Anatomical and physiological characteristics of vestibular neurons mediating the horizontal vestibulo-ocular reflex of the squirrel monkey

The anatomical characteristics of vestibular neurons, which are involved in controlling the horizontal vestibulo-ocular reflex, were studied by injecting horseradish peroxidase (HRP) into neurons whose response during spontaneous eye movements had been characterized in alert squirrel monkeys. Most of the vestibular neurons injected with HRP that had axons projecting to the abducens nucleus or the medial rectus subdivision of the oculomotor nucleus had discharge rates related to eye position and eye velocity. Three morphological types of cells were injected whose firing rates were related to horizontal eye movements. Two of the cell types were located in the ventral lateral vestibular nucleus and the ventral part of the medial vestibular nucleus (MV). These vestibular neurons could be activated at monosynaptic latencies following electrical stimulation of the vestibular nerve; increased their firing rate when the eye moved in the direction contralateral to the soma; had tonic firing rates that increased when the eye was held in contralateral positions; and had a pause in their firing rate during saccadic eye movements in the ipsilateral or vertical directions. Eleven of the above cells had axons that arborized exclusively on the contralateral side of the brainstem, terminating in the contralateral abducens nucleus, the dorsal paramedian pontine reticular formation, the prepositus nucleus, medial vestibular nucleus, dorsal medullary reticular formation, caudal interstitial nucleus of the medial longitudinal fasciculus, and raphé obscurus. Eight of the cells had axons that projected rostrally in the ascending tract of Deiters and arborized exclusively on the ipsilateral side of the brainstem, terminating in the ipsilateral medial rectus subdivision of the oculomotor nucleus and, in some cases, the dorsal paramedian pontine reticular formation or the caudal interstitial nucleus of the medial longitudinal fasciculus. Two MV neurons were injected that had discharge rates related to ipsilateral eye position, generated bursts of spikes during saccades in the ipsilateral direction, and paused during saccades in the contralateral direction. The axons of those cells arborized ipsilaterally, and terminated in the ipsilateral abducens nucleus, MV, prepositus nucleus, and the dorsal medullary reticular formation. The morphology of vestibular neurons that projected to the abducens nucleus whose discharge rate was not related to eye movements, or was related primarily to vertical eye movements, is also briefly presented.     

Mesodiencephalic projections to the vestibular complex in the cat

The distribution of cells in the rostral medial mesencephalon and caudal diencephalon which project to the vestibular complex was mapped in the cat by using retrograde axonal transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Subsequent experiments using anterograde transport of WGA-HRP clarified the position of the terminations of the mesodiencephalic-derived afferents in the vestibular complex. After large injections which involved the entire vestibular complex, retrogradely labeled cells were seen in both the ipsilateral and contralateral interstitial nucleus of Cajal (INC) and were more numerous in its rostral pole. Labeled cells also occurred in the perifascicular region, both immediately adjacent to the fasciculus retroflexus and rostroventral to it. Fusiform midline cells of the Edinger-Westphal nucleus were also labeled, as well as a number of cells in the adjacent somatic portion of the oculomotor complex (OMC). Another group of labeled cells was observed within the contralateral medial terminal nucleus of the accessory optic tract (MTN) and in the posterior hypothalamic nucleus. Injections limited to subregions of the vestibular complex resulted in similar but slightly varying distributions and numbers of retrogradely labeled cells. After injections covering the caudal half of the medial vestibular nucleus (MVN) and descending vestibular nucleus (DVN), labeled cells in the INC and tegmentum dorsal to it were especially prominent, but none was seen in the MTN or OMC. Injections placed in the rostral MVN, lateral vestibular nucleus, y group, and superior vestibular nucleus resulted in a distribution of labeled cells similar to that seen following global vestibular injections, but these cells were fewer in number. After an injection confined to the y group, a small number of retrogradely labeled cells were seen in the rostral pole of the INC and immediately ventral to the fasciculus retroflexus. Projections from the rostral medial mesencephalon and caudal diencephalon to the MVN, DVN, and y group were confirmed by using anterograde transport of WGA-HRP. Direct projections from the INC-perifascicular regions and somatic neurons of the OMC to the caudal vestibular complex could play a role in eye-head coordination. Those projections from the rostral INC and MTN to the rostral vestibular complex may play a role in vertical eye movements and responses to visual stimuli which move in the vertical plane.     

Anatomical connections of the prepositus and abducens nuclei in the squirrel monkey

The primary goal of this investigation was to identify the areas of the brainstem and cerebellum that provide afferent projections to the nucleus prepositus hypoglossi in primates. After horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP) was injected into the prepositus in squirrel monkeys (Saimiri sciureus), the largest populations of retrogradely labeled neurons were found in the vestibular nuclei, the contralateral perihypoglossal nuclei, and the medullary and pontine reticular formation. Unlike the cat, the prepositus in Saimiri received substantial projections from the nucleus raphe dorsalis and the central mesencephalic reticular formation, whereas few or no labeled cells were found in the cerebellar cortex, the superior colliculus, or the nucleus reticularis tegmenti pontis. 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. Anterogradely transported WGA-HRP showed that the major brainstem recipients of prepositus efferents were the vestibular and perihypoglossal nuclei, the inferior olive, the medullary reticular formation, and the extraocular motor nuclei. In the cerebellar cortex, the prepositus projected to restricted regions of crura I and II as well as the caudal vermis and vestibulocerebellum. The many parts of the oculomotor system receiving input from the prepositus and the parallel innervation of the prepositus and the abducens by a large number of premotor centers lend support to the hypothesis that the prepositus may distribute an efference copy of motor activity, and may also play an important role in the process of neural integration.     

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.     

Afferents to the flocculus of the cerebellum in the rhesus macaque as revealed by retrograde transport of horseradish peroxidase

To investigate the afferent projections to the flocculus in a nonhuman primate, we injected horseradish peroxidase into one flocculus of six rhesus macaques (Macaca mulatta) and processed their brains according to the tetramethylbenzidine protocol to reveal retrogradely labeled neurons. Labeled neurons were found in a large set of nuclei within the rostral medulla and the pons. The greatest numbers of labeled neurons were in the vestibular complex and the nucleus prepositus hypoglossi. There were neurons labeled bilaterally throughout all the vestibular nuclei except the lateral vestibular nucleus, but most of the labeled neurons were in the caudal parts of the medial and inferior vestibular nuclei and in the central part of the superior vestibular nucleus; the nucleus prepositus was also labeled bilaterally, primarily caudally. Modest numbers of labeled neurons were found in the y-group, most ipsilaterally, and many neurons were labeled in the interstitial nucleus of the vestibular nerve. No labeled neurons were found in the vestibular ganglion following a large injection into the flocculus. A second large source of afferents to the flocculus was the medial, paramedial, and raphe reticular formation. Dense aggregates of labeled neurons were located in several pararaphe nuclei of the rostral medulla and the rostral pons and in the nucleus reticularis paramedianus of the medulla and several component nuclei of the nucleus reticularis tegmenti pontis bilaterally. Several groups of cells within and abutting upon the medial and rostral aspects of the abducens nucleus were labeled bilaterally. There was a modest projection from two parts of the pontine nuclei. Both a dorsal midline nucleus ventral to the nucleus reticularis tegmenti pontis and a collection of nuclei in a laminar region adjacent to the contralateral middle cerebellar peduncle contained labeled neurons whose numbers, while modest, were large compared to the projections to the flocculus in other animals. This generic difference may be due to the greater development of the smooth pursuit system in monkeys and the consequent need for a more substantial input from the cerebral cortex. As in other genera, the inferior olive projected to the flocculus via the dorsal cap of Kooy and the contiguous ventrolateral outgrowth. The projection was completely crossed and large injections labeled virtually every neuron in the dorsal cap, suggesting that the dorsal cap is the principal source of climbing fiber afferents.(ABSTRACT TRUNCATED AT 400 WORDS)     

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

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

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.     

Morphological identification of nitric oxide sources and targets in the cat oculomotor system.

Nitric oxide (NO) production by specific neurons in the prepositus hypoglossi (PH) nucleus is necessary for the correct performance of eye movements in alert cats. In an attempt to characterize the morphological substrate of this NO function, the distribution of nitrergic neurons and NO-responding neurons has been investigated in different brainstem structures related to eye movements. Nitrergic neurons were stained by either immunohistochemistry for NO synthase I or histochemistry for reduced nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase. The NO targets were identified by cyclic guanosine monophosphate (cGMP) immunohistochemistry in animals treated with a NO donor immediately before fixation of the brain. Connectivity between cells of the NO-cGMP pathway was analyzed by injections of the retrograde tracers horseradish peroxidase or fast blue in different structures. 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. The PH nucleus contained the largest number of nitrergic cell bodies and a rich neuropil, distributed in two groups in medial and lateral positions in the caudal part, and one central group in the rostral part of the nucleus. An abundant cGMP positive neuropil was the only NO-sensitive element in the PH nucleus, where no cGMP-producing neuronal cell bodies were observed. The opposite disposition was found in the marginal zone between the PH and the medial vestibular nuclei, with a large number of NO-sensitive cGMP-producing neurons and almost no nitrergic cells. Both nitrergic and NO-sensitive cell bodies were found in the medial and inferior vestibular nuclei and in the superior colliculus, whereas the lateral geniculate nucleus contained nitrergic neuropil and a large number of NO-sensitive cell bodies. Some of the cGMP-positive neurons in the marginal zone and medial vestibular nucleus projected to the PH nucleus, predominantly to the ipsilateral side. These morphological findings may help to explain the mechanism of action of NO in the oculomotor system.     

Immunocytochemistry of oculomotor afferents in the squirrel monkey (Saimiri sciureus).

Attempts were made to co-define afferents of the oculomotor nuclear complex (OMC) and their putative neurotransmitters in the squirrel monkey. Wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) and wheat germ agglutinin conjugated to enzymatically inactive HRP and coupled to colloidal gold (WGAapoHRP-AU) were used as retrograde tracers in combination with immunocytochemical methods. Primarily unilateral injections were made into portions of the OMC. Stabilized tetramethylbenzidine (TMB) and silver enhanced sections were immunoreacted with antisera for choline acetyltransferase (ChAT), glutamate (GLU), aspartate (ASP), aminobutyric acid (GABA), serotonin (5-HT) and cholecystokinin (CCK). Moderate numbers of ChAT-IR neurons in caudal regions of the medial vestibular nuclei (MVN) projected to the OMC. Tracer labeled ChAT-IR cells in the MVN projected ipsilaterally to the ventral nucleus (medial rectus subdivision) of the OMC and bilaterally with contralateral dominance to other OMC subdivisions. Cholinergic neurons in the dorsal paragigantocellular reticular nucleus (DPG) projected bilaterally to each half of the OMC. 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. Abducens motor neurons were ChAT-IR, but abducens internuclear neurons were not. Cells in caudal parts of the nucleus prepositus (NPP) projecting to the ipsilateral ventral nucleus of the OMC were not ChAT-positive; ChAT-IR cells in rostral NPP did not project to the OMC. Unilateral OMC injections labeled cells ipsilaterally in the RiMLF, contralaterally in the pretectal olivary nucleus, the interstitial nucleus of Cajal and the infracerebellar nucleus and bilaterally in the superior vestibular nucleus, none of which were ChAT-IR. A small number of cells in the locus ceruleus projected ipsilaterally to the OMC. Although large numbers of vestibular neurons were GLU-IR and ASP-IR, only a few tracer labeled ASP-IR neurons in the contralateral MVN projected to the OMC. No other GLU- or ASP-positive neurons were immunoreactive for GABA, 5-HT or CCK, but cells of the lateral vestibular nucleus were surrounded by CCK-IR fibers and terminals.     

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

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

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

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

Internuclear neurons in the ocular motor system of frogs.

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

Central oculomotor circuits

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

Premotor circuits controlling eyelid movements in conjunction with vertical saccades in the cat: I. The rostral interstitial nucleus of the medial longitudinal fasciculus

Saccadic eye movements in the vertical plane are controlled by the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) and the interstitial nucleus of Cajal. Eye movements in the vertical direction are accompanied by concurrent upper eye lid movements. These gaze-related lid movements are produced by the levator palpebrae superioris muscle, whose motoneurons are located in the caudal central subdivision (CCS) of the oculomotor nucleus. The neural circuits that direct such gaze-related lid movements were examined by use of both conventional and dual neuronal tracing methods in the cat. Injections of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) into the area of the CCS revealed a distinctive subset of retrogradely labeled neurons located in the caudomedial portion of the riMLF. This subset of riMLF neurons was not labeled when injections were localized within the oculomotor nucleus proper, without involving the CCS. Injections of biotinylated dextran amine (BDA) that included this caudomedial riMLF region anterogradely labeled axons that projected profusely throughout the CCS. Labeled terminals were seen in close association with retrogradely labeled levator palpebrae motoneurons, which were primarily found contralateral to WGA-HRP muscle injections. Ultrastructural examination revealed that most BDA-labeled terminals contained clear spherical vesicles and formed asymmetrical synaptic contacts, primarily on the proximal dendrites of WGA-HRP-labeled motoneurons. A few had pleiomorphic vesicles. In summary, these results strongly suggest that the caudomedial part of the cat riMLF is a premotor center that monosynaptically controls lid movements in conjunction with vertical saccades.     

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.     

Anatomical connections of the nucleus prepositus of the cat

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

Afferent and efferent connections of the oculomotor region of the fastigial nucleus in the macaque monkey.

Afferent and efferent connections of the fastigial oculomotor region (FOR) were studied in macaque monkeys by using axonal transport of wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP). When injected HRP is confined to the FOR, retrogradely labeled cells appear in lobules VIc and VII of the ipsilateral vermis and in group b of the contralateral medial accessory olive (MAO). In reference to the maps of topographical organization, the extent of the effective site in the fastigial nucleus (FN) could be assessed from the distributions of labeled Purkinje cells (P cells) in the vermis and labeled olivary neurons in the MAO. In contrast to the unilateral nature of the P-cell and climbing-fiber projections, those from the other brainstem regions to the FOR were bilateral. Following the injection of HRP into the FOR, the largest number of retrogradely labeled cells appeared in the pontine nuclei. Although the number of labeled cells was greater on the contralateral side in both the peduncular and dorsomedial pontine nuclei (DMPN), the number of each side was virtually identical in the dorsolateral pontine nucleus (DLPN). In the nucleus reticularis tegmenti pontis (NRTP), labeled cells were located only in its medial and dorsolateral portions bilaterally. In the vestibular complex, labeled cells appeared in the superior (SVN), medial (MVN), and inferior vestibular nuclei (IVN) bilaterally. The lateral vestibular nucleus (LVN), including y group and the ventrolateral vestibular nucleus, were free of labeled cells. Labeled cells appeared also in the perihypoglossal nucleus (PHN) bilaterally. In the pontine raphe (PR) and paramedian pontine reticular formation (PPRF), labeled cells appeared bilaterally in the caudal third of the area between the oculomotor and abducens nuclei. Labeled cells appeared also in the mesencephalic and medullary reticular formation. Tracing of anterogradely labeled axons demonstrated that most fibers from the FOR decussated within the cerebellum and entered the brainstem via the contralateral uncinate fasciculus. Some crossed fibers ascended with the contralateral brachium conjunctivum and terminated in the midbrain tegmentum. A small contingent of fibers advanced further to the thalamus. In the mesodiencephalic junction, labeled terminals were found contralaterally in the rostral interstitial nucleus of medial longitudinal fasciculus (riMLF) and a medial portion of FOrel's H Field. They appeared also in the central mesencephalic reticular formation (cMRF), the periaqueductal gray (PAG), the posterior commissure nucleus, and the superior colliculus. The oculomotor and trochlear nuclei, the red nucleus, and the interstitial nucleus of Cajal were free of labeled terminals.(ABSTRACT TRUNCATED AT 400 WORDS)     

Brainstem afferents to the oculomotor omnipause neurons in monkey

To determine how saccade-related areas in the brainstem address the saccade generator, we examined the afferents to the nucleus raphe interpositus. This region contains the omnipause neurons, which are pivotal in the generation of saccades. Horseradish peroxidase injected iontophoretically into the nucleus raphe interpositus retrogradely labeled a variety of brainstem nuclei. The greatest numbers of labeled neurons were in the paramedian pontomedullary reticular formation, in the nuclei reticularis gigantocellularis, and paragigantocellularis lateralis. Labeling was more modest but consistent in the interstitial nucleus of Cajal and the adjacent mesencephalic reticular formation, the middle gray of the superior colliculi, the region dorsolateral to the nucleus reticularis tegmenti pontis, and the medial vestibular nucleus. A few neurons were labeled around the habenulopeduncular tract and in the medial portion of the nucleus of the fields of Forel, in the nucleus reticularis medullaris ventralis, and in the spinal nucleus of the trigeminal nerve, the cochlear nucleus, and the superior olivary complex. The distribution and density of labeling suggest that omnipause neurons in the monkey are more intimately connected with other oculomotor structures than those in the cat. In addition, the rhombencephalic reticular afferents to the monkey omnipause neurons are more concentrated in their immediate vicinity than in the cat. The label consistently found dorsolateral to the nucleus reticularis tegmenti pontis may be a newly discovered link in saccade generation.     

Abducens internuclear neurons and their role in conjugate horizontal gaze

Experimental evidence suggests that brain stem lesions producing paralysis of lateral gaze and dissociation of conjugate horizontal eye movements have certain common features. Both of these disturbances involve abducens internuclear neurons (Abd IN) or their projections. Attempts were made to determine the course and terminal distribution of Abd IN in the monkey by autoradiographic techniques. Tritiated amino acids injected in the abducens nucleus (Abd N) labeled: (1) root fibers ipsilaterally, and (2) fibers that ascended in medial parts of the contralateral medial longitudinal fasciculus (MLF). In the opposite oculomotor complex (OMC) silver grains were profuse over the ventral nucleus (VN, medial rectus muscle) and patchy over caudal parts of the dorsal nucleus (DN, inferior rectus muscle). Labeling of cells in the reticular formation nucleus to Abd N resulted in transport ipsilaterally, outside the MLF, to the rostral interstitial nucleus of the MLF (RiMLF), a cell group considered to be concerned with vertical eye movements. Bilateral labeling of Abd N and cells of the nucleus prepositus (NPP) resulted in bilateral: (1) transport of isotope via root fibers and the MLF, and (2) selective distribution of silver grains in the OMC. Bilateral silver grain distribution in the OMC suggested profuse terminations in VN, patchy terminations in DN and vertical, linear terminations in caudal parts of the medial nucleus (MN, superior rectus muscle). Comparisons with more discrete unilateral labeling of cells in Abd N suggested that cells of the NPP project selectively to terminations in MN, and may be related to upward eye movements. Two conclusions were drawn: (1) The paresis of ocular adduction which occurs in both anterior internuclear ophthalmophlegia and in paralysis of lateral gaze results from involvement of Abd IN or their ascending projections, and (2) the NPP appears to project selectively to parts of MN of the OMC, a cell group said to provide crossed innervation for the superior rectus muscle.     

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

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