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PubMed1987

Evidence for parallel processing in the frog's auditory thalamus.

We have conducted anatomical and physiological experiments to investigate the functional organization of the dorsal thalamus in the northern leopard frog (Rana pipiens pipiens). Our studies provide evidence for parallel auditory processing at this level of the frog's brain. Acoustically evoked potentials were recorded from the posterior and central thalamic nuclei and several differences in sound-evoked activity were noted between them: the amplitude of acoustically evoked potentials (AEPs), in response to a standard search stimulus, was always greater in the central, as opposed to the posterior, nucleus; the posterior, but not central, nucleus exhibited the phenomenon of nonlinear summation when 350-Hz and 1,700-Hz tones were presented simultaneously rather than individually; and the central, but not posterior, nucleus showed selectivity for the repetition rate of pulsed sound signals. The posterior and central thalamic nuclei also possessed distinct innervation patterns as revealed by the HRP transport patterns arising from these structures. The central nucleus was reciprocally connected with the major auditory relay stations along the frog's central auditory pathway including the superior olive, nucleus of the lateral lemniscus, and the torus semicircularis. Major projections to the lateral thalamic nucleus, ventral hypothalamus, and the telencephalic striatal complex were also observed. The posterior nucleus, on the other hand, established reciprocal connections primarily with the medial reticular nucleus, ventral midbrain tegmentum, and structures constituting of the ventral thalamic nuclei, particularly the nucleus of Bellonci. Thus, time and frequency cues contained within the species mating call, and conveying information concerning species identity, appear to be processed independently within the frog's thalamus with separate neural channels for each.

Animals↗
PubMed1990

Development of connections to and from the visual cortex in the wallaby (Macropus eugenii).

The time course of the development of connections between the visual cortex and the main subcortical visual structures, as well as intrahemispheric and interhemispheric connections, has been studied in the marsupial wallaby (Macropus eugenii) to compare its development with that of placental mammals. Pouch young are born prior to retinal innervation of the primary visual centers and spend a protracted period of development in the pouch, making them ideal for visual, developmental studies. Horseradish peroxidase conjugated to wheatgerm agglutinin was injected into either the presumptive visual cortex or the superior colliculus in young of varying ages. Thalamocortical projections from the dorsal lateral geniculate and lateral posterior nuclei reach the presumptive visual cortex between 12 and 15 days after birth. Descending cortical connections form later. Corticogeniculate axons are first detected in the geniculate and lateral posterior nucleus at 48 days after birth, while corticocollicular axons first reach the superior colliculus at 71 days and, by 81 days, have innervated the superficial layers. Intrahemispheric and interhemispheric connections form even later. By 99 days intrahemispheric axons from area 17 have accumulated in visual association areas but are yet to invade layers III and IV, their major termination zones in adult, while axons projecting back to area 17 have also reached their target area. At this time interhemispheric axons from area 17 have begun to accumulate in the opposite visual cortex, although they have not invaded the cortical layers. By 111 days cortical cells projecting to the opposite visual cortex are first labelled. These have a more widespread distribution in area 17 at 111 and 122 days compared to the adult, where they are confined to the 17/18 border. The results show that the marsupial wallaby has a timetable of similar sequence, but different relative timing, in the formation of cortical connections compared to that of placental mammals. In the first half of the period between conception and eye opening, the timing in the wallaby precedes considerably that in placental mammals. Ascending connections from the thalamus develop relatively earlier in the wallaby but descending collicular connections are delayed until the same relative time that they appear in placental mammals.

Afferent Pathways↗
PubMed1990

Topography and synaptology of mamillary body projections to the mesencephalon and pons in the rat.

The anterograde and retrograde transport of horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP) was used to study the anatomical organization of descending projections from the mamillary body (MB) to the mesencephalon and pons at light and electron microscopic levels. Injections of WGA-HRP into the medial mamillary nucleus resulted in dense anterograde and retrograde labeling in the ventral tegmental nucleus, while injections in the lateral mamillary nucleus resulted in dense anterograde labeling in the dorsal tegmental nucleus pars dorsalis and dense anterograde and retrograde labeling in the pars ventralis of the dorsal tegmental nucleus. Anterogradely labeled fibers in the mamillotegmental tract diverged from the principal mamillary tract in an extensive dorsocaudally oriented swath of axons which extended to the dorsal and ventral tegmental nuclei, and numerous axons turned sharply ventrally and rostrally to terminate topographically in the dorsomedial nucleus reticularis tegmenti pontis and rostromedial pontine nuclei. The anterograde labeling in these two precerebellar relay nuclei was distributed near the midline such that projections from the lateral mamillary nucleus terminated mainly dorsomedial to the terminal fields of projections from the medial mamillary nucleus. In the dorsal and ventral tegmental nuclei, labeled axon terminals contained round synaptic vesicles and formed asymmetric synaptic junctions primarily with small diameter dendrites and to a lesser extent with neuronal somata. A few labeled terminals contained pleomorphic vesicles and formed symmetric synaptic junctions with dendrites and neuronal somata. Labeled axon terminals were also frequently found in synaptic contact with retrogradely labeled dendrites and neuronal somata in the dorsal and ventral tegmental nuclei. These findings indicate that neurons in the dorsal and ventral tegmental nuclei are reciprocally connected with MB projection neurons. In the nucleus reticularis tegmenti pontis and medial pontine nuclei, labeled axon terminals contained round synaptic vesicles and formed asymmetric synaptic junctions primarily with small diameter dendrites. The present study demonstrates that projections from the medial and lateral nuclei of the MB are topographically organized in the mesencephalon and pons. The synaptic morphology of mamillotegmental projections suggests that they may have excitatory influences primarily on the distal dendrites of neurons in these brain regions.

Afferent Pathways↗
PubMed1990

Organization of the visual system in larval lampreys: an HRP study.

The organization of the visual system of larval lampreys was studied by anterograde and retrograde transport of HRP injected into the eye. The retinofugal system has two different patterns of organization during the larval period. In small larvae (less than 60-70 mm in length) only a single contralateral tract, the axial optic tract, is differentiated. This tract projects to regions in the diencephalon, pretectum, and mesencephalic tegmentum. In larvae longer than 70-80 mm, there is an additional contralateral tract, the lateral optic tract, which extends to the whole tectal surface. In addition, ipsilateral retinal fibers are found in both small and large larvae. Initially, the ipsilateral projection is restricted to the thalamus-pretectum, but it reaches the optic tectum in late larvae. Changes in the organization of the optic tracts coincide with the formation of the late-developing retina and consequently, the origin of the optic tracts can be related to specific retinal regions. The retinopetal system is well developed in all larvae. Most retinopetal neurons are labeled contralaterally and are located in the M2-M5 nucleus of the mesencephalic tegmentum, in the caudolateral mesencephalic reticular area and adjacent ventrolateral portions of the optic tectum. Dendrites of these cells are apparent, especially those directed dorsally, which in large larvae extend to the optic tectum overlapping with the retino-tectal projection. These results indicate that in lampreys, visual projections organize mainly during the blind larval period before the metamorphosis, their development being largely independent of visual function.

Afferent Pathways↗
PubMed1992

Lamina I spinocervical tract terminations in the medial part of the lateral cervical nucleus in the cat.

The terminations of spinocervical tract fibers in the lateral cervical nucleus (LCN) of the cat were examined with anterogradely transported Phaseolus vulgaris leucoagglutinin (PHA-L) in order to analyze their organization relative to the most medial part and the main body (the lateral two-thirds) of the LCN, which have differential projections and physiological characteristics. Iontophoretic injections of PHA-L in laminae I-V of the spinal dorsal horn yielded dense labeling in somatotopically appropriate regions of the main body of the LCN, and, as seen previously with horseradish peroxidase, additional terminations were present in the medial LCN after injections at either cervical or lumbar spinal levels. The morphological characteristics of the PHA-L labeling in these two parts of the LCN were different. Terminations in the lateral LCN consisted of dense clusters of thick fibers bearing large numbers of boutons. The terminal axons in the medial part of the LCN displayed a reticulated network of longitudinally oriented, fine fibers with well-spaced varicosities. Some of the fine fibers in the medial LCN appeared to be collaterals of thicker fibers that terminated in the lateral LCN. Injections of PHA-L that were restricted to lamina I resulted in terminal labeling only in the medial LCN. The labeling was more sparse than that observed in the medial LCN after larger dorsal horn injections but displayed the same morphological characteristics. Lamina I terminations were seen in the medial LCN after cervical or lumbar injections on both the ipsilateral and contralateral sides. The PHA-L observations were corroborated by the presence of many retrogradely labeled lamina I cells at both cervical and lumbar spinal levels, following injections of cholera toxin subunit b or rhodamine-labeled microspheres in the medial LCN. In addition, double-immunofluorescent labeling for PHA-L and substance P was performed in a few cases, since substance P immunoreactivity is present in fibers in the medial LCN and also in cell bodies in lamina I; however, very few spinocervical fibers displayed immunoreactivity for both antigens. These observations indicate that the medial part of the LCN receives input from lamina I neurons, and probably from lamina III-V neurons as well, at cervical and lumbar spinal levels. The lamina I input to the medial LCN provides a basis for the small population of nociceptive neurons that differentiate the medial LCN. The lamina I input could also be responsible for the general inhibition of lateral LCN neurons by wide-field noxious stimulation, via activation of GABAergic interneurons in the medial LCN.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent Pathways↗
PubMed1994

Development of the retinofugal projections in the embryonic and larval zebrafish (Brachydanio rerio).

Studies of the projection from the vertebrate retina have contributed significantly to current concepts of neural development. The zebrafish has recently become a favored system for the study of development in general and neural development in particular. Although the development of both the optic nerve and the retinotectal projection of the zebrafish has been described, the retinofugal projection in its entirety has not. This paper describes it and also addresses the issue of projectional exuberance: i.e., transient projections to targets that are not innervated in the adult. The retinofugal projection of embryonic and larval zebrafish (32 hours to 7 days post-fertilization) was labeled by intraocular injection of DiI (1,1'-dioctadecyl-3,3,3',3',tetramethylindocarbocyanine perchlorate) and then studied in wholemounts and sections. The first optic axons crossed the chiasm at 32 hours post-fertilization and projected in a straight line to reach the tectum at about 44 hours. At 48 hours, a few optic axons deviated along either the tract of the posterior commissure or the tract of the postoptic commissure. By 72 hours (about the time of hatching) optic axons arborized in ten distinct regions, termed arborization fields. At 6-7 days post-fertilization, the same ten arborization fields (nine contralateral, one bilateral) were evident. Most of the arborization fields were located in the superficial neuropil and were not associated with morphologically identifiable clusters of somata. On the basis of various landmarks, the ten arborization fields are identified as precursors of retinorecipient nuclei previously described in other adult cypriniform fishes. The development was characterized by the nearly complete absence of any transient projections. Thus, the idea that axonal outgrowth is initially exuberant and trimmed back later is not supported by these results.

Afferent Pathways↗
PubMed1995

Parallel pathways and convergence onto HVc and adjacent neostriatum of adult zebra finches (Taeniopygia guttata).

The structure and connectivity of the forebrain nucleus HVc, a site of sensorimotor integration in the song control system of oscine birds, were investigated in adult zebra finches. HVc in males comprises three cytoarchitectonic subdivisions: the commonly recognized central region with large and medium-sized darkly staining cells, a ventral caudomedial region with densely packed small and medium-sized cells, and a dorsolateral region with oblong cells and rows of cells. All three subdivisions project to area X and the robust nucleus of the archistriatum, with more complexity in the classes and distribution of cells than previously reported. In females, HVc is very small and has a cytoarchitecture distinct from that of the three male subdivisions. The structure of HVc in females treated with estradiol at 15 days of age is similar to male HVc. Tracer studies in males with fluorescent and biotinylated dextrans demonstrate non-topographic projections onto HVc that may carry auditory information, including type 1 and type 2 neurons in subdivisions L1 and L3 of the field L complex, a class of neurons in nucleus interface, nucleus uvaeformis, the caudal neostriatum ventral to HVc, and intrinsic HVc connections. These data are interpreted in terms of HVc's functional properties. Additionally, the neostriatum immediately ventral to HVc receives projections from field L, ventral hyperstriatum, and caudal neostriatum, and projects to a region surrounding RA and near to or into area X. The similarity of the connectivity of HVc and adjacent neostriatum suggests the possibility that they share a common origin.

Afferent Pathways↗
PubMed1994

Ataxic hemiparesis.

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Afferent Pathways↗
PubMed1981

Laminar organization of efferent cells in the parietal cortex of the Virginia opossum.

The size, shape and laminar position of efferent neurons in the parietal cortex of the Virginia opossum were identified using the method of retrograde transport of horseradish peroxidase (HRP). Injection of HRP into the spinal cord, dorsal column nuclei or pontine nuclei leads to labeling of cells in layer V and occasionally in layer VI, while a large injection of HRP in the dorsal thalamus labels many cells in layer VI, with fewer cells in layer V. HRP injections in the SSM cortex label cells in layers II-VI of ipsilateral and contralateral cortical areas. However, the majority of these cortico-cortical cells are found in the supragranular layers. Examination of the size, shape and laminar position of retrogradely labeled layer V neurons after injections in each of these areas suggests that none of these features can be used to predict accurately the projection target of individual neurons. We conclude that the laminar organization of efferent cells of the opossum parietal cortex is very similar to that seen in the neocortex of other mammals, despite the complete coalescence of somatic sensory (SI) and motor (MI) areas in the opossum.

Animals↗
PubMed2005

A central back-coupling hypothesis on the organization of motor synergies: a physical metaphor and a neural model.

We offer a hypothesis on the organization of multi-effector motor synergies and illustrate it with the task of force production with a set of fingers. A physical metaphor, a leaking bucket, is analyzed to demonstrate that an inanimate structure can show apparent error compensation among its elements. A neural model is developed using tunable back-coupling loops as means of assuring error compensation in a task-specific way. The model demonstrates non-trivial features of multi-finger interaction such as delayed emergence of force stabilizing synergies and simultaneous stabilization of the total force and total moment produced by the fingers. The hypothesis suggests that neurophysiological structures involving short-latency feedback may play a central role in the formation of motor synergies.

Afferent Pathways↗
PubMed1996

Cortico-thalamic interplay and the security of operation of neural assemblies and temporal chains in the cerebral cortex.

Much evidence suggests that the mammalian thalamus is not merely a set of nuclei relaying signals to the cerebral cortex, but is engaged in two-way interplay with it. Three important features constrain ideas about the nature of this interplay: (i) thalamic projection neurones lack local axon collaterals; (ii) most corticothalamic projections have very long axonal conduction time; (iii) in the waking state the membrane potential of thalamic projections cells appears to be poised just beneath threshold for firing. It is proposed that corticothalamo-cortical pathways represent connections between different cortical loci which have higher security than the direct cortico-cortical route. Thus each thalamo-cortical projection neurone can have a singular and pivotal role in the activation of one or more cortical neural assemblies. The long delays of cortico-thalamic conduction suggest that the cortico-thalamo-cortical loop also plays a crucial role in the operation of time-structured neural assemblies ('synfire chains'; Abeles), by providing a high-security link from one active node of the chain to nodes activated at a later time in the sequence. It is predicted that, in the waking animal, thalamic projection cells should have a response selectivity to complex percepts and concepts, conferred on them by the cortical assemblies in whose activation they participate.

Afferent Pathways↗
PubMed2000

Mucosa-associated lymphoid tissue in human efferent tear ducts.

The recent description of primary marginal zone lymphomas in human efferent tear ducts with typical features of lymphomas arising in mucosa-associated lymphoid tissue (MALT) infers the presence of MALT in the human efferent tear ducts. To date, studies have not established clearly whether organised MALT occurs in normal human efferent tear ducts. To elucidate this problem, efferent lacrimal pathways from unselected body donors with unknown prior history of efferent tear duct, ocular, or nasal disease were examined for the presence of organised MALT. Organised lymphoid tissue was found with the cytomorphological and immunophenotypic features of MALT in 41% of the cases examined. These findings suggest that MALT is a feature that, although it need not be present in normal efferent tear ducts, is acquired during life in a proportion of apparently asymptomatic individuals.

Adult↗
PubMed1981

On climbing fibre projections to cerebellar paramedian lobule activated from mesencephalon in the cat.

Climbing fibre projections to cerebellar paramedian lobule activated from structures in medial midbrain tegmentum were studied in chloralose anaesthetized cats. Three sagittal zones were identified, from medial to lateral c1, c2 and d1 zones. The c1 and d1 zones were activated, at the same short latency, from a dorsal tegmental region with a location similar to the composite bundle called the central tegmental tract. The d1 zone is probably activated via rubro-olivary fibres and the c1 zone via some other fibres in this bundle. The path to the c2 zone seemed to proceed only partly within this dorsal tegmental region. The d1 zone was also activated, at longer latency, from a region ventral to the red nucleus.

Afferent Pathways↗
PubMed1982

The tecto-olivo-cerebellar pathway in the rat.

The olivo-cerebellar projection to the vermis of cerebellar lobule VII, and the projection of the midbrain tectum onto the inferior olive (IO), were examined in the rat employing, respectively, retrograde and anterograde transport of horseradish peroxidase (HRP). Olivary neurons afferent to lobule VII vermis are located in a restricted region of the contralateral, caudomedial, medial accessory olive (MAO). The termination of the olivary projection demonstrated after HRP placement in the superior colliculus (SC), corresponds closely with this region.

Afferent Pathways↗
PubMed1983

Supraspinal connections of neurones in the thoracic spinal cord of the cat: ascending projections and effects of descending impulses.

Single unit electrical activity has been recorded extracellularly from 103 neurones in the thoracic spinal cord of decerebrate cats. The responses of these neurons to electrical stimulation of cutaneous and visceral afferent fibres, their projection through ascending sensory pathways and the effects of descending impulses on the neurones have been studied. Of the 103 neurones recorded, 45 (43.7%) responded only to activation of cutaneous afferent fibres ('Somatic' neurones). Their recording sites were located mainly in laminae II, III and IV of the dorsal horn. The remaining 58 neurones (56.3%) responded to stimulation of cutaneous and visceral afferent fibres ('Viscero-somatic' neurones). Their recording sites were located in laminae I, V, VII and VIII of the grey matter. Sixteen neurones had axons projecting through ascending pathways: 6 were post-synaptic dorsal column cells (PSDC), 2 were spino-cervical tract cells (SCT), 5 projected through the contralateral ventro-lateral funiculus (VLQ) and 3 through the ipsilateral dorso-lateral funiculus (DLF). All PSDC cells were somatic and all VLQ neurones were viscero-somatic. Reversible spinalization of the animals by cold block resulted in a selective increase of the responses of viscero-somatic neurones to cutaneous and visceral C-fibre input. In some viscero-somatic neurones, cold block induced a reduction or abolition of the visceral input suggesting its mediation via supraspinal loops. Electrical stimulation of the ipsilateral DLF evoked non-specific inhibitions of all inputs to viscero-somatic neurones. These results are discussed in relation with the mechanisms of visceral sensation.

Afferent Pathways↗
PubMed1984

The projection to the mesencephalon from the dorsal column nuclei. An anatomical study in the cat.

The terminal areas and cells of origin of the projection from the dorsal column nuclei to the mesencephalon were investigated by the intra-axonal transport method. Following injection of wheat germ agglutinin-horseradish peroxidase conjugate into the dorsal column nuclei, anterograde labeling was observed in several regions of the midbrain. The main terminal area was situated at the level of transition between the superior and inferior colliculus on the side contralateral to the injection site and comprised the intercollicular nucleus and part of the external and pericentral nuclei of the inferior colliculus and of the nucleus of the brachium of the inferior colliculus, but there were also projections to the caudal half of the deep and intermediate gray layers of the superior colliculus, the anterior and posterior pretectal nuclei, the nucleus of Darkschewitsch and nucleus ruber. Injections restricted to either the gracile nucleus or the cuneate nucleus revealed a somatotopic termination pattern in the intercollicular nucleus, superior colliculus and pretectal nuclei. The retrograde labeling seen after injection of tracer into the midbrain terminal areas showed that the cells of origin were located mainly in the rostral and caudal parts of the dorsal column nuclei, whereas the middle cell nest neurons were unlabeled, thus supporting previous observations that the neurons projecting to the midbrain constitute a population separate from that projecting to the thalamus. Cell counts revealed that the midbrain projection is of a considerable magnitude, involving between 10,000 and 15,000 neurons; its functional significance is, however, largely unknown.

Afferent Pathways↗
PubMed1984

Direct projections from the substantia nigra to the posterior thalamic regions in the cat.

The substantia nigra (SN) of the cat was shown, by the anterograde and retrograde horseradish peroxidase methods, to contain neurons which send their axons to the caudomedial portions of the suprageniculate nucleus and/or lateroposterior nucleus of the thalamus; these neurons were located in the restricted region in the lateral part of the SN, which corresponds to the pars lateralis of the SN.

Afferent Pathways↗