Organization of the spinocervicothalamic pathway in the rat

We used silver degeneration techniques to examine the termination of the spinocervical and cervicothalamic tracts in rats. Lesions of the dorsal portion of the lateral funiculus (DLF) of the spinal cord produced degeneration of a relatively small number of ascending fibers that were seen within the most lateral portion of the DLF rostral to the lesion. Within the lateral cervical nucleus, the degeneration was more extensive mediolaterally and of a finer caliber. Such labeling is attributable to the degeneration of fine fibers and terminals. Degenerating processes could be seen in apposition to neurons in the lateral cervical nucleus. At all levels of the cord, the lateral spinal nucleus was devoid of terminal labeling following lesions of the DLF. No terminal degeneration could be seen within the DLF at levels rostral to the lateral cervical nucleus. Lesions of the DLF at either midcervical or lower thoracic levels produced degeneration throughout the lateral cervical nucleus. This finding suggests that the lateral cervical nucleus of the rat is not somatotopically organized. Lesions of the lateral cervical nucleus produced degeneration of a small number of fibers within the contralateral midbrain and thalamus. Within the mesencephalon, degenerating fibers and terminals were seen primarily in the intercollicular region and the deep layers of the superior colliculus. Less degeneration was found in the lateral portion of the central gray. Within the diencephalon, a small area of termination was located in the ventromedial part of the rostral portion of the medial geniculate nucleus. A prominent termination was present in a restricted area within the caudal fourth of the ventrobasal complex.     

Diencephalic projections from the pontine reticular formation: autoradiographic studies in the cat

Ascending projections to the diencephalon from the pontine reticular formation were studied in the cat by autoradiographic techniques. Projections from both rostral and caudal pontine regions ascend to the caudal diencephalon and divide into two components; a dorsal leaf terminates primarily in the thalamic intralaminar complex and a ventral leaf terminates in the subthalamic region. The relative densities of the two terminal regions vary with the injection site. Fibers originating in the caudal pons (nucleus reticularis pontis caudalis) terminate relatively heavily in the intralaminar nuclei of the dorsal thalamus, particularly the centre median, central lateral, central dorsal and paracentral nuclei, and also the dorsal medial nucleus. Relatively sparse termination occurs in the subthalamic region. In contrast, fibers from the rostral pons (nucleus reticularis pontis oralis) terminate relatively heavily in the subthalamic region, including the zona incerta, the fields of Forel, the ventral part of the thalamic reticular complex, and the lateral hypothalamus. Relatively sparse termination occurs in the dorsal thalamus, but includes the centre median, parafascicular, central lateral, paracentral and dorsal medial nuclei. These data are discussed with regard to reticular control of forebrain activity and the role of the classic dorsal and ventral components of ascending reticular projections.     

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.     

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.     

Reticulospinal fibers of the opossum, Didelphis virginiana. II. Course, caudal extent and distribution

Lesions were placed in the deep midbrain tegmentum and within areas of the pontine and medullary reticular formation which were determined by the retrograde degeneration method to give rise to reticulospinal fibers. The location of each lesion was verified histologically and the spinal cords processed by either the Nauta or Fink-Heimer method. Control material for the origin of degenerated fibers was provided by previous studies on corticospinal, tectospinal and rubrospinal fibers in the same form. Fibers degenerating as a result of mesencephalic tegmental lesions followed the ipsilateral ventral funiculus and could be traced as far caudal as midthoracic levels. They ended in Rexed's laminae VII and VIII. Lesions of the pontine reticular formation resulted in the degeneration of fibers within the ventral funiculus of the spinal cord. The great majority of these fibers followed an ipsilateral course. Such pontospinal fibers were present throughout the length of the spinal cord; their terminal distribution appears to take place mainly within laminae VII and VIII. At certain levels, however, such fibers could be traced into lamina IX. Lesions in the medial medullary reticular formation resulted in fiber degeneration chiefly within the ipsilateral ventral and lateral funiculi. However, a few fibers were present on the side opposite the ablation. This fiber system extends throughout the length of the spinal cord and distributes itself in laminae VII and VIII and, at certain levels, also in lamina IX. Lesions isolated to the lateral, parvocellular portion of the medullary reticular formation resulted in fiber degeneration located within the propriospinal bundles of the lateral funiculus which could be traced to rostral cervical levels. Such lesions also interrupted the rubrospinal tract in the its medullary trajectory.     

Corticobulbar projections of the marsupial phalanger (Trichosurus vulpecula). I. Projections to the pons and medulla oblongata

A total of 27 adult phalangers was employed to investigate the pattern of neocortical projections to the pontine and medullary portions of the brain stem. Lesions restricted to neocortical areas rostral to the orbital sulcus resulted in fiber degeneration which distributed mainly to midline and medial areas of the pontine and medullary reticular formation. The greatest amount of fiber degeneration was located within the superior central nucleus, the nucleus of the pontine raphe, the nucleus pontis centralis oralis and the nucleus pontis centralis caudalis. However, a few degenerating fibers were present within the nucleus gigantocellularis and the magnocellular portion of the medullary raphe. In contrast, lesions which were located just caudal to the orbital sulcus resulted in fiber degeneration chiefly within the more lateral parvocellular reticular formation and within the subnucleus dorsalis of the nucleus medullae oblongatae centralis. In such cases, additional degenerating fibers were present within the dorsal column nuclei and within more medial areas of the reticular formation. In those brains with ventral parietal ablations, degenerating fibers were present within the chief sensory and spinal nuclei of the trigeminal complex and the closely adjacent reticular formation. All of the above neocortical lesions resulted in fiber degeneration within the basilar pontine gray. In those specimens subjected to caudal (striate and peristriate) or ventrocaudal (temporal) lesions, degenerating fibers were present within the basilar pontine gray, but not within other areas of the pons or the medulla oblongata.     

Fastigial efferent projections in the monkey: an autoradiographic study.

Because fastigial efferent fibers partially decussate within the cerebellum and cerebellar corticovestibular projections pass near, or through, the fastigial nucleus (FN), degeneration studies based on lesions in the nucleus leave unresolved questions concerning fastigial projections. Attempts were made to determine fastigial projections in the monkey using autoradiographic tracing technics. Cells in rostral, caudal and all parts of the FN were labeled with [³H] amino acids. Selective labeling of neurons in either rostral or caudal parts of the FN results in transport of isotope primarily via fibers of the contralateral uncinate fasciculus (UF) and the ipsilateral juxtarestiform body (JRB). Fastigial projections to the vestibular nuclei are mainly to ventral portions of the lateral (LVN) and inferior (IVN) vestibular nuclei, are nearly symmetrical and are quantitatively similar on each side. Fastigiovestibular projections to cell groups f and x arise from all parts of the FN and are mainly crossed; modest projections to the medial vestibular nucleus are uncrossed. No fastigial efferent fibers end in the superior vestibular nucleus on either side, or in dorsal regions of the LVN. Crossed fibers descending in IVN terminate in the nucleus parasolitarius. Fastigioreticular fibers arise predominately from rostral regions of the FN, are entirely crossed and project mainly to: (1) medial regions of the nucleus reticularis gigantocellularis, (2) the dorsal paramedian reticular nucleus and (3) the magnocellular part of the lateral reticular nucleus. Fastigiopontine fibers, emerge with the UF, bypass the vestibular nuclei and terminate upon the contralateral dorsolateral pontine nuclei. Crossed fastigiospinal fibers separate from fastigiopontine fibers and descend in the ventrolateral tegmentum beneath the spinal trigeminal tract; in the medulla and upper cervical spinal cord these fibers are intermingled with those of the vestibulospinal tract. Fastigiospinal fibers terminate in the anterior gray horn at C-1 and probably descend further. Ascending fastigial projections arise from caudal parts of the FN, are entirely crossed and ascend in dorsal parts of the midbrain tegmentum. Label is transported bilaterally to the superior colliculi and the nuclei of the posterior commissure. Contralateral fastigiothalamic projections terminate in the ventral posterolateral (VPLc and VPLo) and in parts of the ventral lateral (VLo) thalamic nuclei. The major region of termination of fastigiothalamic fibers is in VPLo. Fastigiothalamic projections, probably conveying impulses concerned with equilibrium and somatic proprioception, appear to impinge upon thalamic neurons receiving inputs from less specialized receptors that signal information concerning position sense and body movement. More modest fastigial projections to VLo could directly influence activity of neurons in the primary motor cortex.     

Ascending tracts of the lateral columns of the rat spinal cord: a study using the silver impregnation and horseradish peroxidase techniques.

The location of projection areas and cells of origin of the ascending fiber tracts of the spinal cord lateral columns were examined in the rat. Projection areas were localized after unilateral microtransection of lateral column fibers at C2 or T10, using silver impregnation of preterminal and fiber degeneration. Cells of origin were localized by unilateral microtransection and subsequent application of horseradish peroxidase (HRP). Two groups of fibers projected to the dorsal medulla. One group projected to nucleus intercalatus, commissuralis, and the dorsal column nuclei. The second group projected via the inferior cerebellar peduncle to the vestibular complex, with additional fibers continuing dorsally to the cerebellum. The most extensive system of ascending fibers projected to the reticular formation. Most spinoreticular fibers coursed through the ventral hindbrain and projected to the lateral reticular nucleus, ventral reticular nucleus, nucleus gigantocellularis, and nucleus subceruleus. Spinotectal and spinocentral gray fibers coursed through the ventral portion of the medulla and then dorsally through the pons. Spinocentral gray fibers projected to the caudal portion of the central gray matter, ipsilaterally. Spinotectal fibers projected to the intercollicular nucleus and adjacent portions of the superior colliculus, bilaterally. Two projections to the thalamus were observed after anterolateral column transection. Preterminal degeneration was observed in the ventrobasal complex ipsilaterally, and bilaterally in the intralaminar nuclei. In conjunction with previous results the present HRP data suggest that the cells of origin of spinothalamic tract fibers were situated in laminae IV, V, and VI. The location of spinal cord cells of origin of additional ascending tracts is discussed.     

Cerebellothalamic projections in the rat: An autoradiographic and degeneration study

The purpose of this study was to determine the topographical organization of cerebellothalamic projections in the rat. Following stereotaxic injections of ³H-leucine or electrolytic lesions in the cerebellar nuclei, efferent fibers were observed to emerge from the cerebellum through two discrete routes. Fibers from the fastigial nucleus decussated within the cerebellum, formed the crossed ascending limb of the uncinate fasciculus, ascended in the dorsal part of the midbrain tegmentum, and entered the thalamus. Cerebellothalamic fibers from the interpositus and dentate nuclei coursed in the ipsilateral brachium conjuctivum, decussated in the caudal midbrain, and ascended to the thalamus via the crossed ascending limb of the brachium conjunctivum. Cerebellar terminations were observed in the intralaminar, lateral, and ventral tier thalamic nuclei as well as in the medial dorsal nucleus. Projections to the intralaminar nuclei were more pronounced from the dentate and posterior interpositus than from the anterior interpositus and fastigial nuclei. The lateral thalamic nuclei received a projection from the dentate and posterior interpositus nuclei while the fastigial nucleus projected to the medial dorsal nucleus. Within the rostral ventral tier nuclei fastigiothalamic terminations were localized in the medial parts of the ventral medial and ventral lateral nuclei, whereas dentatothalamic projections were concentrated in the lateral parts of the ventral medial nucleus and the medial half of the ventral lateral nucleus. Terminations from the posterior interpositus nucleus were observed ventrally and laterally within the caudal two-thirds of the ventral medial nucleus and throughout the ventral lateral nucleus, where they were densest in the lateral part of its lateral wing and within the central part of its cap. The anterior interpositus nucleus also projected to the central and lateral parts of the ventral lateral nucleus, but these terminations were considerably less dense than those from the posterior interpositus. A few fibers from the interpositus nuclei terminated in the medial part of the rostral pole of the ventral posterior nucleus. A prominent recrossing of cerebellothalamic fibers from the fastigial, posterior interpositus, and dentate nuclei occurred through the central medial nucleus of the internal medullary lamina. These terminated within the ipsilateral ventral lateral and intralaminar nuclei. These results show that each of the cerebellar nuclei project to the thalamus and that their terminations are topographically organized in the rostral ventral tier nuclei. The clustering of autoradiographic silver grains or terminal degeneration observed in the thalamic nuclei suggests a medial-to-lateral organization of this cerebellothalamic system.     

The efferent connections of the feline nucleus cuneatus

Nucleus cuneatus projections to nucleus ventralis posterolateralis pars medialis (VPLm) and other thalamic as well as midbrain and medullary nuclei were studied in cats using the Fink-Heimer I silver technique. Single electrolytic lesions of very small size were made stereotaxically in different zones of nucleus cuneatus under electrophysiological control. All zones studied projected to contralateral VPLm in a pattern of discrete terminal arborizations or clusters, which were organized in onionskin-like dorso-ventral laminae. The clusters of degeneration varied in size and density according to their dorsoventral location within VPLm. Those in dorsal areas were smaller in diameter (50–125 μ) and contained less dense amounts of degeneration than clusters (150–300 μ) in more ventral regions. The clustered terminal arborizations mirrored the organization of the VPLm neuronal clusters, themselves. Terminations within VPLm were topographically organized, but were completely inverted, i.e. dorsal nucleus cuneatus projected to ventral VPLm and ventral to dorsal, lateral to medial, and medial to lateral VPLm. A ventral zone of nucleus cuneatus, which contained “deep” units, projected to a separate dorsal zone of VPLm. In addition to its classical connection with VPLm, nucleus cuneatus projected to the following contralateral brainstem or thalamic nuclei: medial and dorsal accessory olives, external nucleus of the inferior colliculus, ventrolateral part of the superior colliculus, nucleus ruber, medial geniculate nucleus pars magnocellularis, suprageniculatus, medial and lateral divisions of the posterior thalamic nuclear group, zona incerta, and Fields of Forel. Very sparse amounts of degeneration were also present within nuclei ventralis posteromedialis (caudal pole) and ventralis posterolateralis pars lateralis. The brainstem and thalamic projections of the dorsocaudal part (cell nest region) of the cuneate nucleus were more restricted than those of its rostral and ventral regions. The clusters of both the VPLm neurons and cuneate terminations within VPLm provides an anatomical basis for the functional characteristics of synaptic security, fine grain somatotopia and modality specificity so prominent in the dorsal column nuclei-medial lemniscal system.     

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.     

Quantitative studies of transneuronal atrophy in the dorsal lateral geniculate nucleus of cats and kittens

Persisting reports of uncrossed secondary trigeminothalamic pathways originating from the principal sensory trigeminal nucleus (PrV) are based mainly on data obtained from surgically induced lesions analyzed by methods other than reduced silver impregnation techniques. After complete stereotaxic or direct surgical lesions of PrV, degenerating fibers proceed ventrally and rostrally from the nucleus and cross the midline in the rostral pons, completing their decussation caudal to nucleus interpeduncularis. These fibers form a large trigeminal lemniscus that ascends through the midbrain dorsomedial to the smaller medial lemniscus. PrV fibers terminate in the posterior thalamic nucleus and throughout the medial two-thirds of the ventrobasal thalamic complex (VBm) contralateral to the side of the lesion. Connections with the ventral part of zona incerta were also found similar to those reported for the medial lemniscus. Another fiber system proceeds dorsad from the parvicellular reticular formation in the PrV region and terminates on cells of the ipsi-and contralateral motor trigeminal nuclei and the adjacent contralateral parvicellular reticular formation. Differential lesions indicate that these “intertrigeminal” commissural connections arise chiefly from the parvicellular reticular formation that lies between PrV and the motor trigeminal nucleus. These experiments, which also included selective lesions of dorsal and rostral portions of PrV, failed to produce any evidence supporting the existence of an uncrossed principal sensory trigeminothalamic pathway in the rat. While selective, ventral zona incerta PrV connections can be demonstrated, no PrV intralaminar projections appear to exist. Initial findings show that comparable fully decussated PrV–VBm connections also exist in the guinea pig but that some ipsilaterally ascending fibers occur in monkeys (Smith, '72). These and other recent data demonstrate that phyletic neural variations exist in the PrV-thalamic projections in various mammalian species.     

Cerebellopontine projections in the American opossum. A study of their origin, distribution and overlap with fibers from the cerebral cortex

The origin, course and distribution of cerebellopontine fibers was studied in the opossum by employing the Nauta-Gygax and Fink-Heimer techniques. Our results substantiate and extnd those of Brodal, Destombes, Lacerda and Angaut ('72) concerning the existence of cerebellopontine projections and provide evidence for a hitherto unreported fastigial projection to the basilar pons. Destruction of the caudal, medial division of the fastigial nucleus elicits bilateral degeneration in a restricted area of the medial pontine nucleus. This small terminal field is located in the angle between the medial lemniscus and the pyramidal tract and is found throughout the caudal three-fifths of the pons. The degenerating fibers do not course within the descending brachium conjunctivum, but reach the pons by filtering through the reticular formation from the uncinate fasciculus. Lesions that involve either the interpositus anterior or the dentate nucleus produce degeneration within the contralateral descending brachium conjunctivum and basilar pons. Terminal fields are located within the median, medial (paramedian nucleus of cat), peduncular, ventral and lateral nuclei. The heaviest degeneration is in the medial nucleus. Although cerebellar and cortical projections have different targets in the basilar pons, there is some overlap. Fastigial and preorbital fibers have partial overlap in the dorsal part of the medial nucleus, whereas the peduncular and lateral nuclei are the areas of overlap between the interpositus anterior and dentate projections with those from forelimb (and probably face) cortical areas. This overlap is particularly obvious in the caudal part of the lateral nucleus and occurs between fibers from limb motor-sensory cortex and those arising mainly within the anterior interpositus nucleus. There is no pontine overlap between cerebellar and visual or auditory cortical projections.     

Experimental study of the projections of the nucleus of the tractus solitarius and the area postrema in the cat

Projections from the area postrema and adjacent parts of the medial solitary nucleus are demonstrated with the Nauta method following lesions limited exclusively to these structures. Experiments are controlled with lesions involving adjacent bulbar regions, cerebellum, and spinal cord. Ascending pathways in the dorsal and lateral columns of the spinal cord project ipsilaterally to the area postrema and bilaterally to a para-alar nucleus in the ventral periphery of the nucleus gracilis. Neurons in the area postrema project mainly inspilaterally to the dorsal and medial regions of the medial solitary nucleus. Neurons in the posterior half of the medical solitary nucleus project ipsilaterally to the lateral solitary nucleus, dorsal vagal nucleus, ambigus, retrofacial nucleus, and dorsal and lateral bulbar reticular formation. Projections to nuclei intercalatus and prepositus hypoglossi, bilaterally, and to the ipsilateral dorsal tegmental nucleus by way of the dorsal longitudinal fasciculus are also shown. No direct projections to the diencephalon are demonstrated. Control lesions in the dorsal column nuclei reveal projections to the contralateral inferior olive and thalamic reticular and ventrobasal nuclei, but not to the projection sites of the solitary nucleus. Evidence is given to support the hypothesis that ascening visceral pathways are interruped in the bulbar reticular formation and dorsal tegmental nucleus before reaching the diencephalon. Correlations are suggested with functional aspects of the central autonomic and reticular activating systems.     

Distribution of dorsal root fibers in the medulla oblongata of the cat

Afferent fibers to the medulla oblongata of the cat were studied in 25 animals following section of one or more dorsal roots. Using the Nauta-Laidlaw stain a map was constructed of the distribution of the fibers to the dorsal column nuclei and a survey made of the afferents to other bulbar nuclei: nucleus cuneatus lateralis, lateral reticular nucleus, descending root of the fifth cranial nerve and the nucleus tractus solitarii. In the intermediate segment of the nucleus gracilis and the caudal two thirds of the nucleus cuneatus, there is a somatotopical arrangement with a dorso-ventral and medio-lateral shifting of the fibers from the dorsal roots as one goes from caudal segments to more cranial ones. A bilateral projection is demonstrated in the nucleus gracilis after section of nearly all the coccygeal-sacro-lumbar-thoracic roots; the contralateral degeneration is confined to the rostral pole of the nucleus gracilis. No bilateral degeneration is found in the nucleus cuneatus after section of the roots projecting to it. Degenerated axons in the nucleus cuneatus lateralis and in the lateral reticular nucleus are always present beginning from T6–T7 while in the descending root of the fifth cranial nerve they are recognizable in only one case with a section of the third sacral root.     

Long descending projections of the hypothalamus in the pigeon, Columba livia

An autoradiographic analysis was performed on the descending projections of nucleus periventricularis magnocellularis (PVM) of the hypothalamus in the pigeon. A PVM-medullospinal pathway was observed coursing posteriorly through the lateral hypothalamus, ventrolateral midbrain tegmentum, and into the spinal lemniscus (ls) in the ventrolateral pons and medulla. In the pons, some fibers course dorsomedially from ls and terminate at the lateral border of the locus coeruleus. At medullary levels, fibers from ls sweep dorsomedially in the plexus of Horsley and project to certain regions of the nucleus of the solitary tract (NTS) and the dorsal motor nucleus of the vagus (NX). Specifically, PVM fibers project heavily into NTS subnuclei medialis superficialis, medialis ventralis, and lateralis (sulcalis) dorsalis as well as into the ventral parvocellular subnucleus of NX. Fibers in ls were traced caudally into the lateral funiculus as far as upper cervical levels of the spinal cord. Although autoradiographs of lower cervical or thoracic spinal cord sections were not available, PVM fibers do descend to thoracic spinal cord levels, as evidenced by the retrograde transport of horseradish peroxidase. In addition to the medullospinal pathway, the autoradiographs demonstrated PVM projections to septum, diencephalon, and midbrain. Labeled PVM fibers are found in the lateral septal nucleus, nucleus of the anterior pallial commisure, dorsomedial thalamic nucleus, dorsolateral anterior thalamic nucleus (pars ventralis), median eminence, medial and lateral hypothalamus, medial mammillary area, and nucleus intercollicularis and central gray of the midbrain. The projection of fibers to medullospinal regions and median eminence suggests that PVM is homologous to the mammalian paraventricular nucleus. These projections to specific subnuclei of NTS and NX denote hypothalamic control over certain autonomic functions.     

The organization of subcortical projections of the hamster's visual cortex

The subcortical projections of the hamster's visual cortex were determined by use of injections of tritiated proline and heat lesions placed in different cortical loci. The brains were processed for autoradiography and silver impregnation of degenerating axons. Striate cortex was shown to project ipsilaterally to the dorsocaudal region of the caudate nucleus, a dorsolateral area within the thalamic reticular nucleus (RT), a laterodorsal region of the nucleus lateralis anterior (LA), the rostral half of nucleus lateralis posterior (LP), the whole territory of the dorsal (dLGN) and ventral (vLGN) geniculate nuclei, the anterior (PA) and posterior (PP) pretectal nuclei, the superior colliculus (SC), and the precerebellar pontine nuclei. In addition, the medial visual area (18b) was shown to project to a medial band of LA and part of the caudal half of LP, while the adjoining parietal cortex was seen to terminate in a lateral part of the caudate, a ventral band of LA, and the ventral half of rostral LP. Segregation of different cortical inputs was clear in LA, LP, caudate, and pons. The projections to dLGN, vLGN, SC, LP, and PA were retinotopically organized. Clear evidence of some topography was found within RT, PP, and the pons, although a consisten map could not be derived from the data.     

Organization of the Visual Reticular Thalamic Nucleus of the Rat

The visual sector of the reticular thalamic nucleus has come under some intense scrutiny over recent years, principally because of the key role that the nucleus plays in the processing of visual information. Despite this scrutiny, we know very little of how the connections between the reticular nucleus and the different areas of visual cortex and the different visual dorsal thalamic nuclei are organized. This study examines the patterns of reticular connections with the visual cortex and the dorsal thalamus in the rat, a species where the visual pathways have been well documented. Biotinylated dextran, an anterograde and retrograde tracer, was injected into different visual cortical areas [17; rostral 18a: presumed area AL (anterolateral); caudal 18a: presumed area LM (lateromedial); rostral 18b: presumed area AM (anteromedial); caudal 18b: presumed area PM (posteromedial)] and into the different visual dorsal thalamic nuclei (posterior thalamic, lateral posterior, lateral geniculate nuclei), and the patterns of anterograde and retrograde labelling in the reticular nucleus were examined. From the cortical injections, we find that the visual sector of the reticular nucleus is divided into subsectors that each receive an input from a distinct visual cortical area, with little or no overlap. Further, the resulting pattern of cortical terminations in the reticular nucleus reflects largely the patterns of termination in the dorsal thalamus. That is, each cortical area projects to a largely distinct subsector of the reticular nucleus, as it does to a largely distinct dorsal thalamic nucleus. As with each of the visual cortical areas, each of the visual dorsal thalamic (lateral geniculate, lateral posterior, posterior thalamic) nuclei relate to a separate territory of the reticular nucleus, with little or no overlap. Each of these dorsal thalamic territories within the reticular nucleus receives inputs from one or more of the visual cortical areas. For instance, the region of the reticular nucleus that is labelled after an injection into the lateral geniculate nucleus encompasses the reticular regions which receive afferents from cortical areas 17, rostral 18b and caudal 18b. These results suggest that individual cortical areas may influence the activity of different dorsal thalamic nuclei through their reticular connections.     

Efferent connections of the centromedian and parafascicular thalamic nuclei: an autoradiographic investigation in the cat

The efferent projections of the centromedian and parafascicular (CM-Pf) thalamic nuclear complex were analyzed by the autoradiographic method. Our findings show that the CM-Pf complex projects in a topographic manner to specific regions of the rostral cortex. These fibers distribute primarily to cortical layers I and III; however, the projection to layer I is more extensive. Following an injection into the rostral portion of the CM-Pf complex, label is found within the lateral rostral cortex, particularly within the presylvian, anterior ectosylvian, and anterior lateral sulci, and within the rostral medial cortex where label is present within the cruciate and anterior splenial sulci and anterior cingulate gyrus. An injection into the caudal dorsal portion of the CM-Pf complex results in label within the more ventral portions of the rostral lateral cortex where it is present within the anterior sylvian gyrus, presylvian regions, and gyrus proreus; and within the rostral medial cortex, where it is present within the rostral cingulate gyrus, and within the cruciate sulcus, and an extensive region ventral to the cruciate sulcus which includes the anterior limbic area. Injections into the caudal ventral portion of the CM-Pf complex result in virtually no cortical label, although a few labeled fibers are found in the subcortical white matter. The subcortical projection from the CM-Pf complex terminates within the caudate nucleus, putamen, globus pallidus, subthalamic nucleus, zona incerta, fields of Forel, hypothalamus, thalamic reticular nucleus, and rostral intralaminar nuclei. Prominent silver grain aggregates are also present within the ventral lateral, ventral anterior, ventral medial, and lateral posterior nuclei, and ventrobasal complex. The aggregates in the thalamus appear to be fibers of passage, but whether these are also terminals cannot be determined with the techniques used in the present study.     

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.