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

Endocranial Anatomy of the Giant Extinct Australian Mihirung Birds (Aves, Dromornithidae)

Dromornithids are an extinct group of large flightless birds from the Cenozoic of Australia. Their record extends from the Eocene to the late Pleistocene. Four genera and eight species are currently recognised, with diversity highest in the Miocene. Dromornithids were once considered ratites, but since the discovery of cranial elements, phylogenetic analyses have placed them near the base of the anseriforms or, most recently, resolved them as stem galliforms. In this study, we use morphometric methods to comprehensively describe dromornithid endocranial morphology for the first time, comparing Ilbandornis woodburnei and three species of Dromornis to one another and to four species of extant basal galloanseres. We reveal that major endocranial reconfiguration was associated with cranial foreshortening in a temporal series along the Dromornis lineage. Five key differences are evident between the brain morphology of Ilbandornis and Dromornis, relating to the medial wulst, the ventral eminence of the caudoventral telencephalon, and morphology of the metencephalon (cerebellum + pons). Additionally, dromornithid brains display distinctive dorsal (rostral position of the wulst), and ventral morphology (form of the maxillomandibular [V2+V3], glossopharyngeal [IX], and vagus [X] cranial nerves), supporting hypotheses that dromornithids are more closely related to basal galliforms than anseriforms. Functional interpretations suggest that dromornithids were specialised herbivores that likely possessed well-developed stereoscopic depth perception, were diurnal and targeted a soft browse trophic niche.

The role of connexins and pannexins in orofacial pain

Trigeminal neuralgia is characterized by extensive spreading of pain, referred to as ectopic pain, which describes the phenomenon of the pain passing from the injured regions to uninjured regions. Patients with orofacial pain often show no response to commonly used analgesics, and the exact mechanism of ectopic pain remains unclear, which restricts the development of specific drugs. The present review aims to summarize the contribution of the two families of transmembrane proteins, connexins (Cxs) and pannexins (Panxs), to the induction and spreading of orofacial pain and to provide potential targets for orofacial pain treatment. Cxs and Panxs have recently been shown to play essential roles in intercellular signal propagation in sensory ganglia, and previous studies have provided evidence for the contribution of several subtypes of Cxs and Panxs in various orofacial pain models. Upregulation of the expression of Cxs and Panxs in the trigeminal ganglia is observed in most cases after trigeminal injury, and regulating their expression or activity can improve pain-like behaviors in animals. It is speculated that after trigeminal injury, pain-related signals are transmitted to adjacent neurons and satellite glial cells in the trigeminal ganglia directly through gap junctions and simultaneously through hemichannels and pannexons through both autocrine and paracrine mechanisms. This review highlights recent discoveries in the regulation of Cxs and Panxs in different orofacial pain models and presents a hypothetical mechanism of ectopic pain in trigeminal neuralgia. In addition, the existing problems in current research are discussed.

Visualizing the trigeminovagal complex in the human medulla by combining ex-vivo ultra-high resolution structural MRI and polarized light imaging microscopy

A trigeminovagal complex, as described in some animals, could help to explain the effect of vagus nerve stimulation as a treatment for headache disorders. However, the existence of a trigeminovagal complex in humans remains unclear. This study, therefore investigated the existence of the trigeminovagal complex in humans. One post-mortem human brainstem was scanned at 11.7T to obtain structural (T1-weighted) and diffusion magnetic resonance images ((d)MR images). Post-processing of dMRI data provided track density imaging (TDI) maps to investigate white matter at a smaller resolution than the imaging resolution. To evaluate the reconstructed tracts, the MR-scanned brainstem and three additional brainstems were sectioned for polarized light imaging (PLI) microscopy. T1-weighted images showed hyperintense vagus medullar striae, coursing towards the dorsomedial aspect of the medulla. dMRI-, TDI- and PLI-images showed these striae to intersect the trigeminal spinal tract (sp5) in the lateral medulla. In addition, PLI images showed that a minority of vagus fibers separated from the vagus trajectory and joined the trigeminal spinal nucleus (Sp5) and the sp5. The course of the vagus tract in the rostral medulla was demonstrated in this study. This study shows that the trigeminal- and vagus systems interconnect anatomically at the level of the rostral medulla where the vagus fibers intersect with the Sp5 and sp5. Physiological and clinical utility of this newly identified interconnection is a topic for further research.

Evolutionary Specialization of Tactile Perception in Vertebrates

Evolution has endowed vertebrates with the remarkable tactile ability to explore the world through the perception of physical force. Yet the sense of touch remains one of the least well understood senses at the cellular and molecular level. Vertebrates specializing in tactile perception can highlight general principles of mechanotransduction. Here, we review cellular and molecular adaptations that underlie the sense of touch in typical and acutely mechanosensitive vertebrates.

Piezo2 in Cutaneous and Proprioceptive Mechanotransduction in Vertebrates

Mechanosensitivity is a fundamental physiological capacity, which pertains to all life forms. Progress has been made with regard to understanding mechanosensitivity in bacteria, flies, and worms. In vertebrates, however, the molecular identity of mechanotransducers in somatic and neuronal cells has only started to appear. The Piezo family of mechanogated ion channels marks a pivotal milestone in understanding mechanosensitivity. Piezo1 and Piezo2 have now been shown to participate in a number of processes, ranging from arterial modeling to sensing muscle stretch. In this review, we focus on Piezo2 and its role in mediating mechanosensation and proprioception in vertebrates.

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.

Neuronal mechanism for acute mechanosensitivity in tactile-foraging waterfowl

Relying almost exclusively on their acute sense of touch, tactile-foraging birds can feed in murky water, but the cellular mechanism is unknown. Mechanical stimuli activate specialized cutaneous end organs in the bill, innervated by trigeminal afferents. We report that trigeminal ganglia (TG) of domestic and wild tactile-foraging ducks exhibit numerical expansion of large-diameter mechanoreceptive neurons expressing the mechano-gated ion channel Piezo2. These features are not found in visually foraging birds. Moreover, in the duck, the expansion of mechanoreceptors occurs at the expense of thermosensors. Direct mechanical stimulation of duck TG neurons evokes high-amplitude depolarizing current with a low threshold of activation, high signal amplification gain, and slow kinetics of inactivation. Together, these factors contribute to efficient conversion of light mechanical stimuli into neuronal excitation. Our results reveal an evolutionary strategy to hone tactile perception in vertebrates at the level of primary afferents.

The anatomy of the bill tip of kiwi and associated somatosensory regions of the brain: comparisons with shorebirds

Three families of probe-foraging birds, Scolopacidae (sandpipers and snipes), Apterygidae (kiwi), and Threskiornithidae (ibises, including spoonbills) have independently evolved long, narrow bills containing clusters of vibration-sensitive mechanoreceptors (Herbst corpuscles) within pits in the bill-tip. These 'bill-tip organs' allow birds to detect buried or submerged prey via substrate-borne vibrations and/or interstitial pressure gradients. Shorebirds, kiwi and ibises are only distantly related, with the phylogenetic divide between kiwi and the other two taxa being particularly deep. We compared the bill-tip structure and associated somatosensory regions in the brains of kiwi and shorebirds to understand the degree of convergence of these systems between the two taxa. For comparison, we also included data from other taxa including waterfowl (Anatidae) and parrots (Psittaculidae and Cacatuidae), non-apterygid ratites, and other probe-foraging and non probe-foraging birds including non-scolopacid shorebirds (Charadriidae, Haematopodidae, Recurvirostridae and Sternidae). We show that the bill-tip organ structure was broadly similar between the Apterygidae and Scolopacidae, however some inter-specific variation was found in the number, shape and orientation of sensory pits between the two groups. Kiwi, scolopacid shorebirds, waterfowl and parrots all shared hypertrophy or near-hypertrophy of the principal sensory trigeminal nucleus. Hypertrophy of the nucleus basorostralis, however, occurred only in waterfowl, kiwi, three of the scolopacid species examined and a species of oystercatcher (Charadriiformes: Haematopodidae). Hypertrophy of the principal sensory trigeminal nucleus in kiwi, Scolopacidae, and other tactile specialists appears to have co-evolved alongside bill-tip specializations, whereas hypertrophy of nucleus basorostralis may be influenced to a greater extent by other sensory inputs. We suggest that similarities between kiwi and scolopacid bill-tip organs and associated somatosensory brain regions are likely a result of similar ecological selective pressures, with inter-specific variations reflecting finer-scale niche differentiation.

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.

Trigeminal and telencephalic projections to jaw and other upper vocal tract premotor neurons in songbirds: sensorimotor circuitry for beak movements during singing

During singing in songbirds, the extent of beak opening, like the extent of mouth opening in human singers, is partially correlated with the fundamental frequency of the sounds emitted. Since song in songbirds is under the control of "the song system" (a collection of interconnected forebrain nuclei dedicated to the learning and production of song), it might be expected that beak movements during singing would also be controlled by this system. However, direct neural connections between the telencephalic output of the song system and beak muscle motor neurons in the brainstem are conspicuous by their absence, leaving unresolved the question of how beak movements are affected during singing. By using standard tract tracing methods, we sought to answer this question by defining beak premotor neurons and examining their afferent projections. In the caudal medulla, jaw premotor cell bodies were located adjacent to the terminal field of the output of the song system, into which many premotor neurons extended their dendrites. The premotor neurons also received a novel input from the trigeminal ganglion and an overlapping input from a lateral arcopallial component of a trigeminal sensorimotor circuit that traverses the forebrain. The ganglionic input in songbirds, which is not present in doves and pigeons that vocalize with a closed beak, may modulate the activity of beak premotor neurons in concert with the output of the song system. These inputs to jaw premotor neurons could, together, affect beak movements as a means of modulating filter properties of the upper vocal tract during singing.

Magnetic field changes activate the trigeminal brainstem complex in a migratory bird

The upper beak of birds, which contains putative magnetosensory ferro-magnetic structures, is innervated by the ophthalmic branch of the trigeminal nerve (V1). However, because of the absence of replicable neurobiological evidence, a general acceptance of the involvement of the trigeminal nerve in magnetoreception is lacking in birds. Using an antibody to ZENK protein to indicate neuronal activation, we here document reliable magnetic activation of neurons in and near the principal (PrV) and spinal tract (SpV) nuclei of the trigeminal brainstem complex, which represent the two brain regions known to receive primary input from the trigeminal nerve. Significantly more neurons were activated in PrV and in medial SpV when European robins (Erithacus rubecula) experienced a magnetic field changing every 30 seconds for a period of 3 h (CMF) than when robins experienced a compensated, zero magnetic field condition (ZMF). No such differences in numbers of activated neurons were found in comparison structures. Under CMF conditions, sectioning of V1 significantly reduced the number of activated neurons in and near PrV and medial SpV, but not in lateral SpV or in the optic tectum. Tract tracing of V1 showed spatial proximity and regional overlap of V1 nerve endings and ZENK-positive (activated) neurons in SpV, and partly in PrV, under CMF conditions. Together, these results suggest that magnetic field changes activate neurons in and near the trigeminal brainstem complex and that V1 is necessary for this activation. We therefore suggest that V1 transmits magnetic information to the brain in this migratory passerine bird.

Organization of the Sensory and Motor Nuclei of the Glossopharyngeal and Vagal Nerves in Lampreys

Anterograde and retrograde transport of horseradish peroxidase was used to examine the afferent and efferent projections of the glossopharyngeal and vagal nerves in the lamprey, Lampetra japonica. Except for the ganglion cells and motoneurons, the distribution patterns of HRP-positive elements differed little between the two nerves. Afferent fibers mainly terminated in the ipsilateral cerebellar area, medial octavolateralis nucleus, and between the ventral octavolateralis nucleus and descending tract and nucleus of the trigeminal nerve (dV). In the cerebellar area, most of the labeled fibers were located in the molecular zone, but some penetrated into the granular zone. In the rostral part of the medial octavolateralis nucleus, labeled fibers coursed from the middle to the lateral area, and in the caudal part, they were localized in the dorsal area of the nucleus. In the area between the dV and ventral octavolateralis nucleus, labeled fibers coursed near the dorsal margin of the rostral part of the dV, and in the caudal part, they shifted dorsally. Ganglion cells and motoneurons of each nerve were also labeled.

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 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.

The sensory trigeminal system in birds: input, organization and effects of peripheral damage. A review

The primary sensory trigeminal system in birds comprises the mesencephalic trigeminal nucleus and the trigeminal ganglion with projections to the principal sensory nucleus (PrV) and the descending tract with its subnuclei. Other cranial nerves can contribute to PrV and the descending system that together form the somatosensory system of the head. There is also a proprioceptive component. The somatosensory system comprises a component serving tactile sense and a nociceptive component. The former processes information from many mechanoreceptors in beak and tongue; both PrV and subnuclei of the descending system are involved. The nociceptive component consists of small ganglion cells projecting presumably to layers I and II of the caudal subnucleus of the descending trigeminal system and cervical dorsal horn; this is the only trigeminal region showing immunoreactivity for substance P. The effects of amputation of the tips of the beak of chickens (debeaking) are estimated by fiber counts in electron microscopic preparations of the trigeminal branches innervating that area, and by cell counts in Nissl stained sections of the trigeminal ganglion. Our data indicate that debeaking causes a loss of exteroceptive units, but not of nociceptive units. Comparison of sections stained for the presence of substance P (immunohistochemistry) did not reveal a long-term effect on the nociceptive system suggestive of the occurrence of chronic pain.

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.

The primary afferent pathway of extraocular muscle proprioception in the pigeon

Recent physiological experiments in our laboratory suggest that extraocular muscle proprioceptive signals are involved in oculomotor control in the pigeon [e.g., Knox and Donaldson (1993) Proc. R. Soc. Lond. B 253, 77-82]; the present results provide information about the primary afferent pathway involved in these actions. In other physiological experiments [Hayman et al. (1993) Proc. R. Soc. Lond. B 254, 115-122] we have shown that extraocular muscle afferent signals modify vestibularly driven neck reflexes in the pigeon; the present results suggest an anatomical substrate for these effects. The localization of the cell bodies and of the central terminations of afferent fibres from the extraocular muscles of the pigeon was examined using transport of horseradish peroxidase. The results showed that primary afferent cell somata subserving extraocular muscle proprioception are located within the ipsilateral trigeminal ganglion. The presence of heavily labelled brainstem neurons reported in a previous study [Eden et al. (1982) Brain Res. 237, 15-21] was confirmed; however, these cells were shown to be accessory abducens motoneurons innervating the quadratus muscle, and presumably the pyramidalis muscle also, and not proprioceptive afferent somata as had been suggested. The central projections of extraocular muscle afferent neurons were found consistently in a restricted area of the external cuneate nucleus. This is in contrast to findings in a number of mammals in which the terminal label has been seen to cluster in portions of the spinal trigeminal nucleus. The presence of a lateral trigeminal tract in the pigeon, through which the afferent axons course, which terminates exclusively in the ventral portion of the external cuneate nucleus may explain this finding.

[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.

Trigeminal primary afferent projections to the spinal cord of the frog, Rana ridibunda

The distribution in the spinal cord of the trigeminal primary projections in the frog Rana ridibunda was studied by means of the anterograde transport of horseradish peroxidase (HRP). Upon entering the medulla via the single trigeminal root, a conspicuous descending tract that reaches the cervical spinal cord segments is established. This projection arises in the ophthalmic (V1), maxillary (V2), and mandibular (V3) trigeminal nerve subdivisions. In the spinal cord, only a minor somatotopic arrangement of the trigeminal fibers was observed, with the fibers arising in V3 terminating somewhat more medially than those from V1 and V2. A dense projection to the medial aspect of the spinal cord, above the central canal, primarily involves V3. Each trigeminal branch sends projections at cervical levels to the contralateral dorsal field, and those from V2 are most abundant. Bilateral experiments with HRP application show convergence of primary trigeminal and spinal afferents within the dorsal field of the spinal cord. The pattern of arrangement of the trigeminal primary afferent fibers in the spinal cord of this frog largely resembles that of amniotes. However, the organization seems simpler and the slight somatotopic distribution of V1, V2, and V3 fibers is similar to the condition in other anamniotes.

Afferent and efferent projections of the glossopharyngeal-vagal nerve in the hagfish

Anterograde and retrograde transport of horseradish peroxidase was used to examine the afferent and efferent projections of the glossopharyngeal-vagal nerve in the hagfish Eptatretus burgeri. Anterogradely labeled ganglion cells are scattered in the glossopharyngeal-vagal nerve trunk, in the saccular ganglion, and in the brainstem. Afferent fibers of the glossopharyngeal-vagal nerve terminate in both the vagal lobe and the fasciculus communis. Close observation showed no morphological differentiation between these two structures, indicating that they are not separate entities, but a single, continuous structure that is homologous with the nucleus and tractus solitarius of other vertebrates. The median part of this structure (the commissura infima) is displaced more rostrally than the same part of the solitary nucleus in many other vertebrates. Some of the afferent fibers invade the ventral portion of the trigeminal sensory nucleus, which receives the maxillo-mandibular nerve fibers, and terminate there. Our study showed that the hagfish has only one nucleus in the vagal motor system, i.e., the vagal motor nucleus, which contains both parasympathetic and branchiomotor neurons. The dendrites of the vagal motor neurons in the hagfish are more highly developed than those in other vertebrates. This suggests that the motor reflex arc of the glossopharyngeal-vagal nerve in hagfishes may be simpler than in other vertebrates.

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.

Regional specificity of developing reticulospinal, vestibulospinal, and vestibulo-ocular projections in the chicken embryo

The regional mapping of reticulospinal, vestibulospinal, and vestibulo-ocular neuron groups onto specific axonal pathways was determined in the chicken embryo by retrograde axonal tracing. Experiments were performed on in vitro preparations of the brain stem to allow for precisely localized tracer injections combined with selective lesions of axon tracts. Brain-stem neuron groups were labelled from 3 days of embryonic development, when the first reticulospinal axons reached the cervical spinal cord, to 9 days of embryonic development, when each of the three systems studied had acquired a relatively mature organization. A striking feature at all stages was the spatial segregation of many neuron groups that projected along different trajectories. Examples of such segregation were found for neuron groups projecting in the same tract on different sides of the brain stem, in different tracts on the same side of the brain stem, and rostrally versus caudally. The occurrence of this segregation from early stages suggests that the choice of projection pathway by many brain-stem neurons is in some way linked to cell position. In some regions of the brain stem, neuron groups projecting along different pathways are intermingled. At least some of this intermingling, however, appears to occur subsequent to the initial establishment of axon projection patterns. Comparison of the mapping patterns at progressively older stages, and with previous mapping in the 11-day-old embryo (Glover and Petursdottir, 1988; Petursdottir, 1990) suggests that these projections are established with little error. The one obvious example of remodelling involved the pontine reticulospinal projection, in which an early contralateral axon population appeared to retract from spinal to medullary levels over the course of a few days. A similar phenomenon may be involved in the elimination of part of the contralateral reticulospinal projection from the midmedulla.

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.

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.

Central distribution of the efferent cells and the primary afferent fibers of the trigeminal nerve in Pleurodeles waltlii (Amphibia, Urodela)

As part of a study on the organization of the brainstem in a primitive group of vertebrates, the efferent cells and primary afferent fibers of the urodele amphibian Pleurodeles waltlii were examined by means of retrograde and anterograde axonal transport and anterograde degeneration. The trigeminal motor nucleus is located in the periventricular gray just medial to the sulcus limitans. Its rostral part is a band of pear-shaped cells lying parallel to the wall of the ventricle, whereas its caudal part is a round mass consisting of polygonal cells. In addition, a small group of scattered neurons is situated ventral to the rostral part of the nucleus. The primary afferent fibers enter the brainstem in the dorsal two-thirds of the trigeminal root. They diverge into a short ascending and a long descending tract. The former distributes its axons to the principal sensory trigeminal nucleus, which is an ill-defined cell group located at the ventrolateral edge of the periventricular gray. In the descending tract, the fibers of the ophthalmic nerve are predominantly located ventromedially, and those of the maxillomandibular nerve dorsolaterally. A fascicle of the ophthalmic nerve leaves the descending tract and, apparently, makes contact with the accessory abducens nucleus. The descending tract extends caudally into the three upper cervical segments of the spinal cord. The mesencephalic trigeminal nucleus consists of conspicuous large cells, which are scattered through the tectum of the mesencephalon. The cells with peripheral branches in the ophthalmic nerve are mainly located in the caudal half of the tectum, and those with peripheral branches in the maxillomandibular nerve in the rostral half. Collaterals of the central branches of the mesencephalic trigeminal system were traced to an area of the periventricular gray situated between the motor nucleus and the principal sensory nucleus of the trigeminus.

Topography and organization of cranial nerve nuclei in the sand lizard, Lacerta agilis

Cobaltic-lysine complex compound was used to label cranial nerves of the ventrolateral (branchiomotor) and dorsomedial (somatomotor) nuclear columns in the sand lizard, Lacerta agilis. The dendritic arborizations and axonal trajectories of neurons of the respective nuclei were reconstructed from serial sections. A fairly uniform neuronal morphology was found in the nuclei of the ventrolateral column: a spindle-shaped perikaryon gave rise to dorsomedial and ventrolateral dendritic trees, the latter arborizing in a characteristic broomlike manner within a narrow region in the lateral white matter. Axons of all neurons converged upon the medial longitudinal fasciculus and after making a hairpin turn formed the corresponding motor roots. A group of small neurons constituted a separate subnucleus within the V motor nucleus. The VII and IX nuclei were fused into a single nuclear complex. The nucleus ambiguus was found dorsal to the XII nucleus and lateral to the dorsal vagal nucleus. The latter nucleus extended rostrally to the caudal pole of the VI nucleus, and its neurons sent axons to the VII, IX, and X nerves. The term "dorsal visceromotor column" designates the extended dorsal vagal nucleus. A number of small polygonal neurons lying scattered in the lateral part of the medulla were labeled via the VII, IX, and X nerves. This loose aggregate of labeled neurons was termed the "lateral visceromotor area." On the basis of nuclear topography and cellular morphology, the existence of a bulbar XI nucleus was excluded. Three different types of neurons could be distinguished in the dorsomedial nuclear column. Neurons with oval or spherical perikarya and radially oriented dendrites constituted the nuclei innervating external eye muscles. Except for the IV nucleus, axons followed a ventral trajectory. The accessory VI nucleus was composed of a second type of neuron with elongated soma and dorsoventral dendrite orientation; the dorsally directed axon turned ventrally at the VI nucleus. The XII nucleus contains a third type of neuron with strongly decussating dendrites. The distinct differences in the neuronal morphology did not support the classical assumption that all of the nuclei of the dorsomedial motor column supply muscles derived from somitic mesoderm. Sensory fibers of the trigeminal nerve formed the familiar spinal tract, which partially decussated in the medullospinal transition zone and could be followed as far as the lumbar segments on the ipsilateral side of the spinal cord. Neurons of the mesencephalic root of the trigeminal nerve were localized in the optic tectum; their descending fibers joined the medial aspect of the spinal tract.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

The motor complex and primary projections of the trigeminal nerve in the monitor lizard, Varanus exanthematicus

The sensory projections and the motor complex of the trigeminal nerve of the reptile Varanus exanthematicus were studied with the methods of anterograde degeneration and anterograde and retrograde axonal transport. The primary afferent fibers diverge in the brainstem into a short ascending and a long descending tract. The former distributes its fibers to the principal sensory trigeminal nucleus, where nerves V1, V2, and V3 are represented along a lateromedial axis. The fibers of the descending tract enter the nucleus of this tract and the reticular formation. Both in the tract and its nucleus, nerves V1, V2 and V3 occupy successively more dorsal positions. A small contingent of nerve V1 fibers course to the accessory abducens nucleus. The descending tract extends caudally into the first and second cervical segments of the spinal cord. The trigeminal motor complex consists of dorsal, ventral, and dorsomedial nuclei. The m. adductor mandibulae externus (the main jaw closer) is represented in the dorsal nucleus, predominantly in its rostral part. The muscles innervated by nerve V3 are represented in the ventral nucleus, mainly in its caudal part. All three divisions of the trigeminal nerve contain peripheral branches of the mesencephalic trigeminal system. Collaterals of the central branches of this system were traced to the ventral motor and the principal sensory trigeminal nuclei.

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.

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

The efferent and intranuclear connections of the nuclei of the descending trigeminal tract of the mallard have been studied with lesion methods, and by axonal transport techniques following injections of tritiated leucine, and of horseradish peroxidase. The large subnucleus oralis neurons, including those belonging to the nucleus of the ascending glossopharyngeal tract, have proven to be the sole origin of trigeminocerebellar connections. The cerebellar afferents are of the mossy fiber type, and terminate predominantly in lobules V, VI and VII, and possibly, lobule IV. Trigeminocerebellar projections are ipsilateral except for the vermal area. Subnucleus interpolaris is the main source of intratrigeminal fibers that terminate in subnucleus oralis and the ventral part of the main sensory nucleus. These intranuclear connections are bilateral, but the medium-celled caudal part of subnucleus interpolaris in particular contains the majority of bi- and/or contralaterally projecting neurons. Additionally, the small cells in the rostral part of subnucleus interpolaris project ipsilaterally upon the parabrachial region, and upon the lateral reticular formation. Projections upon the parabrachial region furthermore emanate bilaterally from layer I of the rostral subnucleus caudalis. A minor part of layer I neurons sends its axons contralaterally along with those of the dorsal column nuclei toward the thalamic nucleus dorsolateralis posterior. Associated with the medial lemniscus, contralateral termination is also present in the lateral part of the ventral lamella of oliva caudalis, in the marginal zone of nucleus mesencephalicus lateralis, pars dorsalis and immediately surrounding intercollicular grey and, finally, in the nucleus intercalatus thalami. Furthermore, a bilaterally descending projection from subnucleus caudalis upon layers I and II of the rostral cervical cord was observed. Close to their origin subnucleus caudalis neurons project upon the adjoining caudal part of the lateral reticular formation.