Spinothalamic projections in the pigeon

The Medial Ventrothalamic Circuitry: Cells Implicated in a Bimodal Network

Previous avian thalamic studies have shown that the medial ventral thalamus is composed of several nuclei located close to the lateral wall of the third ventricle. Although the general connectivity is known, detailed morphology and connectivity pattern in some regions are still elusive. Here, using the intracellular filling technique in the chicken, we focused on two neural structures, namely, the retinorecipient neuropil of the n. geniculatus lateralis pars ventralis (GLv), and the adjacent n. intercalatus thalami (ICT). We found that the GLv-ne cells showed two different neuronal types: projection cells and horizontal interneurons. The projection cells showed variable morphologies and dendritic arborizations with axons that targeted the n. lentiformis mesencephali (LM), griseum tectale (GT), ICT, n. principalis precommissuralis (PPC), and optic tectum (TeO). The horizontal cells showed a widespread mediolateral neural process throughout the retinorecipient GLv-ne. The ICT cells, on the other hand, had multipolar somata with wide dendritic fields that extended toward the lamina interna of the GLv, and a projection pattern that targeted the n. laminaris precommissuralis (LPC). Together, these results elucidate the rich complexity of the connectivity pattern so far described between the GLv, ICT, pretectum, and tectum. Interestingly, the implication of some of these neural structures in visuomotor and somatosensory roles strongly suggests that the GLv and ICT are part of a bimodal circuit that may be involved in the generation/modulation of saccades, gaze control, and space perception.

A pathway for predation in the brain of the barn owl (Tyto alba): projections of the gracile nucleus to the "claw area" of the rostral wulst via the dorsal thalamus

The Wulst of birds, which is generally considered homologous with the isocortex of mammals, is an elevation on the dorsum of the telencephalon that is particularly prominent in predatory species, especially those with large, frontally placed eyes, such as owls. The Wulst, therefore, is largely visual, but a relatively small rostral portion is somatosensory in nature. In barn owls, this rostral somatosensory part of the Wulst forms a unique physical protuberance dedicated to the representation of the contralateral claw. Here we investigate whether the input to this "claw area" arises from dorsal thalamic neurons that, in turn, receive their somatosensory input from the gracile nucleus. After injections of biotinylated dextran amine into the gracile nucleus and cholera toxin B chain into the claw area, terminations from the former and retrogradely labeled neurons from the latter overlapped substantially in the thalamic nucleus dorsalis intermedius ventralis anterior. These results indicate the existence in this species of a "classical" trisynaptic somatosensory pathway from the body periphery to the telencephalic Wulst, via the dorsal thalamus, one that is likely involved in the barn owl's predatory behavior. The results are discussed in the context of somatosensory projections, primarily in this and other avian species.

Efferent connections of the dorsomedial thalamic nuclei of the domestic chick (Gallus domesticus)

Small iontophoretic injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin were placed in the thalamic anterior dorsomedial nucleus (DMA) of domestic chicks. The projections of the DMA covered the rostrobasal forebrain, ventral paleostriatum, nucleus accumbens, septal nuclei, Wulst, hyperstriatum ventrale, neostriatal areas, archistriatal subdivisions, dorsolateral corticoid area, numerous hypothalamic nuclei, and dorsal thalamic nuclei. The rostral DMA projects preferentially on the hypothalamus, whereas the caudal part is connected mainly to the dorsal thalamus. The DMA is also connected to the periaqueductal gray, deep tectum opticum, intercollicular nucleus, ventral tegmental area, substantia nigra, locus coeruleus, dorsal lateral mesencephalic nucleus, lateral reticular formation, nucleus papillioformis, and vestibular and cranial nerve nuclei. This pattern of connectivity is likely to reflect an important role of the avian DMA in the regulation of attention and arousal, memory formation, fear responses, affective components of pain, and hormonally mediated behaviors.

Bilaterally projecting neurons in the two visual pathways of chicks

By means of a double-labeling technique, we have investigated the organization of the bilateral thalamo-Wulst and tecto-rotundal projections in 2-day old chicks. After injecting fluorogold (FG) into one side of the visual Wulst and rhodamine B isothiocyanate (RITC) into the other side of the visual Wulst, the labeled neurons in the nucleus geniculatus lateralis pars dorsalis (GLd) were examined. Although the distribution areas of ipsilaterally and contralaterally labeled neurons overlap partly, very few double-labeled neurons were found (only 0.01% double-labeled neurons). This suggests that the ipsilateral and contralateral projections to the Wulst come from different neuronal populations of the thalamus. The FG and RITC were also injected into the rotundal nuclei (Rt) on each side of the thalamus and the labeled neurons in the optic tectum (TeO) were examined. In the TeO, the distribution areas of the neurons labeled ipsilaterally and contralaterally to Rt overlap completely and we found that up to 45% of the tectal cells were double-labeled by both FG and RITC. Therefore, many tectal neurons have axon collaterals so that they project to the Rt on both sides of the thalamus and must send information simultaneously to both sides of the brain. The differences in the structural organization of the two visual pathways are discussed with reference to the transmission of information to higher centers on both sides of the brain.

Visual and somatosensory inputs to the avian song system via nucleus uvaeformis (Uva) and a comparison with the projections of a similar thalamic nucleus in a nonsongbird, Columba livia

Nucleus uvaeformis (Uva), previously identified as a component of song control circuitry in songbirds, and nucleus dorsolateralis posterior thalami, pars caudalis (DLPc) in pigeon, were compared with respect to their relative positions in the dorsolateral part of the posterior thalamus, their cell types, and their afferent and efferent projections. Both nuclei are closely related to the habenulointerpeduncular tract, have similar cell types, and receive a dense projection from deep layers of the optic tectum, predominantly ipsilaterally, and a distinct projection from the dorsal column and external cuneate nuclei, predominantly contralaterally. Recordings of multiple unit activity evoked by visual and somatosensory stimuli were used to guide injections of tracer into either DLPc or Uva, and the projections to the telencephalon were charted. Both nuclei were found to have a major terminal field in the medial part of the ipsilateral neostriatum intermedium (NI), known as nucleus interfacialis (NIf) in songbirds, and a minor terminal field in the roof of the neostriatum caudale (NC). In pigeon, the DLPc terminations in NC were within a region known as neostriatum dorsale (Nd), and, in male songbirds, the Uva terminations were in the high vocal center (HVC). Recordings of visual and somatosensory evoked activity were then used to guide injections of tracer into NI, and the afferent and efferent projections were again compared in pigeon and songbirds. The projections from either DLPc or Uva were confirmed, and terminal fields were observed either in Nd in pigeon, the dorsolateral part of NC in female songbirds, or HVC in male songbirds. Injections of tracer into either Nd or HVC confirmed their sources of afferents in DLPc or Uva, respectively, and in NI, but there was incomplete overlap of the distribution of retrogradely labelled cells in NI and the terminal fields of DLPc or Uva. It is concluded that DLPc and Uva are comparable nuclei having similar afferent and efferent projections relaying visual and somatosensory information to the telencephalon. The possible role of this information in vocal control is discussed.

Spinothalamic projections in amphibians as revealed with anterograde tracing techniques

Direct spinothalamic pathways were demonstrated in anurans (Rana ridibunda, Xenopus laevis) and in the ribbed newt, Pleurodeles waltl. With the powerful anterograde tracers Phaseolus vulgaris leucoagglutinin and biotinylated dextran amine, rather extensive spinothalamic projections were found, including the ventromedial thalamic nucleus, the dorsal thalamus and several posterior diencephalic nuclei (anurans), and the neuropil lateral to the pars ventralis thalami as well as to the anteroventral and posterodorsal zones (P. waltl), respectively.

New subdivision schema for the avian torus semicircularis: neurochemical maps in the chick

Chemoarchitectonic subdivisions in the chicken torus semicircularis were mapped by means of acetylcholinesterase histochemistry and immunocytochemical labeling of leucine-enkephalin, choline acetyltransferase, neuropeptide Y, and calbindin/calretinin in adjacent sections. The torus semicircularis was found to consist of three main divisions: intercollicular area, toral nucleus, and preisthmic superficial area. All three appear variously subdivided. The intercollicular area is a mid-mesencephalic ventral periventricular region and appears subdivided into core and shell intercollicular regions. The toral nucleus is formed by a large caudal periventricular cytoarchitectonic complex, consisting of a periventricular lamina subdivided into core and shell regions, a pericentral, diffuse external nucleus, a central nucleus subdivided into core and shell regions, a caudomedial shell nucleus, a paracentral nucleus, and a posterior hiliar nucleus, apart from other minor parcellations. The preisthmic superficial area extends superficially at the caudomedial end of the toral nucleus, reaching the paramedian dorsal brain surface just rostral to the isthmo-optic nucleus. It is subdivided into core and shell regions. This previously unnoticed area is distinguished here from the intercollicular area and from the caudomedial shell and paracentral nuclei, all of which are frequently mixed in the literature under the concept "intercollicular nucleus." The revised terminology and subdivision for the avian torus clarifies many chemoarchitectonic and hodological mappings reported in the literature. It also suggests new research subjects and eliminates some causes of confusion.

The evolution of the dorsal thalamus of jawed vertebrates, including mammals: cladistic analysis and a new hypothesis

The evolution of the dorsal thalamus in various vertebrate lineages of jawed vertebrates has been an enigma, partly due to two prevalent misconceptions: the belief that the multitude of nuclei in the dorsal thalamus of mammals could be meaningfully compared neither with the relatively few nuclei in the dorsal thalamus of anamniotes nor with the intermediate number of dorsal thalamic nuclei of other amniotes and a definition of the dorsal thalamus that too narrowly focused on the features of the dorsal thalamus of mammals. The cladistic analysis carried out here allows us to recognize which features are plesiomorphic and which apomorphic for the dorsal thalamus of jawed vertebrates and to then reconstruct the major changes that have occurred in the dorsal thalamus over evolution. Embryological data examined in the context of Von Baerian theory (embryos of later-descendant species resemble the embryos of earlier-descendant species to the point of their divergence) supports a new 'Dual Elaboration Hypothesis' of dorsal thalamic evolution generated from this cladistic analysis. From the morphotype for an early stage in the embryological development of the dorsal thalamus of jawed vertebrates, the divergent, sequential stages of the development of the dorsal thalamus are derived for each major radiation and compared. The new hypothesis holds that the dorsal thalamus comprises two basic divisions--the collothalamus and the lemnothalamus--that receive their predominant input from the midbrain roof and (plesiomorphically) from lemniscal pathways, including the optic tract, respectively. Where present, the collothalamic, midbrain-sensory relay nuclei are homologous to each other in all vertebrate radiations as discrete nuclei. Within the lemnothalamus, the dorsal lateral geniculate nucleus of mammals and the dorsal lateral optic nucleus of non-synapsid amniotes (diapsid reptiles, birds and turtles) are homologous as discrete nuclei; most or all of the ventral nuclear group of mammals is homologous as a field to the lemniscal somatosensory relay and motor feedback nuclei of non-synapsid amniotes; the anterior, intralaminar and medial nuclear groups of mammals are collectively homologous as a field to both the dorsomedial and dorsolateral (including perirotundal) nuclei of non-synapsid amniotes; the anterior, intralaminar, medial and ventral nuclear groups and the dorsal lateral geniculate nucleus of mammals are collectively homologous as a field to the nucleus anterior of anamniotes, as are their homologues in non-synapsid amniotes. In the captorhinomorph ancestors of extant land vertebrates, both divisions of the dorsal thalamus were elaborated to some extent due to an increase in proliferation and lateral migration of neurons during development.(ABSTRACT TRUNCATED AT 400 WORDS)

The evolution of the dorsal pallium in the telencephalon of amniotes: cladistic analysis and a new hypothesis

The large body of evidence that supports the hypothesis that the dorsal cortex and dorsal ventricular ridge of non-mammalian (non-synapsid) amniotes form the dorsal pallium and are homologous as a set of specified populations of cells to respective sets of cells in mammalian isocortex is reviewed. Several recently taken positions that oppose this hypothesis are examined and found to lack a solid foundation. A cladistic analysis of multiple features of the dorsal pallium in amniotes was carried out in order to obtain a morphotype for the common ancestral stock of all living amniotes, i.e., a captorhinomorph amniote. A previous cladistic analysis of the dorsal thalamus (Butler, A.B., The evolution of the dorsal thalamus of jawed vertebrates, including mammals: cladistic analysis and a new hypothesis, Brain Res. Rev., 19 (1994) 29-65; this issue, previous article) found that two fundamental divisions of the dorsal thalamus can be recognized--termed the lemnothalamus in reference to predominant lemniscal sensory input and the collothalamus in reference to predominant input from the midbrain roof. These two divisions are both elaborated in amniotes in that their volume is increased and their nuclei are laterally migrated in comparison with anamniotes. The present cladistic analysis found that two corresponding, fundamental divisions of the dorsal pallium were present in captorhinomorph amniotes and were expanded relative to their condition in anamniotes. Both the lemnothalamic medial pallial division and the collothalamic lateral pallial division were subsequently further markedly expanded in the synapsid line leading to mammals, along with correlated expansions of the lemnothalamus and collothalamus. Only the collothalamic lateral pallial division--along with the collothalamus--was subsequently further markedly expanded in the non-synapsid amniote line that gave rise to diapsid reptiles, birds and turtles. In the synapsid line leading to mammals, an increase in the degree of radial organization of both divisions of the dorsal pallium also occurred, resulting in an 'outside-in' migration pattern during development. The lemnothalamic medial division of the dorsal pallium has two parts. The medial part forms the subicular, cingulate, prefrontal, sensorimotor, and related cortices in mammals and the medial part of the dorsal cortex in non-synapsid amniotes. The lateral part forms striate cortex in mammals and the lateral part of dorsal cortex (or pallial thickening or visual Wulst) in non-synapsid amniotes. Specific fields within the collothalamic lateral division of the dorsal pallium form the extrastriate, auditory, secondary somatosensory, and related cortices in mammals and the visual, auditory, somatosensory, and related areas of the dorsal ventricular ridge in non-synapsid amniotes.

Sensory properties and afferents of the N. dorsolateralis posterior thalami of the pigeon

According to previous studies, the avian n. dorsolateralis posterior thalami (DLP) receives visual and somatosensory afferents. While some authors (e.g., Gamlin and Cohen: J. Comp. Neurol. 250:296-310, '86) proposed a distinction between a visual caudal (DLPc) and a somatosensory rostral (DLPr) part, other authors (e.g., Wild: Brain Res. 412:205-223, '87) could not confirm such a differentiation. The aim of the present experiment was to study with physiological and anatomical methods the proposed parcellation of the DLP into various components dealing with different modalities. The physiological properties of the DLP of the pigeon were analysed with extracellular single unit recordings. With the same approach, neurons of the n. dorsalis intermedius ventralis anterior (DIVA), a somatosensory relay nucleus in the dorsal thalamus, were also analysed. The afferents of the DLP were studied by using anatomical tract tracing techniques with retrograde and anterograde tracers. The sensory properties of DLP cells revealed that somatosensory, visual, and auditory modalities affect the neuronal firing frequency in this nucleus. All three modalities were present throughout the full caudorostral extent of the DLP. Cells recorded in DIVA responded nearly exclusively to somatosensory stimulation. Unlike the DLP, single units in DIVA generally had smaller receptive fields encompassing only one extremity. The analysis of afferent connections of the DLP by using injections of retrograde and anterograde tracers (HRP, WGA-HRP, Fast Blue, and Rhodamine-beta-isothiocyanate) demonstrated extensive projections from the nuclei gracilis et cuneatus (GC) and more sparse projections from the nucleus tractus descendens trigemini (TTD), and the nucleus cuneatus externus (CE). Brainstem afferents of the DLP came from different vestibular nuclei, various areas of the brainstem reticular formation, and the optic tectum. Prosencephalic afferents originated in the n. posteroventralis thalami (PV), the n. ventromedialis posterior thalami (VMP), the n. dorsalis intermedius ventralis anterior (DIVA), and the nucleus reticularis superior pars dorsalis and ventralis (RSd and RSv). Telencephalic afferents of the DLP came from the hyperstriatum accessorium (HA) and a group of cells at the borderline between the hyperstriatum intercalatus superior (HIS) and the hyperstriatum dorsale (HD). The somatosensory afferents of the DLP probably originate from the GC, TTD, and CE, whereas it is likely that the visual input is mediated by the optic tectum. The anatomical source for the acoustic input is unclear. The very long latencies of auditory DLP neurons make it likely that the acoustic input originates at least partly in the reticular formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Avian somatosensory system: II. Ascending projections of the dorsal column and external cuneate nuclei in the pigeon

The ascending projections of the dorsal column and external cuneate nuclei (DCN/CuE) in the pigeon were investigated in anterograde tracing experiments by using autoradiography or wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). The results show that the majority of ascending projections decussate via internal arcuate fibers to form a contralateral medial lemniscus which ascends in a ventral position. In the brainstem, terminal fields were observed in the ventral lamella of the inferior olive (OI), the parabrachial nuclei (PB) of the dorsolateral pons, the intercollicular nucleus (ICo) of the midbrain, and the nucleus pretectalis diffusus (PD). In the diencephalon there were terminal fields in the strata cellulare externum and internum (SCE and SCI) of the caudal hypothalamus; in the intercalated (ICT), ventrolateral (VLT), and reticular nuclei of the ventral thalamus; in the nuclei principalis precommissuralis (PPC), spiriform medialis (SpM), and dorsolateralis posterior, pars caudalis (cDLP) of the caudal thalamus; and in the nuclei dorsalis intermedius ventralis anterior (DIVA), dorsolateralis posterior, pars rostralis (rDLP), dorsolateralis anterior (DLA), and dorsolateralis anterior, pars medialis (DLM) of the rostrodorsal thalamus. The origins of these projections within the DCN/CuE complex were verified in retrograde tracing experiments with WGA-HRP and were found to be partly differentiable with respect to their targets. The projections to DIVA, rDLP, DLA, DLM, cDLP, and SpM arise from all rostrocaudal levels of the DCN/CuE complex; those to ICo arise from caudomedial nuclear regions, while those to the hypothalamus and ventral thalamus arise from rostrolateral nuclear regions. Projections to PB arise from lamina I neurons of the dorsal horn of upper cervical spinal cord segments and from CuE. No evidence was found of a projection to the cerebellum. The distribution of the cells of origin of the medial lemniscus (ML) within the DCN/CuE complex was found to be largely coextensive with the areas of termination of primary spinal (Wild: J. Comp. Neurol. 240:377-395, '85) and some trigeminal (Dubbledam and Karten: J. Comp. Neurol. 180:661-678, '78) afferents. Furthermore, the areas of termination of the ML within the rostrodorsal and caudal thalamus are also either coextensive or closely associated with nuclei which provide a somatosensory projection to separate regions of the telencephalon (Wild: Brain Res. 412:205-223, '87). There are thus clear similarities in the overall pattern of somatosensory projections in the pigeon and in many mammalian species.

Somatosensory areas in the telencephalon of the pigeon. II. Spinal pathways and afferent connections

There are two somatosensory areas in the telencephalon of the pigeon which receive an input from the spinal somatosensory system: one in the rostral Wulst which consists of the three hyperstriatal layers (h. accessorium (HA), h. intercalatus superior (HIS) and h. dorsale (HD] and one in the caudal telencephalon (neostriatum caudale (NC), neostriatum intermedium (NI) and hyperstriatum ventrale (HV]. Recordings of evoked single unit or multi unit activity and of field potentials before and after lesions of spinal pathways at a high cervical level (C4) were made to determine the contribution of these pathways to the transmission of somatosensory signals to these telencephalic areas. The rostral Wulst area receives somatic signals only through dorsal tracts contralateral to the recording site. Inputs from the wing arise mainly through the dorsal columns (DC) and those from the leg largely through the dorsolateral funiculus (DLF). The spinal projection pathway to the caudal neostriatal area includes the dorsal tracts and parts of the lateral funiculi on both sides. There was no difference in response form between the wing and leg responses. Signals transmitted through the lateral pathways were found to elicit the earliest responses (6-13 ms, electrical stimulation) in the caudal forebrain, while signals travelling through the DC arrive later in the caudal area (about 14 ms for wing stimulation) than in the rostral Wulst area (about 9 ms). The afferent thalamic and intratelencephalic connections of the two somatosensory areas in the telencephalon of the pigeon were investigated with retrograde transport of the neuronal tracers horseradish-peroxidase (HRP) or wheatgerm agglutinated HRP (WGA-HRP), Fast Blue (FB) and Rhodamine-isothiocyanat (RITC). Small tracer-injections were made under electrophysiological control at somatosensory responsive locations. These investigations confirm the projection of the caudal part of the nucleus dorsolateralis posterior (DLPc) to the caudal area and of the nucleus dorsalis intermedius ventralis anterior (DIVA) to the rostral area. In addition, it could be shown that the NI/NC projects to the HV thus confirming the electrophysiological results reported in a companion paper (Funke 1989) that the HV is a secondary area. The integrative function of HV is supported by connections to other sensory and motor telencephalic areas. Combined injections of FB and RITC revealed a topographic projection from the DIVA to the anterior Wulst.(ABSTRACT TRUNCATED AT 400 WORDS)