The organization of monoamine neurons within the brainstem of the north american opossum (didelphis virginiana)

The distribution of monoamine-containing neurons within the brain of the opossum is described using the Falck-Hillarp histofluorescence technique. Catecholamine-containing neurons are organized into four groups. The medulla contains one group which is located dorsolateral to the lateral reticular nucleus and ventrolateral to the dorsal vagal nucleus. The second collection is found within the pons and includes both the locus coeruleus and a region continuous with it referred to as the nucleus coeruleus, pars α. The third aggregate includes the substantia nigra, the ventral tegmental area, and the mesencephalic reticular formation and a fourth group is located within the periventricular and dorsal paraventricular nuclei of the hypothalamus. The indoleamine-containing cell bodies are distributed within the nuclei raphe obscurus, pallidus, magnus, dorsalis, and the nuclei linearis and superior centralis except at certain pontine levels where they appear laterally within the reticular formation. A number of small intensely fluorescent (SIF) cells are present within the connective tissue surrounding the brain and its blood vessels as well. Although certain differences are present, the didtribution of monoamine neurons in the American opossum conforms generally to that described for the placental mammals studied to date.     

Developmental plasticity of ascending spinal axons: Studies using the north american opossum,Didelphis virginiana

The objectives of the present study were to determine if axons of all ascending tracts grow through the lesion after transection of the thoracic spinal cord during development in the North American opossum, and if so, whether they reach regions of the brain they normally innervate. Opossum pups were subjected to transection of the mid-thoracic cord at PD5, PD8, PD12, PD20, or PD26 and injections of Fast Blue (FB) into the lower thoracic or upper lumbar cord 30–40 days or 6 months later. In the PD5 transected cases, labeled axons were present in all of the supraspinal areas labeled by comparable injections in unlesioned, age-matched controls. In the experimental cases, however, labeled axons appeared to be fewer in number and in some areas more restricted in location than in the controls. When lesions were made at PD8, labeled axons were present in the brain of animals allowed to survive 30–40 days prior to FB injections but they were not observed in those allowed to survive 6 months. When lesions were made at PD12 or later, labeled axons were never found rostral to the lesion. It appears, therefore, that axons of all ascending spinal pathways grow though the lesion after transection of the thoracic cord in developing opossums and that they innervate appropriate areas of the brain. Interestingly, the critical period for such growth is shorter than that for most descending axons, suggesting that factors which influence loss of developmental plasticity are not the same for all axons.     

Observations on the development of brainstem-spinal systems in the north american opossum

The North American opossum is born 12 to 13 days after conception and is available for 90 days or more in an external pouch where it can be observed and experimentally manipulated. It is of particular interest that the hindlimbs of the newborn opossum are very immature and remain immobile for a week or more after birth. Degeneration techniques reveal that immature brainstem axons are present within the marginal zone of the lumbosacral cord before hindlimb movements begin (our stage I) and material processed for formaldehyde induced fluorescence shows that some of them transport monoamines. Several lines of evidence suggest that part of the fluorescent axons arise within the nucleus locus coeruleus. At this early stage the electron microscope reveals that all brainstem-spinal axons are small (0.1-0.4 μm in diameter) and unmyelinated. By the time random hindlimb movements can be observed (stage II), brainstem axons, including those transporting monoamines, can be demonstrated to have grown into limited areas of the intermediate zone of the lumbosacral cord and to arise from most of the areas contributing to them in the adult animal (horseradish peroxidase technique). Such axons are still immature and it is not yet clear that they have formed synaptic terminals. Brainstem axons continue to grow into the intermediate zone of the lumbosacral cord for some time and come to occupy all of their adult territories before thoracic transection produces obvious change in hindlimb motility (beginning of stage III). It is still another 20 days or so before thoracic transection produces spinal shock comparable to that in the adult animal. The relatively mature use of the hindlimbs and the full expression of spinal shock correlate with changes in the technique and survival time needed to demonstrate degenerating brainstem axons in experimental material.     

The distribution of ganglion cells in the retina of the north american opossum (Didelphis virginiana)

The distribution of ganglion cells in the retina of the opossum was determined from whole-mounted retinae stained with cresyl violet. Isodensity lines were approximately circular with a peak density of 2,000 to 2,700 cells/mm² in superior temporal retina (area centralis). The total number of retinal ganglion cells was estimated to be 72,000 to 135,000 (mean 101,026) in retinae ranging from 125 to 187 mm² in total area. Three groups of ganglion cells were distinguished on the basis of soma size and retinal topography. Large cells (24 to 32 μm diameter) were fairly evenly distributed across the retina. Medium cells (12 to 23 μm diameter) were more numerous in the superior temporal quadrant than in other regions of the retina. Small cells (7 to 11 μm diameter) were prominent in all retinal regions, but particularly in nasal and inferior retina. An analysis of topographical differences in soma size distribution suggests that the medium size cells can be further subdivided into small-medium and large-medium groups.     

Developmental sequence in the origin of descending spinal pathways. Studies using retrograde transport techniques in the North American opossum (Didelphis virginiana)

The origin of descending pathways to thoracic and cervical levels of the spinal cord has been investigated with retrograde tracing techniques in a series of pouch young and adult opossums. The opossum was chosen because it is born in a very immature state, 12-13 days after conception, and has a protracted development in an external pouch. A few neurons in the pontine reticular formation and nucleus coeruleus were labeled by horseradish peroxidase (HRP) injections of the thoracic cord as early as postnatal day (PND) 3. By PND 5, similar injections labeled neurons in the same areas as well as in the medullary reticular formation, the raphe nuclei of the caudal pons and medulla, the spinal trigeminal nuclei, the vestibular complex, the accessory oculomotor nuclei and the interstitial nucleus of Cajal. When Nuclear Yellow (NY) was employed, neurons were also labeled in the red nucleus, the hypothalamus and possibly in the nucleus of the solitary tract. Regardless of the technique employed, neurons in the dorsal column nuclei were not labeled by thoracic injections until at least PND 14. Axons from the nucleus ambiguus, the fastigial and interposed nuclei of the cerebellum as well as the intermediate and deep layers of the superior colliculus reach cervical levels of the cord, where they are specifically targeted, by at least PND 17. They do not significantly overgrow those levels during development. Corticospinal axons are the last of the major descending pathways to innervate the spinal cord. Cortical neurons cannot be labeled by cervical injections of either HRP or NY until at least PND 30. Evidence for transient brainstem-spinal and corticospinal projections was obtained.     

Efferent tectal pathways of the opossum (Didelphis virginiana)

Efferent tectal pathways have been determined for the opossum, Didelphis virginiana, by employing the Nauta-Gygax technique ('54) on animals with tectal lesions of varying sizes. The superior colliculus projected tectothalamic fascicles to the suprageniculate nucleus, the central nucleus of the medial geniculate body, the lateral posterior thalamus, the pretectal nucleus, the ventral lateral geniculate nucleus, the fields of Forel and zona incerta, the parafascicular complex, the paracentral thalamic nucleus and in some cases to restricted areas of the anterior thalamus. Degenerating fibers from superior collicular lesions showed profuse distribution to the deeper layers of the superior colliculus on both sides and to the midbrain tegmentum, but only minimally to the red nucleus and substantia nigra. Fibers of tectal origin did not distribute to the motor nuclei of the oculomotor or trochlear nerves. At pontine levels, efferent fascicles from the superior colliculus were present as an ipsilateral tectopontine and tectobulbar tract and as a crossed predorsal bundle. The tectopontine tract ended mostly within the lateral and ventral basal pontine nuclei, whereas the ipsilateral tectobulbar tract distributed to certain specific areas of the reticular formation throughout the pons and medulla, minimally to the most medial portion of the motor nucleus of the facial nerve and to the nucleus of the inferior olive. The predorsal tract contributed fascicles to certain nuclei of the pontine raphe, extensively to the medial reticular formation of the pons, to the central and ventral motor tegmental nuclei of the reticular formation within the pons and medulla, to the paraabducens region, minimally to cells within restricted portions of the motor nucleus of the facial nerve, to certail specific regions of the caudal medulla and to the cervical cord as far caudally as the fourth segment. The tectospinal fascicles were few but some ended related to the spinal accessory nucleus and the ventral medial nucleus of the ventral horn. Lesions of the inferior colliculus resulted in degenerating fibers which distributed rostrally to the rostral nucleus of the lateral lemniscus and parabrachial region, to the suprageniculate nucleus, the parabigeminal nucleus and to the central nucleus of the medial geniculate body. The inferior colliculus also contributed fibers to the ipsilateral tectopontine and tectobulbar tracts. The latter bundle was traced as far caudally as the medulla and may arise from cells of the superior colliculus which are situated dorsal to the nucleus of the inferior colliculus.     

The organization of olivo-cerebellar projections in the opossum, Didelphis virginiana, as revealed by the retrograde transport of horseradish peroxidase

The retrograde transport of horseradish peroxidase was utilized to map olivo-cerebellar projections in the Virginia opossum. The spinal cerebellum (anterior lobe, paramedian lobule and pyramis) receives input from several separate regions in the dorsal accessory nucleus, the medial accessory nucleus and portions of the principal nucleus. Evidence is present for a topographical organization whereby specific regions of the olive project to restricted longitudinal zones. The visual-auditory region of the posterior vermis receives input from small areas within the caudal part of the medial accessory nucleus. From a distinctly separate region of the caudal medial accessory nucleus (as well as the principal nucleus), axons project to the uvula. The vestibulo-cerebellum is the recipient of axons from the cap of Kooy and from two spatially separate regions of the medial accessory nucleus. The cerebellar hemisphere (Crus I and II, lobus simplex) is the target of axons from parts of all three olivary nuclei and it is possible that the projections from the different nuclei are targeted upon separate zones. The paraflocculus was found to receive an input from the rostral part of the medial accessory nucleus and from the principal nucleus. The present results suggest that a distinct olivary region may project to several widely separate areas of the cerebellum, and that one cerebellar region may receive input from several areas of the olive. The organization of the olivocerebellar projection is highly complex, but when considered in light of known inputs to the olive, certain patterns emerge.     

The vestibular complex of the American opossum didelphis virginiana. II. Afferent and efferent connections.

We have demonstrated the connectivity of the opossum's vestibular nuclei using degeneration, autoradiographic and horseradish peroxidase techniques and have found it to be generally comparable to that reported for the cat. Apart from the primary input described in Part I of our study, the cerebellum provides the major source of afferent connections to the vestibular complex. Axons from the cerebellar cortex distribute mainly to vestibular areas which receive no primary afferent projections, e.g., the dorsal part of the lateral vestibular nucleus, the dorsolateral margin of the inferior vestibular nucleus as well as cell groups comparable to "f" and "x." In contrast, fastigial fibers show considerable overlap with primary vestibular input, particularly in the ventral part of the lateral nucleus, the central part of the inferior nucleus and the medial nucleus. Axons of fastigial origin also distribute to the superior vestibular nucleus, to subnuclei "f" and "x" and to the parasolitary region. Although spinal fibers are diffuse within the main vestibular nuclei, they ramify quite densely within subnucleus "x." Most of the spinovestibular projection appears to arise in the cervical spinal cord of the opossum. Ipsilateral connections from the interstitial nucleus of Cajal and surrounding areas end predominantly, but not exclusively, in the medial vestibular nucleus. A crossed midbrain projection has been verified from the red nucleus to cell group "x" and the lateral part of the inferior nucleus, as well as to an area possibly comparable to cell group "z," as described for the cat. In Part I of our study we have shown that the major targets of primary vestibular fibers are the central part of the superior nucleus, a portion of the parabrachial complex possibly comparable to subnucleus "y"," the ventral part of the lateral nucleus and the medial nucleus. All of these primary zones give rise to fibers supplying extraocular nuclei and surrounding areas (present study). The ascending midbrain fibers from the superior nucleus end mainly ipsilaterally, whereas those from the putative subnucleus "y" and the medial vestibular nucleus distribute contralaterally for the most part. Although the dorsal part of the lateral vestibular nucleus has no primary vestibular input, it does receive a major projection from the cerebellar cortex. This same region of the lateral nucleus projects to the spinal cord, but not to extraocular nuclei. The ventral part of the lateral nucleus, and perhaps the medial nucleus, also relay to the spinal cord. Additional projections from all vestibular nuclei to the reticular formation provide indirect routes through which the vestibular nuclei can potentially effect multiple systems, including those innervating the spinal cord. Finally, commissural vestibular connections of the opossum are shown to arise within all four major nuclei.     

The lateral reticular nucleus of the opossum (Didelphis virginiana). II. Connections

Conformational, histochemical and histofluorescent studies reveal that the entire lateral reticular nucleus (LRN) of the Virginia opossum is positive for cholinesterase activity and that its rostral portion is rich in fluorescent varicosities of the catecholamine type. Although neocortical-LRN connections are relatively meagre, projections to the LRN from the red nucleus are extensive and topographically organized. Rubral-LRN fibers arise from large-medium sized neurons and distribute to the trigeminal division of the LRN as well as to specific portions of its external and internal divisions. Certain areas of the midbrain and pontine reticular formation, as well as the vestibular nuclei, project to the LRN and there is some evidence that reticular neurons adjacent to the LRN provide additional input. Although a relatively small fastigial-LRN projection exists, no evidence was found for a contribution from any of the other deep cerebellar nuclei. Spinal-LRN connections are extensive and topographically organized. Each of the inputs to the LRN have specific terminal targets, but there are varying degrees of overlap. Most of the LRN projects in an organized fashion to the anterior lobe of the spinal cerebellum, whereas only relatively restricted areas relay to the paramedian lobule and the pyramis. Lateral reticular axons distribute to specific longitudinal zones in such areas and the available material suggests that both convergence and divergence exist. The LRN also relays to the lobus simplex, and perhaps to crus I, as well as to visual-auditory areas of the cerebellar vermis. The distribution of the various afferent connections of the LRN is interpreted in light of LRN-cerebellar connections. Although we have described details that have not been elucidated in other species, where comparisons can be made it appears that the connectivity of the opossum LRN is comparable in most respects to that of placental mammals.     

The basilar pontine gray of the opossum (Didelphis virginiana). I. Morphology

The basilar pontine nuclei were described for the opossum in Nissl stained, serial, transverse sections, and named (dorsal, ventral, medial, and lateral) with reference to the pontine pyramidal bundle. The details of neuron morphology were determined by employing Nissl stains and the Golgi technique. The neuron cell bodies of the basilar pontine gray were round, oval or spindle in shape and displayed little angularity. Measurements of neuronal perikaryia indicated a size range of from 8 to 20 μ. The Nissl substance appeared finely granular with some cells displaying large discrete granules. Dendrites observed in Golgi preparations either extended some distance from the perikaryon or ended adjacent to the cell body in terminal arborizations. The phylogenetic significance of the nucleur groups and the primitive condition of a limited pre-trigeminal basilar pons is discussed.     

Raphespinal projections in the north american opossum: Evidence for connectional heterogeneity

Retrograde transport studies revealed that the nuclei pallidus, obscurus, and magnus raphae as well as the adjacent reticular formation innervate the spinal cord in the opossum. HRP-lesion experiments showed that a relatively large number of neurons within the nucleus obscurus raphae and closely adjacent areas of the nucleus reticularis gigantocellularis project through the ventrolateral white matter and that many cells within the nucleus magnus raphae, the nucleus reticularis gigantocellularis pars ventralis, and the nucleus reticularis pontis pars ventralis contribute axons to the dorsal half of the lateral funiculi. Neurons within the rostral pole of the nucleus magnus raphae and the adjacent nucleus reticularis pontis pars ventralis may project exclusively through the latter route. Each of the above-mentioned raphe and reticular nuclei contain nonindolaminergic as well as indolaminergic neurons (Crutcher and Humbertson, 1978). When True-Blue was injected into the spinal cord and the brain processed for monoamine histofluorescence evidence for True-Blue was found in neurons of both types. Injections of ³H-leucine centered within the nuclei pallidus and obscurus raphae and/or the closely adjacent nucleus reticularis gigantocellularis labeled axons within autonomic nuclei and laminae IV-X. Labeled axons were particularly numerous within the intermediolateral cell column and within laminae IX and X. Injections of the caudoventral part of the nucleus magnus raphae or the adjacent nucleus reticualris gigantocellularis pars ventralis labeled axons in the same areas as well as within laminae I-III. When the injection was placed within the rostal part of the nucleus magnus raphae or the adjacent nucleus reticularis pontis pars ventralis axons were labeled within laminae I-III and external zones of laminae IV-VII, but not within lamina IX. The immunohistofluorescence method revealed evidence for indolaminergic axons in each of the spinal areas labeled by injections of ³H-leucine into the raphe and adjacent reticular formation. They were particularly abundant within the intermediolateral cell column and within laminae IX and X. These data indicate that raphe spinal systems are chemically and connectionally heterogeneous.     

The motor nucleus of the facial nerve in the opossum (Didelphis marsupialis virginiana). Its organization and connections

The organization of the facial nucleus was studied in the opossum by localizing neurons which stin poorly for acetylcholinesterase activity following transection of identified facial rami. The caudal auricular representation is limited to the ventromedial extreme of the nucleus, whereas the neurons contributing to the cervical ramus are situated dorsally and medially. The zygomatic representation extends throughout the intermediate portion of the nucleus, apparently overlapping with that of the palpebral and rostral auricular muscles which is limited to the ventral extreme of the intermediate zone. The buccolabial area is particularly large in the opossum and encompasses most of the lateral facial enlargement. Midbrain-facial projections were identified from the superior colliculus, the midbrain tegmentum (particularly caudal ventromedial areas) and the red nucleus. The location of terminal degeneration in the facial nucleus following lesions within each of these areas was plotted and interpreted in light of facial organization. Of particular note is the fact that the fibers of rubral origin distribute preferentially to the zygomatic and, to some extent, buccolabial areas, whereas the ventromedial tegmental system distributes most strongly to the areas of caudal auricular, cervical, palpebral and rostral auricualar representation. The medial and intermediate regions of the facial nucleus receive a denser midbrain projection than does the lateral (buccolabial) area. In contrast, evidence was obtained for an extensive facial projection from the parvocellular reticular formation and the caudal spinal trigeminal nucleus which strongly favors the buccolabial enlargement. The possibility exists that the medial pontine and medullary reticular formation as well as portions of the dorsal column nuclei also have a facial projection. Spino-facial fibers arise rostral to the cervical enlargement and show a predilection for the medial facial enlargement (cervical and caudal auricular areas). Although some systems distribute preferentially to specific areas of the facial nucleus, overlap is present suggesting considerable integration.     

The basilar pontine gray of the opossum (Didelphis virginiana). II. Experimental determination of neocortical input

The pattern of neocortical input into the basilar pons of the opossum was determined by employing the Nauta-Gygax technique ('54) on the brains of animals previously subjected to neocortical lesions. The results indicate that every neocortical area projects to some portion of the basilar pons in this form. Degenerating fibers resulting from more rostral cortical lesions (frontal and preorbital areas) terminated profusely within the medial and ventral nuclei and, to a lesser extent, in the smaller dorsal nucleus. Fascicles from more caudal areas (striate and peristriate cortices) ended abundantly in the lateral nuclear group, and in the lateral part of the ventral nucleus. Every neocortical region studied projected to some portion of the ventral pontine nucleus. Degenerating fibers terminated to some extent within each nucleus of the basilar pons as a result of lesions in midcortical regions (paramarginal, postorbital and parietal cortices). A few fibers of frontal, orbital and parietal origin terminated in the contralateral basilar pontine gray.     

The origin of efferent fibers to the inner ear in a turtle (Terrapene ornata). A horseradish peroxidase study

The origin of efferent acoustic and vestibular fibers was determined in the turtleTerrapene ornata. After injection of an aqueous solution of horseradish peroxidase (HRP) into either the cochlear duct or into the ampullae of the horizontal and anterior semicircular canals, neurons in the medullary reticular formation were labeled by the reaction product of retrogradely transported HRP. These neurons were located bilaterally in the medial reticular nucleus. The majority were found ipsilateral to the injection site. There was no demonstrable difference in size, shape, and labeling pattern between efferent acoustic and efferent vesticular neurons. The crossed component of efferent acoustic fibers, however, was rather sparsely developed.     

Thalamocaudate projections in the macaque monkey (a horseradish peroxidase study).

The distribution of thalamocaudate neurons was studied in four macaque monkeys (Macaca mulatta) using the method of retrograde horseradish peroxidase (HRP Sigma VI) transport following previous aspiration of part of the frontal cortex and subcortical white matter covering the head of the caudate nucleus. After HRP injections into the head of the caudate nucleus we found retrogradely labelled neurons mainly in the intralaminar nuclei (nc. paracentralis, nc. centralis lateralis, nc. parafascicularis) and in the midline nuclei (nc. parataenialis, nc. centralis pars densocellularis, nc. centralis superior, nc. centralis intermedialis, nc. centralis inferior, nc. reuniens). Some labelled neurons were seen in the anterior thalamic nuclei (nc. anterodorsalis, nc. anteromedialis, nc. anteroventralis). In the ventral thalamus labelled neurons were seen in the nc. ventralis anterior (pars magnocellularis) and in the nc. ventralis lateralis (pars medialis). Some labelled neurons were distributed in the marginal parts of the nc. mediodorsalis. As for the caudal portion of the thalamus, labelled neurons were found in the nc. pulvinaris medialis and nc. limitans. The thalamocaudate projection is an exclusively ipsilateral projection system. The position of the thalamocaudate neurons coincides, to a certain degree, with the termination intrathalamic area of the nigrothalamic and tectothalamic projections arising from the deep layers of the superior colliculus.     

The lateral reticular nucleus of the opossum (Didelphis virginiana). I. Conformation,cytology and synaptology

When viewed in Nissl preparations, the lateral reticular nucleus (LRN) of the opossum can be divided into three subgroups: a medial internal portion, a lateral external portion and a rostral trigeminal division. Neurons within the internal division measure 13-45 μ in their greatest dimension whereas those within the external and trigeminal portions measure 11-32 μ and 14-27 μ respectively. Golgi impregnations reveal that many neurons in all three subdivisions display a radial dendritic pattern although some of the nerve cells within the external division have dendrites which orient mainly in a ventromedial to dorsolateral direction. The cell bodies of LRN neurons are relatively spine-free. However, a small percentage of neurons exhibit clusters of sessile spines on proximal and more distal dendritic segments. No locally ramifying axons or axon collaterals were found within the LRN. Synaptic terminals within the LRN were divided into four categories: (1) small terminals measuring 2.5 μ or less containing agranular spherical vesicles; (2) small terminals (2.5 μ or less) with agranular pleomorphic synaptic vesicles, i.e., a mixture of spherical and elliptical synaptic vesicles; (3) small terminals (2.5 μ or less) containing agranular spherical or pleomorphic vesicles with a variable number (4-27) of dense core vesicles; and (4) large terminals (greater than 2.5 μ) which contain agranular spherical synaptic vesicles and a variable number of dense core vesicles (1-17). Dendritic diameters were measured from Golgi impregnations and correlated with cross-sectioned profiles in electron micrographs to help determine the post-synaptic distribution of synaptic endings. Small terminals containing agranular spherical or pleomorphic synaptic vesicles contact the soma and entire dendritic tree in each portion of the nucleus, whereas the small terminals containing dense core vesicles are usually located on distal dendrites or spines. Some large terminals make multiple synaptic contacts with a cluster of spines, others contact groups of small (distal) dendrites. In order to identify two of the major afferent systems to the LRN, 15 adult opossums were subjected to either a cervical spinal cord hemisection or a stereotaxic lesion of the red nucleus. Two days subsequent to spinal hemisection, large terminals in the caudal part of the ipsilateral LRN exhibit either an electron dense or filamentous reaction. Their postsynaptic loci are spines and shafts of proximal dendrites or a number of distal dendrites and spines. In addition, small terminals containing spherical agranular synaptic vesicles undergo an electron dense reaction in the same areas. Their postsynaptic loci are proximal or distal dendrites. Two days subsequent to rubral lesions, small terminals containing agranular spherical synaptic vesicles undergo a dark reaction in rostral portions of the contralateral nucleus. They contact intermediate or distal dendrites and occasionally spines.     

Rubrobulbar projections of the opossum (Didelphis virginiana)

The course and distribution of rubral pathways to the pons and medulla were determined for the opossum by employing the Nauta-Gygax and Fink-Heimer techniques on the brain stems of animals with lesions either within the red nucleus or involving the fibers emanating from it. Control material was provided by previous studies on corticomesencephalic and tectal efferent pathways and by the brains of specimens subjected to deep midbrain lesion which did not involve the red nucleus. A predominantly crossed rubrobulbar pathway coursed through the brain stem as described by Voris and Hoerr ('32) and distributed to the nucleus “K” of Meessen and Olszewski ('49), to neurons interspersed between the fiber bundles of the motor root of the trigeminal nerve, to the parabrachial nucleus of the brachium conjunctivum, the parvocellular reticular formation, the ventral and medial portions of the spinal trigeminal nucleus (nucleus oralis and interpolaris), the lateral and intermediate portions of the motor nucleus of the facial nerve, the lateral reticular nucleus, the ventral external arcuate nucleus and the subnucleus reticularis dorsalis medullae oblongatae. The possible significance of these connections in the opossum is discussed.