The efferent connections of the nuclei of the descending trigeminal tract in the mallard (Anas platyrhynchos L.)

A neural circuit perspective on brain aromatase

This review explores the role of aromatase in the brain as illuminated by a set of conserved network-level connections identified in several vertebrate taxa. Aromatase-expressing neurons are neurochemically heterogeneous but the brain regions in which they are found are highly-conserved across the vertebrate lineage. During development, aromatase neurons have a prominent role in sexual differentiation of the brain and resultant sex differences in behavior and human brain diseases. Drawing on literature primarily from birds and rodents, we delineate brain regions that express aromatase and that are strongly interconnected, and suggest that, in many species, aromatase expression essentially defines the Social Behavior Network. Moreover, in several cases the inputs to and outputs from this core Social Behavior Network also express aromatase. Recent advances in molecular and genetic tools for neuroscience now enable in-depth and taxonomically diverse studies of the function of aromatase at the neural circuit level.

In Search for the Avian Trigeminal Magnetic Sensor: Distribution of Peripheral and Central Terminals of Ophthalmic Sensory Neurons in the Night-Migratory Eurasian Blackcap (Sylvia atricapilla)

In night-migratory songbirds, neurobiological and behavioral evidence suggest the existence of a magnetic sense associated with the ophthalmic branch of the trigeminal nerve (V1), possibly providing magnetic positional information. Curiously, neither the unequivocal existence, structural nature, nor the exact location of any sensory structure has been revealed to date. Here, we used neuronal tract tracing to map both the innervation fields in the upper beak and the detailed trigeminal brainstem terminations of the medial and lateral V1 subbranches in the night-migratory Eurasian Blackcap (Sylvia atricapilla). The medial V1 subbranch takes its course along the ventral part of the upper beak to innervate subepidermal layers and the mucosa of the nasal cavity, whereas the lateral V1 subbranch runs along dorsolateral levels until the nostrils to innervate mainly the skin of the upper beak. In the trigeminal brainstem, medial V1 terminals innervate both the dorsal part and the ventral, magnetically activated part of the principal sensory trigeminal brainstem nuclei (PrV). In contrast, the lateral V1 subbranch innervates only a small part of the ventral PrV. The spinal sensory trigeminal brainstem nuclei (SpV) receive topographically ordered projections. The medial V1 subbranch mainly innervates rostral and medial parts of SpV, whereas the lateral V1 subbranch mainly innervates the lateral and caudal parts of SpV. The present findings could provide valuable information for further analysis of the trigeminal magnetic sense of birds.

A newly identified trigeminal brain pathway in a night-migratory bird could be dedicated to transmitting magnetic map information

Night-migratory songbirds can use geomagnetic information to navigate over thousands of kilometres with great precision. A crucial part of the magnetic 'map' information used by night-migratory songbirds is conveyed via the ophthalmic branches of the trigeminal nerves to the trigeminal brainstem complex, where magnetic-driven neuronal activation has been observed. However, it is not known how this information reaches the forebrain for further processing. Here, we show that the magnetically activated region in the trigeminal brainstem of migratory Eurasian blackcaps (Sylvia atricapilla) represents a morphologically distinctive neuronal population with an exclusive and previously undescribed projection to the telencephalic frontal nidopallium. This projection is clearly different from the known trigeminal somatosensory pathway that we also confirmed both by neuronal tracing and by a thorough morphometric analysis of projecting neurons. The new pathway we identified here represents part of a brain circuit that-based on the known nidopallial connectivities in birds-could potentially transmit magnetic 'map' information to key multisensory integration centres in the brain known to be critically involved in spatial memory formation, cognition and/or controlling executive behaviour, such as navigation, in birds.

A novel relay nucleus between the inferior colliculus and the optic tectum in the chicken (Gallus gallus)

Processing multimodal sensory information is vital for behaving animals in many contexts. The barn owl, an auditory specialist, is a classic model for studying multisensory integration. In the barn owl, spatial auditory information is conveyed to the optic tectum (TeO) by a direct projection from the external nucleus of the inferior colliculus (ICX). In contrast, evidence of an integration of visual and auditory information in auditory generalist avian species is completely lacking. In particular, it is not known whether in auditory generalist species the ICX projects to the TeO at all. Here we use various retrograde and anterograde tracing techniques both in vivo and in vitro, intracellular fillings of neurons in vitro, and whole-cell patch recordings to characterize the connectivity between ICX and TeO in the chicken. We found that there is a direct projection from ICX to the TeO in the chicken, although this is small and only to the deeper layers (layers 13-15) of the TeO. However, we found a relay area interposed among the IC, the TeO, and the isthmic complex that receives strong synaptic input from the ICX and projects broadly upon the intermediate and deep layers of the TeO. This area is an external portion of the formatio reticularis lateralis (FRLx). In addition to the projection to the TeO, cells in FRLx send, via collaterals, descending projections through tectopontine-tectoreticular pathways. This newly described connection from the inferior colliculus to the TeO provides a solid basis for visual-auditory integration in an auditory generalist bird. J. Comp. Neurol. 525:513-534, 2017. © 2016 Wiley Periodicals, Inc.

Embryonic stages in cerebellar afferent development

The cerebellum is important for motor control, cognition, and language processing. Afferent and efferent fibers are major components of cerebellar circuitry and impairment of these circuits causes severe cerebellar malfunction, such as ataxia. The cerebellum receives information from two major afferent types – climbing fibers and mossy fibers. In addition, a third set of afferents project to the cerebellum as neuromodulatory fibers. The spatiotemporal pattern of early cerebellar afferents that enter the developing embryonic cerebellum is not fully understood. In this review, we will discuss the cerebellar architecture and connectivity specifically related to afferents during development in different species. We will also consider the order of afferent fiber arrival into the developing cerebellum to establish neural connectivity.

Afferents from vocal motor and respiratory effectors are recruited during vocal production in juvenile songbirds

Learned behaviors require coordination of diverse sensory inputs with motivational and motor systems. Although mechanisms underlying vocal learning in songbirds have focused primarily on auditory inputs, it is likely that sensory inputs from vocal effectors also provide essential feedback. We investigated the role of somatosensory and respiratory inputs from vocal effectors of juvenile zebra finches (Taeniopygia guttata) during the stage of sensorimotor integration when they are learning to imitate a previously memorized tutor song. We report that song production induced expression of the immediate early gene product Fos in trigeminal regions that receive hypoglossal afferents from the tongue and syrinx (the main vocal organ). Furthermore, unilateral lesion of hypoglossal afferents greatly diminished singing-induced Fos expression on the side ipsilateral to the lesion, but not on the intact control side. In addition, unilateral lesion of the vagus reduced Fos expression in the ipsilateral nucleus of the solitary tract in singing birds. Lesion of the hypoglossal nerve to the syrinx greatly disrupted vocal behavior, whereas lesion of the hypoglossal nerve to the tongue exerted no obvious disruption and lesions of the vagus caused some alterations to song behavior. These results provide the first functional evidence that somatosensory and respiratory feedback from peripheral effectors is activated during vocal production and conveyed to brainstem regions. Such feedback is likely to play an important role in vocal learning during sensorimotor integration in juvenile birds and in maintaining stereotyped vocal behavior in adults.

Localization of cerebellin-2 in late embryonic chicken brain: implications for a role in synapse formation and for brain evolution

Cerebellin-1 (Cbln1), the most studied member of the cerebellin family of secreted proteins, is necessary for the formation and maintenance of parallel fiber-Purkinje cell synapses. However, the roles of the other Cblns have received little attention. We previously identified the chicken homolog of Cbln2 and examined its expression in dorsal root ganglia and spinal cord (Yang et al. [2010] J Comp Neurol 518:2818-2840). Interestingly, Cbln2 is expressed by mechanoreceptive and proprioceptive neurons and in regions of the spinal cord where those afferents terminate, as well as by preganglionic sympathetic neurons and their sympathetic ganglia targets. These findings suggest that Cbln2 may demonstrate a tendency to be expressed by synaptically connected neuronal populations. To further assess this possibility, we examined Cbln2 expression in chick brain. We indeed found that Cbln2 is frequently expressed by synaptically connected neurons, although there are exceptions, and we discuss the implications of these findings for Cbln2 function. Cbln2 expression tends to be more common in primary sensory neurons and in second-order sensory regions than it is in motor areas of the brain. Moreover, we found that the level of Cbln2 expression for many regions of the chicken brain is very similar to that of the mammalian homologs, consistent with the view that the expression patterns of molecules playing fundamental roles in processes such as neuronal communication are evolutionarily conserved. There are, however, large differences in the pattern of Cbln2 expression in avian as compared to mammalian telencephalon and in other regions that show the most divergence between the two lineages.

Interconnections between the dorsal column nucleus and the cerebellum in a reptile

Interconnections between the dorsal column nucleus and the cerebellum were examined in one group of reptiles, Caiman crocodilus. After anterograde tracer injections into the dorsal column nucleus, efferents terminated nearly exclusively in the white matter and ventral portion of the granule cell layer of the ipsilateral cerebellum. Subsequent to deposition of a retrograde tracer into the cerebellum, neurons in the central and ventral half of the dorsal column nucleus were labeled. When compared with the origin of midbrain and spinal cord projecting cells in Caiman, cerebellar projecting neurons arose from a more rostral location in the dorsal column nucleus than did neurons that terminated in either of these two other targets. The results of the present and previous experiments suggest that the dorsal column nucleus in this reptilian group is organized into sectors based on efferent target in a fashion similar to what has been described in certain mammals. Furthermore, the presence of this circuit in crocodilians and turtles suggests that his pathway from the dorsal column nucleus to the cerebellum arose early in amniote evolution.

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.

Comparative Morphology of the Avian Cerebellum: II. Size of Folia

Despite the highly conserved circuitry of the cerebellum, its overall shape varies significantly among and within vertebrate classes. In birds, one of the most prominent differences among orders is the relative size of the cerebellar folia. The enlargement/reduction of individual folia is thought to relate to specific behavioral differences among taxa, but this has not been adequately tested. Here, we survey variation in cerebellar folia size among 96 species of birds and test for phylogenetic effects and correlations with behavior using a combination of conventional and phylogeny-based statistics. Overall, we found that phylogenetic history accounts for a significant amount of variation in the relative size of individual folia. Order membership, in particular, accounted for more than half of the interspecific variation in folia size. There are also complex relationships among folia such that the expansion of one folium is often accompanied by a reduction in other folia. With respect to behavioral correlates: (1) we did not find any significant correlations between folia size and reliance on trigeminal input; (2) there was some evidence supporting a correlation between strong hindlimbs and an expansion of the anterior lobe; and (3) there were significant reductions in folia I-III and expansions in folia VI and VII in species classified as strong fliers. This expansion likely reflects increased visual processing requirements in species with rapid and/or agile flight. It therefore appears that folium size is a product of both phylogenetic history and behavior in birds.

Two optic flow pathways from the pretectal nucleus lentiformis mesencephali to the cerebellum in pigeons (Columba livia)

Neurons in the pretectal nucleus lentiformis mesencephali (LM) are involved in the analysis of optic flow. LM provides mossy fiber inputs to folia VI-VIII of the posterior cerebellum and IXcd of the vestibulocerebellum. Previous research has shown that the vestibulocerebellum is involved in visual-vestibular integration supporting gaze stabilization. The function of folia VI-VIII in pigeons is not well understood; however, these folia receive input from a tectopontine system, which is likely involved with analyzing local motion as opposed to optic flow. We sought to determine whether the mossy fiber input from LM to IXcd differs from that to VI-VIII. Fluorescent retrograde tracers were injected into these folia, and the pattern of labeling in LM was observed. Large multipolar neurons were labeled throughout the rostrocaudal extent of LM. There was a clear mediolateral difference: 74.3% of LM neurons projecting to IXcd were located in the lateral subnucleus of LM (LMl), whereas 73.8% of LM neurons projecting to VI-VIII were found in medial LM (LMm). This suggests that the subnuclei of LM have differing roles. In particular, the LMl-IXcd pathway is involved in generating the optokinetic response. We suggest that the pathway from LMm to VI-VIII is integrating optic flow and local motion to support various oculomotor and visuomotor behaviors, including obstacle avoidance during locomotion.

Rostrocaudal nuclear relationships in the avian medulla oblongata: a fate map with quail chick chimeras

We present a correlative fate map of the nonsegmented caudal hindbrain down to the medullospinal boundary (medulla oblongata), as a companion to a previous fate mapping study of the hindbrain rhombomeres r2-r6 in quail chick chimeras at stages HH10/11 [Marín and Puelles (1995) Eur J Neurosci 7:1714-1738]. For reproducibility and equivalent precision of analysis, successive portions of the medulla-called pseudorhombomeres "r7" to "r11"-were delimited by transverse planes through the center of adjacent somites at stages HH10/11. These units were each grafted homotopically and isochronically from quail donors into chick hosts. The chimeric specimens were fixed at stages HH35/36 and alternate Nissl-stained sagittal sections were compared to adjacent sections in which quail cells were detected immunocytochemically. This analysis in general showed that there is little intermixing between adjacent pseudorhombomeric domains, although some neuronal populations in the vestibular and trigeminal columns, as well as in the reticular formation and pontine nuclei, do migrate selectively into the host hindbrain. Contralateral migration was scarce up to the stages examined. Several motor nuclei, i.e., the vagal motor complex, or sensory nuclei, i.e., the medial vestibular nucleus, show cytoarchitectonic limits that coincide with pseudorhombomeric ones; however, most conventional grisea were found to originate across several pseudorhombomeres. The inferior olivary complex originated between "r8" and "r11" (between the centers of somites 1 and 5). The medullospinal boundary coincided precisely with the center of the fifth somite, slightly caudal to the obex and the end of the choroidal roof, and correlated with the end of many medullary cytoarchitectonic units. In contrast, the dorsal column nuclei and the caudal subnucleus of the descending trigeminal column fell within the spinal cord. On the whole, the patterns observed were very similar to those found before within the overtly segmented part of the hindbrain, suggesting that some underlying common mechanism may account for the transverse cytoarchitectonic boundaries.

Trigeminal projections to the dorsal thalamus in a lacertid lizard, Podarcis hispanica

Trigeminothalamic projections in the lizard Podarcis hispanica were investigated by means of biotinylated dextranamine (BDA) injections into different nuclei of the dorsal thalamus. Previous studies of lizards found a projection from the sensory trigeminal nuclei in the brainstem to the nuclei ventromedialis and ventrolateralis of the ventral thalamus. The present results show that, in addition to these projections to ventral thalamic nuclei, neurons of the nucleus of the descending tract and the principal sensory nucleus project contralaterally to the pretectal nucleus lentiformis thalami and bilaterally to the nucleus dorsolateralis anterior thalami of the dorsal thalamus. The contralateral projection to the nucleus dorsolateralis anterior is more developed than its ipsilateral counterpart and appears to be topographically organized. Since the nucleus dorsolateralis thalami has ascending projections to the cortex telencephali, this nucleus may be the thalamic relay of trigeminal sensory information to the telencephalon.

Location of reticular premotor areas of a motor center innervating craniocervical muscles in the mallard (Anas platyrhynchos L.)

The supraspinal nucleus (SSp) in the mallard, which lies in the rostral spinal cord and caudal brainstem, is a motor nucleus that forms the rostral continuation of the ventral horn. It contains part of the motoneurons innervating the craniocervical muscles. Injections with horseradish peroxidase (HRP) and wheat germ agglutinin conjugated to HRP (WGA) in the SSp were used to localize the craniocervical premotor neurons in the medullary reticular formation. A mixture of WGA and HRP (WGA/HRP) or biotinylated dextran amine (BDA) were injected in the different reticular areas to test the results. Small numbers of craniocervical premotor neurons were found bilaterally in the ventromedial part of the parvocellular reticular formation (RPcvm) and in the caudal extension of RPcvm, the nucleus centralis dorsalis of the medulla oblongata, and the gigantocellular reticular formation (RGc). In a second series of experiments, WGA/HRP and BDA injections in these reticular areas were used to visualize afferent fibers and terminals in the SSp. The combination of the two types of experiments shows that RPcvm and RGc contain modest numbers of craniocervical premotor neurons. Because the reticular formation also contains jaw and tongue premotor neurons and receives a variety of sensory projections, the present results suggest that the medullary reticular formation plays a role in the coordination of complex movements (e.g., feeding). The pattern of afferent and efferent connections of the reticular formation is used to redefine its subdivisions in the myelencephalon of the mallard.

Trigeminal disynaptic circuit mediating corneal afferent input to M. depressor palpebrae inferioris motoneurons in the pigeon (Columba livia)

Corneal afferent projections to the trigeminal brainstem nuclear complex (TBNC) and associated structures, as determined by transganglionic transport of various tracers, were found to be predominantly concentrated in two distinct patches in the dorsolateral medulla at periobex levels. One was in the external cuneate nucleus, and the other was in the ventralmost part of the ophthalmic division of the TBNC. The projections of putative second-order neurons in these regions, as determined by injections of wheat germ agglutinin conjugated to horseradish peroxidase into the dorsolateral medulla, were found to include the dorsal trigeminal motor nucleus (Vd), which innervates the M. depressor palpebrae inferioris. Electrical stimulation of Vd, which elicited lower eyelid movements, was then used to guide injections of tracer into Vd, which retrogradely labeled clusters of neurons in the corneal afferent recipient regions of the dorsolateral medulla. The lower eyelid of pigeons, unlike the nictitating membrane and upper lid, does not appear to be appreciably involved in either reflex blinking in response to relatively mild stimulation of the cornea (e.g., air puff), or in eye closure during the saccade-like head movements associated with walking, or in eye closure during pecking; but in response to a stimulus that makes corneal contact, an upward movement of the lower lid follows descent of the nictitating membrane and upper lid as part of a defensive eye-closing mechanism. The anatomical results thus appear to define a dedicated disynaptic trigeminal sensorimotor circuit for the control of lower eyelid motility in response to mechanical or noxious stimuli of the cornea. Injections of tracers into the lower and upper eyelids labeled palpebral sensory afferents that terminated predominantly in maxillary and ophthalmic portions, respectively, of the dorsal horn of upper cervical spinal segments. These terminal fields were therefore largely separate from those of corneal afferents. There were no specific corneal afferent projections upon accessory abducens motoneurons that innervate the two muscles controlling the nictitating membrane.

Cerebellar and spinal projections of the coeruleus complex in the duck: a fluorescent retrograde double-labeling study

The double fluorescent retrograde tracing technique was used to identify, within the coeruleus complex (Co complex) of the duck, the nerve cells projecting to the cerebellar cortex and to the spinal cord. This technique was also used to investigate the possibility that the cerebellar and spinal projections of the Co complex are collaterals of the same axons. In the same animal, nuclear Diamidino yellow dihydrochloride (DY) fluorescent tracer was placed into the cerebellar cortex of folia V-VII, and cytoplasmic fluorescent Fast blue (FB) dye was injected into C3-C4 spinal cord segments. FB labeled multipolar somata and DY fluorescent nuclei were intermingled within the dorsal caudal region of the locus coeruleus (LCo) and within the dorsal division of the nucleus subcoeruleus (dSCo). Moreover, in the LCo, a low proportion of double-labeled neurons (about 3-4% of labelings) was evidenced among single-labeled neurons. In the ventral division of the nucleus subcoeruleus (vSCo), occasional DY labeled nuclei were found, whereas FB-labeled cells were frequently present. The present findings reveal the location of the coeruleocerebellar and coeruleospinal projecting neurons within the Co complex of the duck. They are intermingled in the caudal portion of the LCo and along the rostrocaudal extent of the subjacent dSco. The LCo and the dSCo are the major source of the projections to the folia V-VII, whereas the vSCo contributes very slightly to the innervation of the cerebellar injected areas. Moreover, the double-labeling study demonstrates that in the duck a low percentage of neurons within the ventrolateral portion of the caudal region of the LCo projects both to the cerebellar cortex of folia V-VII and to C3-C4 spinal cord segments via collaterals. Therefore, these neurons simultaneously influence the cerebellar cortex and spinal cord. The possibility that the projections studied are noradrenergic and that they play a role in feeding is discussed.

Central connections of the nucleus mesencephalicus nervi trigemini in the mallard (Anas platyrhynchos L.)

BACKGROUND

In the mallard duck, functionally distinct groups of jaw muscles are each innervated by a different subnucleus of the main trigeminal (mV) or facial (mVII) motor nucleus. The other subnuclei of mV and mVII innervate several head muscles, including lingual muscles. The reticular premotor cells of the trigeminal and facial jaw motor subnuclei occupy different areas in the parvocellular reticular formation (RPc). The cell bodies of jaw muscle spindle afferents are situated in the mesencephalic nucleus (MesV). In the present study, the central connections of MesV with jaw motor subnuclei and their premotor areas are investigated.

METHODS

In a first series of experiments, horseradish peroxidase (HRP) injections were made in electrophysiologically identified trigeminal and facial subnuclei. In a second series of experiments, HRP was delivered iontophoretically at different parts of RPc. Anterograde tracing with tritiated leucine was used to confirm the central connections of MesV. Double labeling with fluorescent tracers was used to investigate whether MesV collaterals reach both the rostral and caudal parts of RPc.

RESULTS

MesV projects to only two of the five different subnuclei of the trigeminal motor nucleus. The subnuclei that receive spindle afferents innervate jaw adductor muscles (mV2) or pro- and retractors of the mandible (pterygoid muscles; mV1). The three other subnuclei innervate jaw-opener muscles or other head muscles. MesV fibers also project to the rostral part of the dorsolateral RPc (RPcdl), which serves as a premotor area for the motor subnuclei of adductor and pterygoid muscles. The intermediate part of RPcdl does not contain premotor cells of mV or mVII, and a clear projection of MesV to this area is absent. The caudal part of RPcdl projects to the mV and mVII subnuclei that innervate jaw-opener muscles. This part of RPc receives a projection from the same MesV cells as the rostral RPcdl. The MesV projection to RPc does not include premotor cells of mV and mVII in the ventromedial part of RPc (RPcvm).

CONCLUSIONS

Spindle afferents from jaw-closer muscles project only to mV subnuclei innervating jaw-closer muscles (mV1, mV2) and to a population of premotor cells in the rostral RPcdl that innervates these subnuclei. The mixed population of premotor cells in RPcvm, which innervates both jaw-opener and jaw-closer subnuclei, does not receive a MesV projection. However, a premotor area for jaw-opener subnuclei in the caudal part of RPcdl does receive MesV input and may serve as a relay through which proprioceptive information from jaw closer spindles can reach jaw opener muscles.

Cloning and expression analysis of cadherin-10 in the CNS of the chicken embryo

A full-length cDNA of a novel cadherin of chicken (cad10) was cloned. The deduced amino acid sequence of the putative cytoplasmic domain of this molecule is highly homologous to a previously published cytoplasmic fragment of human cadherin-10, a type II cadherin. An in situ hybridization analysis in chicken embryos shows that cad10 expression starts at about 4 days' incubation (E4) and persists at least until the hatching stage. In the central nervous system (CNS), cad10 expression is spatially restricted at all stages of development. At early stages, expression reflects the neuromeric organization of the brain. For example, in the alar plate of the diencephalon, cad10 expression is restricted to the dorsal thalamic neuromere. A number of cad10-expressing brain nuclei are formed in this neuromeric domain during later development. Specific cad10-expressing gray matter structures are also found in all other major divisions of the brain. Many of these structures are known to be functionally connected to each other. The cad10 expression pattern is distinct from that of other cadherins. These results support the idea that cadherins provide a molecular code for the regionalization of the embryonic CNS at the different stages of development.

The avian somatosensory system: the pathway from wing to Wulst in a passerine (Chloris chloris)

The organization of the wing component of the dorsal column-medial lemniscal pathway, and somatosensory projections from the thalamus to the Wulst, are described for an oscine member of the major group of birds, the Passeriformes. Wing primary afferents terminate throughout the cervical spinal cord, but between the brachial enlargement and the spino-medullary junction, they are confined to medial lamina V. Within the medulla, terminations extend rostrally and laterally to occupy the cuneate (Cu) and external cuneate nuclei (CuE). Ascending projections from Cu and CuE form the contralateral medial lemniscus, which has extensive projections to the midbrain and to the thalamus. In the midbrain the projections surround the central auditory nucleus densely, and terminate more sparsely within it. In the thalamus, specific terminations were observed in nucleus uvaeformis and in the nucleus dorsalis intermedius ventralis anterior (DIVA). DIVA projects to the ipsilateral rostral Wulst where it terminates in the intercalated hyperstriatum accessorium, in a distinct, regular patchy fashion. The somatosensory projections to the telencephalon in green finch are similar to those in pigeon, but dissimilar to those in budgerigar.

Restricted expression of R-cadherin by brain nuclei and neural circuits of the developing chicken brain

Cadherins are a family of Ca(2+)-dependent cell-cell adhesion molecules regulating morphogenesis by a preferentially homophilic binding mechanism. We have previously shown that the expression of R-cadherin in the early chicken forebrain (embryonic days E3-E6) is restricted to particular neuromeres or parts of neuromeres. R-cadherin-expressing neuroblasts born in these areas accumulate in the mantle zone and aggregate in particular (pro-) nuclei (Gänzler and Redies [1995] J. Neurosci. 15:4157-4172). In the present study, these findings are extended to later developmental stages (embryonic days E8, E11, and E15). By immunohistochemical and in situ hybridization techniques, we show that, at these stages of development, R-cadherin expression remains restricted to particular developing gray matter regions and fiber tracts. The R-cadherin-positive fiber tracts connect some of the R-cadherin-positive gray matter areas to form parts of particular neural circuits in the visual, auditory, somatosensory, and motor systems. Moreover, R-cadherin expression reflects the morphologic differentiation of gray matter regions. As brain nuclei become morphologically more distinct, the expression of R-cadherin shows a clearer demarcation of the nuclear boundaries. In addition, R-cadherin expression in some nuclei becomes restricted to particular subregions or to clusters of neurons. In the cerebellum, R-cadherin is expressed in parasagittal stripes. These results suggest that R-cadherin expression reflects the functional and morphologic maturation of gray matter structures and of information processing circuits in the embryonic chicken brain.

Central projections and somatotopic organisation of trigeminal primary afferents in pigeon (Columba livia)

Injections of cholera toxin B-chain conjugated to horseradish peroxidase into individual peripheral branches of the trigeminal nerve or into the trigeminal ganglion showed that an ascending trigeminal tract (TTA) terminated in distinct ventral and dorsal divisions of the principal sensory nucleus (PrVv and PrVd, respectively), and a descending tract (TTD) terminated within pars oralis, pars interpolaris, and pars caudalis divisions of the nucleus of TTD (nTTD) and within the dorsal horn of the first six cervical spinal segments. In PrVd, mandibular, ophthalmic, and maxillary projections were predominantly located dorsally, ventrally, and medially, respectively. In nTTD, mandibular projections lay dorsomedially, ophthalmic projections lay ventrolaterally, and maxillary projections lay in between. At caudal medullary and spinal levels, mandibular projections were situated medially, ophthalmic projections were situated laterally, and maxillary projections were situated centrally. The terminations within the dorsal horn were most dense in laminae III and IV and were least dense in lamina II, with laminae III-IV also receiving topographically organised contralateral projections. Extratrigeminal projections were mainly to the external cuneate nucleus by way of a lateral descending trigeminal tract (lTTD; Dubbeldam and Karten [1978] J. Comp. Neurol. 180:661-678) and to the region of the tract of Lissauer and lamina I of the dorsal horn. Other projections were to a region medial to the apex of pars interpolaris, to the nuclei ventrolateralis anterior (Vla) and presulcalis anterior (Pas) of the solitary complex, and sparsely to the lateral reticular formation (plexus of Horsley) ventral to TTD. No projections were seen to the trigeminal motor nuclei or to the cerebellum.

[Fine structure of the trigeminal nerve nucleus of the domestic fowl]

The trigeminal nerve nuclei are examined light- and electron-microscopically in the adult domestic fowl. The nucleus sensibilis principalis nervi trigemini is formed by scarce, medium-sized, round-to-ovoid polygonal neurons. The Nissl bodies are concentrated around the nucleus and consist of short cisterns of the rough endoplasmic reticulum densely bordered with ribosomes. The nucleus tractus spinalis nervi trigemini extends to the first segments of the cervical cord. The rostral part of the nucleus is characterized by medium-sized polygonal neurons. Their cell bodies are densely packed with coarse Nissl bodies. Small multiforme cell types with large nuclei frequently showing two nucleoli predominate in the caudal part. The motorical main portion, nucleus motorius nervi trigemini consists of medium-sized as well as great polygonal neurons. The accessory portion, nucleus motorius dorsalis nervi trigemini, consists of medium-sized polygonal neurons. Both nuclei show the typical motoneuron cytomorphology. In the neuropil, the axodendritic synapses can be differentiated into five types. Occasionally, densely packed glial lamellae and giant mitochondria occur.

Convergence of somatosensory and auditory projections in the avian torus semicircularis, including the central auditory nucleus

Projections of dorsal column, spinal, and cochlear nuclei upon the central nucleus of the torus semicircularis (otherwise known as nucleus mesencephalicus lateralis, pars dorsalis, or MLd) and upon other toral nuclei were investigated in pigeon by anterograde and retrograde tracing and electrophysiological methods. The anatomical results showed that caudal regions of the dorsal column nuclei and medial lamina V of the upper four cervical spinal segments have extensive projections upon the contralateral central auditory nucleus and upon other nuclei of the torus, in particular the core portion of the preisthmic superficial area of Puelles et al. (L. Puelles, C. Rrobles, M. Martiez-de-la-Torre, and S. Martinez, 1994, J. Comp. Neurol. 340:98-125). The projections of nucleus angularis were found to terminate throughout most of the contralateral central nucleus except the dorsomedial portion at rostral levels, where the majority of the projections of nucleus laminaris were concentrated. Nucleus angularis (and to a lesser extent nucleus laminaris) was also found to have substantial projections to certain noncentral toral nuclei, in particular to the caudomedial shell nucleus of Puelles et al. (1994). As shown positively with both Nissl and cytochrome oxidase staining and negatively with substance P labeling, this nucleus is a medial extension of more caudal regions of the central nucleus, and it is suggested that it should be included as part of the auditory midbrain. The electrophysiological results confirmed the anatomical findings by showing that evoked potentials and multiunit activity can be recorded throughout the central and noncentral toral nuclei by using electrical stimulation of the radial nerve and auditory click stimuli. The core portion of the preisthmic superficial area, however, can be regarded as a distinct somatosensory nucleus of the midbrain. It is concluded that there is substantial convergence of somatosensory and auditory inputs within both central auditory and noncentral nuclei of the torus semicircularis in pigeon.

Morphological fate of rhombomeres in quail/chick chimeras: a segmental analysis of hindbrain nuclei

Quail rhombomeres two to six (r2-r6) were individually grafted homotopically into the hindbrain of chick embryos at 2 days of incubation. Nine to 10 days after the operation the chimeric embryos were fixed and processed for parallel cytoarchitectural and immunocytochemical study (with an anti-quail antibody) in order to map the anatomical fate of the grafted tissue. Emphasis was placed on conventionally identified and distinct neuronal populations composing the sensory and motor longitudinal columns. Grafted rhombomeres consistently developed as complete transverse slices of the chimeric hindbrain. Interrhombomeric cell migration was either sparse or restricted to specific nuclei. The cranial nerve motor nuclei showed rhombomeric origins consistent with the patterns described in early embryos. Unexpectedly, alar r2 was found to form the auricular part of the cerebellum. As regards the cochlear nuclei, we found that nucleus angularis derives from r3 to r6, nucleus laminaris from r5 to r6, nucleus magnocellularis from r6 to r7 and nucleus olivaris superior from r5. The nuclei of the lateral lemniscus originated between r1 and r3. We also delimited the respective rhombomeric subdivisions of the sensory vestibular and trigeminal columns, both of which extend from r1 caudalwards throughout the hindbrain. There were consistently some interrhombomeric neuronal migrations inside the vestibular column, some motor nuclei and the reticular formation, involving only one rhombomere length. The pontine nuclei, which extended from r1 to r7, showed neuronal migrations that crossed several rhombomeres. On the whole, these results represent the first anatomical analysis of the mature avian hindbrain in terms of rhombomere-derived domains.

Proposed pathways for vocal self-stimulation: met-enkephalinergic projections linking the midbrain vocal nucleus, auditory-responsive thalamic regions and neurosecretory hypothalamus

In this study, we have investigated the neuroanatomical pathways that may underlie the influence of a female bird's vocal behavior upon her own reproductive endocrine response. We traced the ascending efferent projections of the midbrain vocal control nucleus, the intercollicularis (ICo), using an anterograde tracer, PHAL, delivered by iontophoretic application. We found labelled terminal fields in the anterior regions of the hypothalamus that contained luteinizing hormone releasing hormone- (LHRH) immunoreactive neurons. We injected into the LHRH-rich anterior medial hypothalamus (AM) the retrograde tracer, fluoro-gold, to verify the results of PHAL anterograde tracing and examine whether retrogradely labelled neurons in the ICo can be stained with met-enkephalin antiserum by the immunohistochemical method. Of the retrogradely labelled neurons in the medial division of ICo (mICo), between 5% and 15% were found to be met-enkephalin-immunoreactive positive perikarya. Our data suggest that axonal projections into the anterior medial hypothalamus may arise in part from enkephalin-immunoreactive neurons in the medial ICo. The mICo neurons distributed along the medial border of the midbrain auditory nucleus give rise to projections into the posterior medial hypothalamus (PMH) via synapses within the shell region of thalamic auditory nucleus, ovoidalis (Ov). We conclude that in the ring dove, the medial division of the vocal control nucleus, by virtue of its connection with the auditory thalamus and neurosecretory hypothalamus, is in a position to exert influence on endocrine response partly through enkephalinergic systems. Implications of similar connections in other species are discussed.

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.

Descending projections of the songbird nucleus robustus archistriatalis

The descending, efferent projections of nucleus robustus archistriatalis were investigated in male zebra finches and greenfinches with injections of either biotinylated dextran amine or cholera toxin B-chain conjugated to horseradish peroxidase. The results show that in addition to the well-known projections to the tracheosyringeal motor nucleus and the dorsomedial nucleus of the intercollicular complex, there are other projections of comparable density to the ipsilateral nucleus ambiguus and nucleus retroambigualis. Within nucleus ambiguus, robustus axons terminate in close proximity to laryngeal motoneurons which were retrogradely labelled in the same bird by injections of cholera B-chain into the laryngeal muscles; and within nucleus retroambigualis robustus axons terminate in relation to bulbospinal neurons previously shown to project to regions of spinal cord containing motoneurons innervating abdominal expiratory muscles (J.M. Wild, Brain Res. 606:119-124, 1993). These projections of nucleus robustus thus seem well placed to coordinate syringeal, laryngeal, and expiratory muscle activity during vocalization. Other relatively sparse, but distinct, projections of nucleus robustus were found to nucleus dorsolateralis anterior thalami, pars medialis, to a narrow region between the superior olivary nucleus and the spinal lemniscus, and to the rostral ventrolateral medulla. Neurons in these last two locations were retrogradely labelled bilaterally following injections of cholera B-chain into nucleus retroambigualis of one side. Together with sparse contralateral projections of nucleus robustus to all brainstem targets receiving ipsilateral projections, potential pathways are thus identified by which the respiratory-vocal activity controlled by one side of the lower medulla can be influenced by the nucleus robustus of either side, thereby possibly bringing about bilateral coordination of respiratory-vocal output.

Avian locomotion activated by brainstem infusion of neurotransmitter agonists and antagonists. I. Acetylcholine excitatory amino acids and substance P

Previous studies have demonstrated that focal electrical stimulation of regions within the brainstem of a decerebrate bird will elicit all the normal patterns of avian locomotion. However, electrical stimulation can activate a variety of neuronal elements within the radius of effective current spread, including axons of passage traversing the stimulation point. To restrict activation to neuronal cell bodies within the immediate vicinity, we have utilized direct intracerebral injection of neurotransmitters, their agonists and antagonists, into identified brainstem locomotor regions. To undertake these studies, birds (geese or ducks) were placed in a stereotaxic frame and decerebrated under halothane anesthesia. After completion of surgery, several discrete locomotor regions were first identified with electrical microstimulation. Acetylcholine (ACh) and excitatory amino acid (EAA) agonists and antagonists, as well as Substance P were then slowly infused into each brainstem region. Any change in locomotor behavior was recorded by electromyographic techniques. When injected into a variety of sites, carbachol (an ACh nicotinic (AChN) and muscarinic (AChM) agonist) and pilocarpine (an AChM agonist) evoked locomotion, whereas atropine (an AChM antagonist) blocked locomotion. N-methyl-D-aspartate NMDA), but not glutamate, also elicited locomotion or reduced the current intensity threshold for electrically-evoked locomotion. The NMDA-induced locomotion evoked locomotion. The NMDA-induced locomotion could be blocked by the injection of glutamic acid diethyl ester (GDEE, an EAA antagonist) or D-2-amino-5-phosphonopentanoic acid (AP5) into the same site. Finally. Substance P also evoked locomotion. The above observations strongly suggest that brainstem electrically-stimulated locomotion in decerebrate birds is not due to activation of fibers traversing a brainstem locomotor region, but instead, is due to the activation of receptors located on neuronal cell bodies, dendrites or presynaptic terminals in the immediate vicinity of the micropipette tip. After correlating our findings with similar lamprey and mammalian studies, the comparable discoveries serve to underscore the suggestion that the neuroanatomical substrates underlying the brainstem control of locomotion appear to be highly conserved in all vertebrates.

Organization of the cerebellum in the pigeon (Columba livia): I. Corticonuclear and corticovestibular connections

The projections of the cerebellar cortex upon the cerebellar nuclei and the vestibular complex of the pigeon have been delineated using WGA-HRP as an anterograde and retrograde tracer. Injections into individual cortical lobules (II-IXa) produce a pattern of ipsilateral terminal labeling of both the cerebellar and vestibular nuclei. The pattern of corticonuclear projections indicates both a rostrocaudal and a mediolateral organization with respect to the lobules and is consistent with a division of the cerebellar nuclei into a medial (CbM) and a lateral (CbL) nucleus. The retrograde experiments indicate that these nuclei receive projections, respectively, from Purkinje cells within medial (A) and lateral (C) longitudinal zones, which alternate with longitudinal zones (B, E) projecting upon the vestibular complex. Purkinje cells in (vestibulocerebellar) lobules IXb-X show only limited projections upon the cerebellar nuclei, but do project extensively upon the cerebellovestibular process (PCV), as well as upon the medial, superior, and descending vestibular nuclei. As the injection site shifts from medial to lateral, there is a corresponding shift in focus of the projection within PCV from areas bordering CbM to those abutting CbL. The topographic organization of corticovestibular projections is less clear-cut than those of the corticonuclear projections. Lobules II-X project upon the lateral vestibular nucleus (anterior lobe) or the dorsolateral vestibular nucleus (posterior lobe). These projections originate from either side of the lateral (C) zone. Projections originating from the medialmost (B) zone are interrupted in lobules VI and VII. The anterior and posterior portions of the lateralmost (E) zone overlap along lobules VI and VII. In addition, the E zone of the anterior lobe is the source of projections upon the medial, the descending, and the superior vestibular nuclei. Projections from the auricle and adjacent lateral unfoliated cortex (F zone) focus upon the infracerebellar nucleus, the medial tangential nucleus, and the medial division of the superior vestibular nucleus. The data suggest that the cerebellar cortex of the pigeon, like that of mammals, may be subdivided into a mediolaterally oriented series of longitudinal zones, with Purkinje cells in each zone projecting ipsilaterally to specific cerebellar nuclei or vestibular regions. For cortical regions exclusive of the auricle and lateral unfoliated cortex, three such zones (A, B, and C) are defined that are comparable in their efferent targets with the A, B, and C zones of mammals. There does not appear to be a D zone in the pigeon. The results are discussed in relation to comparative data on amphibians, reptiles, and mammals.

Peripheral and central terminations of hypoglossal afferents innervating lingual tactile mechanoreceptor complexes in Fringillidae

Injections of cholera toxin B subunit conjugated to horseradish peroxidase (CTB-HRP) were made into the lingual branch of the hypoglossal nerve in four species of finch in order to identify the innervation of the mechanoreceptors of the dermal papillae of the tongue, and simultaneously to determine the pattern of central projections of lingual hypoglossal afferents. The results showed that hypoglossal fibers innervate all the Herbst corpuscles and terminal cell receptors of the elaborately organized papillae of the dorsum of the tongue, of the shorter papillae in the ventral tongue, and the loose collection of Herbst corpuscles in the subpapillary region. Labelled fibers were also observed in the intralingual glands, in the intrinsic tongue muscles, and in the posterodorsal epithelium where they formed budlike structures. Retrogradely labelled cell bodies were located in the jugular ganglion and their central processes ascended and descended throughout the brainstem within the descending trigeminal tract (TTD). Terminal fields were observed within the dorsolateral part of the nucleus caudalis of TTD, predominantly ipsilaterally, and within the medial part of the dorsal horn of the first 4-6 cervical segments bilaterally. There were dense patches of termination over a dorsolateral subnucleus of the interpolated nucleus of TTD, and within two regions of the principal sensory trigeminal nucleus: a large one laterally and a small one medially. Terminal fields were also observed within the nucleus ventralis lateralis anterior of the rostral solitary complex, and within adjacent nuclei, which are probably equivalent to the dorsal sensory nuclei of the facial and glossopharyngeal nerves of other avian species. The results are interpreted in the light of the role of the tongue in species-specific patterns of feeding in finches, and the possible requirement for the central integration of touch and taste.

Projections of the parabrachial nucleus in the pigeon (Columba livia)

The ascending and descending projections of the parabrachial nuclear complex in the pigeon have been charted with autoradiographic and histochemical (WGA-HRP) techniques. The ascending projections originate from a group of subnuclei surrounding various components of the brachium conjunctivum, namely, the superficial lateral, dorsolateral, dorsomedial, and ventromedial subnuclei. The projections are predominantly ipsilateral and travel in the quintofrontal tract. They are primarily to the medial and lateral hypothalamus (including the periventricular nucleus and the strata cellulare internum and externum), certain dorsal thalamic nuclei, the nucleus of the pallial commissure, the bed nucleus of the stria terminalis, the ventral paleostriatum, the olfactory tubercle, the nucleus accumbens, and a dorsolateral nucleus of the posterior archistriatum. There are weaker or more diffuse projections to the rostral locus coeruleus (cell group A8), the compact portion of the pedunculopontine tegmental nucleus, the central grey and intercollicular region, the ventral area of Tsai, the medial spiriform nucleus, the nucleus subrotundus, the anterior preoptic area, and the diagonal band of Broca. The parabrachial subnuclei have partially differential projections to these targets, some of which also receive projections from the nucleus of the solitary tract (Arends, Wild, and Zeigler: J. Comp. Neurol. 278:405-429, '88). Most of the targets, particularly those in the basal forebrain (viz., the periventricular nucleus and the strata cellulare internum and externum of the hypothalamus, the bed nucleus of the stria terminalis, and its lateral extension into the ventral paleostriatum, which may be comparable with the substantia innominata), have reciprocal connections with the parabrachial and solitary tract subnuclei and therefore may be said to compose parts of a "visceral forebrain system" analogous to that described in the rat (Van der Kooy et al: J. Comp. Neurol. 224:1-24, '84). The descending projections to the lower brainstem arise in large part from a ventrolateral subnucleus that may be comparable with the Kölliker-Fuse nucleus of mammals. They are mainly to the ventrolateral medulla, nucleus ambiguus, and massively to the hypoglossal nucleus, particularly its tracheosyringeal portion. These projections are therefore likely to be importantly involved in the control of vocalization and respiration (Wild and Arends: Brain Res. 407:191-194, '87). Some of these results have been presented in abstract form (Wild, Arends, and Zeigler: Soc. Neurosci. Abst. 13:308, '87).

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.

Cerebellar connections of the trigeminal system in the pigeon (Columba livia)

Wheat germ-agglutinin conjugated horseradish peroxidase (WGA-HRP) was used to delineate trigeminocerebellar connections in the pigeon. Subnucleus oralis of the nucleus of the descending trigeminal tract (nTTD) is the exclusive origin of trigeminal mossy fibers, which terminate in lobules VIII and IXa. The trigemino-olivary projection originates from subnucleus interpolaris of nTTD, but the existence of an additional pathway relaying in the adjacent lateral reticular formation (i.e. the plexus of Horsley) cannot be excluded. Structures linking the trigeminal cerebellar projections to jaw motoneurons were identified within the cerebellar cortex, the deep cerebellar nuclei and the lateral medullary reticular formation, completing a trigeminocerebellar sensorimotor circuit for the jaw.

Somatotopic organization of the primary sensory trigeminal neurons in the hagfish, Eptatretus burgeri

Primary sensory trigeminal projections were investigated in the hagfish following application of horseradish peroxidase (HRP) to the sensory branches. In our control preparations we were able to distinguish five sensory ganglia and their respective nerves. HRP application confirmed the almost exclusive relation of each of these nerves to their respective ganglia, with very little overlap. In normal frontal sections of the medulla oblongata, five columns of fibers surrounded by neuronal cell bodies could be clearly distinguished, but the number is probably fortuitous, for there was no one-on-one relationship with the five trigeminal ganglia. From their peripheral connections, we surmised that columns 1 and 3 handle general cutaneous sensation, columns 2, 4, and 5 handle taste sensation, and column 5 handles general mucous cutaneous sensation conveyed by utricular ganglion cells. Dorsally located columns received projections from nerves with dorsal peripheral connections, and more ventrally located columns received projections from nerves with ventral peripheral connections. This relation is the reverse of that seen in other vertebrates.

Multisensory convergence in the thalamus of the pigeon (Columba livia)

Neuronal activity was extracellularly recorded from the nucleus dorsolateralis posterior thalami (DLP) of anesthetized pigeons. The sensitivity of individual cells to stimuli of different sensory modalities: somatosensory, visual and auditory was investigated. A substantial population of the DLP neurons responded to mechanical stimulation of large, often bilateral areas of the animal's body surface. No somatotopic organization could be detected. The functional properties of these neurons resembled those described for mammalian nucleus posterior thalami neurons. The visually responsive cells could be driven from large areas of the visual field. Auditory sensitive neurons were optimally activated by wideband noise. Twenty-nine percent of the DLP recorded neurons showed polysensory properties responding either to somatosensory and visual or to somatosensory and auditory stimuli.

The avian somatosensory system: connections of regions of body representation in the forebrain of the pigeon

In order to establish the basic connectivity of physiologically identified somatosensory regions of the thalamus and telencephalon in the pigeon, injections of wheatgerm agglutinin-horseradish peroxidase were made under electrophysiological control and the projections were charted following conventional neurohistochemistry. The physiological recordings generally confirmed the findings of Delius and Bennetto (Brain Research, 37 (1972) 205-221) of somatosensory sites within the dorsal thalamus, anterior hyperstriatum and caudomedial neostriatum, and the anatomical results show that the thalamic cells of origin of the projections to the two telencephalic regions are largely separate: a rostral cell group comprising nucleus dorsalis intermedius ventralis anterior projects to the anterior hyperstriatum accessorium (HA), whilst a caudal cell group comprising caudal regions of nucleus dorsolateralis posterior (DLP) projects to the medial neostriatum intermedium and caudale (NI/NC). Caudal DLP is also the origin of a visual projection to NI/NC, and its terminal field also approximates that of the thalamic auditory nucleus ovoidalis. Since the anterior HA and NI/NC were here shown to be reciprocally connected, there is a possibility of multimodal input to both telencephalic regions. HA was also further defined as the origin of the basal branch of the septomesencephalic tract, and hence potentially provides an outlet for both telencephalic somatosensory regions. The results are discussed within a comparative context.

The avian somatosensory system. I. Primary spinal afferent input to the spinal cord and brainstem in the pigeon (Columba livia)

The process of transganglionic transport was used to determine the pattern of primary afferent projections to the spinal cord and brainstem in the pigeon by (1) applying horseradish peroxidase (HRP) to various peripheral nerves in the leg or wing, (2) by injecting HRP-lectin into feather follicles of the wing or tail, and (3) by injecting HRP-lectin into various muscles of the leg or wing. In the spinal cord major peripheral nerves were represented heavily throughout the dorsal horn laminae but sparsely in more ventral laminae. The representations of these different nerves tended to be located in different mediolateral regions of the dorsal horn. Cutaneous nerves and feather follicles were represented predominantly in laminae I and II, and different sets of follicles were represented in different mediolateral regions of these laminae. Afferent labelling from muscles of the leg and wing was located in the lateral portion of the dorsal horn, predominantly in laminae I, II, and IV. In the caudal medulla the representation of the leg within the gracile nucleus was medial to and separate from that of the wing within the cuneate nucleus (Cu). The wing representation, however, extended laterally throughout the external cuneate nucleus (CuE) and lateral regions of the descending trigeminal tract. There was less evidence of separation of the limb representations at more rostral medullary levels where they both occupied predominantly CuE. Afferent labelling from cutaneous nerves and feather follicles was distributed lightly throughout Cu and CuE, and from muscles of both limbs primarily throughout CuE. There was also a small but specific projection from the limbs to the nucleus of the solitary tract, and from the wing to the principal sensory trigeminal nucleus. These results are discussed within a comparative context with a view to highlighting the similarities and differences in the pattern of primary afferent central projections in different vertebrates.

Primary sensory ganglion cells projecting to the principal trigeminal nucleus in the mallard, Anas platyrhynchos

The trigeminal and glossopharyngeal ganglia of the adult mallard were studied following HRP injections into the principal trigeminal nucleus (PrV). The PrV consists of the principal trigeminal nucleus proper (prV) and the principal glossopharyngeal nucleus (prIX). After an injection into the prV, the labeled cells were found in the ipsilateral trigeminal ganglion. After an injection into the prIX, labeled cells were found in the ipsilateral distal glossopharyngeal ganglion, but not in the proximal ganglion of the IX and X cranial nerve (pGIX + X). In Nissl preparations, two types of ganglion cells in the trigeminal ganglion, pGIX + X, and distal ganglion of N IX could be distinguished: larger light cells and smaller dark cells. We could not determine whether the HRP-labeled cells belonged to both types or to one of them; but because all the labeled cells were over 20 microns, we concluded that the smallest cells (10-19 microns) in the trigeminal ganglion and distal ganglion of N IX did not project to the PrV. The labeling of the cells in the distal ganglion of N IX (average 34.5 microns) was uniformly moderate. In the trigeminal ganglion there were two types of labeled cells: heavily labeled cells (average 29.1 microns) and moderately labeled cells (average 35.1 l microns). These two types of labeling (moderate and heavy) may reflect two types of primary sensory neurons: cells with ascending, nonbifurcating axons, and cells with bifurcating axons. We speculate that the former are proprioceptive neurons and the latter tactile neurons. Labeled bifurcating axons in the sensory trigeminal complex gave off collaterals to all parts of the descending trigeminal nucleus except to the caudalmost laminated spinal part.

Telencephalic connections of the trigeminal system in the pigeon (Columba livia): a trigeminal sensorimotor circuit

A combination of autoradiography and horseradish peroxidase histochemistry was used to identify telencephalic structures linking the sensory and motor components of the trigeminal system in the pigeon. A direct telencephalic projection from the principal trigeminal sensory nucleus upon the nucleus basalis via the quintofrontal tract was confirmed. Nucleus basalis projects upon a belt of neurons within the overlying neostriatum. This region (neostriatum frontale, pars trigeminale: NFT) gives rise to the fronto-archistriate tract which terminates both in the archistriatum intermedium and in the overlying neostriatum caudale, medial to the ventricle (neostriatum caudale, pars trigeminale: NCT). NCT projects, in turn, upon a region of archistriatum intermedium containing cell bodies of the occipito-mesencephalic tract. This pathway provides a link between the telencephalon and premotor areas within the lateral (parvicellular) reticular formation of the lower brainstem. The trigeminal sensorimotor circuit defined in these experiments has been implicated by neurobehavioral studies in the control of pecking, grasping, and feeding in the pigeon.

The subnuclei and primary afferents of the descending trigeminal system in the mallard (Anas platyrhynchos L.)

The descending trigeminal tract and its nuclei were described in the mallard (Anas platyrhynchos L.). The borders of the system were established in Fink-Heimer preparations after unilateral lesions placed in the Gasserian ganglion using the distribution of degenerated particles as a criterion. Adjacent sections, stained with the Nissl, Kluver-Barrera and Haggqvist methods were used in the description of cyto- and fibroarchitecture of the descending trigeminal system and surrounding structures. Descending fibers of the trigeminal root could be traced from the sensory root, ventral to the main sensory nucleus, into the descending tract and its nuclei. Its fibers pass into the spinal cord, but not farther than the third cervical segment. Seven subdivisions (parts a-g) were recognized, but could be combined into four subnuclei, viz. in the terminology of Olszewski: subnucleus oralis containing parts a and b; subnucleus interpolaris parts c and d; subnucleus caudalis part f; dorsal horn part g, etc. No primary trigeminal fibers could be traced to structures outside the main sensory nucleus and nuclei of the descending trigeminal tract; all projections were ipsilateral with the exception of a slight bilateral projection caudal to the obex. Partial lesions in the Gasserian ganglion showed a distribution of the mandibular, maxillary and ophthalmic fibers from dorsal to ventral respectively in the subnuclei oralis and interpolaris, and from medial to lateral in the subnuclei caudalis and dorsal horn. Afferents from the petrosal ganglion project upon the medial part of subnucleus interpolaris and upon a small cell group (nucleus of the ascending glossopharyngeal tract) that may be functionally part of the subnucleus oralis. The subnucleus caudalis receives afferents from the jugular ganglion. These differences in afferentation are used in a tentative functional interpretation of the subdivisions of the nucleus of the descending trigeminal system.