Non-cerebellar visual afferents to the vestibular nuclei involving the prepositus hypoglossal complex: an autoradiographic study in the rat

Organization of visual mossy fiber projections and zebrin expression in the pigeon vestibulocerebellum

Extensive research has revealed a fundamental organization of the cerebellum consisting of functional parasagittal zones. This compartmentalization has been well documented with respect to physiology, biochemical markers, and climbing fiber afferents. Less is known about the organization of mossy fiber afferents in general, and more specifically in relation to molecular markers such as zebrin. Zebrin is expressed by Purkinje cells that are distributed as a parasagittal array of immunopositive and immunonegative stripes. We examined the concordance of zebrin expression with visual mossy fiber afferents in the vestibulocerebellum (folium IXcd) of pigeons. Visual afferents project directly to folium IXcd as mossy fibers and indirectly as climbing fibers via the inferior olive. These projections arise from two retinal recipient nuclei: the lentiformis mesencephali (LM) and the nucleus of the basal optic root (nBOR). Although it has been shown that these two nuclei project to folium IXcd, the detailed organization of these projections has not been reported. We injected anterograde tracers into LM and nBOR to investigate the organization of mossy fiber terminals and subsequently related this organization to the zebrin antigenic map. We found a parasagittal organization of mossy fiber terminals in folium IXcd and observed a consistent relationship between mossy fiber organization and zebrin stripes: parasagittal clusters of mossy fiber terminals were concentrated in zebrin-immunopositive regions. We also describe the topography of projections from LM and nBOR to the inferior olive and relate these results to previous studies on the organization of climbing fibers and zebrin expression.

Two optic flow pathways from the pretectal nucleus lentiformis mesencephali to the cerebellum in pigeons (Columba livia)

Neurons in the pretectal nucleus lentiformis mesencephali (LM) are involved in the analysis of optic flow. LM provides mossy fiber inputs to folia VI-VIII of the posterior cerebellum and IXcd of the vestibulocerebellum. Previous research has shown that the vestibulocerebellum is involved in visual-vestibular integration supporting gaze stabilization. The function of folia VI-VIII in pigeons is not well understood; however, these folia receive input from a tectopontine system, which is likely involved with analyzing local motion as opposed to optic flow. We sought to determine whether the mossy fiber input from LM to IXcd differs from that to VI-VIII. Fluorescent retrograde tracers were injected into these folia, and the pattern of labeling in LM was observed. Large multipolar neurons were labeled throughout the rostrocaudal extent of LM. There was a clear mediolateral difference: 74.3% of LM neurons projecting to IXcd were located in the lateral subnucleus of LM (LMl), whereas 73.8% of LM neurons projecting to VI-VIII were found in medial LM (LMm). This suggests that the subnuclei of LM have differing roles. In particular, the LMl-IXcd pathway is involved in generating the optokinetic response. We suggest that the pathway from LMm to VI-VIII is integrating optic flow and local motion to support various oculomotor and visuomotor behaviors, including obstacle avoidance during locomotion.

Nucleus prepositus

The cytoarchitecture and the histochemistry of nucleus prepositus hypoglossi and its afferent and efferent connections to oculomotor structures are described. The functional significance of the afferent connections of the nucleus is discussed in terms of current knowledge of the firing behavior of prepositus neurons in alert animals. The efferent connections of the nucleus and the results of lesion experiments suggest that it plays a role in a variety of functions related to the control of gaze.

Efferent pathways of the nucleus of the optic tract in monkey and their role in eye movements

To clarify the role of the pretectal nucleus of the optic tract (NOT) in ocular following, we traced NOT efferents with tritiated leucine in the monkey and identified the cell groups they targeted. Strong local projections from the NOT were demonstrated to the superior colliculus and the dorsal terminal nucleus bilaterally and to the contralateral NOT. The contralateral oculomotor complex, including motoneurons (C-group) and subdivisions of the Edinger-Westphal complex, including motoneurons (C-group) and subdivisions of the Edinger-Westphal complex, also received inputs. NOT efferents terminated in all accessory optic nuclei (AON) ipsilaterally; contralateral AON projections arose from the pretectal olivary nucleus embedded in the NOT. Descending pathways contacted precerebellar nuclei: the dorsolateral and dorsomedial pontine nuclei, the nucleus reticularis tegmenti pontis, and the inferior olive. Direct projections from NOT to the ipsilateral nucleus prepositus hypoglossi (ppH) appeared to be weak, but retrograde tracer injections into rostral ppH verified this projection; furthermore, the injections demonstrated that AON efferents also enter this area. Efferents from the NOT also targeted ascending reticular networks from the pedunculopontine tegmental nucleus and the locus coeruleus. Rostrally, NOT projections included the magnocellular layers of the lateral geniculate nucleus (lgn); the pregeniculate, peripeduncular, and thalamic reticular nuclei; and the pulvinar, the zona incerta, the mesencephalic reticular formation, the intralaminar thalamic nuclei, and the hypothalamus. The NOT could generate optokinetic nystagmus through projections to the AON, the ppH, and the precerebellar nuclei. However, NOT also projects to structures controlling saccades, ocular pursuit, the near response, lgn motion sensitivity, visual attention, vigilance, and gain modification of the vestibulo-ocular reflex. Any hypothesis on the function of NOT must take into account its connectivity to all of these visuomotor structures.

Pretectofugal fibers from the nucleus of the optic tract in monkeys

The nucleus of the optic tract (NOT) is the visuo-motor relay between the retina and preoculomotor structures in the pathway mediating optokinetic nystagmus (OKN). NOT lesions in monkeys produce no OKN toward the lesioned side. Then, efferent fibers from the NOT course through the brainstem and may reach the vestibular nucleus, which is proposed to be the final nucleus to the motor nucleus. In the present study, the tracer was injected through a micropipette in the NOT in four monkeys. Labeled terminals were observed ipsilaterally in the parabigeminal nucleus, superficial layers of the superior colliculus, dorsal and lateral terminal nuclei of the accessory optic system and pretectal nuclei and contralaterally in the NOT and superficial layers of the superior colliculus. Descending fibers from the NOT consisted of two major pathways: (1) fibers descended medially from the injection site through the reticularis pontis oralis to reach the lateral part of the ipsilateral nucleus reticularis tegmenti pontis; (2) fibers projecting into the dorsal cap of inferior olive, by far the greatest number of labeled fibers, descended ventrally along the lateral border of the reticularis pontis oralis and reached the medial lemniscus where they descended further and branched into the dorsolateral pontine nucleus, the lateral part of the nucleus reticularis tegmenti pontis, the peduncular pontine nucleus, the lateral pontine nucleus, the nucleus prepositus hypoglossi, the medial vestibular nucleus and finally the dorsal cap of the inferior olive. Consistent with the physiological data, the direct terminals to the medial vestibular nucleus could serve to drive the storage mechanisms and to produce OKN in the monkey.

Independent efferent populations in the nucleus of the optic tract: an anatomical and physiological study in rat and cat

The efferent projections of the nucleus of the optic tract (NOT) and dorsal terminal nucleus of the accessory optic system (DTN) to the contralateral NOT-DTN, ipsilateral inferior olive (IO), ipsilateral nucleus prepositus hypoglossi (NPH), and ipsilateral dorsal lateral geniculate nucleus (LGNd) were examined in pigmented rats and in cats by using anterograde and retrograde tract tracing, as well as extracellular recording and electrical stimulation. Anterograde tracing in the rat revealed a dense termination field of NOT-DTN efferents throughout the homologous contralateral territory. In both species three different cell populations, projecting to the contralateral NOT-DTN, ipsilateral IO, and ipsilateral LGNd, respectively, were distinguished by means of multiple retrograde tracing. No clear topographical segregation of the different NOT-DTN relay cell populations was observed. On the other hand, a large proportion (at least 60%) of NOT-DTN neurons projecting to the ipsilateral NPH were found to bifurcate upon the IO in the rat. Electrophysiologically, NOT-DTN neurons projecting to the IO were identified by their directionally selective responses. Such neurons were never activated by electrical stimulation of either the contralateral NOT-DTN or the ipsilateral LGNd. Neurons antidromically activated from the contralateral NOT-DTN could not be activated from the ipsilateral LGNd. Thus, in both cat and rat the NOT-DTN includes at least three independent relay cell populations. As a consequence, the NOT-DTN must serve functions additional to the generation of eye movements during optokinetic nystagnus, a function subserved by the directionally selective NOT-DTN cells.

The reticulovestibular projection in the rabbit: an experimental study with the retrograde horseradish peroxidase method

The reticulovestibular projections of the brainstem in the rabbit were studied by the retrograde transport of horseradish peroxidase (HRP). After selective iontophoretic injections of the tracer into various subdivisions of the vestibular nuclear complex (VNC), labeled neurons were found in defined regions of the reticular formation (RF) of the caudal pons and the rostral medulla. The results indicate that all four vestibular nuclei receive projection from RF. This projection is bilateral with a contralateral predominance. The major projection originates from dorsal and dorsolateral regions of the caudal pontine reticular nucleus (RPc) and the gigantocellular reticular nucleus (RGc) at the transitional level between them. A modest projection originates from pars alpha of the caudal pontine reticular nucleus (RPc alpha), the parvocellular reticular nucleus (Rpc) and pars alpha of the parvocellular nucleus (Rpc alpha), mostly from their ventral regions. A small projection arises from pars alpha of the gigantocellular reticular nucleus (RGc alpha), as well as from the ventral reticular subnucleus (Rv) and cell group a in the caudal aspect of the medulla. No clear-cut topical relationship was noted between the location of neurons in RF and projection site in VNC. The superior vestibular nucleus (SV) and the medial vestibular nucleus (MV) receive projections exclusively from RPc and RGc, whereas the lateral reticular nucleus (LV) and the inferior vestibular nucleus (IV) receive additional projections from the remaining RF nuclei. The termination areas of reticular fibers within SV and IV seem to be diffuse but in MV and LV there is a clear preponderance to the regions located ventrally. The present study has established cells of origin for the reticulovestibular projections from the pontomedullary RF to individual VNC nuclei in the rabbit.

The nucleus of the optic tract. Its function in gaze stabilization and control of visual-vestibular interaction

1. Electrical stimulation of the nucleus of the optic tract (NOT) induced nystagmus and after-nystagmus with ipsilateral slow phases. The velocity characteristics of the nystagmus were similar to those of the slow component of optokinetic nystagmus (OKN) and to optokinetic after-nystagmus (OKAN), both of which are produced by velocity storage in the vestibular system. When NOT was destroyed, these components disappeared. This indicates that velocity storage is activated from the visual system through NOT. 2. Velocity storage produces compensatory eye-in-head and head-on-body movements through the vestibular system. The association of NOT with velocity storage implies that NOT helps stabilize gaze in space during both passive motion and active locomotion in light with an angular component. It has been suggested that "vestibular-only" neurons in the vestibular nuclei play an important role in generation of velocity storage. Similarities between the rise and fall times of eye velocity during OKN and OKAN to firing rates of vestibular-only neurons suggest that these cells may receive their visual input through NOT. 3. One NOT was injected with muscimol, a GABAA agonist. Ipsilateral OKN and OKAN were lost, suggesting that GABA, which is an inhibitory transmitter in NOT, acts on projection pathways to the brain stem. A striking finding was that visual suppression and habituation of contralateral slow phases of vestibular nystagmus were also abolished after muscimol injection. The latter implies that NOT plays an important role in producing visual suppression of the VOR and habituating its time constant. 4. Habituation is lost after nodulus and uvula lesions and visual suppression after lesions of the flocculus and paraflocculus. We postulate that the disappearance of vestibular habituation and of visual suppression of vestibular responses after muscimol injections was due to dysfacilitation of the prominent NOT-inferior olive pathway, inactivating climbing fibers from the dorsal cap to nodulouvular and flocculoparafloccular Purkinje cells. The prompt loss of habituation when NOT was inactivated, and its return when the GABAergic inhibition dissipated, suggests that although VOR habituation can be relatively permanent, it must be maintained continuously by activity of the vestibulocerebellum.

Cortical and subcortical connections of the pars compacta of the anterior pretectal nucleus in the rat

The efferent and afferent connections of the dorsal part of the anterior pretectal nucleus, pars compacta (APc), were studied experimentally in the rat by using neurotracers. A restricted number of structures supply afferents to the anterior pretectal nucleus: the visual cortex (areas 17, 18 and 18a), ventral lateral geniculate nucleus and superficial layers of the superior colliculus. Additional afferents have been demonstrated originating from the Darkschewitsch nucleus, periaqueductal gray, zona incerta and anterior cingulate cortex. Efferent fibers are distributed to a sector of the deep mesencephalic nucleus just dorsolateral to the red nucleus, the basilar pontine gray, posterior and olivary pretectal nuclei, superficial layers of the superior colliculus, lateral posterior thalamic nucleus, ventral lateral geniculate nucleus and zona incerta. These anatomical observations indicate that the pars compacta of the anterior pretectal nucleus is closely related to visual centers, suggesting an involvement of this nucleus in visually mediated behavior.

Discharge of noradrenergic locus coeruleus neurons in behaving rats and monkeys suggests a role in vigilance

Recordings from noradrenergic locus coeruleus (LC) neurons in behaving rats and monkeys revealed that these cells decrease tonic discharge during sleep and also during certain high arousal behaviors (grooming and consumption) when attention (vigilance) was low. Sensory stimuli of many modalities phasically activated LC neurons. Response magnitudes varied with vigilance, similar to results for tonic activity. The most effective and reliable stimuli for eliciting LC responses were those that disrupted behavior and evoked orienting responses. Similar results were observed in behaving monkeys except that more intense stimuli were required for LC responses. Our more recent studies have examined LC activity in monkeys performing an "oddball" visual discrimination task. Monkeys were trained to release a lever after a target cue light that occurred randomly on 10% of trials; animals had to withhold responding during non-target cues. LC neurons selectively responded to the target cues during this task. During reversal training, LC neurons lost their response to the previous target cue and began responding to the new target light in parallel with behavioral reversal. Cortical event-related potentials were elicited in this task selectively by the same stimuli that evoked LC responses. Injections of lidocaine, GABA, or a synaptic decoupling solution into the nucleus paragigantocellularis in the rostral ventrolateral medulla, the major afferent to LC, eliminated responses of LC neurons to sciatic nerve stimulation or foot- or tail-pinch. This indicates that certain sensory information is relayed to LC through the excitatory amino acid (EAA) input from the ventrolateral medulla. The effect of prefrontal cortex (PFC) activation on LC neurons was examined in anesthetized rats. Single pulse PFC stimulation had no pronounced effect on LC neurons, consistent with our findings that this area does not innervate the LC nucleus. However, trains of PFC stimulation substantially activated most LC neurons. Thus, projections from the PFC may activate LC indirectly or through distal dendrites, suggesting a circuit whereby complex stimuli may influence LC neurons. The above results, in view of previous findings for postsynaptic effects of norepinephrine, are interpreted to reveal a role for the LC system in regulating attentional state or vigilance. The roles of major inputs to LC from the ventrolateral and dorsomedial medulla in sympathetic control and behavioral orienting responses, respectively, are integrated into this view of the LC system. It is proposed that the LC provides the cognitive complement to sympathetic function.

Macaque accessory optic system: II. Connections with the pretectum

Connections of the accessory optic system (AOS) with the pretectum are described in the macaque monkey. Injections of tritiated amino acids in the pretectum demonstrate a major contralateral projection to the dorsal (DTN), lateral (LTN), and medial (MTN) terminal nuclei of the AOS and a sparser projection to the ipsilateral LTN. Injections of retrograde tracers, Fast Blue (FB), or wheat germ agglutinin horseradish peroxidase (WGA-HRP) plus nonconjugated horseradish peroxidase (HRP) in the LTN show that the pretectal-LTN projection originates from two nuclei. The main source of pretectal efferents to the LTN is from the pretectal olivary nucleus (OPN) and is entirely contralateral. This projection, which appears unique to primates, originates from the large multipolar cells of the OPN. In addition to this projection, the nucleus of the optic tract (NOT) projects to the ipsilateral LTN, as in nonprimates. Injection of WGA-HRP in the pretectum shows a reciprocal predominantely ipsilateral projection from the LTN to the pretectum. Retinas were observed after injection of FB in the LTN. The retinal ganglion cells projecting to the AOS are mainly distributed near the fovea and in the nasal region of the contralateral eye, suggesting a nasotemporal pattern of decussation. The demonstration of a direct connection between LTN and OPN forces to a reconsideration of the functional role of the AOS. Previous descriptions of luminance responsive cells in the LTN support a possible participation of this nucleus in the control of the pupillary light reflex.

The perihypoglossal projection to the superior colliculus in the rhesus monkey

The projection of the perihypoglossal (PH) complex to the superior colliculus (SC) in the rhesus monkey was investigated using the retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Following physiological identification by electrical stimulation and multiunit recording, small injections of the tracer were placed within the SC of three monkeys. The largest numbers of retrogradely labeled neurons within the PH complex were found in the contralateral nucleus prepositus hypoglossi (NPH), in the laterally adjacent medial vestibular nucleus, and in the ventrally adjacent reticular formation (the nucleus reticularis supragigantocellularis). These labeled neurons are strikingly heterogeneous in size and morphology. The nuclei supragenualis and intercalatus also contain numerous labeled neurons in the 2 cases in which the injections involve the caudal SC. Large numbers of retrogradely labeled neurons as well as anterogradely transported WGA-HRP are observed also throughout the pontine and medullary reticular formation, including the midline raphe. The PH complex, particularly the NPH, is known to be involved in the coding of eye position and has been hypothesized to be a critical component of the "neural integrator." Our data demonstrate the existence of a robust projection from the PH complex to the contralateral SC in the rhesus monkey. This projection may serve as the anatomical substrate by which a corollary of eye position could reach the SC. Such a signal is a prerequisite for the computation, at the collicular level, of saccadic motor error signals observed in the SC of rhesus monkeys.

A double-labeling investigation of the pretectal visuo-vestibular pathways

The projections of the nucleus of the optic tract were studied in the cat by simultaneous use of two distinct retrograde tracers (Fast Blue and Diamidino Yellow) injected in the inferior olive and the prepositus hypoglossi nucleus. Following injections of diamidino yellow in one structure and fast blue in the other, a significant number of retrogradely labeled neurons projecting to either target were observed dispersed in the nucleus of the optic tract. Three populations of labeled cells were found: one which projected to the inferior olive, a second to the nucleus prepositus hypoglossi, and a third which projected by means of a bifurcating axon to both of these structures. Quantification of these results reveals that 72% of the total number of labeled neurons are labeled by the IO injection, the remaining cells being labeled by the NPH injection. Double-labeled neurons represent more than 7% of the total number of the labeled cells. Tentative inferences as to the electrophysiological properties of the nucleus of the optic tract are discussed in the context of the optokinetic system.

Projections of the nucleus of the optic tract to the nucleus reticularis tegmenti pontis and prepositus hypoglossi nucleus in the pigmented rat as demonstrated by anterograde and retrograde transport methods

The visual pathways from the nucleus of the optic tract (NOT) to the nucleus reticularis tegmenti pontis (NRTP) and prepositus hypoglossi nucleus (ph) were studied following injections of tritiated leucine into the NOT of pigmented rats. The cell bodies of origin of the pretectal-NRTP, NRTP-ph, and pretectal-ph projections were determined using retrograde horseradish peroxidase (HRP) technique. The pretectum projects strongly to the rostral two-thirds of the central and pericentral subdivisions of the NRTP and sends a remarkably smaller projection to the ph. Both are entirely ipsilateral. The fibers destined for the ph travel with the NOT-NRTP bundle, pass through the NRTP, traverse the medial longitudinal fasciculus, and are distributed to the rostral one-half of the ph. The retrograde HRP studies confirm these pathways. The pretectal projections to the NRTP arise from neurons in the rostromedial NOT; those to the ph are located primarily in the rostral NOT although small numbers are found within the anterior, posterior, and olivary pretectal nuclei. Of major importance is the fact that the ph injections retrogradely label neurons within the NRTP and the adjacent paramedian pontine reticular formation. This NRTP-ph projection is entirely bilateral and arises from parts of both subdivisions of the nucleus targeted by NOT afferents. Both the direct NOT-ph and indirect NOT-NRTP-ph connections provide the anatomical basis for the relay of visual (optokinetic) information to the perihypoglossal complex and, presumably, by virtue of reciprocal ph-vestibular nuclear connections, to the vestibular nuclei itself. Such pathways confirm previous physiological studies in rat and, in particular, clarify the contrasting effects of electrolytic lesions of NRTP in rat which completely abolishes optokinetic nystagmus (OKN) (Cazin et al., 1980a) vs kainic acid lesions which produce only minor effects on OKN slow velocity (Hess et al., 1988). Given these differential effects, one concludes that the critical pathway for OKN passes in relation to, but is not significantly relayed by, the neurons of the NRTP or adjacent pontine tegmentum. The present studies suggest that one such fiber system is the NOT-ph bundle. How this relatively small projection compares to other possible fiber of passage systems remains to be determined electrophysiologically.

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.

The projection of the nucleus reticularis tegmenti pontis and adjacent regions of the pontine nuclei to the central cerebellar nuclei in the cat

The projection of the nucleus reticularis tegmenti pontis (NRTP) and the pontine nuclei (NP) to the central cerebellar nuclei (CCN) was investigated by means of anterograde transport of tritiated leucine. Although termination was found in all the CCN, it was most pronounced in the lateral nucleus and the lateral aspect of the posterior interposed nucleus. The extreme lateral aspect of the anterior interposed nucleus and the caudal part of the fastigial nucleus received a projection of modest intensity. Termination in the infracerebellar nucleus and group Y is likely to be present but could not be confirmed with certainty from the light microscopical material. The contribution from the NP was small and originated from the dorsolateral and dorsal paramedian subdivisions of the NP. Within the NRTP the total area giving rise to projections to the CCN was extensive, and the origin of the projections to the individual CCN overlapped considerably. The projection of the NRTP to the ventrocaudal part of the lateral nucleus was found in conjunction with a projection to the ventrolateral part of the posterior interposed nucleus. Both projections seemed to branch off the fiber bundle terminating in the ventral paraflocculus. Similar correlations could be established in the projection of the NRTP to the dorsal paraflocculus and crus II of the ansiform lobule with other parts of the lateral and posterior interposed nuclei. It was concluded that the transverse, lobular organization of mossy fibers, which differs fundamentally from the longitudinal, modular organization of climbing fibers, is maintained in the collateral projection to the CCN. The results are further discussed in relation to the corticonuclear projection and the engagement of the NRTP and different parts of the CCN in pontocerebellar circuits.

Autoradiographic study of descending pathways from the pontine reticular formation and the mesencephalic trigeminal nucleus in the rat

Descending projections were studied in autoradiographically prepared material after injections of tritiated leucine in the pontine tegmentum of rats. Injections involving the medial pontine reticular formation resulted not only in labeling commissural fibers, the medial reticulospinal tract, and the dorsal cap of the inferior olive, but also, in two cases, in labeling a cerebellar projection that originated from a region near the midline and clearly dorsal to the nucleus reticularis tegmenti pontis. The labeled fibers passed ventral in the midline to the pontine gray, then laterally through the gray and into the middle cerebellar peduncle to terminate as mossy fibers primarily in the flocculus, lobulus simplex, and Crus I of the ansiform lobule. Injections involving the mesencephalic nucleus of the trigeminal nerve (Vmes), resulted in labeling of Probst's tract, which descends in the dorsolateral reticular formation. Probst's tract gave off extensive terminal branches to the lateral medullary reticular formation and weaker projections to restricted portions of the descending trigeminal nucleus, the solitary nucleus, and the hypoglossal nucleus. In one case, fibers could be traced into the dorsal horn of the upper cervical cord.

The nucleus reticularis tegmenti pontis and the adjacent rostral paramedian reticular formation: differential projections to the cerebellum and the caudal brain stem

The projection of the nucleus reticularis tegmenti pontis and the adjacent tegmental area, to the caudal brain stem and the cerebellum were investigated by means of anterograde transport of tritiated leucine. The nucleus reticularis tegmenti pontis was found to be exclusively connected with the cerebellum. Mossy fiber terminals were absent only from lobule X and most abundant in lobule VII and the hemispheres with a slight contralateral predominance. The paramedian pontine reticular formation projects with bilateral symmetry to the cerebellar lobules VI, VII and the crura I and II, and heavily to the medial aspect of predominantly the ipsilateral reticular formation in the lower brain stem including specific targets as the nucleus reticularis paramedianus, the nucleus prepositus hypoglossi, the nucleus intercalatus, the nucleus of Roller, the nucleus supragenualis and the dorsal cap of the inferior olive. The nucleus vestibularis medialis receives a very weak projection. The connections are discussed in the light of their possible involvement in pathways for the execution of voluntary and reflex eye movements.

Lesions in the cat prepositus complex: effects on the optokinetic system

The effects of bilateral lesions within and around the prepositus hypoglossi (p.h.) nuclei on the optokinetic system were studied. The pure optokinetic nystagmus (o.k.n.) was evoked by a step of velocity (60 deg/s, 30 s duration) of the surrounding. The visual-vestibular interaction was investigated by measuring the gain and phase of the vestibulo-ocular reflex (v.o.r.) as a function of frequency before and after lesion under three different conditions of testing: basic v.o.r. tested in the dark, v.o.r. tested in the light and v.o.r. suppressed by vision. The tested amplitude was +/- 20 deg. A posterior vermectomy was performed for controls in two cats. A bilateral electrolytic p.h. lesion including the rostral pole of this nucleus was added to the posterior vermectomy in three cats. A lesion similar but sparing the rostral pole of the nucleus was carried out in three other cats. In one cat a bilateral electrolytic lesion of the medial vestibular nuclei (m.v.n.) was combined with a posterior vermectomy. In two cats the medulla was cut on the mid line after a posterior vermectomy. The posterior vermectomy affected neither the optokinetic response nor the visual-vestibular interactions. In cats where p.h. lesion included its rostral pole and in the cat with m.v.n. lesion, all the tested optokinetic effects (step o.k.n., and visual-vestibular interactions) were abolished. In the three cats where p.h. lesion spared its rostral pole, the optokinetic effects were quite normal in one cat, mildly reduced in the second one, and seriously affected but not completely abolished in the third one. The surgical cut of the medulla on the mid line did not dramatically disturb the various optokinetic effects. The most marked deficit was the loss of the optokinetic after-nystagmus (o.k.a.n.). From the comparison of these results with the neuroanatomical data and with the Robinson's model concerning the optokinetic processing, it was suggested that: (a) the rostral p.h. could be the location of the o.k.n. integrator or could be an essential link on the o.k.n. pathway, (b) the posterior four-fifths of the p.h. could not be an essential relay on the o.k.n. pathway, (c) the loss of o.k.a.n. after mid-line lesion could be due to the interruption of the positive feed-back loop formed by the reciprocal inhibitory connexions between the two m.v.n.

Anatomical studies on the nucleus reticularis tegmenti pontis in the pigmented rat. II. Subcortical afferents demonstrated by the retrograde transport of horseradish peroxidase

The subcortical nuclear groups projecting to the nucleus reticularis tegmenti pontis (NRTP) were studied in pigmented rats with the aid of the retrograde horseradish peroxidase (HRP) technique. Small iontophoretic injections of HRP were placed in the medial regions of the NRTP, an area that has been shown in several species to be involved in eye movements. Other large injections in the NRTP or small injections placed just outside the nucleus were used to clarify the projections to the NRTP. Results indicate that the NRTP receives afferents from visual relay nuclei, including the nucleus of optic tract, the superior colliculus, and the ventral lateral geniculate nucleus; oculomotor-associated structures including the zona incerta, the H1 and H2 fields of Forel, the nucleus subparafasciculus, the interstitial nucleus of Cajal, the visual tegmental relay zone of the ventral tegmental area of Tsai, the mesencephalic, pontine, and medullary reticular formations, the nucleus of the posterior commissure, and a portion of the periaqueductal gray termed the supra-oculomotor periaqueductal gray; cerebellar and pontomedullary nuclei, including the superior, lateral, and medial vestibular nuclei, the deep cerebellar nuclei, and NRTP interneurons, and nuclei related to limbic functions including the lateral habenula, the mammillary nuclei, the hypothalamic nuclei, the preoptic nuclei, and the nucleus of diagonal band of Broca. A surprisingly large number of afferents to the medial regions of the NRTP arise from visual- or eye-movement-related nuclei. The projection from the nucleus of the optic tract (NOT) confirms previous anatomical and physiological studies on the pathways involved in horizontal optokinetic nystagmus, but the number of NOT afferents is small in relation to other areas potentially related to visuomotor pathways such as the zona incerta, ventral lateral geniculate nucleus, fields of Forel, perirubral area, and subparafasciculus. The NRTP may also relay information related to vertical visuomotor reflexes (e.g., vertical optokinetic nystagmus) given the strong projections from the medial terminal nucleus of the accessory optic system, visual tegmental relay zone, supra-oculomotor periaqueductal gray, interstitial n. of Cajal, and midbrain reticular formation. The presence of significant NRTP projections from the superior colliculus and the mesencephalic and pontine reticular formations suggests that these nuclei may provide the pathways for the noted saccade-related activity of NRTP neurons. In addition, projections from the vestibular nuclei were found that provide the anatomical basis for head velocity signals recorded in NRTP neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Projections from the nuclei prepositus hypoglossi and intercalatus to the superior colliculus in the cat: an anatomical study using WGA-HRP

In view of the recognized role of the superior colliculus (SC) in eye and head movement, and following recent physiological studies, the presence of afferents to the SC from the vestibular complex and adjacent cellular groups were sought using axonal transport techniques. Injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) were made in the SC of 9 adult cats. Retrogradely labelled cells were observed in the nuclei prepositus hypoglossi (PH) and intercalatus (INT) predominantly contralaterally with only occasional cells in the medial (MVN) and descending (DVN) vestibular nuclei contralaterally. There appeared to be some topographical differences in anteroposterior distribution of labelled cells in the perihypoglossal nuclei after restricted SC injections. Two cases of intramedullary injection of WGA-HRP which involved the PH, INT, MVN and DVN revealed anterogradely labelled terminations in laminae IV and VI of the contralateral SC. A few additional labelled terminations were found in the medial portion of the contralateral medial geniculate body (MGB) and the nucleus of the optic tract-(NOT). The possible role of the perihypoglossal nuclear complex as a site of convergence of vestibular and neck afferent inputs destined for the SC is discussed.

Vestibular afferents of the inferior olive and the vestibulo-olivo-cerebellar climbing fiber pathway to the flocculus in the cat

The projection of the vestibular nuclei to the inferior olive was investigated by means of anterograde transport of tritiated leucine. Following injections in the medial and descending vestibular nuclei, terminal labeling was found ipsilaterally in the dorsomedial cell column, subnucleus beta and the caudal medial accessory olive, while the latter also received afferents from the nucleus prepositus hypoglossi. At the contralateral side termination in the dorsomedial cell column and the medial accessory olive was found after injections in the nucleus vestibularis superior and group Y. The ventrolateral outgrowth and different parts of the principal olive also received afferents from these two nuclei and also from ventral parts of the lateral cerebellar nucleus. The dorsal cap was labeled exclusively from the contralateral nucleus prepositus hypoglossi. The termination in the inferior olive of the vestibular afferents is compared with the projection from a number of pretectal nuclei. Furthermore the consequences of the divergence and convergence of both types of projections at the level of the inferior olive is discussed in relation to the subsequent climbing fiber projection to the flocculus.

Responses of the nucleus of the optic tract neurons projecting to the nucleus reticularis tegmenti pontis upon optokinetic stimulation in the rabbit

The activity of cells (n = 55) in the nucleus of the optic tract (NOT) in pigmented rabbits was recorded extracellularly. All cells were activated orthodromically from the optic chiasm (latency 2.3 +/- 0.4 ms, mean and S.D., n = 33). The mean conduction velocity of optic nerves to the NOT was 19.0 m/s, and the mean nuclear delay in the NOT was 0.9 ms. Stimulation to the nucleus reticularis tegmenti pontis (Nrtp) antidromically activated 37 cells (latency 1.0 +/- 0.3 ms) (Nrtp(+)cell) but did not antidromically activate the other 18 cells (Nrtp(-) cell). The response properties were studied by spike density histograms constructed during the application of moving visual stimuli (random dot patterns, 0.1-200 degrees/s). The optimal velocity was 5-50 degrees/s for the majority (74%, n = 29) of the total samples tested. Of 10 cells preferring high velocities (75-150 degrees/s), four times as many Nrtp(+) cells (n = 8) preferred high velocities as did Nrtp(-) cells. The optimal velocities of Nrtp(+) cells significantly correlated with the rostro-caudality of recording sites in the NOT, the more rostral cells preferring a slower velocity. Nrtp(+) cells preferred not only a horizontal (53%) but also a vertical or diagonal orientation in contrast to the majority (71%) of Nrtp(-) cells, which preferred a horizontal orientation. The receptive fields (mean size 28 degrees X 15 degrees) scattered horizontally corresponding to the visual streak of the retina.

Analysis of signal content of Purkinje cell responses to optokinetic stimuli in the rabbit cerebellar flocculus by selective lesions of brainstem pathways

The heterogeneous signal content of floccular Purkinje cell responses to optokinetic stimuli was analyzed in alert rabbits by means of selective lesions to brainstem pathways. Extracellular spike activities of Purkinje cells were recorded from rostral areas of the flocculus where local electrical stimulation elicited abduction of the ipsilateral eye. Chronic unilateral destruction of the nucleus reticularis tegmenti pontis, interrupting the visual mossy fiber afferent pathway to the flocculus, reduced the gain of the optokinetic eye movement (OKR) to one-third of the control. Concomitantly, simple spike responses of Purkinje cells to optokinetic stimuli were reduced to less than one-third of the control values. Severance of the visual climbing fiber afferent pathway by rostral inferior olivary lesions reduced the OKR gain little, and decreased the simple spike responses of the Purkinje cells only slightly. Bilateral lesions of the rostral half of the medial vestibular nucleus and rostro-ventral part of the lateral vestibular nucleus, which reduced the eye velocity in the OKR to less than one-third of the control value, did not induce any appreciable change in the simple spike responses of the Purkinje cells. It is concluded that visual mossy fiber signals are the most dominant factor which determines Purkinje cell responses to optokinetic stimuli, while visual climbing fiber signals and eye velocity mossy fiber signals make only subsidiary contributions.

Responses of prepositus hypoglossi neurons to optokinetic and vestibular stimulations in the rat

The responses of 47 nucleus prepositus hypoglossi neurons to vestibular optokinetic stimulations in the horizontal plane were recorded in immobilized, pigmented rats. During sinusoidal vestibular stimulation in the dark, type II (62%) and type I (38%) responses were recorded. In addition to the sinusoidal modulation of firing rate, units often showed fast rhythmic increases or decreases in firing (nystagmic modulation). The mean phase of the response relative acceleration measured at 0.025 and 0.2 Hz were 19 and 84 deg., respectively. Some units (25%) showed larger phase-lags. The sensitivities of unit responses at 0.025 and 0.2 Hz were 1.6 and 0.5 spikes X s-1/deg X s-2, respectively. The responses of NPH neurons to binocular optokinetic stimulation were divided in 2 classes: (i) neurons with unidirectional responses (18%) were excited by stimuli moving towards the side of recording and showed no change in firing on oppositely directed stimulation; all of them showed a type II pattern during vestibular stimulation; (ii) bidirectional responses showed an increase in one direction and a decrease in firing for stimulation in the opposite direction. In every case the optokinetic responses were synergistic with the vestibular responses, which consisted of both type I and type II units. On the basis of the directionality of their optokinetic response, the value of their time constants and the shape of their velocity tuning curves, it is suggested that unidirectional type II NPH neurons could serve as relays in the optokinetic pathways between NRTP (or PT) and vestibular neurons. Some other neurons, having time constants particularly long and different for the rising and falling of the response, probably serve other functions.

The mossy fiber projection of the nucleus reticularis tegmenti pontis to the flocculus and adjacent ventral paraflocculus in the cat

The pontine projection of the flocculus and adjacent ventral paraflocculus was investigated with antegrade and retrograde axonal tracer techniques. Injections of horseradish peroxidase into the floccular complex revealed subsets of labeled neurons in the nucleus reticularis tegmenti pontis, the nucleus raphe pontis and the medial lemniscus. Following injections of tritiated leucine in these subsets, the topographical distribution of labeled mossy fibers in the floccular complex was studied. Cells clustered in the central part of the nucleus reticularis tegmenti pontis project to the rostral flocculus and the rostral part of the caudal flocculus. The terminal field of cells in the nucleus raphe pontis and of cells associated with the lateral aspect of the medial lemniscus covered the same area. The number of mossy fiber terminals arising from these cells is small and concentrated in a medial position. The medial extension of the ventral paraflocculus and its most caudal sublobule do receive a very dense mossy fiber projection from cells associated with the medial edge of the medial lemniscus next to the rostral nucleus reticularis tegmenti pontis and beyond. Concomitantly, a collateral projection terminates in a restricted part of the uvula. Labeled mossy fiber terminals were never observed in the nodulus. The nucleus reticularis tegmenti pontis does not project to any part of the lower brain stem. The connections described in this paper are discussed in relation to the possible role of the nucleus reticularis tegmenti pontis as a relay nucleus in brain stem pathways transmitting visual information. It is concluded that in the cat this nucleus is an exclusively pre-cerebellar relay, not involved as a final link in the non-cerebellar pathway transmitting visual information to the vestibular nuclei.

The perihypoglossal nuclei in the macaque monkey and the chimpanzee

The topography of the perihypoglossal nuclei (nucleus intercalatus, nucleus prepositus, and nucleus of Roller) of the macaque and the chimpanzee was studied in serial Nissl-stained sections through the brainstem. Maps of the nuclei in the two species are presented. Although in both species the perihypoglossal nuclei are organized according to the general mammalian pattern, they show some particular features, reflecting in part, an increasing phylogenetic differentiation. In the chimpanzee the nucleus prepositus is relatively larger, the nucleus of Roller is more of a separate unit, and its cellular composition is more uniform than is the case in the macaque. A cellular connection between the two nuclei is present in the macaque (even more conspicuous in the cat, apparently absent in man), but is barely discernible in the chimpanzee. A conspicuous difference between the chimpanzee and the macaque concerns the nucleus supragenualis nervi facialis, forming a rostral continuation of the prepositus. In the chimpanzee it is a loosely structured region of small cells (as in the cat and in man). In the macaque, however, it appears as a rather well-delimited column of chiefly medium-sized cells. Some comparative anatomical and functional aspects are discussed. Many contingents of afferents to--and efferents from--the nucleus prepositus have their preferential sites of termination or origin in the nucleus, although with considerable overlapping. This indicates the presence of topical patterns in the prepositus. In general the caudal parts appear to be more related to the cooperation with the cerebellum than the rostral part, whereas the latter appears to be more particularly linked with the oculomotor apparatus.