Evolution of facial innervation in anomodont therapsids (Synapsida): Insights from X-ray computerized microtomography.

Anomodontia was the most successful herbivorous clade of the mammalian stem lineage (non-mammalian synapsids) during the late Permian and Early Triassic. Among anomodonts, Dicynodontia stands apart because of the presence of an osseous beak that shows evidence of the insertion of a cornified sheath, the ramphotheca. In this study, fourteen anomodont specimens were microCT-scanned and their trigeminal canals reconstructed digitally to understand the origin and evolution of trigeminal nerve innervation of the ramphotheca. We show that the pattern of innervation of the anomodont "beak" is more similar to that in chelonians (the nasopalatine branch is enlarged and innervates the premaxillary part of the ramphotheca) than in birds (where the nasopalatine and maxillary branches play minor roles). The nasopalatine branch is noticeably enlarged in the beak-less basal anomodont Patranomodon, suggesting that this could be an anomodont or chainosaur synapomorphy. Our analyses suggest that the presence or absence of tusks and postcanine teeth are often accompanied by corresponding variations of the rami innervating the caniniform process and the alveolar region, respectively. The degree of ossification of the canal for the nasal ramus of the ophthalmic branch also appears to correlate with the presence of a nasal boss. The nasopalatine canal is absent from the premaxilla in the Bidentalia as they uniquely show a large plexus formed by the internal nasal branch of the maxillary canal instead. The elongated shape of this plexus in Lystrosaurus supports the hypothesis that the rostrum evolved as an elongation of the subnarial region of the snout. Finally, the atrophied and variable aspect of the trigeminal canals in Myosaurus supports the hypothesis that this genus had a reduced upper ramphotheca.

Molecular basis of tactile specialization in the duck bill.

Tactile-foraging ducks are specialist birds known for their touch-dependent feeding behavior. They use dabbling, straining, and filtering to find edible matter in murky water, relying on the sense of touch in their bill. Here, we present the molecular characterization of embryonic duck bill, which we show contains a high density of mechanosensory corpuscles innervated by functional rapidly adapting trigeminal afferents. In contrast to chicken, a visually foraging bird, the majority of duck trigeminal neurons are mechanoreceptors that express the Piezo2 ion channel and produce slowly inactivating mechano-current before hatching. Furthermore, duck neurons have a significantly reduced mechano-activation threshold and elevated mechano-current amplitude. Cloning and electrophysiological characterization of duck Piezo2 in a heterologous expression system shows that duck Piezo2 is functionally similar to the mouse ortholog but with prolonged inactivation kinetics, particularly at positive potentials. Knockdown of Piezo2 in duck trigeminal neurons attenuates mechano current with intermediate and slow inactivation kinetics. This suggests that Piezo2 is capable of contributing to a larger range of mechano-activated currents in duck trigeminal ganglia than in mouse trigeminal ganglia. Our results provide insights into the molecular basis of mechanotransduction in a tactile-specialist vertebrate.

[Magnetoreception systems in birds: a review of current research].

Currently at least two independent systems of magnetoreception are believed to exist in birds, based on different biophysical principles, located in different parts of their bodies, and having different innervation. One magnetoreceptory system is located in the retina and may be based on photo-induced biradical chemical reactions on the basis of cryptochrome. Information from these receptors is processed in a specialized part of visual Wulst, the so-called Cluster N. There are good reasons to believe that this visual magnetoreceptor processes compass magnetic information which is necessary for migratory orientation. The second magnetoreceptory system is probably iron-based (biogenic magnetite), is located somewhere in the upper beak (its exact location and ultrastructure of receptors remain unknown), and is innervated by the ophthalmic branch of trigeminal nerve. It cannot be ruled out that this system participates in spatial representation and helps forming either a kind of map or more primitive signposts, based on regular spatial variation of the geomagnetic field. The magnetic map probably governs navigation of migrating birds across hundreds and thousands of kilometers. Apart from these two systems whose existence may be considered to be convincingly shown (even if their details are not yet fully clear), there are data on the existence of magnetoreceptors based on the vestibular system. It cannot be ruled out that iron-based magnetoreception takes place in lagena (a structure homologous to cochlea of marsupials and eutherians), and the information perceived is processes in vestibular nuclei. The very existence of this magnetoreception system needs verification, and its function remains completely open.

The magnetite-based receptors in the beak of birds and their role in avian navigation.

Iron-rich structures have been described in the beak of homing pigeons, chickens and several species of migratory birds and interpreted as magnetoreceptors. Here, we will briefly review findings associated with these receptors that throw light on their nature, their function and their role in avian navigation. Electrophysiological recordings from the ophthalmic nerve, behavioral studies and a ZENK-study indicate that the trigeminal system, the nerves innervating the beak, mediate information on magnetic changes, with the electrophysiological study suggesting that these are changes in intensity. Behavioral studies support the involvement of magnetite and the trigeminal system in magnetoreception, but clearly show that the inclination compass normally used by birds represents a separate system. However, if this compass is disrupted by certain light conditions, migrating birds show 'fixed direction' responses to the magnetic field, which originate in the receptors in the beak. Together, these findings point out that there are magnetite-based magnetoreceptors located in the upper beak close to the skin. Their natural function appears to be recording magnetic intensity and thus providing one component of the multi-factorial 'navigational map' of birds.

Avian magnetoreception: elaborate iron mineral containing dendrites in the upper beak seem to be a common feature of birds.

The magnetic field sensors enabling birds to extract orientational information from the Earth's magnetic field have remained enigmatic. Our previously published results from homing pigeons have made us suggest that the iron containing sensory dendrites in the inner dermal lining of the upper beak are a candidate structure for such an avian magnetometer system. Here we show that similar structures occur in two species of migratory birds (garden warbler, Sylvia borin and European robin, Erithacus rubecula) and a non-migratory bird, the domestic chicken (Gallus gallus). In all these bird species, histological data have revealed dendrites of similar shape and size, all containing iron minerals within distinct subcellular compartments of nervous terminals of the median branch of the Nervus ophthalmicus. We also used microscopic X-ray absorption spectroscopy analyses to identify the involved iron minerals to be almost completely Fe III-oxides. Magnetite (Fe II/III) may also occur in these structures, but not as a major Fe constituent. Our data suggest that this complex dendritic system in the beak is a common feature of birds, and that it may form an essential sensory basis for the evolution of at least certain types of magnetic field guided behavior.

Ultrastructural analysis of a putative magnetoreceptor in the beak of homing pigeons.

With the use of different light and electron microscopic methods, we investigated the subcellular organization of afferent trigeminal terminals in the upper beak of the homing pigeon, Columba livia, which are about 5 microm in diameter and contain superparamagnetic magnetite (SPM) crystals. The SPM nanocrystals are assembled in clusters (diameter, approximately 1-2 microm). About 10 to 15 of these clusters occur inside one nerve terminal, arranged along the cell membrane. Each SPM cluster is embedded in a solid fibrous cup, open towards the cell surface, to which the cluster adheres by delicate fiber strands. In addition to the SPM clusters, a second inorganic iron compound has been identified: noncrystalline platelets of iron phosphate (about 500 nm wide and long and maximally 100 nm thick) that occur along a fibrous core of the terminal. The anatomic features suggested that these nerve endings could detect small intensity changes of the geomagnetic field. Such stimuli can induce deformations of the SPM clusters, which could be transduced into primary receptor potentials by mechanosensitive membrane receptor channels. The subepidermal fat cells surrounding the nerve endings prevent the inside from external mechanical stimuli. These structural findings corresponded to conclusions inferred from rock magnetic measurements, theoretical calculations, model experiments, and behavioral data, which also matched previous electrophysiologic recordings from migratory birds.

Mechanothermal nociceptors in the scaly skin of the chicken leg.

Electrophysiological recordings were made from single sensory mechanothermal nociceptive afferent fibres in dissected nerve filaments of the parafibular nerve innervating the scaly skin of the lower leg of the chicken. Two classes of mechanothermal nociceptors were identified consisting of 34 C fibres (conduction velocities 0.45-1.5 m/s, mean 1.08) and nine A-delta fibres (3-15 m/s, mean 6.34). The C fibre afferents had receptive fields which were circular or elliptical in shape and ranged in size from 1 mm in diameter to 4 x 3 mm. Thresholds to mechanical stimulation in the C fibre afferents ranged from 0.3 to 33 g (median 1.5 g) and thermal thresholds were in the range 39-61 degrees C (median 49.4 degrees C). Stimulus-response curves to thermal and/or mechanical stimulation were recorded from 28 C fibre afferents and subjected to a linear regression analysis to determine whether they were best fitted by a linear, log or power function. The results were variable and no single function provided the best fit for all the responses. Of the fibres tested with both stimulus modalities (n=17), only 12 fibres showed the same best fit for both stimuli; in the others the best fit regression lines differed between stimuli. The response of the A-delta fibres to mechanical and thermal stimulation was very similar to the C fibres but the small number of A-delta fibres precluded any detailed statistical analysis. Comparison of the physiological properties of the C fibres in the leg with those previously identified in the beak showed that those in the leg had significantly lower thermal thresholds, but higher mechanical thresholds. The possible functional significance of these differences is discussed. These findings are also discussed in a comparative context to identify similarities and differences between mechanothermal nociceptors in birds and other vertebrates, relating these to their possible evolutionary and functional significance.

Trigeminally innervated iron-containing structures in the beak of homing pigeons, and other birds.

The ophthalmic nerve in the upper beak was labelled with cholera toxin B-chain, and iron was identified using the Prussian Blue reaction. Iron deposits were found in the caudal part of the beak, and some were concentrated in cells that clustered in encapsulated structures densely innervated by ophthalmic nerve fibres. Such structures could form the anatomical basis of a type of mechanoreceptor that transmits magnetic sense information to the brain.

Distribution and putative function of autonomic nerve fibres in the bill skin of the platypus (Ornithorhynchus anatinus).

The electroreceptors located in the bill skin of the platypus are modified secretory glands. The electroreceptive nerve terminals form bare endings in close proximity to the duct of these glands. In this study, we describe the autonomic innervation of the glands and a separate specialized autonomic innervation of the epidermal portion of the glandular duct. A range of immunohistochemical labels showed that the gland cells of the electroreceptors have a non-noradrenergic (putative parasympathetic) innervation. Phalloidin labelling revealed a 'sphincter' of epidermal luminal cells that labelled strongly for actin. These actin-dense keratinocytes were seen to have a noradrenergic (putative sympathetic) innervation. Fine-diameter sensory fibres containing substance P (presumably C-fibre thermoreceptors or polymodal nociceptors) were observed to terminate in the superficial epidermis surrounding the pore of the gland. When the bill of the platypus is dry these pores were closed. However, when room temperature water was washed over the bill, the pores opened. It is proposed that this autonomic and sensory innervation, along with the actin sphincter, mediates the opening and closing of the pores. By doing this, the platypus prevents the desiccation of the bare electrosensory nerve terminals when it is out of the water, and it may also be a way to regulate the impedance of the internal electrical circuit presented to the water at the pores.

Sensory receptors in monotremes.

This is a summary of the current knowledge of sensory receptors in skin of the bill of the platypus, Ornithorhynchus anatinus, and the snout of the echidna, Tachyglossus aculeatus. Brief mention is also made of the third living member of the monotremes, the long-nosed echidna, Zaglossus bruijnii. The monotremes are the only group of mammals known to have evolved electroreception. The structures in the skin responsible for the electric sense have been identified as sensory mucous glands with an expanded epidermal portion that is innervated by large-diameter nerve fibres. Afferent recordings have shown that in both platypuses and echidnas the receptors excited by cathodal (negative) pulses and inhibited by anodal (positive) pulses. Estimates give a total of 40,000 mucous sensory glands in the upper and lower bill of the platypus, whereas there are only about 100 in the tip of the echidna snout. Recording of electroreceptor-evoked activity from the brain of the platypus have shown that the largest area dedicated to somatosensory input from the bill, S1, shows alternating rows of mechanosensory and bimodal neurons. The bimodal neurons respond to both electrosensory and mechanical inputs. In skin of the platypus bill and echidna snout, apart from the electroreceptors, there are structures called push rods, which consist of a column of compacted cells that is able to move relatively independently of adjacent regions of skin. At the base of the column are Merkel cell complexes, known to be type I slowly adapting mechanoreceptors, and lamellated corpuscles, probably vibration receptors. It has been speculated that the platypus uses its electric sense to detect the electromyographic activity from moving prey in the water and for obstacle avoidance. Mechanoreceptors signal contact with the prey. For the echidna, a role for the electrosensory system has not yet been established during normal foraging behaviour, although it has been shown that it is able to detect the presence of weak electric fields in water. Perhaps the electric sense is used to detect moving prey in moist soil.

The sensory world of the platypus.

Vision, audition and somatic sensation in the platypus are reviewed. Recent work on the eye and retinal ganglion cell layer of the platypus is presented that provides an estimate of visual acuity and suggests that platypus ancestors may have used vision, as well as the bill organ, for underwater predation. The combined electroreceptor and mechanoreceptor array in the bill is considered in detail, with special reference to the elaborate cortical structure, where inputs from these two sensory arrays are integrated in a manner that is astonishingly similar to the stripe-like ocular dominance array in primate visual of cortex, that integrates input from the two eyes. A new hypothesis, along with supporting data, is presented for this combined mechanoreceptive-electroreceptive complex in platypus cortex. Bill mechanoreceptors are shown to be capable of detecting mechanical waves travelling through the water from moving prey. These mechanical waves arrive after the electrical activity from the same prey, as a function of distance. Bimodal cortical neurones, sensitive to combined mechanical and electrical stimulation, with a delay, can thus signal directly the absolute distance of the prey. Combined with the directional information provided by signal processing of the thousands of receptors on the bill surface, the stripe-like cortical array enables the platypus to use two different sensory systems in its bill to achieve a complete, three-dimensional 'fix' on its underwater prey.

The absence of neuromas in beaks of adult hens after conservative trimming at hatch.

OBJECTIVE: To determine the effects of the amount of break removed and cauterisation time on neuroma formation in hens. DESIGN: A pathology study with controls. ANIMALS: Twenty domestic fowl were beak-trimmed. Three non-beak-trimmed domestic fowl were used as controls. PROCEDURE: Beaks of two age groups with two levels of beak removal and either 2 s or 4 s cauterisation, were investigated macroscopically and microscopically for deformities. RESULTS: Scattered trauma-associated neuromas were present in the beaks of pullets 10 weeks after moderate trimming at hatch. Neuromas were not present in beaks of adult hens that had been similarly trimmed. Sensory corpuscles were present 10 and 70 weeks after moderate trimming, though fewer in number than in intact control hens. In contrast, trauma-associated neuromas persisted in beaks of 70-week-old hens that had been severely trimmed at hatch. A range of deformities that were absent in moderately trimmed hens, were observed in hens with severely trimmed beaks. Receptors were not seen in severely trimmed beaks. CONCLUSION: Beak-trimming at hatch induces the formation of neuromas, regardless of the amount of tissue removed. There is a critical amount of beak tissue that can be removed, beyond which trauma-associated neuromas will not resolve, but will persist in mature hens.

Sensory nerve endings in the beak skin of Japanese quail.

This study is concerned with the distribution and ultrastructure of sensory nerve endings in the beak skin of adult Japanese quail (Coturnix coturnix japonica). The following nerve endings were found: free nerve endings, clusters of dermal Merkel nerve endings, Herbst corpuscles and Ruffini corpuscles. The latter were found only in the dermis of the tip of the upper beak. The remaining endings were present in the skin of all areas of upper and lower beak. Free nerve endings were supplied by either thin myelinated axons or unmyelinated C-fibers and were localized in the dermis close to the basal layer of the epidermis. Merkel cells formed clusters (up to 50) localized below and between the epidermal cones of the beak skin. Disc-shaped thickenings of nerve endings were squeezed between individual Merkel cells. Small Herbst corpuscles were found in the dermis close to the epidermal cones of the beak skin. Large Herbst corpuscles occurred in deep layers of the dermis. The Ruffini corpuscles were cylindrical in shape (80 microns x 400 microns) and arranged in groups of up to ten corpuscles. Each corpuscle was surrounded by an incomplete fibrous capsule.

Ultrastructure and distribution of epidermal sensory receptors in the beak of the echidna, Tachyglossus aculeatus.

Within the rostral one centimetre of the Echidna beak, three specialised receptors were found: a mucous sensory gland, a rod-like structure, and an innervated epidermal pit. The mucous sensory gland consists of a dermal mucous gland and a modified epidermal portion. Bulbous nerve terminals, similar to those reported for the Platypus, were found within the modified epidermal portion of the mucous gland. The rod-like structure contains four types of nerve terminals: Merkel cells, Paciniform corpuscles, and a central and a peripheral vesicle chain receptor. Apart from minor differences, the rod-like structure is similar to that previously reported for the Platypus. Preliminary results are presented for a third structure: an innervated epidermal pit. Topographical and ultrastructural analyses are used in the context of functional interpretation.

Trigeminal deafferentation and prehension in the pigeon.

During the grasping and manipulation phases of the pigeon's ingestive pecking behavior, jaw opening movements are scaled to the size of the target (food) object. To assess the contribution of beak mechanoreceptor afferents to the control of scaling we examined the effects of bilateral trigeminal deafferentation upon the kinematics of jaw opening trajectories. Deafferented subjects exhibited both a transient reduction in the accuracy of peck localization and a more persistent deficit in the effectiveness of their ingestive pecking response. However, they continued to exhibit the same classes of jaw movement described for the normal pigeon. The functional relation between target size and gape remained unchanged after deafferentation as did the relationships among kinematic variables controlling jaw opening. However, deafferentation produced small but significant increase in the absolute values of peak gape for both grasping and mandibulation which reflects an increase in peak opening velocity. The results are discussed in relation to the problem of sensory control of rapid targeted movements.

Receptors in the bill of the platypus.

1. Afferent responses were recorded from filaments of the trigeminal nerve in each of two platypuses (Ornithorhynchus anatinus) anaesthetized with alpha-chloralose. All receptive fields were located along the lateral border of the upper bill. Discrete receptive fields could be identified as belonging to two distinct classes of sensory receptor. 2. The most prominent response was an irregular resting discharge which could be increased or decreased by weak electric pulses. These receptors were insensitive to moderately strong mechanical stimulation, and it was concluded that they were electroreceptors. 3. Each electroreceptor had a single spot of maximum sensitivity on the bill surface. When the stimulating electrode over this spot was the cathode it excited the receptor for the duration of the stimulating pulse, using stimulus strengths as low as 20 mV. When it was the anode, it inhibited the discharge. Cathodal excitation was followed by rebound inhibition and anodal inhibition by rebound excitation. 4. Receptors responded to cathodal steps with an initial high-frequency burst of impulses, followed by a lower maintained rate of discharge. Rapidly changing pulses were similarly effective in exciting receptors, adding support to the claim that platypuses are able to detect moving prey by the electrical activity associated with muscle contraction. 5. The centres of the receptive fields of two electroreceptors were marked by the insertion of fine entomological pins. Histological examination established the presence of a large mucus-secreting gland at the marked spot. The epidermal duct of the gland contained an elaborate myelinated innervation, with morphologically distinct axon terminals that we identify as the electroreceptors. 6. As well as electroreceptors, the skin of the bill contained three kinds of mechanoreceptors: slow-adapting receptors, rapidly adapting, vibration-sensitive receptors and receptors with an intermediate adaptation rate. The slowly adapting receptors were characterized by their low threshold to mechanical stimuli, irregular discharge and significant dynamic sensitivity. Vibration receptors showed maintained responses to sinusoidal vibration of the skin up to 600 Hz. 7. These experiments confirm an earlier report that the platypus bill is an electrodetector organ. The presence of electroreceptors of a unique structure and supplied by the trigeminal nerve indicates that electroreception has evolved independently in monotremes. This in turn emphasizes that monotremes are a highly evolved group which split off from the main mammalian stem a long time ago.

Deplasticizing of thick Epon sections involving a new adhesive technique and staining of nervous tissue.

The present communication deals with a technique developed for the selective staining of neural tissue in thick (10 micron) Epon sections. A new adhesive method was needed, because the known techniques are only applicable to 0.5-2 micron thin sections. The critical step in the procedure is the adhesion of the sections onto the slides. This is accomplished by heating the sections on top of a uniform layer of albumin glycerol on the slide followed by coating with celloidin. The results after deplasticizing and coagulation with this technique are comparable to those obtained by paraffin or frozen section techniques, but in addition have the advantage of Epoxy resin embedding e.g. the possibility of cutting undecalcified hard tissues and sections for serial reconstruction.

Telencephalic bill projections in the Landes goose.

Electrical activity of trigeminal central projection areas was recorded in anesthetized and chronic awake geese. Evoked potentials of telencephalic structures were studied after stimulation of the bill, quintofrontal tract (QF), and several telencephalic structures (nucleus basalis [Bas], neostriatum frontale [NF], paleostriatum augmentatum [PA], and neostriatum caudale [NC]). Short-latency evoked potentials were recorded in Bas after stimulation of the bill or QF; this finding is consistent with a direct connection between the main sensory trigeminal nucleus and Bas. Short- and long-latency evoked potentials were recorded in PA and NC after stimulation of the posterior QF. These potentials are concluded to be due to two different pathways: The shorter-latency response is produced by fibers leaving QF posteriorly, while the longer-latency response is derived from fibers traveling along QF, relaying first in Bas and then in NF. From Bas and NF, two pathways convey impulses to NC; only one is relayed in PA.

Number and distribution of taste buds in the oral cavity of hatchling chicks.

The location and number of taste buds were mapped in palatal epithelia of one-day old chicks and bud widths measured. Bud counts additionally were recorded for the tongue, and floor of the lower beak. An average of 316 taste buds was observed in the oral cavity of which 69%, 29% and 2% were distributed across oral epithelium in the upper beak (palate), lower beak and posteroventrolateral region of the anterior tongue, respectively. In each oral region, salivary gland ducts lying adjacent as well as gland ductules penetrating through the buds were prevalent. This relation may provide the bio-fluid milieu for receptor stimulation during feeding. Widths of palatal buds were bimodally distributed, peaking at diameters between 40-49 and 60-69 microns. The taste bud-rich oral epithelium in these one-day old chicks is consonant with their precocial nature. The topographic distribution of taste buds appears to be in register with those regions of epithelium contacted by food which is transported anteroposteriorly through the oral cavity by the chicken's prehensile tongue.