Reticular premotor neurons projecting to both facial and hypoglossal nuclei receive trigeminal afferents in rats.

The distribution of premotor neurons projecting to motor nuclei of both the VIIth (VII) and XIIth (XII) nerves was examined in the pontomedullary reticular formation (RF) of the rat by using retrograde double labeling. After injection of two different tracers in the VII and the XII, most of the double labeled neurons were found caudally in the dorsal RF whereas rostrally they were located in the ventral RF. In some experiments, additional injections of an anterograde tracer were made in the sensory trigeminal nuclei. Anterogradely labeled trigeminal boutons were found in contact with retrogradely double labeled neurons throughout the pontomedullary RF. These neurons were mainly encountered ventral to the trigeminal motor nucleus and dorsal to the VII. Functionally, this region is known to be involved in eye protection mechanisms.

Fastigiovestibular projections in the rat: retrograde tracing coupled with gammaamino-butyric acid and glutamate immunohistochemistry.

In this study, a double labeling technique using retrograde tracing with protein-gold complex (gold-HRP) in conjunction with a gammaamino-butyric acid (GABA) and glutamate immunohistochemical procedure was performed to identify GABA (GABA-IR) and glutamate (Glu-IR) immunoreactive neurons in the cerebellar fastigial nucleus (FN) that projects to the vestibular nuclei (VN). The results show that FN neurons projecting to the VN consist of both GABA-IR and Glu-IR neurons with a predominance of glutamatergic ones. Because GABAergic neurons in the cerebellar nuclei project to the inferior olive (IO), double retrograde labeling experiments were performed with injections of gold-HRP in the IO and of biotilynated dextran amine in the VN. This showed that the GABA-IR fastigiovestibular neurons project by axon collaterals to both the VN and the IO.

Trigemino-reticulo-facial and trigemino-reticulo-hypoglossal pathways in the rat.

This study was undertaken to identify premotor neurons in the pontomedullary reticular formation serving as relay neurons between the sensory trigeminal complex and the motor nuclei of the VIIth and XIIth nerves. Trigeminoreticular projections were first investigated after injections of anterogradely transported tracers (biotinylated dextran amine, biocytin) into single subdivisions of the sensory trigeminal complex. The results show that the trigeminoreticular projections were abundant from the pars interpolaris (5i) and caudalis (5c) and moderate from pars oralis (5o) of the spinal trigeminal nucleus. Injections into the 5i and 5c produce dense anterograde labeling (1) in the dorsal medullary reticular field; (2) in the parvocellular reticular field, medially adjacent to the 5i; and (3) more rostral in the region dorsal and lateral to the superior olivary nucleus. Some labeled terminals were also found in the intermediate reticular field, whereas only light anterograde labeling was observed in the gigantocellular and oral pontine reticular formation. The 5o sends fibers and terminals throughout the whole reticular formation, with no clear preferential projections within a particular field. Only light projections originated from the principal nucleus (5P). In a second series of experiments, we examined whether premotor neurons in the reticular formation are afferented by trigeminal fibers. Double labeling was performed by injection of an anterograde tracer in the 5i and 5c and retrograde tracer (gold-horseradish peroxidase complex) into the VII or the XII motor nucleus on the same side. Retrogradely labeled neurons in contact with anterogradely labeled boutons were found throughout the reticular formation with predominance in the parvocellular and intermediate reticular fields. These experiments demonstrate the existence of trigeminal disynaptic influences, via reticular neurons of the pontomedullary reticular formation, in the control of orofacial motor behaviors.

Connections between the trigeminal mesencephalic nucleus and the superior colliculus in the rat.

Retrograde tracing methods are employed here to demonstrate that neurons in the trigeminal mesencephalic nucleus (5me) project to the superior colliculus (SC) in the rat. These neurons, mainly of small size, are situated bilaterally in the caudal part of the nucleus. Anterograde tracing studies demonstrated the existence of SC projections to neurons in 5me. The SC fibers contact 'en passant' small as well as large cell bodies of 5me neurons. These pathways suggested a role of the 5me neurons in oculomotor control and associated oro-facial functions.

Dentatovestibular projections in the rat.

The dentatovestibular connections were investigated using anterograde and retrograde tracing methods. All parts of the cerebellar nucleus lateralis (NL) or dentate nucleus sent fibers onto the ipsilateral vestibular nuclear complex. In spite of their apparently widespread area of termination, dentatovestibular fibers were distributed differentially, according to the subregion of the NL they arose from. Fibers from the main, magnocellular region and the dorsolateral hump (dlh) reached the four main vestibular nuclei, but preferentially the superior (SV) and inferior (IV) vestibular nuclei. The projections to the lateral and the medial vestibular nuclei, which were less abundant, essentially originated from neurons located in the dlh. Fibers arising from the parvocellular subregion of Flood and Jansen terminated within the SV and IV only. Some rare reciprocal vestibulodentate projections were observed. These observations suggest highly integrated activities of dentatovestibular connections related to postural, but also vestibulo-oculomotor functions.

Trigeminal projections to hypoglossal and facial motor nuclei in the rat.

This study was undertaken to identify the trigeminal nuclear regions connected to the hypoglossal (XII) and facial (VII) motor nuclei in rats. Anterogradely transported tracers (biotinylated dextran amine, biocytin) were injected into the various subdivisions of the sensory trigeminal complex, and labeled fibers and terminals were searched for in the XII and VII. In a second series of experiments, injections of retrogradely transported tracers (biotinylated dextran amine, gold-horseradish peroxidase complex, fluoro-red, fluoro-green) were made into the XII and the VII, and labeled cells were searched for in the principal sensory trigeminal nucleus, and in the pars oralis, interpolaris, and caudalis of the spinal trigeminal nucleus. Trigeminohypoglossal projections were distributed throughout the ventral and dorsal region of the XII. Neurons projecting to the XII were found in all subdivisions of the sensory trigeminal complex with the greatest concentration in the dorsal part of each spinal subnucleus and exclusively in the dorsal part of the principal nucleus. Trigeminofacial projections reached all subdivisions of the VII, with a gradual decreasing density from lateral to medial cell groups. They mainly originated from the ventral part of the principal nucleus. In the spinal nucleus, most of the neurons projecting to the VII were in the dorsal part of the nucleus, but some were also found in its central and ventral parts. By using retrograde double labeling after injections of different tracers in the XII and VII on the same side, we examined whether neurons in the trigeminal complex project to both motor nuclei. These experiments demonstrate that in the spinal trigeminal nucleus, neurons located in the pars caudalis and pars interpolaris project by axon collaterals to XII and VII.

Organisation of reciprocal connections between trigeminal and vestibular nuclei in the rat.

In order to study the connection patterns between the sensory trigeminal and the vestibular nuclei (VN), injections of anterogradely and/or retrogradely transported neuronal tracers were made in the rat. Trigeminal injections resulted in anterogradely labelled fibres, with an ipsilateral preponderance, within the VN: in the ventrolateral part of the inferior nucleus (IVN), in the lateral part of the medial nucleus (MVN), in the lateral nucleus (LVN) with a higher density in its ventral half, and in the superior nucleus (SVN), more in the periphery than in the central part. Moderate trigeminal projections were observed in the small vestibular groups f, x and y/l and in the nucleus prepositus hypoglossi. Additional retrogradely labelled neurones were seen in the IVN, MVN, and LVN, in the same regions as those receiving trigeminal afferents. Morphological analysis of vestibular neurones demonstrated that vestibulo-trigeminal neurones are relatively small and belong to a different population than those receiving projections from the trigeminal nuclei. The trigeminovestibular and vestibulo-trigeminal relationships were confirmed by tracer injections in the VN. The results show that, in the VN, there is sensory information from facial receptors in addition to those reported from the neck and body. These facial afferents complement those from the neck and lower spinal levels in supplying important somatosensory information from the face and eye muscles. The oculomotor connections of the respective zones of the VN receiving trigeminal afferents suggest that sensory inputs from the face, including extraocular proprioception, may, through this pathway, influence the vestibular control of eye and head movements.

Primary trigeminal afferents to the vestibular nuclei in the rat: existence of a collateral projection to the vestibulo-cerebellum.

Projections from the mesencephalic trigeminal nucleus to the vestibular nuclei were analyzed using retrograde and anterograde tracing methods. The results show that neurons in the caudal part of the trigeminal mesencephalic nucleus project mainly to the medial, inferior and lateral vestibular nuclei and moderately to the peripheral part of the superior vestibular nucleus. Using the double-labeling technique we demonstrate that individual neurons of the mesencephalic nucleus send collaterals to the vestibular nuclei and the vestibulo-cerebellum. These results suggest that these anatomical connections are involved in mechanisms of eye-head coordination.

Brainstem efferents from the interface between the nucleus medialis and the nucleus interpositus in the rat.

In a previous report (Buisseret-Delmas et al. [1993] Neurosci. Res. 16:195-207), the authors identified the interface between the cerebellar nuclei medialis and interpositus as the origin of the nuclear output from cortical zone X. They named this nuclear interface the interstitial cell group (icg). In this study, the authors analyzed the icg efferents to the brainstem by using the anterograde and retrograde tracer biotinylated dextran amine. The main targets of these efferents are from rostral to caudal: 1) the accessory oculomotor nuclear region, essentially, the interstitial nucleus of Cajal; 2) the caudoventral region of the red nucleus; 3) a dorsal zone of the nucleus reticularis tegmenti pontis; 4) restricted regions of the four main vestibular nuclei; and 5) three restricted areas in the inferior olive, one that is caudal in the medial accessory subnucleus and two others that are rostral and caudal in the dorsal accessory subnucleus, respectively. These data support the notion that the icg contributes to the control of gaze-orientation mechanisms, particularly those that are related to the vestibuloocular reflex.

Identification of nerve endings in cat extraocular muscles.

BACKGROUND: The aim of the present study was to identify the varieties of sensory and motor nerve endings in cat extraocular muscles. METHODS: Sensory terminals were identify by injecting neuronal tracers (fast blue, biocytin, or peroxidase) into the trigeminal ganglion, which contains the sensory cells innervating the eye muscles. Motor terminals were identified by injections of horseradish peroxidase or DiI, a fluorescent carbocyanin dye, into either the oculomotor nerve or the IIIrd nuclei. RESULTS: Injections into the trigeminal ganglion anterogradely labelled three types of sensory nerve endings for each neuronal tracer used: (1) the well-known "palisade" endings at the myotendinous junction of each extraocular muscle; (2) "compact" endings consisting of a dense terminal arborization extending up to 60 microm in length on striated muscle fibres 10-15 microm in diameter; and (3) "complex" endings on muscle fibres 15-20 microm in diameter. The complex ending issued from multiple collateral branches of the parent nerve fibre, which stretched and turned around the muscle fibre and gave off numerous terminal varicosities over a distance of about 140 microm. The sensory complex and compact endings presented strong similarities with some "atypical muscle spindles" previously described. In addition to the classic motor "plate" and "grape," we found evidence for the existence of motor "spiral" endings with each tracer. CONCLUSIONS: The sensory nature of the palisade endings was demonstrated, and two other types of sensory terminals were identified and described. The spiral nerve terminals were demonstrated to be motor in nature, and a possible function in the microsaccadic movements associated with fixation is suggested.

Projection from trigeminal nuclei to neurons of the mesencephalic trigeminal nucleus in rat.

The mesencephalic trigeminal nucleus contains cell bodies of primary somatic sensory neurons that innervate the head region. The neurons resemble dorsal root ganglion cells but a striking difference is the presence of synaptic boutons in the nucleus. The present report demonstrates with anterograde tracers, the existence of a direct trigeminal projection from secondary sensory neurons of the principal and spinal nuclei to the mesencephalic nucleus. Our observations strongly suggest that synaptic contact may be established on the cell bodies as well as on the neurites of the mesencephalic neurons. These pathways could play a modulatory role in the processing of sensory afferent information and in the control of orofacial and/or oculomotor functions.

Trigeminocerebellar and trigemino-olivary projections in rats.

Retrograde and anterograde neuronal tracers (HRP, biocytin, biotinylated dextran-amine) were used to study the organisation of trigeminocerebellar and trigemino-olivary connections, focusing on the connectivity between trigeminal nuclear regions and the sagittal zones of the rat cerebellar cortex. Trigeminocerebellar projections were bilateral, but mostly ipsilateral. Direct trigeminocerebellar fibres originated mostly in the principal trigeminal nucleus (VP) and pars oralis (Vo), pars interpolaris (Vi), and to a lesser extent in pars caudalis (Vc) of the spinal trigeminal nucleus. Consistent projections were found from the Vc to cerebellar lobules IX and X. The trigeminal fibres terminated in the cerebellum in an organised fashion. The ventral part of the VP, Vo and Vi projected to the medial A zone and the C3 and D2 subzones, whereas the dorsal part of the nuclei projected to the lateral A zone and the C2, D0 and D1 subzones. In lobules IX and X, the organisation was different. The medial half of the VP, Vo, Vi and Vc projected to the lateral aspects of these lobules whereas their lateral part projected to their medial aspects. Trigeminal projections to the deep cerebellar nuclei were also present. Projections to a given sagittal zone concomitantly reached its corresponding nuclear target. Trigemino-olivary projections were principally contralateral. The Vo, Vi and Vc projected to the rostromedial dorsal accessory olive, the adjacent dorsal leaf and the dorsomedial part of the ventral leaf of the principal olive, which are known to project subzones C3, D0 and D1 of the rat cerebellar cortex.

Nucleus medialis-nucleus interpositus interface: its olivary and cerebellocortical projections in the rat.

The nuclear target of the X zone of the cerebellar cortex was identified in rats as clusters of neurons scattered at the interface between the nuclei medialis (NM) and interpositus (NI). In a previous study, we had outlined these target neurons and termed them "interstitial cell groups" (icg). In order to determine whether the icg should be considered as part of either the NM or the medial NI, we analyzed two efferent pathways from the icg: their nucleocortical and nucleoolivary projections. These were compared to their homologues from the NM and the NI. This analysis is based on mapping retrograde cell labeling and anterograde terminal labeling following microinjections of tracers in either the cerebellar cortex, the cerebellar nuclei, or the inferior olive. Nucleocortical projections originating from the icg are of the three types described previously: a "reciprocal" projection to the ipsilateral X zone, a "nonreciprocal" projection to the ipsilateral A zone, and a "symmetrical" projection to the contralateral X zone. These features can be considered as the summed characteristics of the nucleocortical projections from the NM and from the medial NI. Nucleoolivary projections from the icg target the lateral-rostral portion of the dorsal accessory olive as well as the centrocaudal part of the medial accessory olive. These pathways converge with the nucleoolivary projections from the medial NI and from the NM, respectively. The icg receives olivary afferents from both the regions of the dorsal and medial accessory olives to which it projects. On the basis of similarities shown here between the two types of efferents originating from the icg and those from the NM as well as the medial NI, the icg may be regarded as a "mosaic" of neuron clusters alternately belonging to the NM and the medial NI. Therefore, the icg would be reciprocally connected with the inferior olive.

Cerebellar afferences from the mesencephalic trigeminal nucleus in the rat.

The aim of the present study was to test for and characterize the organization of a direct projection from neurones of the trigeminal mesencephalic nucleus (Vme) to the cerebellum. WGA-HRP was used as a retrograde tracer following injections in the cerebellar cortex. The extent of each injection site within the sagittal zones was determined according to corticonuclear and olivocortical connections. Retrogradely labelled neurons were observed in the caudal part of the ipsilateral Vme only following vermal injection. The Vme projections reached exclusively the ipsilateral sagittal zone X in the anterior lobe, lobule VI and lobule IX. This identification was confirmed by anterograde labelling of mossy fibre terminals following a biocytin injection restricted to the Vme.

On the caudal extension of the X zone in the cerebellar cortex of the rat.

Following a selective injection of biotinylated dextran amine in the nuclear target (the interstitial cell groups, icg) of the X zone of the rat cerebellum, retrogradely labelled Purkinje cells (PCs) were found within a longitudinal strip of cortex, 250 microns in width, 1000 microns lateral to midline. This labelling delineates two compartments in the X zone, one rostral through lobules II-VI, and one caudal through lobules VIII-X. The whole rostrocaudal extent of the icg appears to be the target of PCs from both compartments without any apparent topographical organization.

The sensitive period for strabismic amblyopia in humans.

PURPOSE: In order to assess the sensitive period for strabismic amblyopia, the period of susceptibility to monocular occlusion was investigated in 407 children who ranged in age from 21 months to 12 years. METHODS: Patients were treated between 1975 and 1990 by occlusion of the best eye. The efficiency of the treatment was measured as the ratio of reduction of the amblyopia at the end of the occlusion. RESULTS: The efficiency of the occlusion is shown to depend on the age of the onset of the treatment: recovery of acuity of the amblyopic eye was maximum when the occlusion was initiated before 3 years of age, decreased as a function of age and was about null by the time the patient was 12 years of age. CONCLUSION: This is assumed to be an indication of the sensitive period for strabismic amblyopia in humans. The results are discussed on the basis of the neurophysiological mechanisms of amblyopia established in animals.

The X zone and CX subzone of the cerebellum in the rat.

The existence of an X zone (lateral to the A zone) and a CX subzone (lateral to the C1 subzone) was documented within the anterior lobe and lobule VI in cats and primates. On the basis of their respective efferent and climbing fibre (CF) afferent connections, delineation of these two cortical subdivisions has been investigated here, in the rat, using small injections of WGA-HRP in the cerebellar cortex. We observe that both X and CX are "fractured" into a rostral and a caudal compartment. The rostral compartment of the X zone extends over lobules IV, V and VI and its caudal compartment over lobules VIII, IX and X. The rostral compartment of the CX subzone seems to be restricted to lobules V and VI, its caudal compartment cannot be topographically distinguished, over lobules IX and X, from the caudal compartment of the X zone. The olivary afferents to the X zone and the CX subzone arise from the horizontal and vertical lamellae of the medial accessory olive: subnucleus a projects into the rostral compartment and lobule VIII of the X zone. Subnuclei b and c project into the rostral compartment of both X and CX. The dorsomedial cell column selectively projects onto the caudal compartment of both X and CX over the vestibulo-cerebellum. The corticonuclear projections of the X zone have been found within the junctional region between the nucleus medialis and the nucleus interpositus (NI), here defined as the interstitial cell groups (icg), the corticonuclear projections of the CX subzone within the medial NI. It is suggested that the icg correspond to clusters of neurones dissociated from the medial aspect of the NI. We therefore consider the X zone and CX subzone of the rat, on the basis of their corticonuclear efferents, as "medial C1" and "lateral C1" subzone, respectively, although both may be regarded as part of the A zone on the basis of their olivary afferents.