Isthmic afferent neurons identified by the retrograde HRP method in a teleost, Navodon modestus

Isthmic afferent neurons were investigated by the retrograde horseradish peroxidase (HRP) method in a teleost, Navodon modestus. Following HRP injections into the nucleus isthmi, large pyriform neurons are labeled in the ipsilateral optic tectum. Very large and multipolar neurons are also labeled in the ipsilateral nucleus pretectalis. No labeled neurons were found in other areas.     

Columnar projections from the cholinergic nucleus isthmi to the optic tectum in chicks (Gallus gallus): a possible substrate for synchronizing tectal channels

The cholinergic division of the avian nucleus isthmi, the homolog of the mammalian nucleus parabigeminalis, is composed of the pars parvocellularis (Ipc) and pars semilunaris (SLu). Ipc and SLu were studied with in vivo and in vitro tracing and intracellular filling methods. 1) Both nuclei have reciprocal homotopic connections with the ipsilateral optic tectum. The SLu connection is more diffuse than that of Ipc. 2) Tectal inputs to Ipc and SLu are Brn3a-immunoreactive neurons in the inner sublayer of layer 10. Tectal neurons projecting on Ipc possess "shepherd's crook" axons and radial dendritic fields in layers 2-13. 3) Neurons in the mid-portion of Ipc possess a columnar spiny dendritic field. SLu neurons have a large, nonoriented spiny dendritic field. 4) Ipc terminals form a cylindrical brush-like arborization (35-50 microm wide) in layers 2-10, with extremely dense boutons in layers 3-6, and a diffuse arborization in layers 11-13. SLu neurons terminate in a wider column (120-180 microm wide) lacking the dust-like boutonal features of Ipc and extend in layers 4c-13 with dense arborizations in layers 4c, 6, and 9-13. 5) Ipc and SLu contain specialized fast potassium ion channels. We propose that dense arborizations of Ipc axons may be directed to the distal dendritic bottlebrushes of motion detecting tectal ganglion cells (TGCs). They may provide synchronous activation of a group of adjacent bottlebrushes of different TGCs of the same type via their intralaminar processes, and cross channel activation of different types of TGCs within the same column of visual space.     

Cytoarchitecture and fiber connections of the superficial pretectum in a teleost,Navodon modestus

Fiber connections of the so-called nucleus geniculatus lateralis (or the nucleus pretectalis superficialis pars parvocellularis) in a teleost, Navodon modestus, were examined by means of the horseradish peroxidase (HRP) tracing method. The nucleus receives fibers from the contralateral retina, ipsilateral optic tectum and nucleus isthmi, and projects bilaterally to the nucleus intermedius of Brickner and ipsilaterally to the optic tectum and raphe nuclei. The fiber connections suggest that the nucleus relays mainly visual information to the inferior lobe (hypothalamus) but not to the telencephalon. The nucleus is not a homologous structure to the lateral geniculate nucleus in other vertebrate classes.     

A retinopetal nucleus in the preoptic area in a teleost,Navodon modestus

Following horseradish peroxidase (HRP) applications to the optic nerve of a teleost (Navodon modestus), a retinopetal nucleus was identified in the contralateral preoptic area. The nucleus was composed of small (7–10 μm) round cells. Centrifugal fibers from the nucleus were traced to the inner nuclear layer of the ipsilateral retina by both orthograde HRP and Fink-Heimer methods. The cells in the nucleus showed no neurosecretion. The retinopetal nucleus or neurons could not be found inCarassius carassius. No retinopetal neurons were found in the optic tectum in both species.     

Laminar segregation of GABAergic neurons in the avian nucleus isthmi pars magnocellularis: A retrograde tracer and comparative study

The isthmic complex is part of a visual midbrain circuit thought to be involved in stimulus selection and spatial attention. In birds, this circuit is composed of the nuclei isthmi pars magnocellularis (Imc), pars parvocellularis (Ipc), and pars semilunaris (SLu), all of them reciprocally connected to the ipsilateral optic tectum (TeO). The Imc conveys heterotopic inhibition to the TeO, Ipc, and SLu via widespread γ‐aminobutyric acid (GABA)ergic axons that allow global competitive interactions among simultaneous sensory inputs. Anatomical studies in the chick have described a cytoarchitectonically uniform Imc nucleus containing two intermingled cell types: one projecting to the Ipc and SLu and the other to the TeO. Here we report that in passerine species, the Imc is segregated into an internal division displaying larger, sparsely distributed cells, and an external division displaying smaller, more densely packed cells. In vivo and in vitro injections of neural tracers in the TeO and the Ipc of the zebra finch demonstrated that neurons from the external and internal subdivisions project to the Ipc and the TeO, respectively, indicating that each Imc subdivision contains one of the two cell types hodologically defined in the chick. In an extensive survey across avian orders, we found that, in addition to passerines, only species of Piciformes and Rallidae exhibited a segregated Imc, whereas all other groups exhibited a uniform Imc. These results offer a comparative basis to investigate the functional role played by each Imc neural type in the competitive interactions mediated by this nucleus. J. Comp. Neurol. 521:1727–1742, 2013. © 2012 Wiley Periodicals, Inc.     

A banded distribution of retinal afferents within layer 9A of the normal frog optic tectum

A banded distribution of retinal ganglion cell axons within layer 9A of the superficial tectal neuropil in Rana pipiens was revealed through anterograde labeling with horseradish peroxidase. Layer 9A previously has been demonstrated to mediate binocular vision through a polysynaptic pathway by way of the nucleus isthmi^5,8,9. This nucleus interconnects analogous regions of the two tectal lobes such that isthmic axons retinotopically map the visual world of the ipsilateral eye within tectal layers 9A and 8^9,10. Thus, we have found that a pattern of retinal ganglion cell bands occurs in binocular regions of normal frogs. This pattern is similar, but not identical, to the experimentally produced stripes previously observed in the doubly innervated tecta of 3-eyed and single tecta frogs^2,12–14. Qualitative and quantitative comparisons of these two types of afferent segregation patterns have implicated several structural and functional parameters which might be involved in band formation.     

Cytoarchitecture, synaptic organization and Fiber Connections of the nucleus olfactoretinalis in a teleost (Navodon modestus)

Cytoarchitecture, synaptic organization and fiber connections of the nucleus olfactoretinalis (NOR) in a teleost, Navodon modestus, have been studied light- and electron-microscopically using an HRP or HRP-degeneration combined method. Following HRP injections into the optic nerve, most contralateral and a few ipsilateral neurons in the NOR were labeled. There are two types of neurons in NOR. Type I neurons have a medium-sized spindle-shaped soma with a round nucleus, and type II neurons have a large oval soma with an invaginated nucleus and contain cored vesicles (80-130 nm in diameter). Afferent terminals which form synaptic contacts with cell bodies of NOR neurons were classified into 3 types according to their morphological characteristics; S, F1 and F2 terminals. S terminals originated in ipsilateral area ventralis telencephali pars supracommissuralis (Vs). These terminals contain both spherical and cored vesicles, and make synaptic contacts with both type I and type II neurons. F1 terminals, which originated in ipsilateral area dorsalis telencephali pars posterior (Dp), are large in profile, and contain flat vesicles and mitochondria with irregularly arranged cristae. These terminals make synaptic contacts only with type I neurons. F2 terminals are small in profile, and contain flat vesicles, cored vesicles and small mitochondria with regularly arranged cristae. F2 terminals make synaptic contacts with both type I and type II neurons. The functional significance of NOR and the relationship between NOR and the ganglion of the nervus terminalis are discussed.     

Two distinct visual pathways through the superficial pretectum in a percomorph teleost

The connections of the superficial pretectum and of nucleus isthmi were examined in a percomorph teleost, Lepomis cyanellus. Horseradish peroxidase was injected either with a pin into the parvicellular nucleus of the superficial pretectum or pressure injected into nucleus isthmi; the isthmal injections retrogradely labelled the neurons of the magnocellular nucleus of the superficial pretectum. Two main visual pathways can be recognized: The first projects from the retina to the parvicellular nucleus, and then to the intermediate nucleus of the superficial pretectum, the inferior raphe nucleus, and the trochlear nucleus. The second projects from the retina via the optic tectum to the magnocellular nucleus of the superficial pretectum, and from there to nucleus isthmi and the lateral thalamic nucleus; nucleus isthmi and the lateral thalamic nucleus project back to the optic tectum, and nucleus isthmi also projects back to the magnocellular nucleus. The two pathways are interconnected to some extent because both nucleus isthmi and the optic tectum project to the parvicellular nucleus; nevertheless, we suggest that they may be functionally and evolutionarily distinct. Compared to percomorphs, the first pathway appears reduced in cyprinid teleosts such as goldfish. Furthermore, the magnocellular nucleus of the second pathway is completely different in cyprinids, both in cellular architecture and in efferent connections. A phylogenetic analysis suggests that cyprinid ancestors went through a period of reduced vision and that the magnocellular nucleus of the superficial pretectum in modern cyprinids has been either extensively modified from the primitive condition or lost entirely and replaced by a superficially similar structure.     

Nucleus isthmi provides most tectal choline acetyltransferase in the frogRana pipiens

Up to 9 weeks following the removal of unilateral retinal input, choline acetyltransferase (ChAT) activity in the de-afferented tectal lobe is not significantly different from the intact tectal lobe. At 14 weeks, there is a 29% increase in the de-afferented side compared to the intact side. Following unilateral lesion of nucleus isthmi, ChAT activity in the tectal lobe ipsilateral to the lesion is approximately 30% of that measured in the contralateral lobe. Following bilateral n. isthmi lesion, ChAT activity in each tectal lobe is reduced by approximately 94% from intact tectal lobe controls. Thus, nucleus isthmi is the principal source of cholinergic input to the tectum.     

Evidence of intrinsic cholinergic circuits in the optic tectum of teleosts

Choline acetyltransferase (CAT) was assayed in the optic tectum of 4 teleost species with different visual powers. The results showed a close relationship between the enzyme levels in the optic tectum and the development of the visual system. In the more visual species, the trout, CAT activity in the optic tectum was about 30-fold higher than in the catfish, whose visual system is much less developed. Two species with intermediate development of the visual system, the goldfish and the tench, showed intermediate levels of CAT activity.Kainic acid treatment caused a significant decrease of both CAT and acetylcholinesterase (AChE) in the goldfish optic tectum. Concomitant histological examination showed, among other effects, the disappearance of most neurons belonging to the pyramidal and fusiform type in the stratum fibrosum and griseum superficiale of the tectum.The comparative and experimental data therefore suggest that the relationship between cholinergic mechanisms and the visual function is, to a significant extent, connected with the presence of intrinsic cholinergic circuits in the optic tectum. The relevance of these findings, also in relation to the problem of the identification of the retino-tectal transmitter, is discussed.     

Topographic projections between the nucleus isthmi and the tectum of the frog Rana pipiens.

The connections between the nucleus isthmi and the tectum in the frog have been determined by several anatomical techniques: iontophoresis of horseradish peroxidase into the tectum, iontophoresis of 3H-porline into the nucleus isthmi and the tectum, and Fink-Heimer degeneration staining after lesions of the nucleus isthmi. The results show that the nucleus isthmi projects bilaterally to the tectal lobes. The ipsilateral isthmio-tectal fibers are distributed in the superficial layers of the tectum, coincident with the retionotectal terminals. The contralateral isthmio-tectal fibers travel anteriorly adjacent to the lateral optic tract and cross the midline in the supraoptic ventral decussation, where they turn dorsally and caudally; upon reaching the tectum, the fibers end in two discrete layers, layers 8 and A of Potter. The tectum projects to the ipsilateral nucleus isthmi and there is a reciprocal topographic relationship between the two structures. Thus, a retino-tecto-isthmio-tectal route exists which may contribute to the indirect ipsilateral retinotectal projection which is observed electrophysiologically. The connections between the nucleus isthmi and the tectum in the frog are strinkingly similar to the connections between the parabigeminal nucleus and the superior colliculus of mammals.     

Projections of the optic tectum in the longnose gar,lepisosteus osseus

Efferent projections of the optic tectum were studied with the anterograde degeneration method in the longnose gar. Ascending projections were found bilaterally to 3 pretectal nuclei — the superficial pretectal nucleus, nucleus pretectalis centralis and nucleus pretectalis profundus — and to a number of targets which lie further rostrally — the central posterior nucleus, dorsal posterior nucleus, accessory optic nucleus, nucleus ventralis lateralis, nucleus of the ventral optic tract, rostral part of the preglomerular complex, suprachiasmatic nucleus, anterior thalamic nucleus, nucleus ventralis medialis, nucleus intermedius, nucleus prethalamicus and rostral entopeduncular nucleus. Projections of the tectum reach the contralateral side via the supraoptic decussation and are less dense contralaterally than ipsilaterally. Descending projections resulting from tectal lesions include: (1) a tectal commissural pathway to the core of the torus longitudinalis bilaterally and the contralateral tectum and torus semicircularis; and (2) a pathway leaving the tectum laterally from which fibers terminate in the ipsilateral torus semicircularis, an area lateral to the nucleus of the medial longitudinal fasciculus, lateral tegmental nucleus, nucleus lateralis valvulae, nucleus isthmi and the reticular formation. A component of this bundle decussates at the level of the lateral tegmental nucleus to project to the contralateral reticular formation.

On the basis of comparisons of these findings with the pattern of retinal projections in gars and other data, it is argued that the nuclei previously called the lateral geniculate and rotundus in fish are not the homologues of the nuclei of those names in land vertebrates but are rather pretectal cell groups. The overall organization of both retinal and tectal projections in gars is strikingly similar to that in land vertebrates; at present, the best candidate for a rotundal homologue is the dorsal posterior nucleus.     

Normal and regenerating optic fibers in goldfish tectum: HRP-EM evidence for rapid synaptogenesis and optic fiber-fiber affinity

The distribution of normal and regenerating retinal fibers and synapses was studied on tectum in goldfish by light (LM) and electron microscopy (EM). Since labeling of the early regenerating fibers was previously reported to be difficult, a new 'cold-fill' HRP labeling protocol was developed, which labeled regenerating optic fibers and terminals on tectum as early as 14 days after nerve crush when they first arrive on tectum. In order to characterize the laminar distribution of optic afferents in normal fish and in fish regenerating for 14-240 days, EM photomontages of areas 14 microns wide by 160 microns deep through the HRP-labeled primary optic innervation layer (S-SO-SFGS) were constructed. The time points in regeneration that were examined spanned the period in which others have shown that an initially diffuse retinotopic map becomes spatially restricted. At the LM level regenerating optic fibers were restricted to the optic lamina. They reinnervated tectum in an anterior to posterior sequence as previously seen with autoradiography. In addition, at 14 days, some "pioneer" optic fascicles were found to have already grown to posterior tectum where they gave rise to branches with boutonlike terminations and growth-cone-like processes. Form the ultrastructural analysis it was clear that optic fibers and terminals observed strict laminar boundaries as they partitioned themselves in the optic laminae (S, SO and SFGS) in both normal and regenerating fish. The behavior of optic fibers was lamina specific with respect to synapse formation and the orientation of fiber outgrowth. As early as 14 days regeneration, optic fibers made synapses onto the four types of postsynaptic profiles observed in normal fish. Numerous optic terminals were labeled at 14 days, and there appeared to be no waiting period between fiber ingrowth to the SO and synapse formation in the S and SFGS. At 14-60 days, atypical synaptic contacts which appear to be nascent synapses were made by labeled optic fibers in fascicles and by growth-cone-like processes. By 21-30 days, the density of optic terminals was high and there were many more fasciculated optic fibers in the SFGS than normal as late as 350 days. These findings suggest that optic fiber lamination is highly constrained by tectal cues, that fibers rapidly regenerate many synaptic terminals before retinotopic map refinement is complete, and that fibers have a strong affinity for each other.     

Primary and secondary sensory trigeminal projections in a cyprinid teleost, carp (Cyprinus carpio)

Primary and secondary sensory trigeminal projections were studied by means of tract-tracing methods in a cyprinid teleost, the carp. Tracer injections into the trigeminal nerve root labeled terminals in the ipsilateral principal sensory trigeminal nucleus, descending trigeminal nucleus, medial funicular nucleus, facial lobe, and medial part of posterior lateral valvular nucleus. The principal sensory trigeminal nucleus is considered a major origin of the secondary sensory trigeminal projections in teleosts. To investigate the secondary sensory trigeminal projections, tracer injections were performed into the principal sensory trigeminal nucleus. The present study suggests that the principal sensory trigeminal nucleus projects to the bilateral ventromedial thalamic nucleus, periventricular pretectal nucleus, stratum album centrale of the optic tectum, caudomedial region of lateral preglomerular nucleus, ventrolateral nucleus of semicircular torus, medial part of rostral and posterior lateral valvular nucleus, oculomotor nucleus, trochlear nucleus, trigeminal motor nucleus, facial motor nucleus, superior and inferior reticular formation, descending trigeminal nucleus, medial funicular nucleus, inferior olive, and to the contralateral sensory trigeminal nucleus. These observations indicate that the primary and secondary trigeminal sensory projections of a cyprinid teleost, the carp, are similar to those in percomorph teleosts.     

Caudal topographic nucleus isthmi and the rostral nontopographic nucleus isthmi in the turtle, Pseudemys scripta

Isthmotectal projections in turtles were examined by making serial section reconstructions of axonal and dendritic arborizations that were anterogradely or retrogradely filled with HRP. Two prominent tectal-recipient isthmic nuclei--the caudal magnocellular nucleus isthmi (Imc) and the rostral magnocellular nucleus isthmi (Imr)--exhibited strikingly different patterns of organization. Imc cells have flattened, bipolar dendritic fields that cover a few percent of the area of the cell plate constituting the nucleus and they project topographically to the ipsilateral tectum without local axon branches. The topography was examined explicitly at the single-cell level by using cases with two injections at widely separated tectal loci. Each Imc axon terminates as a compact swarm of several thousand boutons placed mainly in the upper central gray and superficial gray layers. One Imc terminal spans less that 1% of the tectal surface. Imr cells, by contrast, have large, sparsely branched dendritic fields overlapped by local axon collaterals while distally, their axons nontopographically innervate not only the deeper layers of the ipsilateral tectum but also ipsilateral Imc. Imr receives a nontopographic tectal input that contrasts with the topographic tectal input to Imc. Previous work on nucleus isthmi emphasized the role of the contralateral isthmotectal projection (which originates from a third isthmic nucleus in turtles) in mediating binocular interactions in the tectum. The present results on the two different but overlapping ipsilateral tecto-isthmo-tectal circuits set up by Imc and Imr are discussed in the light of physiological evidence for selective attention effects and local-global interactions in the tectum.     

The nucleus isthmi and dual modulation of the receptive field of tectal neurons in non-mammals

The nucleus isthmi in the dorsolateral tegmentum had been one of the most obscure structures in the nonmammalian midbrain for eight decades. Recent studies have shown that this nucleus and its mammalian homologue, the parabigeminal nucleus, are all visual centers, which receive information from the ipsilateral tectum and project back either ipsilaterally or bilaterally depending on species, but not an auditory center as suggested before. On the other hand, the isthmotectal pathways exert dual, both excitatory and inhibitory, actions on tectal cells in amphibians and reptiles. In birds, the magnocellular and parvocellular subdivisions of this nucleus produce excitatory and inhibitory effects on tectal cells, respectively. The excitatory pathway is mediated by glutamatergic synapses with AMPA and NMDA receptors and/or cholinergic synapses with muscarinic receptors, whereas the inhibitory pathway is mediated by GABAergic synapses via GABA_A receptors. Further studies have shown that the magnocellular and parvocellular subdivisions can differentially modulate the excitatory and inhibitory regions of the receptive field of tectal neurons, respectively. Both the positive and the negative feedback pathways may work together in a winner-take-all manner, so that the animal could attend to only one of several competing visual targets simultaneously present in the visual field. Some behavioral tests seem to be consistent with this hypothesis. The present review indicates that the tecto-isthmic system in birds is an excellent model for further studying tectal modulation and possibly winner-take-all mechanisms.     

An ascending visual pathway to the dorsal telencephalon through the optic tectum and nucleus prethalamicus in the yellowfin goby Acanthogobius flavimanus (Temminck & Schlegel, 1845)

Dual visual pathways reaching the telencephalon appear to be an ancient vertebrate trait, but some teleost fish seem to possess only one pathway via the optic tectum. We undertook the present study to determine if and when this loss occurred during evolution. Tracer injection experiments to the optic nerve, the optic tectum, and the dorsal telencephalon were performed in the present study, to investigate ascending visual pathways to the dorsal telencephalon in an acanthopterygian teleost, the yellowfin goby Acanthogobius flavimanus (Temminck & Schlegel, 1845). We confirmed the presence of a nucleus prethalamicus (PTh) in the goby, which has been convincingly identified only in holocentrids, suggesting that this nucleus is present in other acanthopterygians. We found that the optic tectum projects to the PTh bilaterally. The PTh projects in turn to the dorsal telencephalon, ipsilaterally. These results suggest that the yellowfin goby possesses only an extrageniculate‐like pathway, while a geniculate‐like pathway could not be identified. This situation is common with that of holocentrids and may be a character common in acanthopterygians. It is possible that a geniculate‐like system was lost in the common ancestor of acanthopterygians, although the scenario for the evolution of ascending visual systems in actinopterygians remains uncertain due to the lack of precise knowledge in a number of actinopterygian taxons.     

Changes in the topographically organized connections between the nucleus isthmi and the optic tectum after partial tectal ablation in adult goldfish

The projection of the nucleus isthmi to the ipsilateral optic tectum was examined in normal goldfish. This was compared to the projection in animals in which the entire visual field had been induced to compress onto a rostral half tectum by caudal tectal ablation. The isthmo-tectal projection was examined by making localized injections of horseradish peroxidase into the optic tecta and observing the patterns of labeled cells within the nucleus isthmi. The teleost nucleus isthmi consists of a cell sparse medulla covered by a cellular cortex, which is thick on the rostral, medial, and dorsal surfaces of the nucleus. Almost all isthmic cells projecting to the tectum were located in the area of thick cortex. In normal fish, rostral tectal injections labeled cells in the rostroventral portion of the thick cortex; injections midway in the rostrocaudal tectal axis labeled more caudodorsally located cells, and caudal tectal injections labeled cells a little further caudally in extreme dorsal cortex. The rostroventral to caudodorsal isthmic axis was therefore seen to project rostrocaudally along the tectum. This topography contrasts somewhat with the situation seen in amphibia where the rostrocaudal tectal axis receives projections from the rostrocaudal isthmic axis. In fish with half-tectal ablations, injections near the caudal edge of the half tectum (at a site that had originally been midtectal) labeled cells that had previously projected to caudal tectum. Rostral tectal injections in fish with compression of the visual field gave a normal pattern of labeled isthmic cells. The results indicate that a topographically ordered isthmo-tectal projection exists in goldfish that may be induced to compress onto a half tectum.     

Projections of the sensory trigeminal nucleus in a percomorph teleost, tilapia (Oreochromis niloticus)

The sensory trigeminal nucleus of teleosts is the rostralmost nucleus among the trigeminal sensory nuclear group in the rhombencephalon. The sensory trigeminal nucleus is known to receive the somatosensory afferents of the ophthalmic, maxillar, and mandibular nerves. However, the central connections of the sensory trigeminal nucleus remain unclear. Efferents of the sensory trigeminal nucleus were examined by means of tract-tracing methods, in a percomorph teleost, tilapia. After tracer injections to the sensory trigeminal nucleus, labeled terminals were seen bilaterally in the ventromedial thalamic nucleus, periventricular pretectal nucleus, medial part of preglomerular nucleus, stratum album centrale of the optic tectum, ventrolateral nucleus of the semicircular torus, lateral valvular nucleus, prethalamic nucleus, tegmentoterminal nucleus, and superior and inferior reticular formation, with preference for the contralateral side. Labeled terminals were also found bilaterally in the oculomotor nucleus, trochlear nucleus, trigeminal motor nucleus, facial motor nucleus, facial lobe, descending trigeminal nucleus, medial funicular nucleus, and contralateral sensory trigeminal nucleus and inferior olive. Labeled terminals in the oculomotor nucleus and trochlear nucleus showed similar densities on both sides of the brain. However, labelings in the trigeminal motor nucleus, facial motor nucleus, facial lobe, descending trigeminal nucleus, and medial funicular nucleus showed a clear ipsilateral dominance. Reciprocal tracer injection experiments to the ventromedial thalamic nucleus, optic tectum, and semicircular torus resulted in labeled cell bodies in the sensory trigeminal nucleus, with a few also in the descending trigeminal nucleus.     

Long ascending projections of the spinal dorsal horn in a teleost, Sebastiscus marmoratus

Ascending projections of the spinal cord in a teleost, Sebastiscus marmoratus, were studied by means of the horseradish peroxidase tracing method. Projecting fibers were observed in the reticular formation, vagal lobe, octaval nuclei, a dorsomedial portion of the descending nucleus of the trigeminal nerve, corpus cerebelli and nucleus ventromedialis thalami.