Synaptic input from the pontine reticular nuclei to absucens motoneurons and internuclear neurons in the cat

Subsets of extraocular motoneurons produce kinematically distinct saccades during hunting and exploration

Animals construct diverse behavioral repertoires by moving a limited number of body parts with varied kinematics and patterns of coordination. There is evidence that distinct movements can be generated by changes in activity dynamics within a common pool of motoneurons or by selectively engaging specific subsets of motoneurons in a task-dependent manner. However, in most cases, we have an incomplete understanding of the patterns of motoneuron activity that generate distinct actions and of how upstream premotor circuits select and assemble such motor programs. In this study, we used two closely related but kinematically distinct types of saccadic eye movement in larval zebrafish as a model to examine circuit control of movement diversity. In contrast to the prevailing view of a final common pathway, we found that in the oculomotor nucleus, distinct subsets of motoneurons were engaged for each saccade type. This type-specific recruitment was topographically organized and aligned with ultrastructural differences in motoneuron morphology and afferent synaptic innervation. Medially located motoneurons were active for both saccade types, and circuit tracing revealed a type-agnostic premotor pathway that appears to control their recruitment. By contrast, a laterally located subset of motoneurons was specifically active for hunting-associated saccades and received premotor input from pretectal hunting command neurons. Our data support a model in which generalist and action-specific premotor pathways engage distinct subsets of motoneurons to elicit varied movements of the same body part that subserve distinct behavioral functions.

VEGF is an essential retrograde trophic factor for motoneurons

VEGF was initially discovered due to its angiogenic activity and therefore named "vascular endothelial growth factor." However, its more recently discovered neurotrophic activity may be evolutionarily more ancient. Our previous work showed that all the changes produced by axotomy on the firing activity and synaptic inputs of abducens motoneurons were completely restored after VEGF administration. Therefore, we hypothesized that the lack of VEGF delivered by retrograde transport from the periphery should also affect the physiology of otherwise intact abducens motoneurons. For VEGF retrograde blockade, we chronically applied a neutralizing VEGF antibody to the lateral rectus muscle. Recordings of extracellular single-unit activity and eye movements were made in alert cats before and after the application of the neutralizing antibody. Our data revealed that intact, noninjured abducens motoneurons retrogradely deprived of VEGF exhibited noticeable changes in their firing pattern. There is a general decrease in firing rate and a significant reduction in eye position and eye velocity sensitivity (i.e., a decrease in the tonic and phasic components of their discharge, respectively). Moreover, by means of confocal immunocytochemistry, motoneurons under VEGF blockade showed a marked reduction in the density of afferent synaptic terminals contacting with their cell bodies. Altogether, the present findings demonstrate that the lack of retrogradely delivered VEGF renders abducens motoneurons into an axotomy-like state. This indicates that VEGF is an essential retrograde factor for motoneuronal synaptic drive and discharge activity.

Extraocular Motoneurons and Neurotrophism

Extraocular motoneurons are located in three brainstem nuclei: the abducens, trochlear and oculomotor. They control all types of eye movements by innervating three pairs of agonistic/antagonistic extraocular muscles. They exhibit a tonic-phasic discharge pattern, demonstrating sensitivity to eye position and sensitivity to eye velocity. According to their innervation pattern, extraocular muscle fibers can be classified as singly innervated muscle fiber (SIF), or the peculiar multiply innervated muscle fiber (MIF). SIF motoneurons show anatomical and physiological differences with MIF motoneurons. The latter are smaller and display lower eye position and velocity sensitivities as compared with SIF motoneurons.

Transmitter and ion channel profiles of neurons in the primate abducens and trochlear nuclei

Extraocular motoneurons initiate dynamically different eye movements, including saccades, smooth pursuit and vestibulo-ocular reflexes. These motoneurons subdivide into two main types based on the structure of the neuro-muscular interface: motoneurons of singly-innervated (SIF), and motoneurons of multiply-innervated muscle fibers (MIF). SIF motoneurons are thought to provoke strong and brief/fast muscle contractions, whereas MIF motoneurons initiate prolonged, slow contractions. While relevant for adequate functionality, transmitter and ion channel profiles associated with the morpho-physiological differences between these motoneuron types, have not been elucidated so far. This prompted us to investigate the expression of voltage-gated potassium, sodium and calcium ion channels (Kv1.1, Kv3.1b, Nav1.6, Cav3.1-3.3, KCC2), the transmitter profiles of their presynaptic terminals (vGlut1 and 2, GlyT2 and GAD) and transmitter receptors (GluR2/3, NMDAR1, GlyR1α) using immunohistochemical analyses of abducens and trochlear motoneurons and of abducens internuclear neurons (INTs) in macaque monkeys. The main findings were: (1) MIF and SIF motoneurons express unique voltage-gated ion channel profiles, respectively, likely accounting for differences in intrinsic membrane properties. (2) Presynaptic glutamatergic synapses utilize vGlut2, but not vGlut1. (3) Trochlear motoneurons receive GABAergic inputs, abducens neurons receive both GABAergic and glycinergic inputs. (4) Synaptic densities differ between MIF and SIF motoneurons, with MIF motoneurons receiving fewer terminals. (5) Glutamatergic receptor subtypes differ between MIF and SIF motoneurons. While NMDAR1 is intensely expressed in INTs, MIF motoneurons lack this receptor subtype entirely. The obtained cell-type-specific transmitter and conductance profiles illuminate the structural substrates responsible for differential contributions of neurons in the abducens and trochlear nuclei to eye movements.

A review on screening tests for vestibular disorders

Although many studies have reported on tests of the vestibular system a valid and reliable, evidence-based screening battery for easy clinical use remains elusive. Many screening tests attempt to assess the vestibulo-ocular reflex. Therefore, head shaking, the Dix-Hallpike maneuver, the supine roll test, and head impulse tests are discussed. Other tests address the spatial orientation functions of the vestibular system, such as the Bucket Test and the Fukuda Stepping test. Still, other tests are based on the known correlates with balance skills, both static and dynamic, such as tandem walking and the modern variation of the Romberg test, the modified Clinical Test of Sensory Interaction and Balance. This review provides a critical overview of the literature on some of these tests and their value for clinical use and in epidemiological studies.

Functional Diversity of Neurotrophin Actions on the Oculomotor System

Neurotrophins play a principal role in neuronal survival and differentiation during development, but also in the maintenance of appropriate adult neuronal circuits and phenotypes. In the oculomotor system, we have demonstrated that neurotrophins are key regulators of developing and adult neuronal properties, but with peculiarities depending on each neurotrophin. For instance, the administration of NGF (nerve growth factor), BDNF (brain-derived neurotrophic factor) or NT-3 (neurotrophin-3) protects neonatal extraocular motoneurons from cell death after axotomy, but only NGF and BDNF prevent the downregulation in ChAT (choline acetyltransferase). In the adult, in vivo recordings of axotomized extraocular motoneurons have demonstrated that the delivery of NGF, BDNF or NT-3 recovers different components of the firing discharge activity of these cells, with some particularities in the case of NGF. All neurotrophins have also synaptotrophic activity, although to different degrees. Accordingly, neurotrophins can restore the axotomy-induced alterations acting selectively on different properties of the motoneuron. In this review, we summarize these evidences and discuss them in the context of other motor systems.

Clinical presentation of pseudo-abducens palsy

PURPOSE

Pseudo-abducens palsy is a neurologic limitation in abduction with an intact abducens nerve. This rare condition can be observed when voluntary eye movements show impairment in lateral gaze, yet the vestibulo-ocular reflex (VOR) demonstrates full abduction. The intact VOR indicates the integrity of the infranuclear abducens nerve, thus suggesting that a pseudo-abducens palsy is likely caused by supranuclear pathology.

CASE REPORT

A 63-year-old African American man presented with sudden-onset, binocular, horizontal diplopia. Extraocular motilities revealed a complete restriction of abduction OS on pursuits with intermittent spasm of the right medial rectus that was most evident in primary and left gazes. No cyclodeviation or vertical deviation was found. Doll's head maneuver was used to stimulate the VOR, which facilitated complete abduction OS, despite its gross limitation on pursuits. The intact VOR and impaired abduction on pursuits, coupled with contralateral esotropia, yielded a diagnosis of pseudo-abducens palsy.

CONCLUSIONS

Although considered to be a rare condition, it is important for the clinician to differentiate pseudo-abducens palsy from a classic abducens infranuclear palsy. Presented is a case displaying the typical features of a pseudo-abducens palsy, in conjunction with contralateral esotropia, which further supports the theory of a dysfunctional supranuclear vergence pathway.

Complementary actions of BDNF and neurotrophin-3 on the firing patterns and synaptic composition of motoneurons

Neurotrophins, as target-derived factors, are essential for neuronal survival during development, but during adulthood, their scope of actions widens to become also mediators of synaptic and morphological plasticity. Target disconnection by axotomy produces an initial synaptic stripping ensued by synaptic rearrangement upon target reinnervation. Using abducens motoneurons of the oculomotor system as a model for axotomy, we report that trophic support by brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) or a mixture of both, delivered to the stump of severed axons, results in either the prevention of synaptic stripping when administered immediately after lesion or in a promotion of reinnervation of afferents to abducens motoneurons once synaptic stripping had occurred, in concert with the recovery of synaptic potentials evoked from the vestibular nerve. Synaptotrophic effects, however, were larger when both neurotrophins were applied together. The axotomy-induced reduction in firing sensitivities related to eye movements were also restored to normal values when BDNF and NT-3 were administered, but discharge characteristics recovered in a complementary manner when only one neurotrophin was used. This is the first report to show selective retrograde trophic dependence of circuit-driven firing properties in vivo indicating that NT-3 restored the phasic firing, whereas BDNF supported the tonic firing of motoneurons during eye movement performance. Therefore, our data report a link between the synaptotrophic actions of neurotrophins, retrogradely delivered, and the alterations of neuronal firing patterns during motor behaviors. These trophic actions could be responsible, in part, for synaptic rearrangements that alter circuit stability and synaptic balance during plastic events of the brain.

Neurobiological mechanisms for the regulation of mammalian sleep-wake behavior: reinterpretation of historical evidence and inclusion of contemporary cellular and molecular evidence

At its most basic level, the function of mammalian sleep can be described as a restorative process of the brain and body; recently, however, progressive research has revealed a host of vital functions to which sleep is essential. Although many excellent reviews on sleep behavior have been published, none have incorporated contemporary studies examining the molecular mechanisms that govern the various stages of sleep. Utilizing a holistic approach, this review is focused on the basic mechanisms involved in the transition from wakefulness, initiation of sleep and the subsequent generation of slow-wave sleep and rapid eye movement (REM) sleep. Additionally, using recent molecular studies and experimental evidence that provides a direct link to sleep as a behavior, we have developed a new model, the cellular-molecular-network model, explaining the mechanisms responsible for regulating REM sleep. By analyzing the fundamental neurobiological mechanisms responsible for the generation and maintenance of sleep-wake behavior in mammals, we intend to provide a broader understanding of our present knowledge in the field of sleep research.

Firing properties of axotomized central nervous system neurons recover after graft reinnervation

Axotomy produces changes in the electrical properties of neurons and in their synaptic inputs, leading to alterations in firing pattern. We have considered the possibility that these changes occur as a result of the target deprivation induced by the lesion. Thus, we have provided a novel target to axotomized central neurons by grafting embryonic tissue at the lesion site to study the target dependence of discharge characteristics. The extracellular single-unit electrical activity of abducens internuclear neurons was recorded in the alert behaving cat in control, after axotomy, and after axotomy plus the implantation of cerebellar primordium. As recently characterized (de la Cruz et al. [2000] J. Comp. Neurol. 427:391-404), firing alterations induced by axotomy included an overall decrease in firing rate and a loss of eye-related signals, i.e., eye position and velocity neuronal sensitivities, that do not resume to normality with time. The grafting of a novel target to the injured abducens internuclear neurons restored the normal firing and sensitivities as recorded in the majority of units. To study the reinnervation of the implant, we performed anterograde labeling with biocytin combined with electron microscopy visualization. Axons of abducens internuclear neurons grew into the transplant sprouting into granule cell and molecular layers, as characterized by the immunostaining for gamma-aminobutyric acid and calbindin D-28k. Ultrastructural examination of labeled axons and boutons revealed the establishment of synaptic contacts, mainly axodendritic, with different cell types of the grafted cerebellar cortex. Therefore, these data indicate that axotomized central neurons resume to normal firing after the reinnervation of a novel target.

Abduction paresis with rostral pontine and/or mesencephalic lesions: Pseudoabducens palsy and its relation to the so-called posterior internuclear ophthalmoplegia of Lutz

BACKGROUND

The existence of a prenuclear abduction paresis is still debated.

METHODS

In a retrospective design, we identified 22 patients with isolated unilateral (n = 20) or bilateral (n = 2) abduction paresis and electrophysiologic abnormalities indicating rostral pontine and/or mesencephalic lesions. Another 11 patients had unilateral abduction paresis with additional ocular motor abnormalities indicating midbrain dysfunction. Eight of these 11 patients also had electrophysiological abnormalities supporting this location. Electrophysiological examinations in all patients included masseter and blink reflexes (MassR, BlinkR), brainstem auditory evoked potentials (BAEP), and direct current elctro-oculography (EOG).

RESULTS

Unilateral MassR abnormalities in patients with unilateral abduction paresis were seen in 17 patients and were almost always (in 16 of 17 patients) on the side of the abduction paresis. Another 11 patients had bilateral MassR abnormalities. BlinkR was always normal. EOG disclosed slowed abduction saccades in the non-paretic eye in 6 patients and slowed saccades to the side opposite to the abduction paresis in another 5 patients. Re-examinations were done in 27 patients showing normalization or improvement of masseter reflex abnormalities in 18 of 20 patients and in all patients with EOG abnormalities. This was always associated with clinical improvement.

CONCLUSIONS

Electrophysiologically documented or clinically evident rostral pontine and/or mesencephalic lesions in our patients exclude an infranuclear intrapontine 6th nerve lesion and indicate the existence of an abduction paresis of prenuclear origin. An increased tone of the antagonistic medial rectus muscle during lateral gaze either by abnormal convergence or impaired medial rectus inhibition seems most likely.

Discharge characteristics of axotomized abducens internuclear neurons in the adult cat

The aim of the present work was to characterize the axotomy-induced changes in the discharge properties of central nervous system neurons recorded in the alert behaving animal. The abducens internuclear neurons of the adult cat were the chosen model. The axons of these neurons course through the contralateral medial longitudinal fascicle and contact the medial rectus motoneurons of the oculomotor nucleus. Axotomy was carried out by the unilateral transection of this fascicle (right side) and produced immediate oculomotor deficits, mainly the incapacity of the right eye to adduct across the midline. Extracellular single-unit recording of abducens neurons was carried out simultaneously with eye movements. The main alteration observed in the firing of these axotomized neurons was the overall decrease in firing rate. During eye fixations, the tonic signal was reduced, and, on occasion, a progressive decay in firing rate was observed. On-directed saccades were not accompanied by the high-frequency spike burst typical of controls; instead, there was a moderate increase in firing. Similarly, during the vestibular nystagmus, neurons hardly modulated during both the slow and the fast phases. Linear regression analysis between firing rate and eye movement parameters showed a significant reduction in eye position and velocity sensitivities with respect to controls, during both spontaneous and vestibularly induced eye movements. These firing alterations were observed during the 3 month period of study after lesion, with no sign of recovery. Conversely, abducens motoneurons showed no significant alteration in their firing pattern. Therefore, axotomy produced long-lasting changes in the discharge characteristics of abducens internuclear neurons that presumably reflected the loss of afferent oculomotor signals. These alterations might be due to the absence of trophic influences derived from the target.

Effects of botulinum neurotoxin type A on abducens motoneurons in the cat: alterations of the discharge pattern

The discharge characteristics that abducens motoneurons exhibit after paralysis of the lateral rectus muscle with botulinum neurotoxin type A were studied in the alert cat. Antidromically identified motoneurons were recorded during both spontaneous and vestibularly induced eye movements. A single injection of 0.3 ng/kg produced a complete paralysis of the lateral rectus muscle lasting for about 12-15 days, whereas after 3 ng/kg the paralysis was still complete at the longest time checked, three months. Motoneurons recorded under the effect of the low dose showed differences in their sensitivities to both eye position and velocity according to the direction of the previous and ongoing movements, respectively. These directional differences could be explained by post-saccadic adaptation of the non-injected eye in the appropriate direction for reducing ocular misalignment. Thus, backward and forward post-saccadic drifts accompanied on- and off-directed saccades, respectively. The magnitude of the drift was similar to the magnitude of changes in eye position sensitivity. The discharge of the high-dose-treated motoneurons could be described in a three-stage sequence. During the initial 10-12 days, motoneuronal discharge resembled the effects of axotomy, particularly in the loss of tonic signals and the presence of exponential-like decay of firing after saccades. In this stage, the conduction velocity of abducens motoneurons was reduced by 21.4%. The second stage was characterized by an overall reduction in firing rate towards a tonic firing at 15-70 spikes/s. Motoneurons remained almost unmodulated for all types of eye movement and thus eye position and velocity sensitivities were significantly reduced. Tonic firing ceased only when the animal became drowsy, but was restored by alerting stimuli. In addition, the inhibition of firing for off-directed saccades was more affected than the burst excitation during on-directed saccades, since in many cells pauses were almost negligible. These alterations could not be explained by adaptational changes in the movement of the non-injected eye. Finally, after 60 days the initial stages of recovery were observed. The present results indicate that the high dose of botulinum neurotoxin produces effects on the motoneuron not attributable to the functional disconnection alone, but to a direct effect of the neurotoxin in the motoneuron and/or its synaptic inputs.

Are laws that govern behavior embedded in the structure of the CNS? The case of Hering's law

The pattern of axonal terminations of individual premotoneuronal medium lead burst neurons (MLBs) has been elucidated with the help of the intraaxonal recording and horseradish peroxidase injection technique in alert behaving monkeys. These findings indicate that individual MLBs do not influence individual muscles; instead they influence groups of muscles. 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. Also, upward MLBs project to motoneurons innervating both the superior rectus and the inferior oblique muscles of both eyes. Finally, downward MLBs project to ipsilateral motoneurons innervating the ipsilateral inferior rectus muscle and the contralateral superior oblique muscle. All in all, the pattern of oculomotor terminations of MLBs provides experimental support for Hering's law of equal innervation. EBNs also project to the nucleus prepositus hypoglossi while vertical MLBs also project to the interstitial nucleus of Cajal (NIC). These structures are thought to participate in the process of "velocity to position integration", in the horizontal and vertical planes respectively. Intracellular recording from individual neurons of the NIC in alert behaving monkeys followed by biocytin or horseradish peroxidase injections demonstrates that their axons pass through the posterior commissure on their way to vertical extraocular motoneurons. It also demonstrates that these fibers carry a phasic signal related to saccades and a tonic signal related to eye position in a precise quantitative manner. Work is in progress to determine whether their pattern of oculomotor terminations is also appropriate for Hering's law.

Histological identification of premotor neurons for horizontal saccades in monkey and man by parvalbumin immunostaining

The premotor excitatory and inhibitory burst neurons are essential for horizontal saccades. In the monkey, excitatory burst neurons lie in the ipsilateral paramedian pontine reticular formation, and the inhibitory burst neurons lie more caudally in the contralateral nucleus paragigantocellularis dorsalis. For a neuropathological analysis of degenerative changes in saccadic disorders of patients, the histological identification of the burst neuron areas in man is important. Here, we show that this is possible with parvalbumin immunostaining as a histological marker. 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. Then, human brainstem sections were immunoreacted for parvalbumin, and, by comparing the resulting staining pattern to that in the monkey, the homologous burst neuron areas were defined in man. In the monkey, excitatory burst neurons were confirmed to the nucleus reticularis pontis caudalis and did not extend farther rostrally into the nucleus reticularis pontis oralis. All retrogradely labeled cells in both burst neuron areas were parvalbumin positive, and approximately 70% of the parvalbumin-positive cells were retrogradely labeled. Both burst neuron areas were highlighted by their parvalbumin staining pattern and could be outlined in man as well. 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. The location of the burst neuron areas, including the burst neurons themselves, via parvalbumin immunostaining will help in the analysis of clinical cases with slow saccades.

Effects of target depletion on adult mammalian central neurons: functional correlates

The physiological signals and patterns of synaptic connectivity that CNS neurons display after the loss of their target cells were evaluated in adult cats for one year. Abducens internuclear neurons were chosen as the experimental model because of their highly specific projection onto the medial rectus motoneurons of the oculomotor nucleus. Selective death of medial rectus motoneurons was induced by the injection into the medial rectus muscle of ricin, a potent cytotoxic lectin that leaves the presynaptic axons intact. The electrical activity of antidromically identified abducens internuclear neurons was recorded in chronic alert animals, during both spontaneous and vestibularly induced eye movements, before and after target removal. During the three weeks that followed ricin injection, abducens internuclear neurons exhibited several firing-related abnormal properties. There was an overall reduction in firing rate with a corresponding increase in the eye position threshold for recruitment. In addition, neuronal sensitivities to eye position and velocity were significantly decreased with respect to control data. Bursting activity was also altered since low-frequency delayed burst accompanied the saccades in the on-direction and, occasionally, internuclear neurons exhibited low-frequency discharges associated with off-directed saccades. Intracellular recordings carried out seven and 15 days after ricin injection demonstrated no significant changes in their electrical properties, although a marked depression of synaptic transmission was evident. The amplitude of both excitatory and inhibitory postsynaptic potentials of vestibular origin was reduced by 60-85% with respect to controls. However, postsynaptic potentials recorded one month after ricin injection showed normal amplitude values which persisted unaltered one year after target loss. Recovery of synaptic transmission occurred at the same time as the re-establishment of normal eye-related signals in the discharge pattern of abducens internuclear neurons recorded in alert cats from days 25-30 post lesion. The functional restoration of firing properties was maintained in the long term (one year). Conversely, abducens motoneurons showed normal firing and synaptic patterns at all time intervals analysed. These results demonstrate that, after an initial period of altered physiological properties, abducens internuclear neurons survive the loss of their target motoneurons and regain a normal discharge pattern and afferent synaptic connections.

Effects of target depletion on adult mammalian central neurons: morphological correlates

The morphological sequelae induced by target removal were studied on adult cat abducens internuclear neurons at both the somata and terminal axon arborization levels. The neuronal target--the medial rectus motoneurons of the oculomotor nucleus--was selectively destroyed by the injection of toxic ricin into the medial rectus muscle. Retrograde labeling with horseradish peroxidase demonstrated the survival of the entire population of abducens internuclear neurons up to one year after target removal. However, soma size was reduced by about 20% three months postlesion and maintained for one year. At the ultrastructural level, a considerable deafferentation of abducens internuclear neurons was observed at short intervals (i.e. 10 days after lesion). Large regions of the plasmalemma appeared devoid of presynaptic boutons but were covered instead by glial processes. The detachment of synaptic endings was selective on abducens internuclear neurons since nearby motoneurons always showed a normal synaptic coverage. By one month, abducens internuclear neurons recovered a normal density of receiving axosomatic synapses. Anterogradely biocytin-labeled axon terminals of abducens internuclear neurons remained in place after the lesion of medial rectus motoneurons, although with a progressive decrease in density. Ultrastructural examination of the oculomotor nucleus 10 days after the lesion revealed numerous empty spaces left by the dead motoneurons. Targetless boutons were observed surrounded by large extracellular gaps, still apposed to remnants of the postsynaptic membrane or, finally, ensheathed by glial processes. 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. Labeled boutons were never found contacting with the oculomotor internuclear neurons either in control tissue or at short periods after ricin injection. These results indicate that the availability of undamaged neurons close to the lost target motoneurons might support the long-term survival of abducens internuclear neurons. Specifically, the oculomotor internuclear neurons, which likely suffer a partial deafferentation after medial rectus motoneuron loss, constitute a potential new target for the abducens internuclear neurons. The reinnervation of a new target might explain the recovery of synaptic and firing properties of abducens internuclear neurons after medial rectus motoneuron lesion, which occurred with a similar time course, as described in the accompanying paper [de la Cruz R. R. et al. (1994) Neuroscience 58, 81-97.].

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.

Abnormalities of horizontal gaze. Clinical, oculographic and magnetic resonance imaging findings. I. Abducens palsy

Fifty one patients with abnormalities of horizontal gaze were studied with magnetic imaging of the brain (MRI) and eye movement recordings to identify the loci of lesions responsible for isolated abducens palsy, conjugate gaze palsy and different types of internuclear ophthalmoplegias. The lesions responsible for a particular disorder were identified by overlapping enlarged drawings of the individual scans at comparable brain-stem levels and identifying the areas where the abnormal MRI signals intersected. A statistical procedure was devised to exclude the possibility that the areas of overlap occurred by chance. In this paper, the findings in the group of patients with VI nerve palsy are reported since the location of their lesions could be predicted from known anatomy, so validating the procedure. The results were independently obtained with the overlapping technique and the statistical procedure and showed that the lesions were located in a region corresponding to the posterior part of the abducens fasciculus. This confirms that central lesions producing isolated lateral rectus weakness spare the abducens nuclei. The agreement between the procedures used and earlier clinical and experimental results suggest that the method we describe can be applied to locate the site of lesions on MRI scans in other groups of patients with more complex gaze disorders.

Abnormalities of horizontal gaze. Clinical, oculographic and magnetic resonance imaging findings. II. Gaze palsy and internuclear ophthalmoplegia

The site of lesions responsible for horizontal gaze palsy and various types of internuclear ophthalmoplegia (INO) was established by identifying the common areas where the abnormal MRI signals from patients with a given ocular-motor disorder overlapped. 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. Lesions responsible for bilateral gaze palsy involved the pontine tegmental raphe. Since this region contains the saccadic omnipause neurons, this finding suggests that damage to omnipause cells produces slowing of saccades rather than opsoclonus, as previously proposed. All INOs, regardless of the presence of impaired abduction or convergence, had similar MRI appearances. Frequently the lesions in patients with INO, were not confined to the medial longitudinal fasciculus (MLF) but also involved neighbouring structures at the pontine and mid-brain levels. There was a statistically significant association between the clinical severity of the INO and the presence of abnormal abduction or convergence. The findings suggest that the lesions outside the MLF, which may affect abducens, gaze or convergence pathways, are responsible for the presence of features additional to INO, depending on the magnitude of functional disruption they produce.

Subcortical connections of frontal 'oculomotor' areas in the cat

The subcortical connections of the frontal 'oculomotor' areas in the medial wall of the hemisphere under the cruciate sulcus (CRUo), the medial (PREo-m) and the lateral banks (PREo-l) of the presylvian sulcus of cats were investigated using WGA-HRP tracing combined with electrophysiological techniques. Two main modes were identified; one was the common connections to the same targets in the cortico-thalamo-cortical, cerebro-cerebellar and cortico-tectal pathways, and the other was the individual connections to unique portions of the saccade-generating centers in the brainstem. The common reciprocal connections were found in the ventral anterior-ventral lateral complex, principal ventromedial nucleus, rostral intralaminar nuclei, centromedian-parafascicular complex, lateral posterior nucleus, and suprageniculate nucleus. The common efferent projections were in the subthalamic nucleus, lateral habenular nucleus, pretectal nucleus, posterior commisure nucleus, nucleus of Darkschewitsch, pontine nucleus, nucleus reticularis tegmenti pontis, medial accessory inferior olive, and the superior colliculus. The CRUo projected into the ipsilateral field of Forel and paramedial pontine reticular formation (PPRF), the PREo-l projected into the contralateral dorsomedial medullary reticular formation, and the PREo-m projected into the ipsilateral medullary reticular formation and there was only a small degree of projection into the central portion between the abducens nerve rootlets.

Descending projections from the gigantocellular tegmental field in the cat: cells of origin and their brainstem and spinal cord trajectories

The trajectories and the cells of origin of the pontobulbar gigantocellular tegmental field descending pathways were studied in the cat using anterograde WGA-HRP and retrograde HRP techniques. Four main descending pathways and cells of origin were delineated: (1) Predominantly large neurons in the pontine gigantocellular tegmental field (average soma diameter = 43.4 microns) and rostral bulbar gigantocellular tegmental field (41.3 microns) gave rise to reticulospinal fibers descending in the ipsilateral medial longitudinal fasciculus and ventral funiculus and distributed in laminae V-X with an ipsilateral predominance. These were primarily large-diameter fibers. (2) Predominantly large neurons (46.9 microns) in the bulbar gigantocellular tegmental field gave rise to reticulospinal fibers descending in the contralateral medial longitudinal fasciculus and ventral funiculus. These were mainly large-diameter fibers. (3) Neurons of predominantly medium size (29.5 microns) in the pontine gigantocellular tegmental field gave rise to reticuloreticular fibers descending directly to and distributed bilaterally in the bulbar reticular formation. These were small-diameter fibers. (4) Neurons of predominantly medium size (28.9 microns) in the bulbar gigantocellular tegmental field gave rise to reticulospinal fibers descending in the ipsilateral reticular formation and lateral funiculus. These were small-diameter fibers.

Morphological and electrophysiological identification of gigantocellular tegmental field neurons with descending projections in the cat: I. Pons

Two different descending projections from the pontine gigantocellular tegmental field (PFTG) were defined by the use of intracellular recording and intracellular horseradish peroxidase (HRP) techniques in the cat. Type I neurons (reticulospinal neurons) had antidromic spike potentials produced by stimulation of the ipsilateral medial longitudinal fasciculus (MLF) and sent axons to the ipsilateral MLF. Most type I neurons had large ellipsoidpolygonal somata (mean, 59.7 microns), thick axons (average diameter, 3.33 microns), and slightly oblate large dendritic fields. The mean anteroposterior extent of the dendritic field was 1,492 microns, the mean mediolateral extent was 1,784 microns, and the mean dorsoventral extent was 1,562 microns. There were no type I neurons with axon collaterals. In contrast, type II neurons (reticuloreticular neurons) had antidromic spike potentials produced by stimulation of the bulbar reticular formation (BRF) and sent axons directly to the BRF. In comparison with type I neurons, most type II neurons had smaller ellipsoidpolygonal somata (mean, 40.2 microns), thinner axons (average diameter, 2.32 microns), and smaller, slightly oblate dendritic fields. The mean anteroposterior extent of the dendritic field was 1,264 microns; the mean mediolateral extent was 1,511 microns; and the mean dorsoventral extent was 1,226 microns. Also in contrast to type I neurons, 36% of type II neurons had axon collaterals.

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.

Morphology and physiology of abducens motoneurons and internuclear neurons intracellularly injected with horseradish peroxidase in alert squirrel monkeys

Axons of abducens motoneurons and internuclear neurons were penetrated with HRP-filled glass microelectrodes in alert squirrel monkeys. The firing rate of these axons and spontaneous eye movements were recorded and the axons were then injected with HRP for subsequent visualization of the recorded cells. Soma-dendritic and axon and axonal terminal morphology were studied for possible correlation with firing frequency. The physiology of squirrel monkey abducens neurons is qualitatively similar to their counterparts in the rhesus monkey and the cat, being primarily correlated with the position and velocity of the eyes. The locations of moto- and internuclear neurons are similar in the squirrel monkey and cat as are the axonal projections and terminals. However, squirrel monkey abducens cells are smaller than their feline counterparts and have dendrites that are confined to the cellular borders of the abducens nucleus. The size of the soma and proximal dendrites of moto- and internuclear neurons are poorly correlated with either their threshold for recruitment or their tonic eye position sensitivity. However, cells with smaller dendritic trees tended to have higher saccadic eye velocity sensitivity than those with larger trees. Three types of internuclear neurons were distinguishable upon the basis of their axon collaterals. All cells terminated within the medial rectus subdivision of the oculomotor nucleus. One class of cells did not give rise to collaterals before projecting to the oculomotor nucleus and the other classes gave rise to collaterals that terminated in the intermediate and/or caudal interstitial nuclei of the median longitudinal fasciculus. Within the IIIrd nucleus internuclear terminations were usually confined to a single subgroup of medial rectus motoneurons.

Paresis of lateral gaze alternating with so-called posterior internuclear ophthalmoplegia. A partial paramedian pontine reticular formation-abducens nucleus syndrome

A patient with multiple myeloma developed gaze paresis to the left with slowed saccades and gaze-paretic nystagmus, which alternated with abduction palsy in the left eye (with preserved oculocephalic deviation) and dissociated adducting nystagmus in the right eye, suggesting so-called posterior internuclear ophthalmoplegia. At autopsy multiple small infarcts were found with partial destruction of the left paramedian pontine reticula formation (PPRF) extending towards the abducens nucleus, which was involved only in its inferior pole. The medial longitudinal fasciculus and other oculomotor structures were spared. It is suggested that slowing of all ipsilateral saccades with gaze-paretic nystagmus corresponded to partial destruction of the PPRF, and that intermitted abduction palsy in the ipsilateral eye with adduction nystagmus in the fellow eye was due to intermittant dysfunction of the abducens nucleus. Involvement of voluntary saccades, pursuit movements and vestibulo-ocular responses may be dissociated in partial lesions of the abducens nucleus.

Comparison of the morphology of physiologically identified abducens motor and internuclear neurons in the cat: a light microscopic study employing the intracellular injection of horseradish peroxidase

Abducens motoneurons and internuclear neurons were identified electrophysiologically in anesthesized, paralyzed cats and stained by intracellular injection of horseradish peroxidase. Neurons were reconstructed and surface area of selected cells measured by light microscopy. Surface area of motoneurons and internuclear neuron with similar soma size and shape were roughly comparable. Dendrites of motoneurons were highly tapered and highly branched. By contrast, dendrites of internuclear neurons were less tapered and less branched. Axons of motoneurons had no collaterals within the brainstem. Internuclear axons crossed the midline at the level of their parent somata and ascended in the medial longitudinal fasciculus toward the oculomotor nucleus. Approximately 30% of the internuclear axons branched in the contralateral medial longitudinal fasciculus sending a fine collateral caudal toward the prepositus hypoglossi nucleus. The results suggest that, on the average, structural correlates of injected neurons (i.e., soma-dendritic morphology) can account at least in part for the earlier firing and higher intraburst frequencies of internuclear neurons versus motoneurons during on-direction rapid eye movements in alert cats.

Efferent projections of the neonatal superior colliculus: extraoculomotor-related brain stem structures

The development of eye movements is a prolonged process which presumably involves the efferents of the superior colliculus. In the present study we sought to determine which, if any, of the colliculus efferents that influence eye movements in adult cats were present in neonatal kittens. The autoradiographic and orthograde horseradish peroxidase tracing methods were employed in kittens ranging from 6 h to 5 weeks of age and in adult cats. Surprisingly, most of the known projections from the superior colliculus which are believed to be involved in eye movements were already present in the youngest animals studied. These included projections to (a) the ventral central gray matter overlying the oculomotor nucleus, and (b) those portions of the pontine and medullary reticular formation which provide excitatory and inhibitory inputs to abducens neurons. Apparently, the pathways over which the superior colliculus influences eye movements are elaborated quite early in life. However, in the predorsal bundle and pontomedullary reticular areas the density of transported label was less in 1-day-old kittens than in older animals. Thus, anatomical as well as functional development of portions of this circuitry appear to require a significant period of postnatal maturation.

Cat medial pontine reticular neurons related to vestibular nystagmus: firing pattern, location and projection

In alert cats, extracellular spikes of neurons in the medial pontine tegmentum were recorded simultaneously with whole nerve discharges of the abducens and medial rectus nerves during horizontal vestibular nystagmus. Nystagmus-related neurons were classified by their firing patterns in relation to the abrupt cessation of the slow phase nerve activity of abducens or medial rectus nerves. The ipsilateral abducens nucleus was electrically stimulated to examine the axonal projections of physiologically identified examples of each category of neurons. Anatomically, pause units clustered near the midline at the rostral pole of the abducens nucleus. Long- and medium-lead burst units were 1-4 mm rostral to the area for pause units. Most burst-tonic units, clearly distinguished from nearby axons of passage, were found close to the MLF. Physiologically, it was concluded that: (1) some long-lead burst units terminate in the abducens nucleus and may excite motoneurons and/or internuclear neurons; (2) pause units directly inhibit burst inhibitory neurons which terminate slow phase activities of contralateral abducens motoneurons; (3) burst-tonic units fire in a manner very similar to contralateral abducens motoneurons; and (4) some medium-lead burst, long-lead burst and burst-tonic neurons (but not pause neurons) project to the cerebellar flocculus.

Development of the brain stem in the rat. II. Thymidine-radiographic study of the time of origin of neurons of the upper medulla, excluding the vestibular and auditory nuclei

Groups of pregnant rats were injected with two successive daily doses of 3H-thymidine from gestational days 12 and 13 (E12 + 13) until the day before birth (E21 + 22). In radiographs from adult progeny of these rats the proportion of neurons generated on specific days was determined in the major nuclei of the upper medulla, with the exception of the vestibular and auditory nuclei. The neurons of the motor nuclei are generated over a brief period. Neurons of the retrofacial nucleus are produced first, with more than 60% of the cells arising on day E11 or earlier. Peak generation time of abducens neurons is day E12 and of the neurons of the facial nucleus is day E13. In contrast, the neurons of the superior salivatory nucleus are produced late, predominantly on day E15 and some on day E16. The generation of the (sensory relay) neurons of the nucleus oralis of the trigeminal complex takes place over an extended period between days E12 and E15; the last generated cells include the largest neurons of this nucleus. Neurons of the raphe magnus are produced between days E11 and E14, the neurons of the rostral medullary reticular formation between days E12 and E15. The latest generated neurons of the upper medulla (excluding the cochlear nuclei) belong to a structure identified as the granular layer of the raphe. Combining these results with those of the preceding paper (Altman and Bayer, '80a) and with additional data, it is postulated that the laterally and ventrally situated motor nucleus of the trigeminal, the facial nucleus, and the nucleus ambiguous form a single longitudinal zone of branchial motor neurons with a rostral-to-caudal cytogenetic gradient. In contrast, the medially and dorsally situated (juxtaventricular) hypoglossal nucleus and abducens nucleus (together with the other nuclei of the ocular muscles) form a longitudinal somatic motor zone with a caudal-to-rostral gradient. The dorsal nucleus of the vagus and the superior salivatory nucleus may constitute a preganglionic motor zone, also with a caudal-to-rostral cytogenetic gradient.

Absence of REM and altered NREM sleep in patients with spinocerebellar degeneration and slow saccades

The pontine tegmentum contains the neurons responsible for generation of saccadic eye movements and certain phases of sleep. We studied two genetically unrelated patients with spinocerebellar degeneration and slow saccadic eye movements. Multiple all-night sleep studies in both patients disclosed absence of REM and stage 4 sleep with an extremely short stage 3 and long stage 2. Both patients had a sleep stage (X) not previously reported. These are the first awake and ambulatory humans in whom consistent absence of REM sleep has been demonstrated. Both behaved appropriately during wakefulness and showed no overt psychological abnormalities.

Superior colliculus connections with the extraocular motor nuclei in the cat

Direct and indirect projections from the cat superior colliculus to the extraocular motor nuclei were studied using the orthograde autoradiographic tracing method, the retrograde horseradish peroxidase technique, and Golgi methods. The results show that the superior colliculus projects to the central gray matter directly overlying the oculomotor complex. This projection arises almost entirely from the rostral third of the colliculus, and it terminates most heavily over the rostral half of the oculomotor complex. Dendrites of oculomotor cells extend into this tectal termination zone, making direct tecto-oculomotor contacts possible. Central gray cells within this termination zone project bilaterally to the abducens nuclei. It is proposed that the superior colliculus projection to the supraoculomotor central gray matter and the projection from the central gray matter to the abducens nuclei play a role in convergent eye movements. The superior colliculus projects lightly to a cell group directly ventrolateral to the trochlear nucleus. The superior colliculus sends a small direct projection to the contralateral abducens nucleus and a substantial projection to wide regions of the reticular formation that have been shown previously to project, in turn, to the abducens nucleus. Colliculus cells projecting to the abducens nucleus and adjacent reticular formation are located only in the caudal three-fourths of the colliculus, where they become increasingly concentrated at successively more caudal levels. It is proposed that the graded density of the cells of origin of this projection is the basic structural mechanism by which the colliculus generates horizontal foveating saccades of different amplitudes. Laminar analysis of the origin of all the superior colliculus projections to the extraocular motor regions described here revealed that they arise mostly from the stratum griseum intermedium.