Afferents to the trigeminal and facial motor nuclei in pigeon (Columba livia L.): central connections of jaw motoneurons

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.

Androgen receptors in the embryonic zebra finch hindbrain suggest a function for maternal androgens in perihatching survival

Bird embryos are exposed to maternal androgens deposited in the egg, but the role of these hormones in embryonic development and hatchling survival is unclear. To identify possible target organs, we used in situ hybridization to study the distribution of androgen receptor (AR) RNA in the developing zebra finch brain. The first brain expression domain of AR mRNA is in the hindbrain. From embryonic day 7 (E7) onward, when the hypoglossal motor nucleus (nXII) has just formed, there was AR mRNA expression in both its lingual (nXIIl) and its tracheosyringeal (nXIIts) parts, and this was the major site of hindbrain expression at all embryonic stages and in both sexes. From E8 onward, we also found AR mRNA in the supraspinal motor nucleus (nSSp), which innervates neck muscles. Furthermore, the syrinx, the target of the nXIIts, contained AR mRNA by E10, localized principally in the perichondria. Muscle was first evident in the syringeal region at E9, but no AR was detected in syringeal muscles until after hatching. The expression pattern of AR in the zebra finch embryo suggests that maternal androgens act via AR in the brainstem and syrinx to influence hatching as well as acoustic and visual components of food-begging behavior. Maternal androgens seem unlikely to function in the development of sexual dimorphisms in the zebra finch nXIIts and syrinx, insofar as these are not evident until between 10 and 20 days posthatching.

Functional circuitry involved in the regulation of whisker movements

Neuroanatomical tract-tracing methods were used to identify the oligosynaptic circuitry by which the whisker representation of the motor cortex (wMCx) influences the facial motoneurons that control whisking activity (wFMNs). Injections of the retrograde tracer cholera toxin subunit B into physiologically identified wFMNs in the lateral facial nucleus resulted in dense, bilateral labeling throughout the brainstem reticular formation and in the ambiguus nucleus as well as predominantly ipsilateral labeling in the paralemniscal, pedunculopontine tegmental, Kölliker-Fuse, and parabrachial nuclei. In addition, neurons in the following midbrain regions projected to the wFMNs: superior colliculus, red nucleus, periaqueductal gray, mesencephalon, pons, and several nuclei involved in oculomotor behaviors. Injections of the anterograde tracer biotinylated dextran amine into the wMCx revealed direct projections to the brainstem reticular formation as well as multiple brainstem and midbrain structures shown to project to the wFMNs. Regions in which retrograde labeling and anterograde labeling overlap most extensively include the brainstem parvocellular, gigantocellular, intermediate, and medullary (dorsal and ventral) reticular formations; ambiguus nucleus; and midbrain superior colliculus and deep mesencephalic nucleus. Other regions that contain less dense regions of combined anterograde and retrograde labeling include the following nuclei: the interstitial nucleus of medial longitudinal fasciculus, the pontine reticular formation, and the lateral periaqueductal gray. Premotoneurons that receive dense inputs from the wMCx are likely to be important mediators of cortical regulation of whisker movements and may be a key component in a central pattern generator involved in the generation of rhythmic whisking activity.

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.

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.

Neural sites and pathways regulating food intake in birds: a comparative analysis to mammalian systems

The paper reviews hypotheses explaining the regulation of food intake in mammals that have addressed specific anatomical structures in the brain. An hypothesis, poikilostasis, is introduced to describe multiple, homeostatic states whereby the regulation of metabolism and feeding occur in birds. Examples are given for both wild and domestic avian species, illustrating dynamic shifts in homeostasis responsible for the changes in body weights that are seen during the course of an annual cycle or by a particular strain of bird. The following neural structures are reviewed as each has been shown to affect food intake in birds or in mammals: ventromedial hypothalamic nucleus (n.), lateral hypothalamic area, paraventricular hypothalamic n., n. tractus solitarius and area postrema, amygdala, parabrachial n., arcuate n. and bed n. of the stria terminalis. Two neural pathways are described which have been proposed to regulate feeding. The trigeminal sensorimotor pathway is the most complete neural pathway characterized for this behavior and encompasses the mechanics of pecking, grasping and mandibulating food particles from the tip of the bill to the back of the buccal cavity. A second pathway, the visceral forebrain system (VFS), affects feeding by regulating metabolism and the balance of the autonomic nervous system. Wild, migratory birds are shown to exhibit marked changes in body weight which are hypothesized to occur due to shifts in balance between the sympathetic and parasympathetic nervous systems. Domestic avian species, selected for a rapid growth rate, are shown to display a dominance of the parasympathetic nervous system. The VFS is the neural system proposed to effect poikilostasis by altering the steady state of the autonomic nervous system in aves and perhaps is applicable to other classes of vertebrates as well.

Neural pathways for the control of birdsong production

As in humans, song production in birds involves the intricate coordination of at least three major groups of muscles: namely, those of the syrinx, the respiratory apparatus, and the upper vocal tract, including the jaw. The pathway in songbirds that controls the syrinx originates in the telencephalon and projects via the occipitomesencephalic tract directly upon vocal motoneurons in the medulla. Activity in this pathway configures the syrinx into phonatory positions for the production of species typical vocalizations. Another component of this pathway mediates control of respiration during vocalization, since it projects upon both expiratory and inspiratory groups of premotor neurons in the ventrolateral medulla, as well as upon several other nuclei en route. This pathway appears to be primarily involved with the control of the temporal pattern of song, but is also importantly involved in the control of vocal intensity, mediated via air sac pressure. There are extensive interconnections between the vocal and respiratory pathways, especially at brain-stem levels, and it may be these that ensure the necessary temporal coordination of syringeal and respiratory activity. The pathway mediating control of the jaw appears to be different from those mediating control of the syrinx and respiratory muscles. It originates in a different part of the archistriatum and projects upon premotor neurons in the medulla that appear to be separate from those projecting upon the syringeal motor nucleus. The separateness of this pathway may reflect the imperfect correlation of jaw movements with the dynamic and acoustic features of song. The brainstem pathways mediating control of vocalization and respiration in songbirds have distinct similarities to those in mammals such as cats and monkeys. However, songbirds, like humans, but unlike most other non-songbirds, have developed a telencephalic vocal control system for the production of learned vocalizations.

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.

Projections of the dorsomedial nucleus of the intercollicular complex (DM) in relation to respiratory-vocal nuclei in the brainstem of pigeon (Columba livia) and zebra finch (Taeniopygia guttata)

Injections of neuronal tracers were made into the dorsomedial nucleus of the intercollicular complex (DM) of pigeons and zebra finches in order to investigate the projections of this nucleus which has long been implicated in respiratory-vocal control. Despite the fact that pigeons are nonsongbirds and zebra finches are songbirds, the projections were very similar in both species. Most descended throughout the brainstem, taking ventral and dorsal trajectories, which merged in the medulla. Those descending ventrally terminated upon the ventrolateral parabrachial nucleus (PBvl), the nucleus infraolivaris superior, a nucleus of the rostral ventrolateral medulla (RVL), and the nucleus retroambigualis (RAm). Those taking a dorsal trajectory via the occipitomesencephalic tract terminated in the tracheosyringeal part of the hypoglossal nucleus (XIIts), the suprahypoglossal region, and nucleus retroambigualis. There were also substantial projections throughout an arc extending between XIIts and RVL rostrally, and XIIts and RAm caudally. Neurons throughout this arc, which include inspiratory premotor neurons at levels straddling the obex and expiratory premotor neurons more caudally (in RAm), were retrogradely labeled from spinal injections. The DM projections were predominantly ipsilateral, but there were distinct contralateral projections to all the homologous nuclei in both species. All but the projections to PBvl and XIIts were reciprocal. In summary, the projections of DM suggest that it is able to influence all the key motor and premotor nuclei involved in patterned respiratory-vocal activity.

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.

Organization of afferent and efferent projections of the nucleus basalis prosencephali in a passerine, Taeniopygia guttata

The connections of nucleus basalis (NB) of the rostral forebrain of the zebra finch were investigated electrophysiologically and with anterograde and retrograde tracing methods to determine their functional organization, the sources of their pontine afferents, and the targets of their telencephalic efferents. The nucleus was found to be partitioned into three major components, a rostral lingual part that received a hypoglossal projection via a lateral subnucleus of the principal sensory trigeminal nucleus (PrV), a middle beak part that received a trigeminal projection via a medial subnucleus of PrV, and a caudal auditory part that received a short latency auditory projection via the intermediate nucleus of the lateral lemniscus. Beak NB also received a projection from a paralateral lemniscal nucleus, and the dorsocaudal part of auditory NB and the medially adjacent neostriatum also received a projection from a lateral subnucleus of the superior vestibular nucleus (VS). The efferent projections of each of the three major parts of NB were mainly to the adjacent neostriatum frontale (NF), which then provided projections to the lobus parolfactorius (exclusive of area X), the lateral archistriatum intermedium (Ail), and the lateral neostriatum caudale (NCl). Ail received a projection from NCl and provided terminal fields to the contralateral NCl and the NF. The major projections of Ail, however, descended bilaterally through the brainstem via the occipitomesencephalic tracts, with dense terminations in the medial spiriform nucleus and with extensive bilateral terminations throughout the lateral reticular formation of the pons and medulla. For the most part, jaw, tongue, and tracheosyringeal motor nuclei did not receive terminations. The results suggest that NB in zebra finch, like NB in pigeon and duck, is likely to be a major component of trigeminal sensorimotor circuitry involved in feeding and in other oral-manipulative behaviors. Results also show that the auditory component of NB is not directly linked to the vocal control system at telencephalic levels, but the possibility remains that the lingual, beak, and auditory parts of NB play a role in vocalization by multisynaptic influences on cranial nerve motor nuclei innervating various parts of the vocal tract.

[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.

Early development of efferent projections from the chick tectum

The early development of the uncrossed tectobulbar and the crossed tectospinal tracts was studied. These two projections arise from the same structure, the mesencephalon, and develop during the same time period, but follow divergent courses. We have traced the pathways followed by these projections and identified the positions at which axon guidance decisions are made. The first neurons differentiate either side of the entire rostrocaudal extent of the dorsal midline and initiate axons that extend dorsoventrally across the surface of the tectum. At the ventral edge of the tectum these axons turn abruptly and fasciculate to form a caudal descending projection to the hindbrain. These axons extend to the caudal hindbrain and do not project to the periphery along cranial nerve roots. We therefore consider this tract to be the tectobular, rather than the mesencephalic division of the trigeminal. While the tectobulbar projection is still developing, a second wave of axons is initiated, which arises from only the rostral part of the tectum. These axons grow beyond the tectobulbar turn point and continue toward the ventral midline, where they cross the floor plate, before turning caudally at the lateral edge of the main descending hindbrain tract, the ventrolateral tract. We discuss the development of these tracts with reference to possible guidance cues mediating their course.

Organization of the avian "corticostriatal" projection system: a retrograde and anterograde pathway tracing study in pigeons

Birds have well-developed basal ganglia within the telencephalon, including a striatum consisting of the medially located lobus parolfactorius (LPO) and the laterally located paleostriatum augmentatum (PA). Relatively little is known, however, about the extent and organization of the telencephalic "cortical" input to the avian basal ganglia (i.e., the avian "corticostriatal" projection system). Using retrograde and anterograde neuroanatomical pathway tracers to address this issue, we found that a large continuous expanse of the outer pallium projects to the striatum of the basal ganglia in pigeons. This expanse includes the Wulst and archistriatum as well as the entire outer rind of the pallium intervening between Wulst and archistriatum, termed by us the pallium externum (PE). In addition, the caudolateral neostriatum (NCL), pyriform cortex, and hippocampal complex also give rise to striatal projections in pigeon. A restricted number of these pallial regions (such as the "limbic" NCL, pyriform cortex, and ventral/caudal parts of the archistriatum) project to such ventral striatal structures as the olfactory tubercle (TO), nucleus accumbens (Ac), and bed nucleus of the stria terminalis (BNST). Such "limbic" pallial areas also project to medialmost LPO and lateralmost PA, while the hyperstriatum accessorium portion of the Wulst, the PE, and the dorsal parts of the archistriatum were found to project primarily to the remainder of LPO (the lateral two-thirds) and PA (the medial four-fifths). The available evidence indicates that the diverse pallial regions projecting to the striatum in birds, as in mammals, are parts of higher order sensory or motor systems. The extensive corticostriatal system in both birds and mammals appears to include two types of pallial neurons: 1) those that project to both striatum and brainstem (i.e., those in the Wulst and the archistriatum) and 2) those that project to striatum but not to brainstem (i.e., those in the PE). The lack of extensive corticostriatal projections from either type of neuron in anamniotes suggests that the anamniote-amniote evolutionary transition was marked by the emergence of the corticostriatal projection system as a prominent source of sensory and motor information for the striatum, possibly facilitating the role of the basal ganglia in movement control.

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.

Connections of the auditory forebrain in the pigeon (Columba livia)

Ascending auditory efferents in birds terminate mainly within Field L2, a cytoarchitectonically distinct region of the caudomedial telencephalon. The organization of Field L2, and that of its flanking regions, L1 and L3, was investigated with 14C-2-deoxyglucose (14C-2-DG), cytochrome oxidase, and both retrograde and anterograde tracing techniques. Field L2 was found to contain a high concentration of cytochrome oxidase. Following auditory stimulation, 14C-2-DG autoradiography revealed that Field L2 consists of two adjacent but seemingly discontinuous zones, designated Field L2a, which lies ventromedially, and Field L2b, which lies dorsolaterally. Termination of thalamic efferents: The thalamic auditory nuclei ovoidalis (Ov) and semilunaris parovoidalis (SPO) project predominantly upon Field L2, and possibly sparsely upon L1, L3 and the overlying hyperstriatum ventrale (HV). Ov subnuclei project upon L2a and SPO projects predominantly upon L2b. The topography of the projections is inverted along the ventromedial-to-dorsolateral axis of L2, and is in accord with an inverted tonotopic representation of frequencies; high frequencies (< 3.5 kHz) being found in the more ventromedial parts of L2a, and low frequencies and broad band responses in L2b. Intra- and extratelencephalic connections: Field L2a also receives a substantial projection from HV, but the efferent projections of L2a appear confined to adjacent "neostriatal" regions. The subsequent projections of L2b were not identified in this study. L1 and L3 project predominantly to the dorsal neostriatum (Nd) caudolateral to Field L, and have fewer projections to the caudomedial paleostriatum and anterior hyperstriatum accessorium. Nd projects massively upon the ventromedial nucleus of the intermediate archistriatum (Aivm), which has bilateral projections upon the caudomedial telencephalon and is the origin of a major descending pathway having dense terminations surrounding the ovoidalis complex (Ov and SPO), MLd, the lateral lemniscal nuclei, and sparse terminations within SPO itself. It is suggested that within the telencephalon the major components of the auditory pathway consist of cell groups which collectively correspond to the populations of neurons found within the auditory cortex of mammals.

Grasping in the pigeon: control through sound and vibration feedback mediated by the nucleus basalis

Pigeons were trained to detect auditory and vibratory stimuli in two separate experiments using an instrumental conditioning procedure. The discriminative stimuli became effective as the subjects grasped a probe with the beak. The pigeons learned to suppress responding upon this grasp-contingent stimulation. Bilateral lesions of the nucleus basalis prosencephali (Bas), known to be involved in the motor control of pecking and to receive short latency input of cochlear and trigeminal origin, eliminated the behavioral stimulus detection. The performance of a control color discrimination was not affected by the Bas lesions, demonstrating that these had a specific effect. The processing of peck-related feedback by the nucleus basalis during the normal food uptake of pigeons is discussed.

Visuomotor feeding perturbations after lateral telencephalic lesions in pigeons

Pigeons with lesions of the lateral part of the telencephalon, visual Wulst, and fronto-archistriatal tract were compared with sham-operated controls in 2 procedures. In one of them the time it took the pigeons to grasp and eat a certain number of grains was recorded. In the other experiment the number of grains was counted that the pigeons consumed out of a mixture of grains and pebbles within a fixed time interval. Only the pigeons with lateral telencephalic lesions were impaired. While in the first experiment the lateral ablated birds improved with time there was no recovery of performance in the second experiment.

Neuroanatomical substrates involved in the control of food intake

Five neural pathways were reviewed regarding their specific role in the control of food intake in birds. The five pathways included the trigeminal sensorimotor system, the visual system/basal ganglia pathway, the gustatory system, the olfactory pathway, and the autonomic nervous system/parasympathetic pathway. The trigeminal system is the pathway best understood among the five systems associated with feeding. It begins with sensory nerves innervating the upper and lower mandibles and buccal cavity and ends with nerves projecting to jaw muscles. The function of the pathway is to control the grasping and mandibulation of pellets or seeds. The visual system includes both the tectofugal and thalamofugal pathways. Both visual pathways interact with the avian paleostriatal complex. The latter is equivalent to the mammalian basal ganglia. The second pathway is important in food recognition as well as in orienting the body with respect to its position in three-dimensional space. The third neural circuit involves the sense of taste. Approximately 300 taste buds have been identified within the buccal cavity of the chicken, suggesting that the gustatory system is better developed than once thought. The fourth pathway involves the olfactory system; as in the visual system, more than one pathway has been identified. The dominant pathway appears to project to the piriform cortex, a structure that may play a role in monitoring essential amino acid contents of the brain. The fifth pathway involves an interaction of the hypothalamus and the dorsal motor nucleus of the vagus. This pathway is important in activating the parasympathetic nervous system and in preparing an organism to feed. All five pathways play different roles in controlling food intake in birds.

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.

Prehension in the pigeon. I. Descriptive analysis

Eating in the pigeon involves a series of jaw movements some of which serve a prehensile function; i.e., they are utilized in the grasping and manipulation of objects. These prehensile behaviors are extremely brief (30-80 ms), produce an adjustment of jaw opening amplitude to the size of the food object, are mediated by an effector system involving a relatively small number of muscles and are amenable to both "reflexive" and "voluntary" control. This combination of structural simplicity and functional complexity suggests that the pigeon's jaw movements may provide a useful "model system" for the study of motor control mechanisms in targeted movements. The present report provides a classification of jaw opening movements occurring during eating and a preliminary determination of the extent to which each movement class is scaled to the size of the food object. Jaw movements were monitored during responses to spherical food pellets of six different sizes (3.2-11.1 mm in diameter) using a transducing system which produces a continuous record of gape (i.e., interbeak distance). Assignment to movement classes was then carried out using a computer-assisted scoring program. Functions relating jaw opening amplitude to target size were determined for each movement class. Four jaw movement classes were identified: Prepecks (just prior to pecking), Grasps (opening movements made during pecking but prior to contact with the target), Mandibulations (movements serving to position and transport the object within the buccal cavity) and Swallows. For two of these movement classes (Grasps, Mandibulations) jaw opening amplitude is scaled to pellet size but the scaling functions differ in ways that reflect the functional requirements of the two behaviors.(ABSTRACT TRUNCATED AT 250 WORDS)

Projections of the nucleus of the tractus solitarius in the pigeon (Columba livia)

With the aid of autoradiographic and histochemical (WGA-HRP) tracing techniques, the projections of the nucleus of the tractus solitarius (nTS) in the pigeon have been delineated and related to the viscerotopic organization of the nucleus. As in mammals, nTS projects to both brainstem and forebrain structures. At medullary levels, projections were seen to nTS itself, to the dorsal motor nucleus of the vagus and to the subjacent and more ventral reticular formation. There is a substantial projection to the parabrachial nuclear complex with terminations in all its subnuclei and minor projections to locus coeruleus and several mesencephalic areas, including the ventral area of Tsai, the nucleus of the ascending brachium conjunctivum, and the compact portion of the tegmental pedunculopontine nucleus. At diencephalic levels, projections to the hypothalamus (magnocellular periventricular nucleus, stratum cellulare internum and externum) and dorsal thalamus were seen. Terminal fields within the basal telencephalon included the nucleus of the pallial commissure, the bed nucleus of the stria terminalis, and the nucleus accumbens. The organization of nTS projections in pigeons is correlated with the pattern of inputs to specific nTS subnuclei. Lateral tier subnuclei receiving cardiovascular and pulmonary inputs project upon the ventrolateral reticular formation and the ventrolateral parabrachial complex. Medial tier subnuclei receiving gustatory and gastrointestinal inputs project upon dorsal and medial parabrachial nuclei. Transparabrachial projections arise from nTS subnuclei receiving little or no primary input from the viscera.

Vestibular, olfactory, and vibratory responses of nucleus basalis prosencephali neurons in pigeons

Evoked multiple unit responses were recorded through chronically implanted electrodes from the nucleus basalis prosencephali of the pigeon (Columba livia) upon vibratory stimulation of the beak-tip and airborne auditory stimulation, thus confirming earlier anatomical and physiological findings. Electrical stimulation of the olfactory nerve led to similar short latency responses. A specific directional sensitivity to rotatory vestibular stimulation was observed. Pitch motions of the head in the downward direction evoked the most pronounced multi-unit responses. These results support the suggestion that the nucleus basalis prosencephali is a sensorimotor coordinator of the pigeon's pecking/feeding behaviour.

The organization of the nucleus basalis-neostriatum complex of the mallard (Anas platyrhynchos L.) and its connections with the archistriatum and the paleostriatum complex

The pattern of connections between the nucleus basalis, neostriatum, hyperstriatum ventrale, paleostriatum complex and archistriatum in the mallard has been analysed using Nissl material and a combination of neuroanatomical tracing procedures (autoradiography, horseradish peroxidase and horseradish peroxidase-wheat germ agglutinin histochemistry, lesion/degeneration technique). The frontal part of the mallard's telencephalon is characterized by its multilayered organization and the predominantly vertical arrangement of the connecting fiber systems. The nucleus basalis, endstation of the ascending sensory trigeminal system, is a large laminar cell area with a dorsal and a ventral layer. The overlying neostriatum frontale can be subdivided into a medial, a dorsal and a ventral intermediate, and a lateral area. The nucleus basalis has distinct connections with the ventral layer and sparse connections with the dorsal layer of the intermediate neostriatum, and abundant reciprocal connections with the ventral layer of the hyperstriatum ventrale. The ventral intermediate neostriatum also has reciprocal connections with the hyperstriatum ventrale; its projections overlap partly with those from the nucleus basalis. The ventral layer of the intermediate neostriatum frontale has a distinct projection upon the paleostriatum augmentatum. The dorsal layer sends fibers to the lateral neostriatum, to the rostral "sensorimotor" part of the archistriatum and to the lateral zone of the lobus parolfactorius. Another source of archistriatal afferents is the paleostriatum ventrale, an area that may also send fibers to the brainstem. Figure 21 summarizes the connections described in this paper. The functional significance of this organization is discussed in relation to its possible role in the guidance of pecking and other feeding behaviors in the mallard. Differences in the organization of the systems in pigeon and mallard are related to the differing degrees of visual and tactile (trigeminal) contributions to feeding in the two birds. It is suggested that the pattern of reciprocal connections between the hyperstriatum ventrale and the nucleus basalis and ventral intermediate neostriatum frontale forms the neuroanatomical substrate for a "comparator-system".

Sensory inputs to the nucleus basalis prosencephali, a feeding-pecking centre in the pigeon

Evoked potentials were recorded from the nucleus basalis prosencephali (Bas) of the pigeon through chronically implanted electrodes. The auditory sensitivity of the Bas was assessed by the amplitude of the potentials. Audiograms thus obtained were comparable to those similarly measured from stations of the orthodox auditory pathway and resembled those obtained by others with behavioural techniques from the same species. The sensitivity to vibration applied to the beak was also measured. The vibrogram revealed two separate optima, one located in the lower frequency and another in the higher frequency region. These were shown to be due to trigeminal mechanoreceptive sensitivity and to bone/cochlea mediated sound sensitivity, respectively. Evoked potentials of the Bas in response to vestibular stimulation are described for the first time. The possibility that they were artefacts was excluded with several control procedures. These findings confirm recent anatomical evidence of a direct pathway from the vestibular nucleus to the nucleus basalis prosencephali. All afferents to the Bas are discussed in conjunction with the probable function of the nucleus as a sensorimotor coordinator of the pigeon's pecking/feeding behaviour.

Central afferent connections and origin of efferent projections of the facial nerve in the chicken (Gallus gallus domesticus)

The central afferent connections and origin of efferent projections of the facial nerve in the adult domestic chicken were studied by anterograde and retrograde transport of horseradish peroxidase from the geniculate ganglion. Ipsilateral afferent projections were traced caudal to the level of entrance of the facial nerve and into tractus solitarius (TS), located dorsomedial to the spinal trigeminal nuclear complex. At several rostrocaudal levels in the medulla, fibers exited from TS and terminated in n. sensorius N. facialis (SVII), and nn. ventrolateralis anterior (Vla) and intermedius anterior (Ia) solitarii. Some axons were followed to n. presulcalis anterior (Pas) solitarii. A separate component terminated in subnucleus interpolaris (ip) of n. descendens nervi trigemini or its medially adjacent reticular formation either by exiting from TS or coursing caudally through the trigeminal complex from entering facial rootlets. Another diffuse component of facial axons ascended dorsally and rostrally from the level of entrance of the facial nerve; these projections dissipated in the pons--some on the dorsomedial border of n. principalis N. trigemini (PrV). Ipsilateral efferent projections were traced through the main genu of the facial nerve to retrogradely labelled somata of pars dorsalis (FMd), pars intermedia (FMi), and pars ventralis (FMv) of n. motorius nervi facialis. A separate group of smaller, multipolar, and spindle-shaped cells (8-25 microns) wedged between n. olivaris superioris (OS) and the caudal end of FMv, rostrally, and extending caudally in ventrolateral medulla were labelled. These small cells contrast with the larger (21-45 microns), oval, round, and multipolar somata of FMv and may correspond, in part, to a parasympathetic n. salivatorius (Sal).

Grasping in the pigeon (Columba livia): final common path mechanisms

A combination of cinematographic and denervation procedures were used to analyse the mechanisms involved in the adjustment of gape size during grasping in the pigeon. Gape size was found to vary directly with seed size and to reflect the operation of two variables, jaw opening velocity and jaw opening duration. Effects upon duration are mediated, indirectly, by the effect of seed size upon head height, which, in turn, controls the velocity of head descent. The data suggest that the control of gape during grasping may involve two different effector systems (jaw muscles, neck muscles). Analysis of the displacement of individual jaws (maxilla, mandible) during grasping indicates that both opener muscles take part in the control of gape. Denervation experiments (motor nerve section) identified these opener motoneurons as contributors to the final common path for the opening phase of grasping. A comparison of the kinematics of pecking/grasping in pigeons and reaching/grasping in humans reveals a number of similarities in the topography and spatiotemporal organization of these behaviors.

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.

Grasping in the pigeon: mechanisms of motor control

A quantitative analysis of grasping in the pigeon suggests important functional similarities between the visuomotor controls of the avian beak and the primate hand. Beak-opening (gape) during eating is directly proportional to target size and the adjustment is completed prior to contact. The control of gape size involves variations in both the velocity and duration of jaw opening and these parameters are mediated by different effector systems (jaw muscles, neck muscles). Nerve section experiments were used to identify jaw motoneurons which are components of the final common path for grasping. Grasping in the pigeon approximates the functional complexity of mammalian visuomotor behavior but is mediated by a relatively simple effector system.

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.

A trigeminal sensorimotor circuit for pecking, grasping and feeding in the pigeon (Columba livia)

A combination of autoradiographic and histochemical tracing procedures was used to identify telencephalic structures, linking the sensory and motor components of the trigeminal system in the pigeon. In addition to the nucleus basalis, these structures include trigeminal projection areas in the frontal and caudal neostriatum both of which project upon the intermediate archistriatum . Archistriatal output reaches premotor areas in the lateral (parvocellular) reticular formation via a descending pathway, the occipitomesencephalic tract. The trigeminal sensorimotor circuit defined in these experiments has been linked, by neurobehavioral studies, to the control of pecking, grasping and feeding in the pigeon.

An HRP study of the brainstem afferents to the accessory abducens region and dorsolateral pons in rabbit: implications for the conditioned nictitating membrane response

Brain projections to the accessory abducens region and dorsolateral pons were investigated in rabbit using implants of crystalline horseradish peroxidase (HRP). Following implantation of HRP in the accessory abducens region (N = 3), labeled cells were observed in the sensory trigeminal nuclei and other regions implicated in the reflex pathway of the defensive nictitating membrane (NM) response. Neurons in the supratrigeminal zone were also labeled, as were portions of the contralateral red nucleus. Implantation of HRP into the dorsolateral pons (N = 5) revealed ipsilateral projections from deep-cerebellar nuclei in some cases. In addition, the parvocellular reticular formation displayed bilateral labeling of cells and an ipsilateral network of fibers and apparent terminations. Many cells of the contralateral supratrigeminal zone were labeled in these cases. Results were discussed in relation to lesioning and electrophysiological studies implicating the supratrigeminal region and other structures in the control of the classically conditioned NM response. Specifically, the possibility that supratrigeminal neurons are premotor elements responsible for the conditioned response is considered. Alternative hypotheses are discussed, including pathways by which cerebellar nuclei could control conditioned responding.

Exteroceptive and proprioceptive afferents of the trigeminal and facial motor nuclei in the mallard (Anas platyrhynchos L.)

Central pathways converging upon the trigeminofacial motor nuclei of the mallard were studied in order to elucidate neuroanatomically the presumed influence of primary sensory trigeminal afferents upon jaw muscle activity. The techniques used included the Fink-Heimer I method after lesions, and axonal transport labeling following injections of 3H-leucine or of HRP for retrograde identification of the neurons of origin. A general description is given of the trigeminofacial motor complex. Jaw closer muscles are innervated by trigeminal motor neurons, and facial motor neurons innervate the jaw depressor muscles. Two afferents premotor systems, one including the mesencephalic trigeminal nucleus (MesV) and the other the rhombencephalic reticular formation, are distinguished. The proprioceptive neurons of the mesencephalic trigeminal nucleus project upon the ipsilateral trigeminal motor nucleus and upon the nucleus supratrigeminalis. The latter cell group bilaterally projects upon the dorsal and intermediate parts of the facial motor nucleus and upon the dorsal and intermediate parts of the facial motor nucleus and upon part of the trigeminal motor nucleus. Exteroceptive information, relayed through the primary sensory trigeminal column (PrV and nTTD), ultimately reaches the motor nuclei via the reticular formation. The reticular formation forms the final link of three separate circuits: a telencephalic one entered through the principal trigeminal sensory nucleus, a cerebellar one via subnucleus oralis of the descending trigeminal system, and a direct one via subnucleus interpolaris. No direct connections between the principal trigeminal sensory nucleus or subnuclei of the descending trigeminal system and the motor nuclei of the trigeminal (NV) and facial (NVII) nerves have been observed, nor are such direct projections present in the outflow of the presumed telencephalic and cerebellar circuits, viz. of the archistriatum and the central cerebellar nuclei, respectively. The archistriatum projects via the occipitomesencephalic tract upon the lateral rhombencephalic reticular formation as far down as the rostral cervical cord, as well as upon the subnucleus interpolaris of the descending trigeminal system. Similarly, efferents from the central cerebellar nuclei reach the reticular formation, which in turn projects bilaterally upon the motor nuclei. Finally, commissural intermotor connections apparently are mediated by reticular cells surrounding the motor nuclei of NV or NVII, rather than emanating from these nuclei directly.