additional population of motoneurons with "MIF properties" was identified within the boundaries of the abducens nucleus, which could represent the motoneurons innervating MIFs in the orbital layer of lateral rectus muscle.  

Binocular frontal vision implies a perfect parallelism of both eyes, which is ensured in the pons by means of the abducens nucleus, controlling abduction as well as adduction. The pathological example of the "one-and-a-half" syndrome, in which the abducens nucleus and the adjacent medial longitudinal fasciculus are simultaneously impaired, is described.  

The abducens nucleus was located ipsilaterally in a ventrolateral part of the rhombencephalic reticular formation.  

We recorded intracellular potentials from neurons in the IBN region and identified them as IBNs based on their antidromic activation from the contralateral abducens nucleus and short-latency excitation from the contralateral caudal SC and/or single-cell morphology.  

Double labeling studies showed that the medial cell group of the facial nucleus lies between the principal and accessory abducens nuclei in the pons, whereas the lateral group lies adjacent to the accessory abducens nucleus. The facial nucleus of pond turtles largely overlaps the rostrocaudal extent of the accessory abducens nucleus, but extends well beyond it into the medulla.  

We demonstrated for the first time that the number of MNs in the mouse abducens nucleus was also increased by 40%, thus extending known MN populations that are responsive to GDNF.  

1) abducens nucleus injections gave rise to silver grain deposits over all medial rectus subgroups, both twitch and nontwitch.  

Additionally, the principal abducens nucleus receives vestibular inputs, whereas the accessory nucleus receives input from the cochlear nucleus.  

In the abducens nucleus, however, a significantly higher percentage of cells expressed TrkB than the other two receptors, among both motoneurons (81.8%) and internuclear neurons (88.4%).  

Lateral eye movements are organized in the pons, with paralysis of adduction (and preservation of convergence) when the lesion affects the medial longitudinal fasciculus (internuclear ophthalmoplegia), paralysis of conjugate lateral eye movements when the lesion affects the abducens nucleus (VI) and the "one-and-a-half" syndrome when both these structures are involved.  

Galectin-1 mRNA was predominantly observed in the cell bodies of neurons such as oculomotor nucleus (III), trochlear nucleus (IV), trigeminal motor nucleus (V), abducens nucleus (VI), facial nucleus (VII), hypoglossal nucleus (XII), red nucleus, and locus ceruleus.  

Finally, the wobbler abducens nucleus displayed neither vacuolated neurones, nor NADPH-d reactivity nor CB immunoreactivity. Sparing of oculomotor neurones in other diseases has been attributed to their relatively high PV expression, which we also observed in the abducens nucleus of both wobblers and littermates, and to a lesser extent in the other motor nuclei too.  

In the brainstem of symptomatic SOD1(G93A) transgenic mice, significantly increased immunoreactivity for MnSOD was observed in abducens nucleus, facial nucleus, dorsal motor nucleus of vagus, hypoglossal nucleus, medullary and pontine reticular formation, superior and inferior olivary nucleus, and cochlear nucleus.  

We demonstrate that in r5, the combined mutation of Hoxa3 and Hoxb3 result in a loss of Pax6- and Olig2-expressing progenitors that give rise to somatic motoneurons of the abducens nucleus.  

Finally, boutons and neurons were found in the rhombencephalic reticular formation surrounding the abducens nucleus.  

Semithin sections of the abducens nucleus were immunostained against calretinin, to identify abducens internuclear neurons, and either synaptophysin (SF), to label synaptic terminals, or glial fibrillary acidic protein (GFAP) to detect the astrocytic reaction.  

We argue that BT neurons in the nucleus prepositus hypoglossi/medial vestibular nucleus play an important role in the generation of unequal eye movements during disjunctive saccades, and carry appropriate information to shape the saccadic discharges of the abducens nucleus neurons to which they project..  

In this report, we provide the first characterization of abducens nucleus neuron (ABN) discharge dynamics during horizontal disjunctive saccades.  

The distribution of SC neurons influencing the abducens nucleus, thus, was nonuniform.  

The projection of the magnocellular red nucleus (RNm) to the region of the accessory abducens nucleus (AABD) was traced in rabbit using the bidirectional tracer wheat germ agglutinin-horseradish peroxidase (WGA-HRP).  

In most cases, the abducens nucleus and nerve are absent or hypoplastic, and the lateral rectus muscle is innervated by a branch of the oculomotor nerve.  

Computer-averaged evoked responses were recorded from far-field electrodes placed on the scalp and from near-field electrodes stereotaxically positioned in or near the inferior colliculus and abducens nucleus. Direct electrical stimulation of the cochlear nerve elicited a four-wave, auditory brainstem response-like extracranial response, strong activity in the inferior colliculus, and weak activity in the abducens nucleus.  

The peripheral neurons labeled from the lateral rectus muscle surround the medial half of the abducens nucleus: from superior oblique, they form a cap over the dorsal trochlear nucleus; from inferior oblique and superior rectus, they are scattered bilaterally around the midline, between the oculomotor nucleus; from both medial and inferior rectus, they lie mainly in the C-group, on the dorsomedial border of oculomotor nucleus.  

The motor nuclei commanding extraocular muscles did not contain elements of the NO-cGMP pathway, except for some scattered nitrergic neurons in the most caudal part of the abducens nucleus.  

A population of ventral horn neurons in the spinal cord, hypoglossal nucleus, dorsal motor nucleus of the vagus, facial motor nucleus, nucleus ambiguus, abducens nucleus and trigeminal motor nucleus exhibited irU-II of varying intensities.  

Our results showed that: (i) TeNT blocked selectively the afferent inhibitory signals on abducens neurons only when used at a low dose, whereas both excitatory and inhibitory synaptic drive was lost after the high dose treatment; (ii) all effects were reversible within one month; and (iii) strikingly, the internuclear neurons of the abducens nucleus showed similar discharge alterations to the motoneurons, suggesting a TeNT action on shared common afferences..  

The goal of this study was to examine and compare the number and size of motoneurons in the cat and squirrel monkey abducens nucleus. We also examined medial rectus muscle motoneuron compartmentalization in the squirrel monkey oculomotor nucleus and compared those cells to abducens nucleus motoneurons. Cat abducens nucleus motoneurons showed a wide range of cell sizes (26.0-66.0 microm, mean = 37.2 +/- 6.2 microm), four or more dendrites per cell and an average of 1,418 cells within a relatively loosely packed nucleus. Squirrel monkey abducens nucleus motoneurons were significantly smaller than those in the cat with a narrower range of cell sizes (20.0-44.0 microm, mean = 31.7 +/- 3.8 microm), four or more dendrites per cell and an average of 2,473 cells densely packed within the nucleus. MRa motoneurons comprise the primary innervation for the medial rectus muscle and were similar in size to abducens nucleus motoneurons while the MRc subgroup cells were significantly smaller in size.  

CONCLUSION: DRS is characterized by abnormal development of the cells in the abducens nucleus (CN VI), resulting in restricted or absent abduction and erroneous innervation of the lateral rectus by branches emanating from oculomotor nuclei (CN III).  

The isolated whole brain (IWB) preparation of the guinea pig was used to investigate the floccular modulation of vestibular-evoked responses in abducens and oculomotor nerves and abducens nucleus; for identification of flocculus target neurons (FTNs) in the vestibular nuclei and intracellular study of some of their physiological properties; to search for possible flocculus-dependent plasticity at the FTN level by pairing of vestibular nerve and floccular stimulations; and to study the possibility of induction of long-term depression (LTD) in Purkinje cells by paired stimulation of the inferior olive and vestibular nerve. Floccular stimulation significantly inhibited the vestibular-evoked discharges in oculomotor nerves on both sides and the inhibitory field potential in the ipsilateral abducens nucleus while the excitatory responses in the contralateral abducens nerve and nucleus were free from such inhibition. None of them led to long-term modification of responses in the abducens nucleus or oculomotor nerve evoked by activation of vestibular afferents.  

The responsible pontine lesion involved the ipsilateral abducens nucleus and the adjacent medial longitudinal fasiculus (MLF), as demonstrated by magnetic resonance imaging (MRI).  

In rat, [ (125)I]-U-II binding density was highest in the abducens nucleus of brainstem (139.6+/-14 amol mm(-2)).  

The distance of the rostral portion of facial colliculus from obex was longer than the distance of the rostral pole of abducens nucleus from obex in every examined specimen (by 0.7 mm on average).  

Immunoreactive fibres were observed in the following; the inferior central nucleus; the pontine gray nuclei; the K├Âlliker-Fuse nucleus; the motor trigeminal nucleus, the anteroventral cochlear nucleus; the abducens nucleus; the retrofacial nucleus; the superior, lateral, inferior, and medial vestibular nuclei; the lateral nucleus of the superior olive; the external cuneate nucleus; the nucleus of the trapezoid body; the postpyramidal nucleus of the raphe; the medial accessory inferior olive; the dorsal accessory nucleus of the inferior olive; the nucleus ambiguus; the principal nucleus of the inferior olive; the preolivary nucleus; the nucleus ruber; the substantia nigra; and in the area postrema.  

The abducens nucleus was negative for both gfralpha1 and gfralpha2.  

Two further groups of GABAir interneurons, periventricular and lateral, were located in the abducens nucleus among the cells of the caudal rectus and the ventral rectus motor subnuclei, respectively.  

Neurons in the abducens nucleus of monkeys were first studied during saccades made with the head stationary.  

The lateral rectus muscle was innervated by the abducens nucleus, which was composed by two subgroups of labeled cells, respectively observed in the principal and accessory abducens subnuclei, whereas efferents to the inferior oblique muscle originated from both the ventral and intermediate oculomotor subnuclei.  

It is due to a unilateral lesion of the dorsal pontine tegmentum, involving the ipsilateral paramedian pontine reticular formation, internuclear fibres of the ipsilateral medical longitudinal fasciculus and, usually, the abducens nucleus.  

Measurements from auditory nuclei of experimental animals were corrected against an unaffected control area (abducens nucleus) and compared with measurements taken from animals in the sham group.  

The present study extends these observations to the ultrastructural localization of the excitatory amino acid neurotransmitters, glutamate and aspartate, using a postembedding immunogold procedure combined with the pre-embedding immunoperoxidase localization of anterogradely transported biocytin from the abducens nucleus and the ventral lateral vestibular nucleus.  

Intracellular responses to SC stimulation were analyzed in lateral rectus (LR) and medial rectus (MR) motoneurons and internuclear neurons in the abducens nucleus (AINs). Transneuronally labeled neurons were mainly distributed ipsilaterally in the paramedian pontine reticular formation (PPRF) rostral to retrogradely labeled LR motoneurons and the vestibular nuclei, and contralaterally in the paramedian pontomedullary reticular formation (PPMRF) caudomedial to the abducens nucleus and the vestibular nuclei.  

In the abducens nucleus they were restricted to the caudal pole of the nucleus, which is located ventrolaterally to the genu of the facial nerve.  

In addition, in the brainstem, the oculomotor nucleus, trochlear nucleus, mecencephalic and motor nuclei of trigeminal nerve (N), abducens nucleus, facial nucleus, nucleus of the raphe pontis, dorsoral motor nucleus of vagal N, hypoglossal nucleus and ambiguus nucleus showed motopsin mRNA expression.  

Using extracellular recordings, we confirmed that in the presence of pentobarbitone, GYKI-53784 counteracts the effects of AMPA but not of kainate on antidromic field potentials in the abducens nucleus.  

Nov-positive neurons were detected at G28W in the nucleus of the spinal tract of the trigeminal and cuneate nucleus, and at G38W in the abducens nucleus of pons, the red nucleus and the substantia nigra of the midbrain, the ventral posterolateral and the mediodorsal thalamic nucleus.  

Following a 2 h survival time, facial motoneurons in newborn opossum pups (1 PN) exhibited CtB labeling, with their cell bodies localized near the developing cranial abducens nucleus.  

Tracer application to the abducens nucleus anterogradely labeled thick contralateral fibers that specifically contact the medial rectus motor subnucleus by means of large boutons. Tracer application to the abducens nucleus labeled a group of small interneurons in the ipsilateral dorsal rectus motor subnucleus.  

Oculomotor internuclear neurons identified from the abducens nucleus constituted a nonuniform population, because low percentages of the three types of immunostaining were observed, calbindin being the most abundant (28.5%).  

We conclude that during head-free gaze shifts, the head velocity signal carried to the abducens nucleus by primate excitatory burst neurons (EBNs; if EBNs and IBNs carry similar signals) must be offset by other premotor cells..  

This question was investigated by simulating the input signal to medial rectus motoneurons (MR-MNs) from internuclear neurons of the abducens nucleus (INNs).  

At birth both alpha-type and beta-type CGRP messenger RNA were present in the parabrachial nucleus, inferior olive and motor nuclei (except for abducens nucleus), and only alpha-type CGRP messenger RNA in some posterior thalamic nuclei. As development advanced, new nuclei started to express either only alpha-CGRP gene (superior olive, parabigeminal, sagulum, and some hypothalamic and cranial thalamic nuclei) or both genes (abducens nucleus).  

During the whole time-span studied, presynaptic wavelets were present, indicating no affecting of the conduction of afferent volleys to the abducens nucleus.  

As clearly shown by MRI of the brain, the pontine tegmentum, including bilateral abducens nucleus, paramedian pontine reticular formation (PPRF), medial longitudinal fasciculus (MLF), and the facial nerve were affected, but the thalamo-mesencephalic junction, including the rostral interstitial medial longitudinal fasciculus (riMLF) and the nucleus of Cajal, was spared.  

One of these, the abducens nucleus, will not be dealt with here.  

Electron microscopy of motoneurons in the oculomotor and abducens nucleus showed axosomatic and axodendritic synaptic endings containing spheroidal synaptic vesicles establishing chemical, presumed excitatory, synaptic contacts with asymmetric pre- and/or postsynaptic membrane specializations.  

Likely because of the close apposition of the intrapontine facial nerve to the abducens nucleus, lateral rectus EMG activity was highly predictive of seventh nerve injury.  

Based on this hypothesis, we estimated the number of synapses intervening between the SC and motoneurons by applying midflight stimulation to the SC, the BN area, and the abducens nucleus. The mean latencies of the stimulus-evoked eye movements were: 7.9 +/- 1.0 ms (SD; ipsilateral eye) and 7.8 +/- 0.9 ms (SD; contralateral eye) for SC stimulation; 4.8 +/- 0.5 ms (SD; ipsilateral eye) and 5.1 +/- 0.7 ms (SD; contralateral eye) for BN stimulation; and 3.6 +/- 0.4 ms (SD; ipsilateral eye) and 5.2 +/- 0.8 ms (SD; contralateral eye) for abducens nucleus stimulation.  

A patient with a complete unilateral conjugate gaze paralysis caused by a small lesion affecting the region of the right abducens nucleus, documented by magnetic resonance imaging, is reported.  

Likely because of the close apposition of the intrapontine facial nerve to the abducens nucleus, lateral rectus EMG activity was highly predictive of seventh nerve injury.  

As a step toward investigating how system-level linearity might be achieved despite nonlinear components, a distributed model of motor units in the abducens nucleus and lateral rectus muscle was constructed.  

The mean eye velocity sensitivity of these "early" preposito-collicular neurones (1.46 +/- 0.53 spikes/s per degree per second) was quite similar to that calculated from a sample of putative motoneurones or interneurones that have been recorded within abducens nucleus and quantified in the same conditions.  

Two reticulospinal neurons had large multipolar somata either just rostral or ventral to the abducens nucleus.  

Although pathological changes are observed in both the oculomotor nucleus and abducens nucleus in autopsied cases of infantile progressive spinal muscular atrophy, external and internal ocular palsy and ptosis have not been previously reported clinically.  

Extracellular recordings were made from medium-lead burst neurons (MLBNs) in the paramedian pontomedullary reticular formation rostral and caudal to the abducens nucleus in the alert cat.  

The neurons of the rat abducens nucleus that project to the flocculus of the cerebellum were studied by double labelling using the retrograde transport of horseradish peroxidase (HRP) and choline acetyltransferase (ChAT) immunohistochemistry. These findings show the existence of a bilateral projection from the abducens nucleus to the flocculus which uses acetylcholine as a neurotransmitter.  

In addition, intense v7-3 mRNA expression was found in the motor nuclei including the oculomotor nucleus, abducens nucleus, trigeminal motor nucleus, facial nucleus, hypoglossal nucleus and ventral horn of spinal cord.  

Rabbits were eyeblink conditioned while their accessory abducens nucleus (ACC), facial nucleus (FN), and surrounding reticular formation (RF) were temporarily inactivated with microinjections of muscimol to determine whether these structures are critically involved in acquisition of the conditioned eyeblink response (CR).  

Spontaneous spikes of PNs were suppressed after single shock stimulation of excitatory burst neuron (EBN) area immediately rostral to the abducens nucleus.  

The abducens nucleus is a pontine nucleus directly involved in oculomotion through its connections with the lateral rectus muscle of the eye. The aim of the present study was to investigate the cytoarchitectural organization of the abducens nucleus in man. This finding would suggest that the neuronal relationships of the abducens nucleus supplied by the afferent fibers which pass through or end within it take place almost completely inside the nucleus.  

Extracellular recordings were made from medium-lead burst neurons (MLBNs), omnipause neurons (OPNs) and burster-driving neurons (BDNs) in the paramedian pontomedullary region rostral and caudal to the abducens nucleus.  

The neural circuit consists of neurons in the paramedian pontine reticular formation (burst, tonic and pause cells), the vestibular nucleus, abducens nucleus, oculomotor nucleus, cerebellum, substantia nigra, nucleus reticularis tegmenti pontis, the thalamus, the deep layers of the superior colliculus and the oculomotor plant for each eye.  

Horizontal medium lead burst neurons (EBNs) project to ipsilateral lateral rectus motoneurons directly, and could contact contralateral medial rectus motoneurons indirectly, through the internuclear neurons of the ipsilateral abducens nucleus.  

Stimulation of 41 single, abducens nucleus motoneurons in the cat evoked electromyographic (EMG) and contractile responses in the ipsilateral lateral rectus muscle.  

First, in monkeys, the premotor burst neurons were backlabeled by injections of wheat germ agglutinin-horseradish peroxidase or cholera toxin subunit B into the abducens nucleus or tetanus toxin fragment C into the lateral rectus muscle and shown by double labeling to contain parvalbumin. The putative excitatory burst neuron area in man is in the medial part of the nucleus reticularis pontis caudalis (extending 2.5 mm mediolaterally), immediately rostral (250 microns) to the omnipause neurons and extending 2.2 mm rostrally, and the putative inhibitory burst neuron area lies in the medial part of the paragigantocellular nucleus caudal to the abducens nucleus, extending 1.8 mm caudally.  

Interestingly, among many cranial motor and other brainstem nuclei examined, only the survival of motoneurons from the abducens nucleus was enhanced by BEX.  

It is expressed in: (1) motor nuclei such as the oculomotor nucleus, trochlear nucleus, motor trigeminal nucleus, abducens nucleus, facial nucleus, ambiguus nucleus, dorsal motor nucleus of vagus and hypoglossal nucleus; (2) several sensory-related nuclei like the mesencephalic trigeminal nucleus, ventral nucleus of the lateral lemniscus, lateral and spinal vestibular nuclei, ventral and dorsal cochlear nuclei and nucleus of the trapezoid body; and (3) other regions such as the red nucleus, dorsal raphe nucleus, pontine nuclei, three cerebellar nuclei (medial, interposed and lateral), Purkinje cells, cells in the granular layer of the cerebellum, locus coeruleus, several areas of the reticular nucleus and area postrema..  

Furthermore, the principal abducens nucleus labeled heavily during conditioning. Multiple sites of conditioning are hypothesized, including the cerebellorubral circuitry and brainstem pathways that activate the principal abducens nucleus..  

The lesion of the abducens nucleus was clearly detected with magnetic resonance imaging. We suggest that this phenomenon is useful as a differential diagnosis between lesions of the abducens nucleus and of the paramedian pontine reticular formation..  

Injection of horseradish peroxidase (HRP) into the cerebellar flocculus of the rat was employed to identify neurons in the abducens nucleus that project to the flocculus.  

All neurons exhibited increased phase lead with respect to abducens nucleus neurons during stimuli involving head rotation.  

k increases and r decreases with stimulus frequency, which indicates that the simplest transfer function mediating conversion of abducens nucleus (VI) firing rate to eye position (E) has two poles and one zero.  

Studies of the pathways involved in the vestibulo-ocular reflex have suggested that the projection from the superior vestibular nucleus to the ipsilateral oculomotor nucleus is inhibitory, whereas the medial vestibular nucleus, the abducens nucleus and the contralateral superior vestibular nucleus most likely exert excitatory effects on oculomotor neurons. The abducens nucleus projected contralaterally to the medial rectus subdivision. All terminals derived from the medial vestibular nucleus the abducens nucleus and the contralateral superior vestibular nucleus were non-GABAergic. None of the anterogradely labeled terminals from the studied vestibular nuclei or abducens nucleus were glycinergic. Furthermore, it confirms that the projections from the medial vestibular nucleus, the abducens nucleus and the contralateral superior vestibular nucleus are exclusively non-GABAergic..  

Projection into the superior and the medial nuclei and the ipsilateral abducens nucleus were also observed..  

Retrogradely labeled neurons were observed in the following structures: (1) anterior, tangential and descending nuclei of the octaval column--afferents to these nuclei were mainly ipsilateral for the former and exclusively contralateral for the other two; (2) cerebellum; (3) rhombencephalic reticular formation, near the abducens nucleus; and (4) nucleus of the medial longitudinal fasciculus.  

We refer to this area, caudal to the abducens nucleus, as the gigantocellular head movement region.  

Injecting WGA-HRP into the abducens nucleus and the surrounding tissue labeled many cells in SC, PPRF, MVN, FN, and nucleus prepositus hypoglossi (NPH).  

Electrophysiological studies were performed using cats anesthetized with alpha-chloralose, to elucidate the 5-hydroxytryptamine (5-HT) receptor subtypes involved in the 5-HT-induced inhibition of the lateral vestibular nucleus (LVN) neurons projecting to or through the abducens nucleus. The effects of 5-HT receptor subtype agonists and antagonist were examined in polysynaptic neurons activated by stimulation of the ipsilateral abducens nucleus (IAN) antidromically, since these neurons are sensitive to 5-HT as shown in our previous study.  

The number and proportion of motoneurons and interneurons present in the rat abducens nucleus was determined by the use of ChAT immunostaining and of HRP staining after retrograde transport from the injected right lateral rectus muscle. After considering and discarding the hypotheses that the lateral rectus muscle could be incompletely filled by HRP, and that other muscles may be innervated by abducens motoneurons, it is concluded that some interneurons of the abducens nucleus of the rat are probably cholinergic..  

Rotatory nystagmus has been rarely observed in the patients with MLF syndrome, and previous authors postulated a lesion in the MLF above the level of the abducens nucleus and below the level of the trochlear nucleus.  

Most neurons in the abducens nucleus decrease their firing rate during convergence, and the source of this vergence signal is unknown. Oculomotor internuclear neurons (OINs) in monkeys project primarily from the medial rectus subdivisions of the oculomotor nucleus to the contralateral abducens nucleus, although there is a smaller ipsilateral projection as well. The latter effect was presumed to be the result of OIN innervation of the contralateral abducens nucleus.  

There were no cells labeled in the region of the abducens nucleus, and no candidate for a nucleus prepositus hypoglossus was identified.  

At longer intervals (> one month), the ultrastructure of the oculomotor nucleus was re-established and labeled boutons were observed contacting either unidentified dendrites within the neuropil or the soma and proximal dendrites of the oculomotor internuclear neurons, that project to the abducens nucleus.  

In each injection experiment, the location of the abducens nucleus of the alert cat was mapped out by recording the antidromic field potentials evoked by the stimulation of the abducens nerve. Two micropipettes were then glued together in such a way that when the tip of the recording micropipette was in the centre of the abducens nucleus the tip of the injection micropipette was in a target area. The twin pipettes were then lowered in the brainstem until the recording micropipette reached the centre of the abducens nucleus.  

In this study, we describe the gaze-related activity of cat premotor "inhibitory burst neurons" (IBNs) identified on the basis of their position relative to the abducens nucleus.  

Numerous theories concerning the etiology and pathogenesis of DRS have been proposed, including agenesis of the abducens nucleus, but the majority of investigators concur that the characteristic findings are best explained by a paradoxical innervation of the lateral rectus muscle, which subsequently causes a cocontraction of the horizontal rectus muscles.  

For example, labeling for beta-1 messenger RNA was found in the anterior olfactory nucleus, cerebral cortex, lateral intermediate septal nucleus, reticular thalamic nucleus, oculomotor complex, vestibular nuclei, deep cerebellar nuclei, trapezoid nucleus, abducens nucleus, ventrolateral pontine and medullary reticular formations, the intermediate gray matter of the spinal cord and in the pineal gland, while beta-2 messenger RNA labeling was strongest in the olfactory bulb, piriform cortex, hippocampal formation, thalamic intralaminar nuclei and cerebellar cortex.  

Motoneurons supplying the accessory rectus muscles were found ventrolateral to the main abducens nucleus, in a position similar to that occupied by the cat accessory abducens nucleus; although others may be present in the main nuclei.  

The single-unit activity of neurons in the vestibular nucleus, the prepositus nucleus, and the abducens nucleus, whose activity was primarily related to horizontal eye movements, was recorded in alert squirrel monkeys that were trained to track a small visual target by generating smooth pursuit eye movements and to cancel their horizontal vestibuloocular reflex (VOR) by fixating a head stationary target.  

Three groups of somas have been identified: Those located in the proximity of the vestibular nuclei, those sandwiched between the facial genu and the IVth ventricle, and those in the RF, surrounding the abducens nucleus.  

In order to test this hypothesis, we disrupted the vestibular commissure of the cat by a parasagittal cut running between an abducens nucleus and the adjacent medial vestibular nucleus.  

The enhanced MRIs revealed a discrete left median pontine tegmental lesion, involving the abducens nucleus, MLF, and facial nerve knee. Among these structures, the area of the abducens nucleus seems to be responsible for the unilateral horizontal gaze palsy. We are not aware of any previous precise neuroradiological documentation of unilateral paralysis of conjugate gaze due to a lesion of the abducens nucleus by sagittal and horizontal MRIs..  

Synaptic vestibular inputs of antidromically identified motoneurons and internuclear neurons in the abducens nucleus were studied electrophysiologically and pharmacologically in the isolated brain of grass frogs (Rana temporaria).  

Intra-axonal HRP labeling showed that visuomotor TRSNs of the X type (n = 3) terminate in the abducens nucleus, with 145-331 boutons terminaux and en passant.  

The accessory abducens nucleus is present in tetrapods except apes and human. The neurons differentiate in situ, they do not migrate from the main abducens nucleus.  

The principal interneurons of abducens nucleus were arranged as a cluster of cells under the floor of the IVth ventricle and more lateral than the principal motoneurons, with no intermingling..  

During reflex activity, neurons in the ipsilateral trigeminal nucleus, principal abducens nucleus, and presumed interneurons ventrolateral to the principal abducens nucleus, labeled with the dye, in addition to areas in the raphe nucleus and reticular formation. In conditions where the long-duration component of the reflex was suppressed, sulforhodamine label was absent in the principal abducens nucleus and in the caudal brainstem. From these data it is hypothesized that the region of interneurons and the accessory abducens nucleus participate in the short-duration component of the reflex. The principal abducens nucleus is postulated to contribute also to the short-duration portion of the reflex, but is primarily involved in the generation of the long-duration component.  

In contrast, scarce GABA immunostaining was associated with the motoneurons and internuclear neurons of the abducens nucleus. Second, oculomotor internuclear neurons were identified following the injection of horseradish peroxidase into the abducens nucleus to determine whether they could give rise to GABAergic terminations in the abducens nucleus. A high percentage (80%) of the oculomotor internuclear neurons projecting to the abducens nucleus showed immunonegative perikarya. It was concluded that the oculomotor internuclear pathway to the abducens nucleus comprises both GABAergic and non-GABAergic neurons and, at least in part, the GABA input to the abducens nucleus originates from this source.  

Furthermore, a few labeled cells were found dorsomedially to the caudal pole of the unlabeled anterior octaval nucleus; (2) the contralateral abducens nucleus. The labeled internuclear neurons were arranged in two groups within and 500 microns behind the caudal subdivision of the abducens nucleus; (3) a few labeled cells were observed in the rhombencephalic reticular formation near the abducens nucleus, most of which were contralateral to the injection site.  

Electrical stimulation of the contralateral superior colliculus evoked monosynaptic field potentials which were largest in the caudal pontine reticular formation rostral to the abducens nucleus and in the rostral part of the medullary reticular formation caudal to the abducens nucleus.  

Extracellular and intracellular techniques were used to study single motor units of the abducens nucleus and lateral rectus muscle in the cat.  

Using an extracellular recording technique, those common interneurons were identified as reticulospinal neurons by showing axonal projection to C2-3 spinal segment; they were also found to distribute mainly to the medial region of the nucleus reticularis pontis caudalis immediately rostral to the abducens nucleus, and not in the medullary reticular formation caudal to the abducens nucleus. The present experiment suggested that integration of tectal and cortical commands for horizontal eye-head coordination are taking place in the reticular formation rostral to the abducens nucleus which has been called the paramedian pontine reticular formation in the cat..  

The MVN sends a contralateral excitatory projection onto the abducens nucleus (second decussation of the lateral smooth pursuit circuitry).  

All of the 100 units described in our study were recorded from regions of the medulla that were prominently labeled after injections of horseradish peroxidase into the abducens nucleus.  

EHV neurons excited during ipsilateral movements received neither direct primary afferent input nor projected to either abducens nucleus. Other types rarely were activated from the eighth nerve or projected to the abducens nucleus.  

These findings also suggest that lesions of non-paralytic pontine exotropia may be located in the paramedian pontine reticular formation rostral to the abducens nucleus with ipsilateral medial longitudinal fasciculus lesion, but further investigation is necessary..  

The firing behavior of burst tonic neurons in the medial vestibular nucleus, the prepositus nucleus, like the cells in the abducens nucleus, was closely related to the eye movements generated when the VOR is cancelled.  

These findings provide evidence that a double decussating pathway mediates smooth pursuit; the first decussation is from the pons to the cerebellum, and the second decussation is from the vestibular nucleus to the contralateral abducens nucleus..  

The abducens nucleus (M VI) contains about 30 neurons with ipsilateral projections only. There is no evidence for an accessory abducens nucleus in Protopterus.  

The two components of the conditioned response may reflect two sequential bursts of activity in the accessory abducens nucleus, the principal site of the motoneurons for the retractor bulbi muscle, or may reflect the synergistic activity of the accessory abducens nucleus and the motor nuclei of the other extraocular muscles.  

Cells of the DPG, considered to contain inhibitory burst neurons impinging upon the contralateral abducens nucleus, were shown to project to virtually all subdivision of the OMC.  

Although the abducens nucleus consists of distinct rostral and caudal subgroups, mean ks and rs values were not significantly different between the two populations.  

In the abducens nucleus, a few labeled cells were still present at 2 weeks of age, but absent by 3 weeks.  

This syndrome is caused by damage to structures within the pontine tegmentum: the medial longitudinal fasciculus, the ipsilateral paramedian pontine reticular formation, or the ipsilateral abducens nucleus.  

In 13 alert, chronically prepared cats, a total of 293 neurons were antidromically identified from either the predorsal bundle (PDB) immediately rostral to abducens nucleus or the ventromedial funiculus of the spinal cord at the level of the first cervical vertebra (C1).  

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. 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. 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.  

It is further suggested that motoneurons in the main abducens nucleus participate in the corneal reflex, permitting several final pathways for conditioned and unconditioned nictitating membrane extension..  

In the majority of LVN polysynaptic neurons that responded to antidromic stimulation of the ipsilateral or contralateral abducens nucleus, orthodromic spikes elicited by vestibular nerve stimulation were inhibited by DR conditioning stimulation and the iontophoretic application of 5-HT. These results indicate that 5-HT derived from the DR inhibits the synaptic transmission of LVN polysynaptic neurons ascending to the abducens nucleus, and suggest that 5-HT derived from the DR is involved in the regulation of the vestibulo-ocular reflex..  

The extraocular stretch receptors were activated by passive eye movement within the pigeon's saccadic range; such movements modified the vestibular responses of all 19 units studied which were all, histologically, in the abducens nucleus. The results show that an afferent signal from the extraocular muscles influences vestibularly driven activity in the abducens nucleus to which it carries information related to amplitude, velocity, plane and direction of eye movement in the saccadic range.  

The central processes of type Aa neurons projected to the following three groups of target nuclei: 1) nuclei functioning as interneurons, including supratrigeminal nucleus (Vsup), intertrigeminal nucleus (Vint), juxta-trigeminal region (Vjux), and parvicellular nucleus of the pontomedullary reticular formation (PcRF); 2) motor nuclei, including the trigeminal motor nucleus (Vmo), accessory facial nucleus (NVIIacs), accessory abducens nucleus (NVIacs), and a small number of labeled axons in the oculomotor nucleus and trochlear nucleus; 3) sensory nuclei, including the dorsomedial part of the principal trigeminal sensory nucleus (Vpdm) and the dorsomedial part of subnucleus oralis of the trigeminal spinal nucleus (Vodm).  

Extracellular single-unit data from the vicinity of the abducens nucleus from rhesus monkeys recorded during horizontal saccades are used as input to an updated oculomotor plant to simulate saccades to test the robustness of the model.  

This band rostromedially continued with the caudal part of the group of internuclear neurons of the abducens nucleus.  

Three groups of EVN somas have been identified within the brainstem; located in the proximity of the vestibular nuclei (I), the facial nerve genu (II) and the abducens nucleus (III).  

Crossing fibers were sectioned at the midline of the medulla caudal to the abducens nucleus in four cynomolgus monkeys.  

They either leave the periaqueductal gray dorsally and pass through the gray matter in the floor of the fourth ventricle towards the abducens nucleus and ventral medulla, or are directed ventrally after passing through either the inferior colliculus or parabrachial nucleus.  

Finally, new information is presented which shows that large numbers of flocculus projecting neurons are located within the medial longitudinal fasciculus at two locations; one just rostral to the hypoglossal nucleus and another group extends 2-3 mm rostral to the abducens nucleus.  

Main findings in our previous studies are as follows: 1) there are three Purkinje cell zones running perpendicular to the long axis of the folia in the cat flocculus, 2) the caudal zone controls activity of the superior rectus (SR) and inferior oblique (IO) extraocular muscles via the y-group and oculomotor nucleus (OMN) neurons, and 3) the middle zone controls activity of the lateral (LR) and medial rectus (MR) muscles via the medial vestibular (MV) and abducens nucleus (ABN) neurons.  

Within the MLF, the tangential axons course posteriorly, forming collaterals that innervate the abducens nucleus, and then proceed to the cervical spinal cord.  

This MRI coincides with the lesion limited to the abducens nucleus and genu of facial nerve. Traditionally, projections from the PPRF to the ipsilateral abducens nucleus and opposite MLF was postulated. Recently, however, experiment on monkey and autopsy cases showed projection from abducens nucleus, instead of PPRF, to the opposite MLF has been proposed.  

Lampreys have a motor nucleus similar to the accessory abducens nucleus previously described only in tetrapods.  

Patients with unilateral gaze palsy had lesions in the paramedian area of the pons, including the abducens nucleus, the lateral part of the nucleus reticularis pontis caudalis and the nucleus reticularis pontis oralis. Patients with abducens nucleus lesions showed additional clinical signs of lateral rectus weakness.  

Some cells in class A and C were antidromically activated from the medial longitudinal fasciculus at the level of abducens nucleus, suggesting that the signals carried by these cells may be sent to the lower brainstem.(ABSTRACT TRUNCATED AT 400 WORDS).  

VOC axons reached the contralateral MLF at the level of the abducens nucleus and immediately divided into an ascending and a descending, usually thicker, branch. Seven VOC axons could be traced to the extraocular motor nuclei; three terminated in the medial aspect of the oculomotor nucleus bilaterally and four terminated in the medial aspect of the contralateral abducens nucleus.  

Both anatomical and physiological studies have shown that pause neurons (PNs) in the medial pontine reticular formation project to two groups of burst neurons (BNs) involved in the genesis of horizontal saccadic eye movements: The excitatory burst neurons (EBNs), which lie rostral to the abducens nucleus, and the inhibitory burst neurons (IBNs), which lie caudal to the abducens.  

The apparent lack of age-related changes in neurons of the abducens nucleus could be a consequence of the high degree of motor activity in the extrinsic eye muscles during both the waking and sleeping states..  

The distribution of abducens nucleus motoneurons and internuclear neurons was determined in the pigeon (Columbia livia) by injecting horseradish peroxidase or fluorochromes into the ipsilateral lateral rectus muscle and/or in the contralateral oculomotor nucleus. Both labelled populations, motoneurons and internuclear neurons, were more numerous at the central part of the abducens nucleus.  

Horseradish peroxidase and the fluorochromes Fast blue and propidium iodide were injected into the lateral rectus and retractor bulbi muscles and/or the oculomotor nucleus of the rabbit to determine the locations and basic morphology of motoneurons and internuclear neurons in the abducens nucleus.  

It is caused by ipsilateral lesion of the caudal part of pontine dorsal tegmentum - medial longitudinal fasciculus and paramedian pontine reticular formation or/and 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. 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. 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. GABA-immunoreactive staining in the abducens nucleus, however, was sparse. 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.  

The abducens nucleus contains two types of neurons which are motor and internuclear neurons. No double-labeled neuron was detected within the abducens nucleus. These results indicate that motor and internuclear neurons in the abducens nucleus have their own projections and no neuron has dual projections..  

The activity of medial vestibular nucleus neurons projecting to the contralateral abducens nucleus (premotor vestibular neurons) has been recorded during spontaneous and vestibular induced eye movements in the alert cat. Recorded neurons were identified by their antidromic activation from the abducens nucleus and by the post-synaptic field potential induced in this nucleus.  

More caudally, numerous neurons were labelled in the contralateral abducens nucleus and the vestibular complex and a few in the nucleus reticularis pontis caudalis. 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.  

The general organization and extension of the dendritic trees depended on the location of the soma within the abducens nucleus.  

They lie interspersed between the fascicles of the medial longitudinal fasciculus (MLF) and paramedian tracts in the caudal pons and medulla, and they also constitute the rostral part of the classical abducens nucleus.  

Intensely-immunostained neuronal cell bodies were further observed in other regions which have been reported to contain neurons sending mossy fibers to the cerebellum; in the dorsal part of the principal sensory trigeminal nucleus, dorsomedial part of the oral subnucleus of the spinal trigeminal nucleus, interpolar subnucleus of the spinal trigeminal nucleus, paratrigeminal nucleus, supragenual nucleus, regions dorsal to the abducens nucleus and genu of the facial nerve, superior and medial vestibular nuclei, cell groups f, x and y, hypoglossal prepositus nucleus, intercalated nucleus, nucleus of Roller, reticular regions intercalated between the motor trigeminal and principal sensory trigeminal nuclei, linear nucleus, and gigantocellular and paramedian reticular formation.  

The retrograde transneuronal labeling of premotor neurons of the abducens nucleus was examined by wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) injections into the lateral rectus muscle in the cat. Native horseradish peroxidase injections into the lateral rectus muscle resulted in labeling of neurons in the ipsilateral abducens nucleus and accessory abducens nucleus only. By contrast, when WGA-HRP injections were made into the lateral rectus muscle, retrogradely labeled cells appeared in the mesencephalic central gray, the oculomotor complex, the pontine reticular nuclei, the medial and lateral vestibular nuclei, and the nucleus prepositus hypoglossi, in addition to strong labeling of cells in the ipsilateral abducens nucleus and the accessory abducens nucleus. The same areas were also labeled by direct injection of a small amount of WGA-HRP into the abducens nucleus.  

The abducens nucleus consists of two distinct subnuclei, one medial in location, the abducens proper, and the other lateral, the abducens accessorius.  

Single neurons in the abducens nucleus were recorded extracellularly in alert rhesus macaques trained to make a variety of eye movements.  

The efferent system is, ipsi- and contralaterally in the brainstem, composed of ventral and dorsal cell groups that extend from the level of the principal abducens nucleus caudally where they overlap with the facial motor nucleus.  

Motoneurons in the principal abducens nucleus of the monitor lizard Varanus exanthematicus were identified by retrograde labeling following application of horseradish peroxidase to the abducens nerve. They may originate from local circuit neurons or from neurons extrinsic to the principal abducens nucleus..  

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. Two or three collaterals arising close to this bifurcation distributed terminals in a relatively wide area in the contralateral abducens nucleus. 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.  

These include the abducens nucleus, the intermediate gray layer of the superior colliculus (SCi), the anterior pretectal nucleus (APN), the ventral lateral geniculate nucleus (LGNv), and regions of the central gray directly bordering the oculomotor nucleus, the interstitial nucleus of Cajal, and the nucleus of Darkschewitsch.  

With colchicine treatment, calcitonin gene-related peptide-like immunoreactive cells were found in more areas of the brain stem such as the abducens nucleus, parabigeminal nucleus, principal oculomotor nucleus, trochlear nucleus and central gray, along with the nuclei which had shown calcitonin gene-related peptide immunoreactivity in the untreated animals.  

The responses of single units in the vestibular nuclei, nucleus praepositus hypoglossi and in the brainstem, deep and posterior to the abducens nucleus, were studied in anaesthetized, paralysed cats. praepositus hypoglossi and 45 in the magnocellular nucleus of the reticular formation posterior and deep to the abducens nucleus.  

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.  

With the use of single-unit recording, the reticular formation immediately caudal to the abducens nucleus was searched for saccadic burst neurons in alert, trained rhesus monkeys. Horseradish peroxidase injected into the abducens nucleus retrogradely labeled cells in the contralateral reticular formation where burst neurons were recorded, showing that cells in this region make crossed monosynaptic connections.  

A fascicle of the ophthalmic nerve leaves the descending tract and, apparently, makes contact with the accessory abducens nucleus.  

Systematic microstimulation in the brainstem during recording of 16 of 19 target neurons of floccular middle zone inhibition revealed that the target neurons projected to the ipsilateral abducens nucleus (ABN), and not to the contralateral ABN nor the oculomotor nucleus.  

The motor output to the lateral rectus eye muscle was studied in decerebrate cats with electromyographic recordings and in alert cats with multi-unit and single neuron recordings from abducens nucleus.  

Intracellular electrophysiological and morphological (horseradish peroxidase) techniques were used to identify neurons just rostral and ventral to the cat abducens nucleus. Cells seen in previous investigations, which responded similarly, were thought to lie within the principal abducens nucleus.  

Stimulation of the accessory abducens nucleus, the abducens nucleus, and the reticular formation at the level of the spinal trigeminal nucleus supported lower, transient levels of conditioning.  

By comparing the afferents to the prepositus with those to the abducens nucleus, we found that all regions projecting to the abducens also projected to the prepositus, without exception.  

In the ipsilateral accessory abducens nucleus, we observed electron-dense degenerating axon terminals that formed asymmetric synaptic contacts with the primary and secondary dendrites of large neurons retrogradely labeled with HRP. No terminal degeneration was found in the contralateral accessory abducens nucleus or in the ipsilateral and contralateral principal abducens nuclei. 1988, 267:370-386), and strongly suggest the existence in Varanus of a monosynaptic, unilateral reflex pathway in which trigeminal fibers, presumably originating from the cornea, synapse with motoneurons of the bursalis and retractor bulbi muscles, which are located in the accessory abducens nucleus.  

The principal abducens nucleus is located just beneath the fourth ventricle laterally adjacent to the medial longitudinal fasciculus and innervates the posterior rectus muscle. The accessory abducens nucleus has a ventrolateral position in the brainstem in close approximation to the ophthalmic fibers of the descending trigeminal tract. The fibers arising in the accessory abducens muscles form a loop in or just beneath the principal abducens nucleus before they join the abducens nerve root.  

Neurons were identified by their antidromic activation from the abducens nucleus. Spikes of these neurons were used to trigger the recording of field potentials in the abducens nucleus. The analysis by post-spike averaging of the field potentials showed the presence of a trifold system of reciprocal (excitatory and inhibitory) direct projections that originated in the above nuclei and terminated in the abducens nucleus with a distinctly graded effectiveness.  

The effects of peripheral and central VIth nerve axotomy on abducens nucleus synaptic potentials of vestibular origin and the ultrastructure of intracellularly labeled abducens motoneurons were examined in the anesthetized cat.  

These interneurones were identified by the criteria that they were fired monosynaptically from both the tectum and the cerebral peduncle and were activated antidromically from the abducens nucleus. Systematic threshold mapping for the antidromic activation in and around the abducens nucleus indicated that they gave off many collateral branches in the nucleus. Such neurones were found in the nucleus reticularis pontis caudalis, being distributed in the area extending 0.8-3 mm rostral to the rostral pole of the abducens nucleus, 1.3-2.7 mm deep from the dorsal surface of the brain stem, and 0.8-1.8 mm lateral from the midline.  

A few neurons in the lateral part of APv were labeled by HRP injections into the abducens nucleus (nVI).  

A class of these identified neurons was antidromically activated from the contralateral excitatory burst neuron (EBN) area immediately rostral to the abducens nucleus.  


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