Efferent projections of the superior colliculus in the opossum

The Fink and Heimer technique was used to study the ascending and descending fiber degenerations resulting from unilateral electrolytic lesions in the superior colliculus of sixteen adult opossums. The ascending fiber degeneration left the colliculus by way of a small parabrachial bundle or with the brachium of the superior colliculus. The former bundle contributed degenerating terminals to the suprageniculate, parabrachial, and magnocellular medial geniculate nuclei and appeared to terminate in the lateral terminal nucleus of Hayhow (Hayhow, '66). The latter bundle contributed fibers to the pretectal and posterolateral nuclei as well as discrete projections to the ventral lateral geniculate nucleus and ventral thalamus. Descending degenerating fibers in the brainstem were destributed along three tracts: (1) a medial predorsal bundle, (2) an intermediate tectoreticular, and (3) a lateral tectopontine. These tracts were seen to terminate mainly in tegmental and reticular centers as well as in the lateral basilar nucleus of the pons. Degenerating fibers of the predorsal bundle were followed to lower medullary levels but could not be traced to spinal levels. The present findings are descussed in relation to data reported for more developed mammalian species.     

An autoradiographic study of the efferent connections of the superior colliculus in the cat

The differential projections of the three main cellular strata of the superior colliculus have been examined in the cat by the autoradiographic method. The stratum griseum superficiale projects caudally to the parabigeminal nucleus and rostrally to several known visual centers: the nucleus of the optic tract and the olivary pretectal nucleus in the pretectum; the deepest C laminae of the dorsal lateral geniculate nucleus; the large-celled part of the ventral lateral geniculate nucleus; the posteromedial, large-celled part of the lateral posterior nucleus of the thalamus. Several of these projections are topographically organized. The stratum griseum profundum gives rise to most of the descending projections of the superior colliculus. Ipsilateral projections pass to both the dorsolateral and lateral divisions of the pontine nuclei, the cuneiform nucleus, and the raphe nuclei, and to extensive parts of the brainstem reticular formation: the tegmental reticular nucleus, and the paralemniscal, lateral, magnocellular, and gigantocellular tegmental fields. Contralateral projections descending in the predorsal bundle pass to the medial parts of the tegmental reticular nucleus and of some of the tegmental fields, the dorsal part of the medial accessory nucleus of the inferior olivary complex, and to the ventral horn of the cervical spinal cord. Ascending projections of the stratum griseum profundum terminate in several nuclei of the pretectum, the magnocellular nucleus of the medial geniculate complex and several intralaminar nuclei of the thalamus, and in the fields of Forel and zona incerta in the subthalamus. The strata grisea profundum and intermediale each have projections to homotopic areas of the contralateral superior colliculus, to the pretectum, and to the central lateral and suprageniculate nuclei of the thalamus. However, the stratum griseum intermediale has few or no descending projections.     

Tectal projections of an infrared sensitive snake,Crotalus viridis

Crotaline snakes have detectors for infrared radiation and this information is projected to the optic tectum in a spatiotopic manner. The tectal projections were examined in Crotalus viridis with the use of silver methods for degenerating fibers and the autoradiographic and horseradish peroxidase tracing methods. Large lesions included all of the tectal layers but not the underlying structures. Projections to the thalamus include a sparse input to the ipsilateral ventral and dorsal lateral geniculate nuclei, the ventromedial nucleus, and nucleus lentiformis thalami. Nucleus rotundus was not detected. The projections to the pretectal nuclei are primarily ipsilateral to the nucleus lentiformis mesencehali and pretectal nucleus. At the level of the mesencephalon, tectal efferents are bilateral to nucleus profundus mesencephali and the tegmentum. There is minimal input to the contralateral deep tectal layers. There are ispilateral terminations in a nucleus identified as the posterolateral tegmental nucleus. Descending fibers include the two major tracts—the ventral tectobulbar tract that terminates in the ipsilateral lateral reticular formation and the predorsal bundle that distributes throughout the contralateral medial reticular formation. Two small descending tracts were noted—the intermediate and dorsal tectobulbar tracts. All of these descending tracts appear to terminate by the time they reach the caudal medulla. After superficial lesions terminals could be found in the ventral lateral geniculate nucleus, the nucleus profundus mesencephali, and the posterolateral tegmental nucleus; the two major descending tracts contained degenerated fibers as well. The areas receiving tectal input in Crotalus were compared to those of other reptiles and discussed.     

Immunocytochemical localization of the choline acetyltransferase containing neuron system in the rat lower brain stem

This distribution of choline acetyltransferase (CHAT) immunoreactivity (CHAT-I) in the rat lower brain stem was analyzed using a highly sensitive avidin-biotin immunocytochemical method and 3-amino-9-ethyl-carbazole visualization. A much wider and more abundant distribution of CHAT-I structures in the lower brain stem was demonstrated than in earlier studies. The following areas were newly identified as areas rich in CHAT-I fibers: the interpeduncular nucleus, medial geniculate body, central gray matter of pons, pontine nucleus, parabigeminal nucleus, dorsal tegmental nucleus of Gudden, lateral trapezoid nucleus, inferior colliculus, dorsal and ventral cochlear nuclei, medial and lateral vestibular nuclei, reticular formation of medulla oblongata, and gelatinosa of caudal trigeminal spinal tract nucleus. In addition to the areas in which they have been known to exist, CHAT-I perikarya were found in the caudal portion of substantia nigra pars reticulata, the area between trigeminal motor nucleus and superior olivary nucleus, the medial and spinal vestibular nucleus, prepositus hypoglossal nucleus, raphe magnus and obscurus, ventromedial portion of solitary tract nucleus and its just ventral reticular formation, and caudal trigeminal spinal tract nucleus.     

Autoradiographic demonstration of retinal projections to the brain stem structures in the rabbit using transneuronal tracing technique with special reference to the retinal projections to the inferior olive

Retinal projections to the brain stem structures in the rabbit were examined autoradiographically using transneuronal tracing technique. Three or four weeks after intraocular injections of tritiated proline and tritiated fucose, significant amounts of silver grains indicating transneuronal labeling of axon terminals were present bilaterally in the visual cortical areas, the dorsal portions of the medial geniculate nucleus, and the suprageniculate nucleus, and contralaterally in the thalamic reticular nucleus, the lateroposterior-pulvinar nuclear complex, the parabigeminal nucleus, the pontine tegmental reticular nucleus of Bechterew, the dorsolateral, lateral, and paramedian pontine nuclei, the pontine reticular formation, and the dorsal cap and beta nucleus of the inferior olive. The label in the pontine regions was probably due to the afferent fibers from the pretectal nuclei and the superior colliculus, and the label in the inferior olive was considered to depend on the uncrossed afferent fibers from the pretectal nuclei and the nuclei of the accessory optic tract.     

Optic tectum of the eastern garter snake, Thamnophis sirtalis. I. Efferent pathways

Extracellular, iontophoretic injections of horseradish peroxidase were used to anterogradely fill axons efferent from the optic tectum in garter snakes. The tectal efferent pathways consist of six axon types with distinct projections and terminal morphologies. Tectogeniculate axons pass into the diencephalon via the optic tract, bearing collaterals that form spatially restricted, rodlike arbors in the pretectum, the ventral lateral geniculate nucleus, and the ventrolateral nucleus. Tectoisthmi axons exit the tectum as a thin-caliber component of the ventral tectobulbar tract. They form spatially restricted, spherical arbors within nucleus isthmi. Tectoisthmobulbar axons also give rise to small, spherical arbors within nucleus isthmi, but the parent axons continue caudally into the pontine and medullary reticular formation issuing many short collateral branches. Tectorotundal axons reach the diencephalon via the tectothalamic tract and give rise to fine terminal collaterals in the nucleus of the tectothalamic tract ipsilaterally and in nucleus rotundus bilaterally. Single axons form sheetlike terminal fields that span the rostrocaudal extent of nucleus rotundus. Ipsilateral tectobulbar axons descend into the midbrain tegmentum where they issue several thick collaterals that terminate widely throughout the nucleus lateralis profundus mesencephali. The parent axon continues caudally giving off several widely spreading collaterals within the pontine and medullary reticular formation. Crossed tectobulbar axons enter the dorsal tectobulbar tract and cross the midline to form the predorsal bundle. Single axons give rise to terminal collaterals in the nucleus lateralis profundus mesencephali bilaterally, the contralateral pontine and medullary reticular formation, and the intermediate gray of the cervical spinal cord.     

Efferent projections of the intergeniculate leaflet and the ventral lateral geniculate nucleus in the rat

The intergeniculate leaflet (IGL) and the ventral lateral geniculate nucleus (VLG) are ventral thalamic derivatives within the lateral geniculate complex. In this study, IGL and VLG efferent projections were compared by using anterograde transport of Phaseolus vulgaris-leucoagglutinin and retrograde transport of FluoroGold. Projections from the IGL and VLG leave the geniculate in four pathways. A dorsal pathway innervates the thalamic lateral dorsal nucleus (VLG), the reuniens and rhomboid nuclei (VLG and IGL), and the paraventricular nucleus (IGL). A ventral pathway runs through the geniculohypothalamic tract to the suprachiasmatic nucleus and the anterior hypothalamus (IGL). A medial pathway innervates the zona incerta and dorsal hypothalamus (VLG and IGL); the lateral hypothalamus and perifornical area (VLG); and the retrochiasmatic area (RCA), dorsomedial hypothalamic nucleus, and subparaventricular zone (IGL). A caudal pathway projects medially to the posterior hypothalamic area and periaqueductal gray and caudally along the brachium of the superior colliculus to the medial pretectal area and the nucleus of the optic tract (IGL and VLG). Caudal IGL axons also terminate in the olivary pretectal nucleus, the superficial gray of the superior colliculus, and the lateral and dorsal terminal nuclei of the accessory optic system. Caudal VLG projections innervate the lateral posterior nucleus, the anterior pretectal nucleus, the intermediate and deep gray of the superior colliculus, the dorsal terminal nucleus, the midbrain lateral tegmental field, the interpeduncular nucleus, the ventral pontine reticular formation, the medial and lateral pontine gray, the parabrachial region, and the accessory inferior olive. This pattern of IGL and VLG projections is consistent with our understanding of the distinct functions of each of these ventral thalamic derivatives.     

Descending pathways from the superior collicullus: an autoradiographic analysis in the rhesus monkey (Macaca mulatta).

The autoradiographic tracing method has been used to identify the various descending tectofugal pathways and their targets in the rhesus monkey (Macaca mulatta). The present data reveal that the majority of descending tectofugal axons arise from collicular laminae which lie ventral to the stratum opticum (layer 3). Such descending axons can be grouped into two major bundles or tracts, i.e., the ipsilateral tectopontine-tectobulbar tract and the crossed tectospinal tract (or the predorsal bundle). There is, in addition to these two major pathways, a smaller, commissural projection. The ipsilateral pathway courses laterally and ventrocaudally to terminate within the parabigeminal nucleus, the mesencephalic reticular formation, the dorsal lateral pontine gray (in several discrete patches), the dorsal lateral wing of the nucleus reticularis tegmenti pontis, and within the nucleus reticularis pontis oralis. Other ipsilateral targets of the deep tectal layers are the cuneiform nucleus and the external nucleus of the inferior colliculus. In several experiments transported protein is also apparent within the substantia nigra. Axons which comprise the tectospinal tract, or the predorsal bundle, cross within the dorsal tegmental decussation and descend within the brainstem in a position slightly lateral to the midline. The most rostral and quite extensive target of the predorsal bundle is the nucleus reticularis tegmenti pontis. As the predorsal bundle courses caudally within the pontine tegmentum, labeled axons enter the dorsal and medial regions of both the oral and the caudal divisions of the nucleus reticularis pontis. At caudal medullary levels, the mojority of the labeled axons comprising the predorsal bundle pass ventrally to end quite profusely with the subnucleus b of the medial accessory nucleus of the inferior olivary complex. Caudal to this only a few scattered, labeled axons can be followed into the cervical spinal cord. Labeled axons also pass to the opposite, or contralateral colliculus via the tectal commissure. Such axons appear to arise and end primarily within the deeper tectal layers. In one experiment, the injection invaded the mesencephalic nucleus of the trigeminal nerve. Labeled axons were apparent within the motor nucleus, the chief sensory nucleus (quite profusely) and within the spinal or descending nucleus of the trigeminal nerve.     

The tecto-thalamic connections in the brain of the rabbit

We have correlated the tectal connections and cytoarchitecture of regions in the rabbit's midbrain and caudal thalamus. The inferior colliculus projects ipsilaterally to the central gray, superior colliculus, and via the brachium of the inferior colliculus to its interstitial nucleus and the parabrachial region of the midbrain tegmentum. From the brachium, fibers fan out to the principal and internal divisions of the medial geniculate. A smaller contralateral pathway sweeps into the contralateral inferior colliculus and in its brachium to the interstitial nucleus, the parabrachial region, and the internal and principal divisions of the medial geniculate. The superior collicular projection is mainly ipsilateral. Medially, fibers terminate in the central gray and pretectal area. Laterally, fibers ascend in the superior brachium to parabrachial region, suprageniculate pretectal nucleus, posterior complex, caudodorsal internal division of the medial geniculate, and to a discrete part of the ventral nucleus of later geniculate. A component of the commissure of Gudden originates in the rostral superior colliculus and terminates in the contralateral ventral lateral geniculate, posterior complex, pretectal area and midbrain tegmentum. Interconnections between the colliculi and overlap of their projections in the parabrachial region, the central gray, and the internal division of the medial geniculate are described.     

Central acoustic tract in an echolocating bat: an extralemniscal auditory pathway to the thalamus

To determine the sources and targets of auditory pathways that bypass the inferior colliculus in the mustache bat, we injected WGA-HRP in the medial geniculate body and related auditory nuclei of the thalamus as well as in the lower brainstem. We used electrophysiological methods to verify that the injection electrode was in an area responsive to sound. The only thalamic injections that produced retrograde transport to cells in auditory nuclei caudal to the inferior colliculus were those that included the suprageniculate nucleus. These injections labeled a group of large multipolar cells lying between the ventral nucleus of the lateral lemniscus and the superior olivary complex. Neurons in this cell group have also been shown to project to the deep layers of the superior colliculus in the mustache bat. The pathway revealed by these studies is almost identical to the "central acoustic tract" in which fibers course medial to the lateral lemniscus and bypass the inferior colliculus to reach the deep superior colliculus and the suprageniculate nucleus.     

Fluorescent histochemical localization of neutral endopeptidase-24.11 (enkephalinase) in the rat brainstem.

Characterization of the distribution of the peptide-degrading enzyme neutral endopeptidase-24.11 (E.C. 3.4.24.11; NEP; enkephalinase) in the rat brainstem was examined by means of a unique fluorescent histochemical method. Enzyme staining was completely blocked by three potent NEP inhibitors (thiorphan, phosphoramidon, and JHF-26) at a concentration of 50 nM, supporting the specificity of this method to visualize sites of NEP activity selectively. At all levels of the brainstem, NEP was localized to cell bodies, cell processes or terminal-like fields and was localized to more than 90 distinct nuclei or subnuclei. In the mesencephalon these included the central gray, cuneiform n., dorsal and lateral tegmental n., inferior colliculus, interpeduncular n., lateral and medial geniculate n., central linear raphe n., mesencephalic n. of the trigeminal nerve, mammillary nuclei, occulomotor n., red n., superior colliculus, ventral n. of the lateral lemniscus, substantia nigra-ventral tegmental area, and the zona incerta. In the pons, NEP staining was restricted to fewer regions or nuclei, including the dorsal and ventral cochlear n., facial n., motor trigeminal n., principal sensory trigeminal n., parabrachial nuclei, pontine n., the oral and caudal pontine reticular n., pontine olivary nuclei, several pontine tegmental nuclei, pontine raphe nuclei, and the trapezoid n. In the cerebellum, staining was localized largely to the granule cell layer of the cerebellar cortex. Scattered staining was observed in the molecular cell layer. The medulla contained extensive NEP staining localized to nuclei that included the ambiguous n., dorsal motor n. of the vagus, hypoglossal n., inferior olivary n., prepositus hypoglossus n., solitary tract n., nuclei of the spinal tract of the trigeminal n., and the lateral, medial, and superior vestibular nuclei. Nuclei of the medullary reticular formation that were also richly stained for NEP included the raphe magnus n., raphe obscurus n., raphe pallidus n., dorsal, lateral, and ventral reticular nuclei of the medulla, and the gigantocellular, lateral paragigantocellular, linear, paramedian and parvicellular reticular nuclei. The widespread distribution of NEP in the brainstem suggests the existence of a number of functional systems, including the pathways involved in the mechanisms of pain and analgesia, which are potential targets of NEP inhibitors. In most regions, the distribution of NEP closely overlapped with that reported for the enkephalins, and showed a more restricted overlap with the reported distribution of substance P.     

Projection systems and terminal localization of dorsal column afferents: an autoradiographic and horseradish peroxidase study in the rat

Projection systems from the gracile nucleus and the cuneate nuclear complex to their terminal sites in the mesencephalon, diencephalon, and cerebellum were examined by means of anterograde autoradiography and retrograde horseradish peroxidase methods. Three projection systems emerge from the dorsal column nuclei, decussate via internal arcuate fibers, and form the contralateral medial lemniscus (ML). At the obex, some fibers split off the ML and course dorsolaterally, forming an ascending lateral system which fits the "lemniscal adjunct channel" (LAC) concept of Graybiel ('72). The ML continues rostrally as the "main lemniscal line channel" (MLLC). At the inferior colliculus, some LAC fibers terminate in the pontine nuclei, parabrachial, dorsal reticular nuclei, and the external and ventral medial part of the central nucleus of the inferior colliculus. More rostrally at the level of the superior colliculus, terminal fields are found in the medial nucleus of the medial geniculate body, the suprageniculate, pretectal, and mesencephalic reticular nuclei, marking the end of the LAC. In the diencephalon, gracile fibers leave the MLLC and form a crescentlike terminal field along the extreme lateral border of the ventral posterior lateral nucleus (VPL) of the thalamus. Cuneate MLLC fibers terminate in a bandlike formation in the VPL medial to the gracile termination. The third fiber system, the cuneocerebellar projection, emerges from the cuneate, the external cuneate nuclei, and the "cellular bridge" and immediately enters the ipsilateral inferior cerebellar peduncle. Upon entering the cerebellum, the major fiber component remains ipsilateral and terminates as vertical bands in vermal and paravermal lobules, and lobules I through IVa. The posterior cerebellar lobe contains terminal bands in lobules VII-IX, the copula pyramidis, and the paramedian lobule. It is concluded that the dorsolateral fiber system conforms to Graybiel's LAC. It is more divergent and probably less modality specific, whereas the medial lemniscal system conforms to the MLLC, which is said to be modality specific, less divergent, and locked to specific sensory-motor response characteristics. The topography of cerebellar terminal bands indicates that there is sensory-motor representation from all parts of the body to all parts of the cerebellum, at least in the rat.     

Distribution of choline acetyltransferase immunoreactive axons and terminals in the rat and ferret brainstem.

A survey was made of the density of the cholinergic innervation of different parts of the brainstem of the rat and ferret. Sections of rat and ferret brainstems were stained for choline acetyltransferase (ChAT) immunoreactivity by using a sensitive immunocytochemical method. Adjacent sections were stained for acetylcholinesterase activity or Nissl substance. The density of the distribution of fine calibre, varicose ChAT-positive axons, assumed to represent cholinergic terminals, was categorised arbitrarily into high, medium, or low. A high density of ChAT-positive terminals was found in all or parts of these structures: interpeduncular nucleus, superficial grey layer of the superior colliculus (ferret), intermediate layers of the superior colliculus, lateral part of the central grey (rat), an area medial to the parabigeminal nucleus (rat), pontine nuclei, ventral tegmental nucleus (rat), midline pontine reticular formation, and an area ventral to the exit point of the 5th nerve (ferret). A medium density of ChAT-positive terminals was observed in all or parts of: the substantia nigra zona compacta (ferret), ventral tegmental area (ferret), superficial grey layer of the superior colliculus, intermediate and deep layers of the superior colliculus, lateral central grey, area medial to the parabigeminal nucleus, inferior colliculus, dorsal tegmental nucleus, ventral tegmental nucleus (ferret), pontine nuclei, ventral nucleus of the lateral lemniscus (ferret), midline pontine reticular formation, ventral cochlear nucleus, dorsal cochlear nucleus, lateral superior olive, spinal trigeminal nuclei, prepositus hypoglossal nucleus, lateral reticular nucleus, paragigantocellular nucleus, and the dorsal column nuclei including the cuneate, external cuneate, and gracile nuclei. A low density of ChAT-positive terminals was seen throughout the remainder of the brainstem of the rat and ferret, but these terminals were absent from the medial superior olive, substantia nigra zona reticulata (rat), and the central part of the ferret lateral superior olive. A pericellular-like distribution of ChAT-positive terminals was observed in the ventral cochlear nucleus and in association with some of the cells of the nucleus of the mesencephalic tract of the trigeminal nerve. A climbing fibre type arrangement of ChAT-positive terminals was found in the substantia nigra zona compacta (ferret) and medial reticular formation. In general, the distribution of staining for AChE activity reflected that of the distribution of ChAT immunoreactivity in the brainstem, except in a few regions where there were also species differences in the distribution of ChAT-positive terminals, e.g., in the superficial grey layer of the superior colliculus and in the substantia nigra.(ABSTRACT TRUNCATED AT 400 WORDS)     

Subcortical connections of frontal 'oculomotor' areas in the cat

The subcortical connections of the frontal 'oculomotor' areas in the medial wall of the hemisphere under the cruciate sulcus (CRUo), the medial (PREo-m) and the lateral banks (PREo-l) of the presylvian sulcus of cats were investigated using WGA-HRP tracing combined with electrophysiological techniques. Two main modes were identified; one was the common connections to the same targets in the cortico-thalamo-cortical, cerebro-cerebellar and cortico-tectal pathways, and the other was the individual connections to unique portions of the saccade-generating centers in the brainstem. The common reciprocal connections were found in the ventral anterior-ventral lateral complex, principal ventromedial nucleus, rostral intralaminar nuclei, centromedian-parafascicular complex, lateral posterior nucleus, and suprageniculate nucleus. The common efferent projections were in the subthalamic nucleus, lateral habenular nucleus, pretectal nucleus, posterior commisure nucleus, nucleus of Darkschewitsch, pontine nucleus, nucleus reticularis tegmenti pontis, medial accessory inferior olive, and the superior colliculus. The CRUo projected into the ipsilateral field of Forel and paramedial pontine reticular formation (PPRF), the PREo-l projected into the contralateral dorsomedial medullary reticular formation, and the PREo-m projected into the ipsilateral medullary reticular formation and there was only a small degree of projection into the central portion between the abducens nerve rootlets.     

Anatomical connections of the nucleus prepositus of the cat

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

Afferent projections to the superior colliculus in the rat, with special attention to the deep layers

Sources of afferent projections to the superior colliculus (SC) of the rat were determined by means of the retrograde horseradish peroxidase (HRP) method. The experiments were conducted first to provide a comprehensive map of all the brain structures which supply input to the SC, and secondly to pinpoint the sources of afferents to the deep collicular layers. Experiments conducted after large HRP deposits invading almost all the collicular layers resulted in the labeling of visual centers (cortical areas 17, 18 and 18a, ventral lateral geniculate nucleus, nucleus of the posterior commissure, nucleus of the optic tract, anterior and olivary pretectal nuclei, parabigeminal nucleus); somatosensory centers (cortical area SmI, principal and spinal tract trigeminal nuclei) auditory centers (auditory cortex, inferior colliculus and nuclei of the lateral lemniscus) and various other centers (zona incerta, substantia nigra, cingulate and motor cortices, and some hypothalamic, thalamic, pontine reticular and deep cerebellar nuclei). Deposits limited to the deep SC layers resulted in the labeling of a smaller number of structures: visual centers (cortical area 18a, nucleus of the posterior commissure, parabigeminal nucleus); somatosensory centers (cortical area SmI, principal and spinal tract trigeminal nuclei); auditory centers (inferior colliculus, nuclei of the lateral lemniscus); and various other centers (zona incerta, substantia nigra, cingulate cortex, some hypothalamic nuclei, posterior thalamic nucleus, central gray, cuneiformis and subcuneiformis nuclei, pontine reticular nucleus pars oralis). The pattern of the afferent connections to the rat's SC appears identical to the general plan of organization in mammals; the sources of afferents are extensive and allow the SC to be influenced by multisensory, limbic and even motor-related systems.     

Localization of GABAA-receptor alpha 1 subunit mRNA-containing neurons in the lower brainstem of the rat

The localization of gamma-aminobutyric acid-A (GABAA) receptors (GABAA-R) in the lower brainstem of the rat was examined by means of in situ hybridization histochemistry using an oligonucleotide probe to the sequence of the alpha 1 subunit (GABAA-R alpha 1). Strongly labeled neurons were found in the cranial motor nuclei, the dorsal motor nucleus of the vagus, reticular formation (large neurons), lateral vestibular nucleus, dorsal nucleus of the lateral lemniscus, central nucleus of the inferior colliculus, intermediate and white layers of the superior colliculus, red nucleus and substantia nigra. In addition, moderately labeled cells were abundant in the nucleus of the solitary tract, medial and inferior vestibular nuclei, parabrachial area, dorsal and ventral tegmental nuclei of Gudden, central gray matter, ventral nucleus of the lateral lemniscus, and reticular formation (small neurons). This study has therefore revealed some of the target neurons of GABA-containing fibers in the lower brainstem.     

Ascending brain stem projections of the anteroventral cochlear nucleus in the rhesus monkey

In a series of seventeen rhesus monkeys attempts were made to produce discrete stereotaxic lesions in the anteroventral cochlear nucleus (Av). Anterograde degeneration was described in detail in four cases with lesions confined within the cochlear complex to Av. Fibers decussating at pontine levels coursed exclusively in the trapezoid body. Degenerated fibers projected: ipsi-laterally to the lateral superior olivary nucleus; bilaterally to the preolivary nuclei; to the lateral side of the ipsilateral medial superior olive and the medial side of the contralateral medial superior olive; and to the contralateral medial trapezoid nucleus. A topographic projection upon the medial superior olive was demonstrated. Projections were bilateral but mainly crossed to the nuclei of the lateral lemniscus and central nucleus of the inferior colliculus; the posterior end of the ipsilateral ventral nucleus of the lateral lemniscus contained an island of profuse degeneration. A few fibers crossed in the commissure of the inferior colliculus. Few if any fibers from Av projected to the contralateral magnocellular medial geniculate.     

Differential projection patterns of superior and inferior collicular neurons onto posterior paralaminar nuclei of the thalamus surrounding the medial geniculate body in the rat

The thalamic nuclei at the medial border of the medial geniculate body (i.e. the suprageniculate nucleus, the medial division of the medial geniculate nucleus, the posterior intralaminar nucleus and the peripeduncular nucleus) which relay sensory information to the amygdala are thought to receive convergent input from multiple sites. In order to delineate the organization of these multimodal thalamic nuclei, the locations of superior and inferior collicular neurons projecting to these nuclei were studied by means of retrograde transport methods. Small injections of the tracer Miniruby were made into single paralaminar thalamic nuclei. Injections of Miniruby into the suprageniculate nucleus labelled predominantly neurons in the stratum opticum of the superior colliculus, whereas injections into the medial division of the medial geniculate body, the posterior intralaminar nucleus and the peripeduncular nucleus labelled predominantly neurons in the deep layers of the superior colliculus. These injections also labelled neurons in the inferior colliculus. The majority of retrogradely labelled neurons were found in the external nucleus of the inferior colliculus and here predominantly in layer 2. Injections focused onto the medial division of the medial geniculate body additionally labelled magnocellular neurons in layer 3 of the external nucleus and a few neurons in the central nucleus. More ventrally located injections, focused onto the posterior intralaminar and peripeduncular nucleus, almost exclusively labelled neurons in layer 1 of the external nucleus and the dorsal part of the dorsal nucleus. After injections into the suprageniculate nucleus, only neurons in layer 2 were found. Neurons in the central nucleus of the inferior colliculus were only found after injections that involved the medial division of the medial geniculate body. The present results suggest that, despite a considerable degree of convergence in this thalamic region, each of these thalamic nuclei receives a unique pattern of projections from the superior and inferior colliculi. It appears that the thalamic nuclei may be concerned mainly, but not exclusively, with a single sensory modality, and give rise to parallel multimodal and unimodal pathways to the amygdala.     

Neocortical projections to the pons and medulla of the nine-banded armadillo (Dasypus novemcinctus)

The pattern of neocortical projections to the pons and medulla was determined by employing the Nauta-Gygax technique ('54) on the brains of armadillos subjected to neocortical ablations. The results of this study indicate that the pretrigeminal basilar pontine gray receives input from a considerable portion of the neocortex. Degenerating fibers resulting from a lesion of the frontal tip of the neocortex terminated within the dorsal medial, the medial and the ventral medial areas of the rostral basilar pontine gray. Corticopontine fibers from the mid-presupraorbital neocortex ended throughout the rostral to caudal extent of the basilar pontine gray, and terminated within the dorsal medial, the medial and the ventral medial areas; whereas degenerating fibers resulting from a lesion of the neocortex immediately rostral to the supraorbital sulcus terminated within the medial, the ventral and the ventral lateral areas of the basilar pontine gray. The neocortex immediately caudal to the supraorbital sulcus distributed corticopontine fibers to the ventral, the ventral lateral, the dorsal lateral and to the dorsal areas of the basilar pontine gray, while degenerating fibers resulting from lesions of the caudal one-third and most caudal tip of the neocortex projected to the ventral and lateral portions of the basilar pontine gray. Neocortical projections to the pontine and medullary reticular formation originated mainly from cortical areas rostral and immediately caudal to the supraorbital sulcus. The neocortex rostral to the supraorbital sulcus distributed to the rostral and medial portions of the pontine reticular formation, whereas corticoreticular fibers from the neocortex immediately caudal to the suprarbital sulcus, also distributed degenerating fascicles to the spinal trigeminal nucleus, the nucleus of the solitary tract and to the nucleus cuneatus. No degenerating fibers were seen to terminate within motor nuclei of cranial nerves located within either the pons or medulla.